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CCS for 6SN7 driver tube

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Posted on June 20, 2016 at 12:15:36
DAK
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 Hi all, i am building a class A push pull kt88 tube amp which consists of 2 SE 6sn7 x kt88 amps into an EICO HF87 output transformer. Input signal will be split with a Lundahl tranny and the positive and negative signal will be sent to each SE amp.
I would like to make sure that each half of the 6sn7 operates as linearly as possible and I am trying to maximize gain from the 6sn7. I believe that a CCS will help achieve that. If you have any ccs circuits that i can try for the 6sn7 it would be great to see it. So any suggestions on this situation would be most appreciated. cheers, Dak

 

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RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 12:38:58
DAK
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 I should have mentioned that the 6sn7 operating condition will be 320v B+ into a l00k plate resistor with the bias resistor of 390 ohms. There will also be a 100k plate to plate RH type or Schade feedback resistor from the kt88 to the driver tube. regards, Dak

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 13:14:54
sideliner
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 With what you're proposing to do, the Interstage transformer will be used in a parafeed arrangement which puts a cap in the signal cap. Also, it will be difficult to apply Schade feedback across the IT due to phase rotation issues (though you could probably add some local feedback between primary of IT and driver plate). If you were to just use the IT in 'series feed' as is usually done, then I would also suggest to look into a tube that is better suited for transformer drive, one with lower output impedance than the SN7.

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 13:18:11
DAK
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 Hi i am not using an interstage trans just an input trans to split the signal.

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 13:38:01
sideliner
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 Sorry, my bad, I misunderstood. I guess the simplest CCS solution would be the IXYS 10M45. Another option could be two DN2540 mosfets in cascode and yet another option is the gyrator. Sorry but I don't have any specific circuits to provide you with. If you go the resistive loading option, Schade feedback would not be advisable since it lowers the input impedance of the driven tube and loads down the driver. Schade works well with pentodes since they act as current sources.

 

RE: can you show me the IXYS 10M45 CCS? nt, posted on June 20, 2016 at 13:55:10
DAK
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 .

 

Here's one (two in cascode) loading a 6n6p, posted on June 20, 2016 at 14:12:31
Tre'
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If you want to use one with a diff amp, with plate resistors, for push pull you might want to use one in the common cathode (as current sink) instead of the resistor.

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Here it is, posted on June 20, 2016 at 14:15:53
sideliner
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 Here's the manufacturer's data sheet

http://ixdev.ixys.com/DataSheet/98704.pdf

Fig. 2 from the data sheet is what you will use to find the current sense resistor for a given current through the 6SN7. For example if you decide on 8 mA as the standing current for the driver, you'd use a 360 Ohm resistor on top of the tube.

Here's a design implementation as seen in Tubelab's SSE

http://www.tubelab.com/images/AssemblyManualSimpleSE/Simple_SE_Amp_Sch_11-08.jpg

Hope this helps.

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 14:19:29
Michael Samra
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 It sounds as tho you are going to use one 6SN7 per output tube as a driver/voltage amp for each KT88 and you are going to split the phase with the Lundahl.Using both sides of each 6SN7 with say a 10M45 will definitely give you more voltage gain,but I'm not sure it will give you a enough.Keep in mind that you will have some losses thru the PS transformer because there is no gain before you split the phase and then you are asking a pair of low mu triodes to handle these duties.Now I see you mentioned in the post below that you are using 100K plate loads and that is set up for high gain..Were you planning on paralleling the triodes in each 6SN7,or using one triode for voltage gain and the other as a driver? I think you could do it the latter way.


"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Here's one (two in cascode) loading a 6n6p, posted on June 20, 2016 at 14:45:25
sideliner
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 Correct, putting a CCS at the tail could be a good idea (although some would disagree) as it will force operation into class A for the driver and make this amplification stage a 'series' P-P amplifier.

Here's a cascoded pair of DN2540 mosfets for use at either the plates or the tail.





