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Driving 6Y6GTs with El Cheapo 12AT7 front end?

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Posted on April 25, 2013 at 10:47:34
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 I've acquired some 6y6GTs that I'd like to try in push-pull. I have read that tubes with low plate resistance and high-ish transconductance are easier to drive than some others. Eli mentioned in a post elsewhere that the 12AT7 with CCS used in the El Cheapo amp could, for example drive the 7591A in push-pull.

The 6y6GT at the operating point I'm considering has a plate resistance of around 18,300 Ohms and a transconductance of around 7,100 uMhos.

Interestingly, an old Acrosound flyer shows a circuit labeled for 6V6/6Y6/EL84 driven by a 12AX7. I have heard that the 12AX7 may run into issues when trying to drive more difficult output tubes, so I'd like to learn what would make a better choice.

Other circuits I have found use 6sn7, 12sn7, 12BZ7 or 6eu7 + 5687 as splitter/drivers, some using them in more than one stage.

Would the El Cheapo 12AT7 CCS splitter/driver work okay with the 6y6GT? If not, what might work better?

Thanks for any insight,
Jeff

 

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CCSs fix a whole host of problems. 12AT7s with CCS will be more than up to the task., posted on April 25, 2013 at 12:02:45
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 I built a little PP 6V6GT that I love and use everyday for TV sound. The wife and kids use that tube amp every day, not many people can make a similar claim.

 

RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?, posted on April 25, 2013 at 17:20:21
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Jeff,

Take a look at the 6Y6 Data sheet. Notice the low max. plate voltage limit. That does not mesh at all well with the 12AT7 LTP front end found in "El Cheapo".

I take it you want to do another PP amp. Consider a CCS loaded 6922 section voltage amplifier DC coupled to an IRFBC20 "concertina" phase splitter. Use "fixed" bias "finals", with an individual trim pot for each tube. Bridge rectify the O/P of your Triad N-77U to get the B+. CLC filter. The rail will come in at approx. 165 V. and you'll have way more current than you can use. :>D Go full pentode mode and regulate g2 B+ with a VR105. LED bias the voltage gain triode and follow the "textbook" O/P trafo to voltage amplifier cathode GNFB setup.

Edcor's CXPP30-MS-5K seems about right, assuming "normal" Class "AB1" operation is chosen for the O/P stage.


Eli D.

 

RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?, posted on April 25, 2013 at 19:03:17
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Eli,

Okay, more complex than I expected. I could just drop a BUNCH of voltage to the finals and use the 12AT7s, but somehow I don't think you would agree that such a notion makes sense. And now I get to learn some new stuff. Thanks, I think? :-)

May I get some clarification just to be sure I'm following you? Are you describing each section of a 6922 (one section per channel) to be ANODE-loaded with a CCS and LED biased? Then the anode at the tube goes straight into the gate of a MOSFET IRFBC20 with no capacitor coupling? Perhaps a stopper resistor?

Also, I have a lot of Amperex 6DJ8s. Might they be up to the task? Frugality would seem to warrant using up some of my tubes that I already have, but I know your penchant for good engineering.

Finally, Is this the kind of splitter you are describing?





One more question: How do I figure a good value to use for the source and drain load resistors on the splitter? I found a source talking about tube cathodynes, but not much on the values for MOSFETs.

Thanks for any further help you can offer!

--Jeff

 

RE: CCSs fix a whole host of problems. 12AT7s with CCS will be more than up to the task., posted on April 25, 2013 at 19:04:32
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Hi, Chip647,

How are you driving your 6Y6 pairs?

Thanks,
Jeff

 

RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?, posted on April 25, 2013 at 20:48:01
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Pentodes in pentode mode don't have Miller capacitance, so their input capacitance is small and it does not take much current in the driver to drive them to clipping. This is not the case in ultralinear or in triode mode.

However, if you need to drive them into grid current on transient peaks (WAY more common than you'd think!) the above kind of goes out the window and the analysis gets all messy and nonlinear - very difficult to generalize.

Triode drivers with high gain need high plate voltage in order to avoid drawing grid current in the driver stage. Usually grid current becomes enough to introduce distortion when the bias minus peak signal is less than 1 volt. Bias is normally 50% to 70% of (plate voltage divided by mu), so you can see why high mu needs high voltage.

