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Opinions please: need recommendations for a Triode driver for a 2a3

50.80.146.114
Posted on January 16, 2017 at 16:54:36
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Recently, I modified my 6c6 pentode driver to work in triode mode. I really like the results. As such, I am seeking a triode driver tube that will work nicely with my 2a3 output tubes. My schematic below represents the 6c6 in pentode mode. I want the ability to heat the filaments with a battery, just like I am doing now with the 6c6.

So, please give me your recommendation(s) on which triode driver you prefer.

Thanks in advance for any input.
Pat


******************************************************************
UPDATE AS OF 2/7/17

1. First of all, thank you to all who have contributed to my initial post. Much appreciated

2. I strapped the 6C6 to triode mode and initially, the output was insufficient. Very weak. I very much liked the transparency and tone of the 6C6 triode, however it did not have enough drive. After trying several adjustments, I tried different values for the plate resistor (which is shown as 75k in the schematic). Inadvertently, the whole time I was using the 6C6 triode mode, the plate resistor was 6.8k ohms. I had mislabeled the resistor and thought it was 68k ohms. A couple of days ago, I reinserted the 75k ohm plate resistor. Now, there is plenty of gain using 6C6 triode mode. The amp sounds the best it has ever sounded at this point. FYI: I put pieces of masking tape on resistors to easily identify their value. In this case, I did a poor job of labeling them and it thus the confusion. Dang it!

So, now I am back to pursuing a DHT tube for the driver. The 6C6 (in triode mode) has a mu of 20 and a resistance of 11k ohms. Where does this leave us? I am still looking for a good alternative driver tube.

END OF 2/7/17 UPDATE

**************************************************************************



 

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Again . . . How About a 27?, posted on February 8, 2017 at 07:32:45
FlaCharlie
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Posts: 940
Location: Gville, FL
Joined: June 1, 2003
 I kinda lost track of this thread after it, predictably, veered off into suggestions for using rare and expensive tubes, setups that employed hundreds of LEDs, the need for complex heater schemes in order to operate quietly, and such stuff.

I'm a big believer in the KISS philosophy so I'll suggest - again - that you try a 27 using battery bias on the grid, or cathode if you prefer. I posted a schematic earlier.

No, it's not directly heated but that's the point - simple to heat using AC with minimal parts count. As long as you are using a preamp with sufficient gain - which it seems you are - it should work well and should give you, sonically, most all the benefits of a DHT, like the 26, without the complexity and related expense.

Others here DO have a much broader and detailed understanding of technical theory. That's not saying much because my technical abilities are essentially nil. So they may offer some objections. Possibly even valid objections.

Try it anyway. =8^)

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on February 7, 2017 at 17:26:02
elblanco
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Posts: 3486
Joined: August 20, 2004
 what is the voltage measured from plate to top of cathode/battery of 6c6?

when you make plate resistor 2x plate resistance good things happen.

is B+ 295v as stated in schematic?

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 26, 2017 at 00:33:55
sser2
Audiophile

Posts: 2571
Location: Pittsburgh USA
Joined: July 30, 2003
 45 or 71A. A 2A3 driver needs to deliver 20-22 Vrms at low output impedance and distortion level same as, or better than 2A3. Few tubes can do it better than other DHT.

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 26, 2017 at 08:20:47
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
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 Its a two stage 2A3 amp ( thank goodness ) so the mu you suggest is WAY too low.

Your suggestion is OK, but it assumes a three stage amp.


Jeff Medwin

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 26, 2017 at 16:25:42
sser2
Audiophile

Posts: 2571
Location: Pittsburgh USA
Joined: July 30, 2003
 Jeff:

Your moniker suggests you should love 71A or 45 as drivers :-)

The purpose of the driver is to provide undistorted voltage swing of high amplitude. Voltage gain can be, and should be done in the preceding cascades.

6H30 or 6N6P have same plate resistance, higher mu, higher plate voltage rating, and excellent linearity. They might be the best choice if one is not crazy about DHT like yours truly.