 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 15:07:21
JKT
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 You may not want to hear this, but in my experience any imbalance, no matter how small, in the two halves of a balanced gain stage will "muddy" the sound. Using SE "halves" there will always be gain differences. No two tubes are identical. The only way in my experience to enforce AC balance is with a differentail pair and the stiffest impedance CCS "tail" you can implement. Also, with the SE approach you have two AC current loops per stage. Each loop takes the signal through the PS (AC current has to flow from the plate to back to the cathode) where it can acquire PS colorations. Also add in the cathode bypass caps. With a differential stage the AC signal current flows from plate to plate and cathode to cathode. One AC current loop with no reference to the PS or ground. AC balance in the two halves is enforced by the high "tail" impedance. Sonically any old CCS will not cut it here. Most of the sonic gains in my system over the years have come from finding better "tail"constant current sinks.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 16:58:19
Michael Samra
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 You may not want to hear this, but in my experience any imbalance, no matter how small, in the two halves of a balanced gain stage will "muddy" the sound. Using SE "halves" there will always be gain differences. No two tubes are identical. The only way in my experience to enforce AC balance is with a differentail pair and the stiffest impedance CCS "tail" you can implement.

I personally would put the voltage gain before PS trafo but having two separate tubes driving the pos and neg of the sine wave will not help with a CCS unless he somehow ties the plates together to same load resistor on the different tubes.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Very Interesting, but..., posted on June 20, 2016 at 16:58:39
DAK
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 No offense gents, I am looking for a tried implementation on a 6sn7 driver tube. I don't want to play with it as i am not all that comfortable using SS devices and invariably hook them up wrong. regards, Dak

 

RE: Here's one (two in cascode) loading a 6n6p, posted on June 20, 2016 at 17:01:50
Michael Samra
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 V1 is a 6N6P? It looks like a pentode in that schematic.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Very Interesting, but..., posted on June 20, 2016 at 17:04:56
Michael Samra
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 Dak
How are you going to implement each 6SN7? Are you to use one of the triodes as a voltage amp and the other as a driver for each tube? You have to make up for the voltage gain you are losing in the earlier stage but not having any.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 17:08:24
DAK
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 Yes, i just got through reading a piece about that. But he was writing about phase splitters. Anyway, i am going to try and see what will happen with using 2 SE amps. I can get very closely matching power tubes to minimize the gain difference. Also i will be using fixed bias so there is not the cathode bias issue you referred to.

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 17:17:42
DAK
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 I was planning to use 1 half of each 6sn7 to drive each power tube. I realize that the 6sn7 has low gain and i could reconfigure the amp with a 6sl7 for more gain.
I have made this SE amp and really like the sound so i had the "clever" idea of making a PP version using the input transformer splitter.

 

RE: Very Interesting, but..., posted on June 20, 2016 at 17:24:25
DAK
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 I am planning to use 1/2 of each 6sn7 for the driver so it will be a very simple 2 stage amplifier. I can also configure it with s 6sl7 driver tube.

 

RE: I sense a lot of head scratching goin on, posted on June 20, 2016 at 17:43:37
DAK
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 Ok. let me back up a little. I built a very nice sounding "RH KT88" single ended stereo amp which uses a single 6sn7 or 6sl7 to drive the 88. So, instead of make a PSE amp to get more power what if i made it push pull and also get some second harmonic cancellation and maybe that will be an interesting and hopefully very nice sounding amp too. Also, i will get to use some HF87 outputs that had just been door stops, too boot. So my intentions are not to get more power but just to try this amp design. I know there must be some kinks to work out but hopefully not insurmountable ones. I have already made the power supply, which is on a seperate chassis, and about halfway through the amplifier section. So, a concern has been to equalize the gain of each single ended stage. thanx for the help, Dak

 

The 6n6p was in my example., posted on June 20, 2016 at 20:25:46
Tre'
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RE: I sense a lot of head scratching goin on, posted on June 20, 2016 at 20:37:10
Tre'
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 "So, a concern has been to equalize the gain of each single ended stage."