A final note - the 6DJ8 is specified for 130v maximum quiescent plate voltage. In practice it has a reputation for early failures if that voltage exceeds 80 or 90 volts. That's pretty low - something like a 5670 with similar gain but higher voltage tolerance is probably a better choice.

 

RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?, posted on April 25, 2013 at 21:11:03
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Paul,

Thanks for the info about these principles, especially about bias parameters. I really appreciate hearing about these things.

--Jeff

 

RE: Driving 6Y6GTs with El Cheapo 12AT7 front end?, posted on April 25, 2013 at 22:06:45
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



" I have a lot of Amperex 6DJ8s. Might they be up to the task? Frugality would seem to warrant using up some of my tubes that I already have, but I know your penchant for good engineering."

I have no idea about how tough your Amperex ECC88s are. That really doesn't matter, as they are worth MONEY. Sell some them off, for all that the traffic will bear. :>D

The Russian 6H23П-EB (6n23p-ev) is a very tough hombre, with OK sonics. The Russian variant "laughs" at the conditions Mr. Joppa indicated makes many 6DJ8 family specimens fail. Tubes labeled ElectroHarmonix (EH) 6922 are, actually, 6H23П-EB.

"May I get some clarification just to be sure I'm following you? Are you describing each section of a 6922 (one section per channel) to be ANODE-loaded with a CCS and LED biased? Then the anode at the tube goes straight into the gate of a MOSFET IRFBC20 with no capacitor coupling? Perhaps a stopper resistor?"

100% correct, especially the gate stopper!

"Is this the kind of splitter you are describing?"

Yes, the graphic describes a "concertina", AKA split load, phase splitting setup. "Cathodyne" doesn't quite "make it", when the object of the exercise is a MOSFET. ;>)

"How do I figure a good value to use for the source and drain load resistors on the splitter? I found a source talking about tube cathodynes, but not much on the values for MOSFETs."

It's not too different than working with a tube. In this DC coupled setup, the gate is (of necessity) at the triode's anode voltage. Assume the FET's source is at the same voltage, even though it actually is slightly lower. Pick an arbitrary drain current value. The IRFBC20 I suggested will loaf at 20 mA., while providing huge transconductance and a LOW O/P impedance. Good old Ohm's Law is what you use to crunch the numbers. Make the total source resistance variable and somewhat larger than the drain resistance. Then, you can bring the amplitudes of the 2 phases into "perfect" congruence.


Eli D.

 

Would any of these triodes also work?, posted on April 26, 2013 at 17:13:45
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 I assessed my small collection of tubes and found the following triodes on hand that seem to operate at around 100 to 150 for anode voltage and have medium gain. Would any of these work as well as the 6922?

Triodes/Twin Triodes on hand:

Type     Va      Gain

8532     150     52.5

7062     100/150 50/46 (two sets of values found in the data sheets)

12AV7   150     41

6J6A      100     31
(150V Parallel)

5BK7A   150     45

3GK5     135     78 (higher gain)

6SN7     90-250  20 at 250V (lower gain)

Combination Triode/Pentodes on hand, triode characteristics:

Type      Va    Gain

5AN8     150    21

5GH8     125    46

The transconductance for these may be lower than the 6922. Is that a big factor? I'm willing to go with the 6922 if necessary, but I thought I'd ask about any of these.

Any insights into how these make good or poor choices for this application would be welcomed. I want to learn.

Thanks!

--Jeff

 

RE: Would any of these triodes also work?, posted on April 26, 2013 at 18:08:07
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Jeff,

High transconductance provides resistance to HF error correction signal induced slew limiting, in designs with loop NFB. Pentode mode "finals" need plenty of NFB, to limit distortion and obtain a satisfactory damping factor. We'll stick with the 6DJ8 family, for reasons of high gm and comparatively low anode voltage requirements. ;>)

Take a good look at the 6922 plate curves. I propose to run each triode with a CCS locked IB of 8 mA. and single red LED for bias. The anode voltage is approx. 90, under those conditions. That's a safe, conservative, operating point. Your Amperex ECC88 stock should work well. :>D A high quality, cascode, CCS is needed. A 10M45S is not good enough.