 

Use a 01A Gen 2 and add gain in the preamp, posted on January 18, 2017 at 02:33:45
andy evans
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Posts: 4378
Joined: October 20, 2000
 There's no doubt in my mind that Ale Moglia's Gen 2 01A preamp is the best single stage I've ever built or heard. It also has filament bias, which is very close to your desire for a clean DC battery supply. In filament bias you use a Rod Coleman reg (see Lyrima) for the filament, and I'd use a choke input supply to that.

If you need more gain, then get it from your preamp. Alternatively, use the Gen 2 preamp and then use the same gyrator circuit with a 4P1L as your driver stage. Again in filament bias. The 4P1L will operate at 25mA so more drive capability. You won't find anything more transparent and detailed than this. If you want "warmth" I don't know what to say to you - the last thing I'd ever do to a sound system is add "warmth". If the sound is detailed, transparent and natural with good timbre to acoustic instruments, then that's the goal of reproduction in my book.

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 12:39:02
hifipaul
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Posts: 735
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Pat

You seem to have a preamp that puts out 15v. That means that all you need is a low mu driver stage, and the 6c6 ain't it. My suggestions; 12B4, 6AH4, 6CK4, in place of the 6c6. They all have low Rp and a mu <10 and indirectly heated, which makes implementation easy. They are also inexpensive and somewhat common. The latter 2 use 8 pin sockets and the 12b4 uses a nine pin socket.

 

How About a 27?, posted on January 17, 2017 at 12:08:24
FlaCharlie
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Posts: 940
Location: Gville, FL
Joined: June 1, 2003



Here's my build that's similar in that the output tube is essentially an indirectly heated 6.3v version of a 2A3. I used battery grid bias on the 27. A lot of people think the 27 sounds very close to the highly regarded 26. It has a mu of 9 and since it is indirectly heated, like the 6A5G, it's a lot simpler to implement and there are no hum issues with AC heating. You can use battery cathode bias too, of course. While I have used this with a simple volume control and a CD player as a source, it's much better if you have a preamp.

Other responders surely have a lot more knowledge than I do - I'm more of a copy and paste builder so there are likely some flaws in my design that they will surely point out. I like it a lot though.

 

I have about 15-20 NOS NIB 27's in red Zeneth boxes..., posted on January 17, 2017 at 17:01:25
Ivan303
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 Biased at 8-9 Volts and driven by an active tube pre-amp, would think it no problem as a driver for my 300B amp IF...

8-9 Volts is a good bias point for linearity and by the data sheet 10-11 Volts is close to ideal?

I have 2.5 VAC supply for running 2.5V filament 300B's so all I would have to do is switch out my octal driver socket and replace with a 5pin.


First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: I have about 15-20 NOS NIB 27's in red Zeneth boxes..., posted on January 18, 2017 at 09:05:53
FlaCharlie
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Posts: 940
Location: Gville, FL
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 I don't have any experience with the 300B so I don't know if the 27 would work well or not. I think I recall hearing that the 300B is harder to drive than a 2A3/6A3/6B4G/6A5G. Maybe some of the guys here who have more expertise will comment on its suitability.

Probably best to start a new thread if your focus is on the 300B as this one concerns a 2A3 amp.

 

You can do it with a 6C6..., posted on January 17, 2017 at 09:20:16
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 ... the problem is you want battery bias.

This limits you to a specific cathode voltage, which will affect the gain of the tube.

IMO you would be better off by using a conventional cathode resistor of the right value and bypassing it with the correct capacitance. This will act a lot like battery bias but will allow you to set up the tube's parameters with a bit greater accuracy. Lower distortion and higher gain should be the result if you do it right.

Just off the top of my head (I work with 6SN7s a lot) if you want battery bias for sure I'm thinking that its going to be around 6-8 volts to be ideal. Also, the 75K plate load is marginal- I would go with 49.9K which will give you sufficient current for bandwidth and no worries driving the input capacitance of the 2A3.

 

RE: You can do it with a 6C6..., posted on January 17, 2017 at 10:01:35
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Hi Ralph,

Thanks for the input. I am currently using a 6C6 and want to move away from it. I want to use a triode (not a pentode wired for triode).