A circuit that consists of two triodes with their plate resistors connected to the same power supply, with one common cathode resistor, with each grid being driven by out of phase signal is a differential amp (diff amp) and it's one differential stage not two single ended stages.

The common cathode resistor causes corrective feedback so the gain in one half is being forced to be the same as the gain in the other. (the feedback at the top of the cathode resistor is negative feedback for one tube, while at the same time, positive feedback for the other)

A CCS (constant current sink), instead of a cathode resistor, improves this action.

Both triodes are forced to operate only in Class A and they are forced to have equal gain (one triode vs. the other).

BTW, in your original post you said linearity and now you say gain.

Gain is just gain, example.....1 volt in.....10 volts out equals a gain of 10, etc.

Linearity has to do with harmonic distortion. A very linear tube operating at a very linear operating point (voltage, current and load impedance) will have very low harmonic distortion.

A constant current source in the plate circuit of a SE stage can help with linearity and gain.

A constant current sink in the tail (the cathode) of a diff amp can help with linearity and balance (matching the gain of each triode in the diff amp). The gain of the diff amp, as a whole, will be determined by the value of the plate resistors and the tube chosen.

Tre'
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RE: I sense a lot of head scratching goin on, posted on June 20, 2016 at 20:46:11
DAK
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 Hi Tre, thank you for stating that information. It sounds like i am the right track at least as far as concept is concerned. I have all the parameters of the differential amp set and just need some advice on the CCS, sink, for the cathode of the driver tube. If you have any suggestions there i would appreciate the contribution. I have used the LM317 for bias of power tubes but i believe that IC is not in the operating range for the cathode of a 6sn7. cheers, Dak

 

RE: I sense a lot of head scratching goin on, posted on June 20, 2016 at 20:58:21
Tre'
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 How much current will your diff amp be drawing? How much bias voltage will there be at the cathode?

It is possible (probable) that you will need a negative supply under the CCS.

Depletion mode MosFets sound much better with more voltage across them.

Without the negative supply the MosFets will have to work with only the bias voltage.

BTW, I edited my post above adding more info.

Tre'


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RE: I dont think this will work for me, posted on June 20, 2016 at 20:58:37
DAK
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 The schematic i am following has plate to plate fb from the power tube to the driver. I am using a 100k resistor for that. I dont know if that will mess up the operation of this design.

 

RE: I dont think this will work for me, posted on June 20, 2016 at 21:02:36
Tre'
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 Can you post your schematic?

Tre'
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RE: I sense a lot of head scratching goin on, posted on June 20, 2016 at 21:12:25
DAK
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 hi Tre, the schematic is here;
http://rh-amps.blogspot.com/2013/05/rh-universal-v2-totally-universal.html

The 6sn7 plugs into the 12at7 spot with the proper pin correction and using the same operating points with the same resistors. The coupling cap i will be increasing to .5uf. I am also using negative voltage on the power tube grid for fixed bias operation.

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 21:52:33
Michael Samra
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 It's too bad they don't have an octal version of a 7247 which is half of a 12AX7 and half of a 12AU7..You need to swing at least 100v of signal to drive the KT88s to full power.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 20, 2016 at 23:03:40
DAK
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 According to AK simulations the amp puts out 12 watts per channel. Which is decent. Really that is plenty for my speakers and the size of my room.

 

6EM7..., posted on June 21, 2016 at 08:06:20
Lew
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 ...is half of a 6SL7 plus half of a 6SN7, in one envelope. MFA Luminescence is the only commercial product that ever used them, so far as I know.

 

I have reread your post on this thread..., posted on June 21, 2016 at 08:52:33
Tre'
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 and I don't see any use for a CCS.

It seems to me that you want to build two SE amps as shown (with the exception of fixed bias on the output tubes) and drive one push pull output transformer with them.

There's nothing wrong with that.

(Sorry, I was originally thinking something very different)

"So, a concern has been to equalize the gain of each single ended stage."