The B+ boost "trick" will be employed. Add a properly phased winding from your VPS24-1800 to each end of the N-77U. The "extra" B+ volts are badly needed. It's NBD if the rail comes in a tad over 200 V. The 6Y6 data sheet anode limit of 200 V. is very conservative design center. Don't "fudge" on the plate dissipation limit, as that IS what leads to early failure.

Heater power for the signal tubes will come from a Hammond 266M6 filament trafo.

The drain load on the IRFBC20 will consist of 2X 1.2 K carbon film parts and a 1.8 K part, wired in series. All 3 resistors are 2 W. rated. The source load consists of a series wired combo of a 1.8 K CF part, a 1.2 K CF part, and a 1.5 K linear taper potentiometer. Some "sticker shock" will occur, when you look the price of a 2 W. rated pot. up.

BTW, the 820 μF./250 WVDC 'lytics on your "pile" will get used too.


Eli D.

 

Grid current, posted on April 26, 2013 at 22:42:04
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 "However, if you need to drive them into grid current on transient peaks (WAY more common than you'd think!) the above kind of goes out the window..."

I've gradually come to believe over the last few years that capacitive coupling to the outputs is a significant handicap. Even if the driver Z isn't sufficiently low to produce a true AB2 or A2 capability, faster recovery time with choke or transformer coupling is an audible improvement.

 

RE: Would any of these triodes also work?, posted on April 27, 2013 at 21:45:46
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Good news! Nebraska Surplus part # (RWV) CTS-45 meets the 2 W./1.5 KOhm pot. need of this project, at "reasonable" cost.


Eli D.

 

B+ PSU Hen Scratches, posted on April 28, 2013 at 00:36:32
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



Uploaded!

You make it purty, Jeff. ;>)


Eli D.

 

Addendum, posted on April 28, 2013 at 01:30:02
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Add a CL-90 inrush current limiting thermistor between the rectifier bridge and the PSU filter.


Eli D.

 

RE: B+ PSU Hen Scratches, posted on April 28, 2013 at 08:48:21
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Hi, Eli,

Thanks for the schematic!

While there are several 1000muH rf chokes available, the few 1500 muH at that level of current capacity appears limited. I found this one @ .68A (see link).

Most of the rest at that inductance are lower current.

Would this work?:



 

RE: B+ PSU Hen Scratches, posted on April 28, 2013 at 09:41:50
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Mouser stock # 542-5900-152-RC is what you need. As part of the Bourns conglomerate, long time coil winder J.W. Miller is still "on the job". :>)

Either UF5408s or the 3 A./600 PIV "GI" diodes McShane uses in Cit. 2 and Cit. 5 amps are fine, for the ground side of the rectifier bridge. 3 A./600 PIV Schottky SiC diodes go on the "hot" side of the bridge. Buy 10 nF. film caps. specifically intended for snubber duty, that are rated for pulses over 1 KV., from Jim.


Eli D.

 

Capacitor Value, posted on April 28, 2013 at 17:50:51
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 I'm very well aware that 820 μF. is overkill, for this application. However, I also know that Jeff has a couple of those "puppies", on his "pile". "Us Cheapskates" DO stick together! :>D


Eli D.

 

Small Signal Hen Scratches, posted on April 28, 2013 at 20:11:16
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



Please do some "lifting", Jeff. ;>) Provide all interested parties with a nice, neat, "professional", graphic.

Add a protective Zener diode between the FET's gate and source electrodes. A 100 Ω CF "stopper" on the 6922's anode is a good idea too. The "hard" sound some people ascribe to the 6922 can be due to parasitic oscillation. If nothing else, the 6DJ8 family is HIGH gm and, therefore, parasitic oscillation prone. The CF plate "stopper" is very low cost "Boogeyman Insurance".

The topology of a voltage amplifier DC coupled to a "concertina" phase splitter is "as old as the hills". Fisher, Scott, Dyna ... used it frequently. We're adding "bells and whistles" to a proven setup. FETs can swing closer to the voltage rail than triodes can and volts are at a premium, in this situation. Not having to deal with heater power and a heater/cathode potential limit is also advantageous.


Eli D.

 

O/P Stage, posted on April 29, 2013 at 08:06:52
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



The bias setup is next.


Eli D.

 

Bias, posted on April 29, 2013 at 12:45:52
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000






Eli D.