WHile the battery bias is ideal, I would certainly forego it if necessary to get a triode driver.

Any thoughts on a 4-pin triode such as a 20B?

Pat

 

the mighty 841_ sadly ... rare~nT, posted on January 22, 2017 at 12:02:36
Cleantimestream
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 ~!
The Mind has No Firewall~ U.S. Army War College.

 

RE: the mighty 841_ sadly ... rare and only generally comparable, posted on January 25, 2017 at 09:51:13
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 The type 841 is fairly low gm, resulting in high plate resistance. The 20B has a lot of gm for a single element( as opposed to a TL304 ). The HY40 comes close, and the 65 Watt cousin HY51 eclipses it.
cheers,
Douglas

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

 

RE: You can do it with a 6C6..., posted on January 20, 2017 at 05:51:26
PakProtector
Audiophile

Posts: 12363
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 Don't rule out pentodes just yet. The type EL84 is a fine signal pentode. Gain is determined by delivered gm at your operating point and the load resistance. Output impedance is approximately the load resistance, but with available gm in that tube, a reasonable gain can be acquired without exceeding 10kOhm...:)
cheers,
Douglas

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

 

20B, posted on January 17, 2017 at 10:32:54
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Hm- complications.

The mu is sufficient... I'd give it a shot- just remember you're having fun, right?

 

RE: 20B fun without Nreaking the Bank..., posted on January 18, 2017 at 19:15:19
drummerwill
Audiophile

Posts: 965
Location: St Louis Mo.
Joined: January 7, 2003
for my latest build I wanted an all DHT front end. I thought
about EML's 20B, but I just couldn't see paying that cost...

My solution was to use the Type 26 DHT. ..........

A single 26 can drive a 2A3 or a 45......caveats are 1) you must use
DC filaments to lower hum. and 2) the gain from a single Type 26 is not
much.

I use 2 stages of Type 26's cap coupled and both 26's on a common filament supply. This arrangement allow 1) Twice the gain for the output
tube and 2) provides Humbucking noise canceling for the filaments..
My amp has all AC filament heating.

Have fun
Willie

 

RE: 20B fun without Nreaking the Bank..., posted on January 19, 2017 at 09:27:34
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 HY40 is very comparable. Mu is a bit higher, and gm a bit lower. Thoriated cathode instead of coated...:)
cheers,
Douglas

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

 

Those HY40's are rarer than Hens Teeth......, posted on January 19, 2017 at 21:12:53
drummerwill
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Posts: 965
Location: St Louis Mo.
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... with the hi plate voltage and filament requirements the HY40 sounds more like an output tube than a input/driver tube.

I think I'll stick with the Type 26..


Willie

 

RE: Those HY40's are rarer than Hens Teeth......, posted on January 20, 2017 at 04:44:10
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 I suppose it pays to collect such stuff as it becomes available...and the only reason I'd worry about the 'power tube' flavor is in its cathode heating requirement. That it will tolerate HV, and substantial plate dissipation is of little consequence. Besides, it is only about 30% bigger than the 20B which was put up as a potential signal tube solution.

Along with the HY40, there is its bigger, higher gm cousin the HY51A and HY51B which are also not easy to find...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: 20B fun without Nreaking the Bank..., posted on January 19, 2017 at 13:39:11
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



I am going to have to agree here with Jeff-Dennis

If you have a low-mu tube you will never get the dynamic quality (dymanic contrasts) it will sound like all other SE amplifiers limp whet noodle?


I discovered this 16 years ago with my then WE 300b tubes and tube amplifier these low-mu 6sn7 tubes just dont have the getup and go like a higher mu 6sl7! or other higher mu tubes.

once you discover this for yourself you will never look back

I am speaking from experience here just like jeff and Dennis

my current SE amplifier uses a very hi mu tube 300!! LED biased and the gm70 final tube is also biased with LEDS!!!


Now there is nothing wrong with LED Bias actually if you look into why you will see several advantages one i will mention here is when you LED bias you get to full mu of the tube! and you remove rk bypasses and the rk!!!