If you make one of the 390 ohm cathode resistors adjustable then you could manually set the AC balance.

That's the best idea I can come up with.

Tre'
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RE: I have reread your post on this thread..., posted on June 21, 2016 at 09:42:27
DAK
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 Hi Tre, and thank you for checking my ideas. One of my concerns was that the input signal since halved for each 6sn7 section would be such a small signal to drive that the tube would be in a bad operating point. Although i have several preamps to drive the amplifier.
Going to an adjustable resistor for one of the cathodes to equalize the AC portion of the drive signal, how would one measure that signal? thank you for your help , regards, DAk

 

RE: CCS for 6SN7 driver tube, posted on June 21, 2016 at 12:09:25
6AS7_6SN7






I would recommend LM 334Z economy current source.
By a 11.5 Ohm resistor you can get a fair 6 mA current in the tail of a differential driver.



 

RE: I have reread your post on this thread..., posted on June 21, 2016 at 14:02:51
Tre'
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 You should have enough gain if you have a preamp.

Like you said, your not needing a great deal of power. You're just going for good sound.




You would measure distortion. Adjust for minimum distortion.

Or, with a signal generator feeding the preamp, measure the AC voltage at each 6sn7 plate (between the plate and ground) and adjust until they are equal.

Tre'
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RE: CCS for 6SN7 driver tube, posted on June 21, 2016 at 14:06:13
Tre'
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 He's not interested in building a diff amp driver.

He wants to try 2 SE amps, bridged at the plates of the output tubes with a push pull output transformer.

I think it's an interesting idea and should turn out to be educational.

Tre'
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RE: CCS for 6SN7 driver tube, posted on June 21, 2016 at 18:29:05
Michael Samra
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 Now keep in mind that the HF=87 output transformers are in the 5k range where the typical KT88 runs at 3.3k to 3.6k. That will explain part of the lower power rating of the circuit especially if you running below 430v B+.
If that's all you are looking to get out of the amp which is 12 watts,your implementation would be fine.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: 6EM7..., posted on June 21, 2016 at 18:31:42
Michael Samra
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 Yes
I forgot about that.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 21, 2016 at 18:43:53
Michael Samra
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 DAK
That link you posted in response to Tre is a SE amp and my understanding is you are building a PP amp with a SE drivers,one to drive the positive half of the cycle and one to drive the negative half of the cycle..Now you said you would one half of each of the 6SN7 driver tubes.I would use one triode of each 6SN7 for voltage gain and the other half as a voltage follower driver.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 21, 2016 at 21:10:26
DAK
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 Right I want to have the 6sn7 operating at its most linear with high output since it will be the only driver, even at the expense of longevity. I thought maybe a CCS on the cathode would be able to help that situation.

 

RE: CCS for 6SN7 driver tube, posted on June 22, 2016 at 03:15:34
6AS7_6SN7



You should read the Morgan Jones book first.
This is an excerpt of page 170 (specifically devoted to 6SN7 tube).
You can bet that differential stage with CCS in the tail is the most linear topology with 99% probability!

 

RE: CCS for 6SN7 driver tube, posted on June 22, 2016 at 09:05:12
Tre'
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 A CCS in the cathode of a single ended 6sn7 stage will lower that stages gain.

The CCS presents a large impedance.

The larger the impedance in the cathode, the greater the amount of degenerative feedback.

Note, a CCS as a plate load will present the highest load impedance to the tube and the tube will work into a horizontal load line and the stage's gain will be at it's theoretical max.

Tre'
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RE: What is a differential driver? , posted on June 22, 2016 at 09:33:00
DAK
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 I thought i new what you were referring to but now, i am not sure.

 

RE: I have reread your post on this thread..., posted on June 22, 2016 at 12:36:26
DAK
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 hI, Tre, so i put a pot on one of the cathodes instead of the a bias resistor, then i can adjust the pot while measuring the AC on the plate and adjust the pot to get equal ACV? Sounds easy enough. cheers, Dak

 

Caution is Advised..., posted on June 22, 2016 at 13:12:51
Triode_Kingdom
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 I'll never believe that inserting a noisy, three-terminal regulator at the cathodes won't make this amp sound like garbage. Every bit of SS sonics from that IC will be directly impressed on the grids.