 

RE: Bias, posted on April 29, 2013 at 19:01:42
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 These hens cratchings are awesome, Eli! I'll start schematics for this soon, and I'll start a BOM, too. Thanks for all the tips and the sketches.

A few questions:

1. Will much of the El Cheapo LTP CCS topology still work for use with the 6922 as an anode CCS? Would it just stand on "top" of the tube instead of under it? Does it still need any kind of voltage supply? If not, is the "old" B- entry point now the anode connection, and the "old" cathode connection "on top" now the B+ connection? Is the 5KOhm 5W resistor in the B- entry leg still used? Are the bypass capacitors adjacent to that B- entry point still needed?

2. Are the bias trim pots to be added between the bias supply "under" the 47KOhm grid resistors on the OP tubes? Or do the trim pots replace the grid resistors?

3. In targeting ~200VDC on the 6Y6GT plates, would about 210 VDC make sense for a B+ target to allow for the drop in the OPT? At an approximate 60 mA plate current, I would predict a drop of 8 - 10 Volts across each section of the OPT.

4. In the bias supply, your diodes have a curly cathode and look like the Schottky symbol. Is this the type to look for?

5. Is 63VDC okay for the all the 'lytics in the bias multiplier, or is a larger rating needed? Would 35WVDC work for the first two?

Again, thanks,

--Jeff

[Edited wording on questions after looking again at the bias schematic.]

 

RE: Bias, posted on April 29, 2013 at 22:57:18
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



SHIT!! I just lost a set of comprehensive answers to your questions. :>( :>(

I'll try again tomorrow, after work and dinner. In the meantime, I've uploaded the crucial parts of Doug P.'s cascode depletion mode MOSFET CCS setup. The spot labeled LTP cathodes gets connected to B2+. The other end feeds the 6922 anode circuitry. Adjust the trim pot. for 0.8 V. of drop across the test resistor.


Eli D.

 

RE: Bias, posted on April 30, 2013 at 19:43:49
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



"In the bias supply, your diodes have a curly cathode and look like the Schottky symbol. Is this the type to look for?"

Yes, use Schottky diodes, for their "noiseless" character.

"Is 63VDC okay for the all the 'lytics in the bias multiplier, or is a larger rating needed? Would 35WVDC work for the first two?"

Obviously, all caps. rated for 63 WVDC is OK. Cheapskate that I am, 25 WVDC in the 1st stage and 50 WVDC in the 2nd stage get the nod. Use a top quality part, like Nichicon Muze, in the reservoir position. Perhaps the exact part McShane puts into the H/K Cit. 2. Good enough for the "Deuce" is certainly good enough here.

"Are the bias trim pots to be added between the bias supply "under" the 47KOhm grid resistors on the OP tubes? Or do the trim pots replace the grid resistors?"

The grid resistors connect to the wipers of the trim potentiometers. The ends of the trim pots. and, perhaps, some additional resistance form adjustable voltage dividers across the C- supply and ground. Please notice that you start with the tube cut off and back off until the desired "idle" current is measured.

"In targeting ~200VDC on the 6Y6GT plates, would about 210 VDC make sense for a B+ target to allow for the drop in the OPT?"

Wherever the B+ rail comes in will have to do. Stop fretting. Instead of the same old/same old, we'll try real Class "AB1" in this design. My starting point for O/P tube "idle" current is 10 mA.


Eli D.

 

RE: B+ PSU Hen Scratches, posted on May 6, 2013 at 03:32:34
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 In drawing up the B+/B2+ supply, I am faced with the need to supply much less votage to the 6922 than to the 6Y6GTs. This could be done by either (1) using dropping resistors after a split "check valve" like El Cheapo, except with much larger resistors (about 6KOhm) or (2) doing a second supply, taking power off the N-77U but not using the booster transformer that the B+ supply uses.

Is there a preference in which way this is done? Using larger resitors seems wasteful of power, but a second bridge and filters seems like overkill. That said, we will need a screen supply anyway, so it could go either way.

Thanks for any thoughts,
Jeff

 

RE: B+ PSU Hen Scratches, posted on May 7, 2013 at 05:44:53
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 As is the case in "El Cheapo", "check valve" decoupling between B+ and B2+ will be employed. Volts to operate the "concertina" are at a premium.

The CCS 6922 anode loads "automagically" get the plate potential right.


Eli D.

 

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