Have a blast i am

Lawrence

 

RE: 20B fun without Nreaking the Bank..., posted on February 7, 2017 at 20:20:48
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 "when you LED bias you get to full mu of the tube! "

You do? How?

The gain of a stage can only reach the mu of the tube when the total load impedance is infinite. A very high total plate load will get you close to the mu. A CCS plate load, either direct coupled to the following stage or RC coupled with a very high grid resistor, will get you close.

So back to my question, what does LED bias have to do with the gain of the stage?

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

 

RE: 20B fun without Nreaking the Bank..., posted on January 20, 2017 at 11:15:17
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 That tube, likely a 6HV5 or one of its cousins has a ridiculous amount of gm; 65 mA/V which for a mu of 300 delivers a plate resistance of around 5kOhm. compared to the 60kOhm plate resistance of a 12AX7...it is quite small.
cheers,
Douglas

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

 

RE: 20B fun without Nreaking the Bank..., posted on January 20, 2017 at 09:45:41
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Lawrence,

Good to hear that others are using LEDs to bias the tubes. Please tell me, how and what specific LED parts are you using the bias the final tubes? Curious how you get enough current for the finals using the LEDs.

Pat

 

RE: 20B fun without Nreaking the Bank..., posted on January 20, 2017 at 15:59:51
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 Hey Pat, just like you did on your driver tube, you needed 4v bias and since some of the the red led's are 2v VF you can get your bias the current will fall into place but if you want to know for sure your trusty DVM in series measuring the Current this will tell you exactly what you need to know and that is the cathode current

In mine I put 55 LED's in series and there is 5 in parallel to bring the slope resistance down, each of these red led's should have a slope resistance of 2ohms or so. even though my added slope resistance is around 20ohms it sounds really good, now i can parallel more say 20 to bring it down around 5ohms and i may try that but you have to be careful if there is not enough current running through the led you can have differing slope resistance (higher) so at or around 7-10 ma each Led is good

I bought my led's for .02$ each so 1000 leds for like 20 bucks!!! very cheap to make very good bias, cheaper then good quality dale or whatever resistor

if you want to know where i got them contact me privately


Lawrence

 

RE: 20B fun without Nreaking the Bank..., posted on January 19, 2017 at 15:03:20
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 And this has what to do with a very fine, thoriated tungsten triode?
cheers,
Douglas

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

 

RE: 20B, posted on January 17, 2017 at 10:36:46
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Yes, Ralph, this is fun and educational at the same time. I think?

Mostly fun.

 

Perhaps you are ready to step up your learning., posted on January 17, 2017 at 12:12:05
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Hi Pat,

It can be fun to just throw money at stuff - try stuff out...

But at some point soon, you will find it more rewarding to do some basic learning (theory) of how tube circuits work so you can work some of this stuff out yourself. You can simply apply someone's advice, or you can try and work out the "why"and "how" yourself, seeking guidance along the way. I reckon the second approach will be more rewarding and better for your learning.

What requirements are needed from your triode driver? What restrictions do you have in its implementation. What sonics are looking for - what would you like to be different from your pentode? You will find it helpful - and waste less of others freely-given time - if you know why these questions should be asked and how to start answering them, even if you can't work all the way to the solution.

The approach I take before a build to ask myself *honestly* what the purpose of the build is: Reliability? Nice looking? What sonic presentation? Do I want it to be "special"? What would I like to learn? Then I plan (too much planning, it must be said), design, build, and check it... If it works, great. Sounds great - even better. If not, I troubleshoot and learn some more. But I will try to work it out and understand what is going on - what I need to know.

I don't have much free time to invest in this stuff - I wish I had more - but you apparently do. I'd love in a few months to be coming to you for advice!

Enjoy,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Perhaps you are ready to step up your learning., posted on January 17, 2017 at 12:34:55
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 91, yes you are correct. I am trying to learn as I go along. I have been experimenting with several modifications of the amp over the past 8 months. Some mods I understand why/what/how and others I am at a loss and ask many questions. I do read quite a bit, but in the end I do rely upon the good folks at AA for much of the learning experience.