 

RE: I have reread your post on this thread..., posted on June 22, 2016 at 16:33:48
Tre'
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 Yes, wire the pot as a variable resistor. Start with it adjusted to 390 ohms and measure the AC at each plate. Adjust until the 2 plates AC voltages are equal.

Tre'
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RE: Caution is Advised..., posted on June 23, 2016 at 09:50:22
DAK
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 Hi TK, it looks like for now, i will just be using the cathode resistor, unbypassed, and get the amp up and running first. It seems like a sensible procedure before augmenting the circuit. cheers, Dak

 

RE: I got a question..., posted on June 23, 2016 at 14:40:14
DAK
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 what if the pot i use can not get the AC signal to equalize? I am thinking that the pot has to be a larger value than the bias resistor. Should its value be twice as much as the resistor or ??
Also i calculate the approximate current dissipation in the 6sn7 using the voltage and resistor values in the article by AK. I calulated that to be around 4ma. Would it be better to operate the driver tube at a little higher current? regards.

 

RE: I got a question..., posted on June 23, 2016 at 15:03:22
Tre'
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 The pot should be larger than 390 so you can reach above and below that number.

A 6sn7 doesn't get real linear until about 8ma.

It's amazingly linear at about 12ma.

Take a look at the curves in the pdf linked below. Bottom graph, page 4.

Look at 14ma. 200 volts plate. No tube gets much better than that.

The problem is, to keep the load line reasonably horizontal you need a large plate resistor (100k) but with a large plate resistor, and that much current flowing, the voltage drop across the plate resistor will be very large.

In that case you would need a lot (like 1500 vdc) of B+.

This is where a CCS plate load comes in. You can have a horizontal load line (that will give very low distortion and the most gain) and not need an abnormal amount of B+.

Tre'
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RE: I got a question..., posted on June 23, 2016 at 15:38:05
DAK
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 So, the current measured at the cathode which i calculated to be about 4 ma is the current you are referring to and calculated in the same way? Or is it the total current consumption of the tube including the plate current? I am not comfortable working around the "theory" side of amp construction and using maths to find your operating point.

 

RE: What about "characteristics and typical operation", posted on June 23, 2016 at 16:23:08
DAK
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 From the GE tube manual, for the 6sn7gta,gtb, Class A amplifier, each section, at 250 plate voltage, -8 grid volts, plate current is 9 ma. I realize that is recommended operating point. But, how far can one stray from there? As i said i have no experience working from the design side and am unsure of how much i can massage the resistor and voltage values around the already designed amplifier.

 

RE: I got a question..., posted on June 23, 2016 at 16:27:20
Tre'
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 Yes, the cathode current and the plate current are the same thing in a triode.

In a pentode you have the current through the plate and the current through the screen grid. So the cathode current would include both.

With a triode the only current path is the plate, that is to say, the current that flows up from ground to the cathode and then across the vacuum to the plate and then to the power supply and then back to ground.

If we allow the control grid to become positive WRT the cathode then we would also have current flowing out of the control grid but this is not something we would normally do.

I know you said that you're not comfortable working around the "theory" side but I would encourage you to learn theory.

IMO this hobby is soo much more fun when you start understand what's really happening with these circuits.

I think the real beginning of understanding is to understand that electronics (and electrons) are real and physical.

When I look at a schematic I can "see" where the electrons and electronic charges are going.

Tre'
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RE: What about "characteristics and typical operation", posted on June 23, 2016 at 16:32:17
Tre'
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 Once you start changing (massaging) resistor and voltage values the design is no longer the design, it's now your design.

As far as the tube it self, you can do anything you want as long as you don't exceed the limits.

Those limits are given on page 2 "maximum ratings".

Tre'
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RE: Another thing to consider...., posted on June 23, 2016 at 17:19:41
DAK
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 is the plate to plate feedback resistor. For the 6sn7 it was specified at 100k ohms from the anode of the kt88 to the 6sn7 anode. How does this feedback resistor affect the operating point of the 6sn7, and the kt88 when i change the plate resistor of the 6sn7 to a smaller value, i am going to start at around 30k.

 

RE: Another thing to consider...., posted on June 23, 2016 at 20:22:28
Michael Samra
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 Dak
Answer a question.Were you planning on using one triode in each 6SN7 for voltage gain while the using the 2nd half as a follower?.If you CCS load one section of the 6SN7,you will get a gain of 20.I'd run it at about 8ma to 10ma.

I know you only want to get 12 watts but you still want it to be very linear and as far as operating points go,that is more affected by the DC voltage settings than the AC signal. Do you have a schematic to that circuit?

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Another thing to consider...., posted on June 23, 2016 at 20:34:58
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010

Hi Mike one section of the 6sn7 will drive the power tube, so, only a 2 stage amp Each 6sn7 will therefore drive the 2 se amps that make up the push pull pair for the output trans. The circuit is located here;
http://rh-amps.blogspot.com/2013/05/rh-universal-v2-totally-universal.html
thank you for your interest.

 

RE: CCS for 6SN7 driver tube, posted on June 23, 2016 at 20:36:45
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Tre
I'm confused on what he is trying to do as I haven't seen a schematic...He said he is using 1/2 of each 6SN7..If he uses it as a CF,he won't get any gain..If he uses the first 1/2 of each 6SN7 as a voltage gain amp and CCS loads the plate,a gain of 20 is no problem.
I realize he is only going for 12 watts and it may have enough drive for that but if sets it up for no voltage gain in the amp,he is going to have to have one hell of a strong preamp considering he will losses thru the PS transformer as well.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

I don't see any circuit with a PP output stage in the link you provided., posted on June 23, 2016 at 21:05:30
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005



I know you want to build a PP amp with the drivers being single ended for half of the cycle to feed each output tube..If you look tho,they are using the 12AU7 as a voltage gain amp driver and I thought you wanted to use a cathode follower driver..Their circuit has gain and this is where I am confused.You need to use half of each 6SN7 as a voltage amp and the other half as a driver..If you paralled the halves of the 6SN7,you are just going to end up with a lower impedance driver or voltage amp,depending on how you use it.
I heard a superb sounding Single Ended KT88 built by a guy named Kegger and it used Edcor iron and it was driving a pair of ADS 780s but it sounded much bigger than the 10 or 12 watts is was rated at.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: I don't see any circuit with a PP output stage in the link you provided., posted on June 24, 2016 at 00:08:47
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 There is no amp as such and i am trying to make one out of 2 of the single ended amps. Phase splitting will be done with a Lundahl input transformer and each mirror imaged signal will be handled by a se amp.
I have built 2 of these that are kt88 type and they sound excellent. So, i was thinking i would make a parallel single ended version to get the power up to 20 watts. Well, that's been done many times and i thought i have not read about anyone taking a se amp and using it as a basis for a push pull amp. So this is where i am now.

 

Errr...6EM7 correction, posted on June 24, 2016 at 05:29:59
PakProtector
Audiophile

Posts: 12364
Joined: May 14, 2002
 The high mu side of the 6EM7 is very like a 6SL7 half. The low mu side is quite different from a 6SN7 section. Its gm is `3x the 6SN7, mu about a quarter, and it is quite capable of use as the power final element.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: CCS for 6SN7 driver tube, posted on June 24, 2016 at 08:00:12
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 My post above has nothing to do with what DAK is trying to do.

I was just adding some info.

DAK wants to build an amplifier that consists of 2 independent SE amps (with a input transformer as a phase inverter at the inputs) bridged at the plates of the output tubes using a push pull OPT.

At least that's what he originally wanted to do, now I'm not so sure. :-)

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: CCS for 6SN7 driver tube, posted on June 24, 2016 at 09:59:41
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 My post above has nothing to do with what DAK is trying to do.