It is fun to learn electronics and apply it.

The purpose of my build is quite simple: I want the best sound possible, to my ears at least. With regards to the current situation, the amp has great dynamics/transients and texture. However, I want to smooth out the sound with a bit of warmth and fullness. So, Dave Slagle was kind enough to point me in the direction of modifying the current 6C6 driver and making it a triode. This was a first step to see if the triode driver moved me in the right direction. Yes, it did indeed. The only issue with the 6C6 driver is that the output is too low, but otherwise I enjoy the sonics. So, I posted this latest post to see what triode driver tubes would do well with the 2A3. It appears that the 20B might be in the right direction. I like the 20B as it would require few modifications from the current schematic and I "might" be able to supply the filament with battery (5V, 1.4A). So, I am learning bits and pieces. Doing research on my own and asking many questions.

I appreciate your response. Keep the good stuff coming.

Pat

 

RE: Perhaps you are ready to step up your learning., posted on January 17, 2017 at 22:19:56
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Hi Pat,

Glad you took my post in the spirit intended - sometimes when rushed I can come off a little abrasive!

Considering your objectives, the 20B would not be my first choice. The 20B should be as clear as a bell, transparent, and direct... probably without etch. But it won't provide that intimate, smooth, tonally dense sound you are are seeking.

The 6C6 should be the ideal tube (gain aside). My system, which also runs a 6C6 driving a 2A3, has a lovely alive, flowing, and textured and natural presentation from the bass and midrange, although there is some high-frequency emphasis (but not etch, ringing or graininess): sibilants seem emphasised on some poorly produced music. But, it is pretty recent and coincides with some other system changes (several actually) - I don't think it is due to the 6C6.

Added later:

I'm curious, can you explain what changes moved you closer to warm and fuller and those that moved you away from it?

Currently, you have some pretty funky stuff going on with the design. I reckon it may have sounded initially appealing/impressive, but may become tiresome with extended listening. I often read of people trying LED and battery bias then returning to more traditional biasing because it sounds more natural. The lack of output tube cathode bypassing greatly increases plate resistance and suggests a much higher tube load is warranted; running a too low a load (like in your current design) can create a rather high distortion, strained, etched sound. The output tubes of both channels is supplied from a single 50uF DC Link capacitor - that seems a little low, even for LSES - I have not used a DC Link, but I'd guess that is not helping your cause either... and the PS might be ringing, which might fake clarity but may become fatiguing with extended listening. But I have not heard these things as a system or independently, so I can't be sure.

My guess is that the sound you are hearing is caused by a few significant issues and layered-on compensation. It is important to make changes progressively and listen to each critically, then relax and listen for an extended period non-critically. Then make a decision as to whether you like the change. Basically, apply reductionist thinking then system thinking.

Looking at the over-all design shown (with 6C6 as pentode) I'd suggest:

Output: trying the Hashimoto at 3.5k load (if it has multiple primary windings) and bypassing the output stage cathode with a balanced-sounding metalised poly cap (NOT Mundorf, except maybe the plain Supreme).

Input/driver: using a larger PIO on the screen bypass (at least 4uF), and trying a traditional cathode 5W R (Mills WW/Kiwame/KOA Speer - if the correct values are avail.) bypassed with a balanced-sounding metalised poly cap.

PS: without some modelling, I can't say. I'd be looking at it though.

I guess what I am suggesting is that you return the amp to "known good" state - one that should satisfy your preferences - then experiment carefully.

All that said, I do wonder about your system's gain structure. If your preamp is outputting circa 17v (that sounds like the max - whether you achieve this depends on the source output and the preamp's gain), then you could drive your 2A3 with a 45 or 2A3! Or just build a different amp specifically designed to meet your needs and wants...

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Perhaps you are ready to step up your learning., posted on January 18, 2017 at 16:29:46
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 91. Thanks for the additional input. You inspired me to re-listen to the 6C6 in pentode mode with a few mods. The link below is an update of where I stand. Your input would be appreciated, if so desired.