I realize that but I wanted a clean place to jump in and address this with the back and forth..After I made this post he responded to my post below. I know the about the separate SE drivers he wants to use for each half of the cycle for the PP output stage...I just have my doubts about not having any voltage gain going into the CF or whatever he plans on using. If he uses one triode of the 6SN7 for voltage gain,he can definitely DC couple the cathode of the second triode to each KT88.I would love to see a schematic of this thing and I can't find it in the link.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 24, 2016 at 13:09:07
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002

We don't need a CF to drive a KT88 in pentode mode, do we?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: CCS for 6SN7 driver tube, posted on June 24, 2016 at 14:07:51
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 We don't need a CF to drive a KT88 in pentode mode, do we?

Not in pentode but we are going to need some local FB. He wants a very linear output stage and I just don't get using a pair of KT88s in PP for a measly 12 watts..If you only need 12 watts,use the most linear pentodes out there which are the 6V6 and EL84 tube types.He will get all the power he needs and they are both easier to drive than a KT88..
That output circuit is SE in the schematic and he is talking a PP output circuit driven by independent tubes in SE fashion for each half of the cycle.He is going to have to implement AC balance so one driver tube doesn't become more dominant than the other because the top and bottom halves of the sinewave have to be pretty even I would guess especially in this type circuit with limited power.
I think he would be much better off using one CCS plate loaded 6SN7 to drive each tube but use the first half for a voltage amp and the second half for a CF DC coupled driver..This is if he uses KT88s.If he uses 6V6s or EL84s,a basic voltage amp/cap coupled driver would be fine.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: CCS for 6SN7 driver tube, posted on June 24, 2016 at 16:18:49
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 A lot of arguments could be made against what he is proposing.

(2 SE amps driven out of phase by means of a input transformer and coupled at the output tubes with a push pull transformer)

But it's what he wants to try.........

BTW the circuit, as drawn, has feedback. Rfb1 and Rfb2 on the schematic.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: But won't he need four SE amps?, posted on June 25, 2016 at 11:49:04
Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001
 A typical PP amp might have a voltage amp coupled to inverter/splitter coupled to output tubes. One could have the inverter/splitter first if desired (transformer or something else).

I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer. Once the SE output transformers are removed (and you have no air gap and core with DC current) you no longer have anything that is "SE" or different from many standard push-pull amps (other than the input transformer doing the splitting).

 

RE: But won't he need four SE amps?, posted on June 25, 2016 at 22:38:34
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
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 "But won't he need four SE amps?"

Yes.

"I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer."

I understand what you mean.

It's just that what he wants to do is not a "normal" push pull amp.

A normal push pull amp would have a diff amp for the output stage and the driver stage. He doesn't want to do that. He wants two SE amps coupled at the input with a input transformer and at the output with a push pull output transformer.

And yes, in the end it's a push pull amp but a very different way of getting there.

Why he wants to do it that way, I have no idea.

I think the results might be interesting.

That's all, nothing more, nothing less.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

But won't he need four SE amps?, posted on June 25, 2016 at 22:57:27
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
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 That's where so much of confusion comes in..He will need two separate 6sn7s for each channel and he is splitting the phase via transformer in the beginning with NO voltage amplification before or after the PS trafo.
I think the best way for him to do this is to CCS plate load a 6SN7 for each tube. He already split the phase with the trafo and now just use the first half of each 6SN7 as a voltage amp and the second half as a DC coupled follower to drive each of the output tubes..He will have a voltage gain of 20 but if he only wants to drive the KT88s to 12 watts,he will probably have enough gain but can't be 100% sure.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: But won't he need four SE amps?, posted on June 27, 2016 at 06:05:35
Triode_Kingdom
Audiophile

Posts: 10047
Location: Central Texas
Joined: September 24, 2006
 "I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer."

Yes, there's a conceptual error in all this. It makes no sense to me either. Seems somewhat akin to building a bicycle by connecting two unicycles together.

 

RE: But won't he need four SE amps?, posted on June 27, 2016 at 10:35:03
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 "Seems somewhat akin to building a bicycle by connecting two unicycles together."

That's a good analogy of what he wants to do.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

Did anyone suggest that you arrange the two sections of the 6SN7s differentailly?, posted on June 28, 2016 at 14:06:07
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 This will work very well for driving the output tubes and will help insure good performance, likely with less distortion if you set it up right. In fact if you did it this way you might not need the transformer at all, but of course to do that the CCS would have to have excellent performance. That is the key to getting differential circuits to work right.

The cathode circuit of the two 6SN7s would then be your high quality CCS.

 

RE: Did anyone suggest that you arrange the two sections of the 6SN7s differentailly?, posted on June 28, 2016 at 15:48:28
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Ralph, that was suggested early on but the amp schematic i will use has "Schade" feedback which is from the anode of the power tube to the anode of the driver tube. I am not sure how that would work with a differential amplifier. Part of my planning is to not use a tube phase splitter. cheers, Dak

 

No matter how you cut it, phase split happens, posted on June 29, 2016 at 07:54:46
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 How a differential amplifier works is one tube section is forced to do the opposite of the other due to the cathodes being coupled together.

As 'phase splitters' go its a pretty elegant approach and does not have the sonic signature associated with most other phase splitter circuits. If you were to drive each half of the differential amplifier with your transformer the result will be lower distortion than if you drove the 6SN7 sections each single-ended.

 

RE: can you show me an example from a schematic?, posted on June 29, 2016 at 10:48:36
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Did you check out the schematic of the amp i will be copying? The amplifier section will be proceeded by a Lundahl phase splitting input transformer. In my amp i will be using a 6sn7 or 6sl7 for the driver tube. I don't think you can get it any simpler or more efficient than that. cheers, Dak

 

RE: can you show me an example from a schematic?, posted on June 29, 2016 at 13:27:49
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Set up the way you are describing probably not. I could draw something up though.

 

RE: CCS for 6SN7 driver tube, posted on October 7, 2016 at 14:53:12
pmaneri@samsontech.com
Manufacturer

Posts: 152
Location: Long Island,NY
Joined: May 22, 2015
 DAK - Have you looked at Max Robinson's site, www.funwithtubes.com yet? He has a section on building amplifier sections and I found a great differential pair using 12AX7's with a 2N3904 CCS on the cathodes. It works very well. Low distortion, perfect symmetry and balance between the sections. If you can't bear to use a transistor, he also has a circuit that uses a pentode for the CCS.
It's almost impossible to use a three term. reg. because the Iq (quiescent current) is at a min. 5 ma. My circuit only required 3 ma total. Even the TO-92 package has too much Iq for low current designs. -Pete

 

RE: CCS for 6SN7 driver tube, posted on October 24, 2016 at 12:27:14
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Pete, i hope you get to read this. i have not gone back to this thread for awhile as i finished my amp a few months ago and i am just listening to it to let its sonic signature sink in.
In terms of the "nuts and bolts" the amp is a success. Sonically, it sounds very good as well with more detail than the SE RH amp that the PP amp is based on.
Thank you for the tip on Max Robinson, i love that guy! He is like the best forum moderator, ever. He is always helpful and encouraging. cheers, Dak

 

RE: 6EM7..., posted on September 28, 2018 at 09:42:30
caligari
Audiophile

Posts: 415
Joined: March 20, 2008
 6EM7 is more like half 6SL7 and half 6BX7 with lower mu.
6DN7 is more like half 6SN7 and half 6BL7.

 

RE: CCS for 6SN7 driver tube, posted on September 28, 2018 at 15:55:36
fiacono@optusnet.com.au
Audiophile

Posts: 31
Location: Melbourne, Australia
Joined: May 21, 2003
 It sounds OK but I would go with cascode CCS, they sound much better. Only down side is the required -ve supply voltage.
Frank

 

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