Thanks,
Pat

 

RE: Perhaps you are ready to step up your learning., posted on January 18, 2017 at 21:45:50
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Thanks Pat.

I won't post to the other thread - I am tight for time at the moment.

One last thing though. Your comparo of 6C6 as pentode versus triode should be taken in context of your current build and system only - don't draw absolute conclusions from it. I suspect the pentode is highlighting deficincies in implementation. For example, pentode have low (no!) power supply ripple rejection; if your PS in noisy - and i suspect it could be - the pentode will sound as you have described. A triode has better PSRR. Also, pentodes are sensitive to screen voltage variation and bypassing quality, which will not be ideal in your design. Not trying to defend pentodes or suggest they are better... only that triodes are more tolerant of implementation and that is what you may be hearing.

Continue to enjoy!

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 04:59:57
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 Since you already have the octal socket and best guess is the triode 6C6 has a gain of about 20 the natural next step would be a 6J5 which is basically 1/2 a 6SN7. (and do search out a round plate as Sony6060 suggested)

The key point here is selecting a tube with the proper amount of gain to avoid a hair trigger volume control and the horrible possibility of not being able to turn it up to 11. The major factor here is to let us know how much voltage you can provide the grid of the input tube. If it is 2V like a typical source / passive combo you will be right at the edge of enough gain but since that is where you are now it seems like a safe bet.

For some odd reason it has always been an "industry standard" to insist you need a linestage that puts out 20V coupled to an amp that needs less than a volt to get to full power. The solution to this odd mix has always been bragging about the precision attenuator in the linestage that is matched at -50dB for normal listening levels :-)

Other factors to make known are what you mean by battery filaments. As 91derlust said you show battery bias which is a whole different story. Also are you open to the possibility of changing sockets?

dave

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on February 7, 2017 at 13:26:49
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave, please see my update in the original post. Thanks, Pat

 

+1 nt, posted on January 17, 2017 at 09:11:38
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 07:15:49
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave,

I forgot to mention the following in my implementation of triode mode with the 6C6 tubes. The output is low. I have to turn up the volume control all the way to get good output. That said, I am still using 2.6V on the bias for the 6C6. I believe it should have more, correct me if I am wrong.

Pat

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 08:06:47
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 OK..

My pre-amp has 27 db / 17.4V RMS output.

That opens up the window to look at tubes with lower gain.

My pre-amp has 27 db / 17.4V RMS output. The pre-amp also has an internal setting whereby I can reduce the output by 6db if necessary. The pre-amp's manual suggests running the output at the lower output level.


Hmmm... if you are only biased at 2.4V and turning it all the way up gives you closer to 15V something is not adding up. Lets assume you get a gain of 15 from your triode 6C6, that menas you need 3V in to get to the 45V bias of the 2A3 but since you are only biased at 2.4V this means your driver will actually hit A2 before it clips the output. This tells me you need more bias and a third battery will start you in the right direction. It doesn't explain the difference between the alleged preamp out and the amp input bias voltage unless your sources only put out a quarter of a volt or less.

I have been using a battery bias on my 6c6 tubes. If possible, I would like to continue to use the battery bias on the new driver tube.

Getting back to the assumption that you can drive this with a clean 10V source. This suggests you can look at the lower gain tubes and possibly even a DHT but as your mu goes down your bias will increase and at some point you *may* find that the stack of batteries becomes a sonic liability rather than an asset. (I found getting over 5V of bias with batteries or LED's to lose the magic and that a tradiitonal RC combo to sound better ymmv)

I would rewire the amp for a 6J5 and see where things end up and then you can explore getting some more bias. As a quickie with the 6C6 you could try lowering the plate resistor. The schematic says 75K and I would cut it to 30-40K so you can keep the current up as you increase bias.

dave

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 08:21:46
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave, not sure how this affects our direction, but I have already lowered the plate resistor to a 27K. So, my current listening reflects the 27K resistor.

Pat

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 08:45:10
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 OK... how much current are you running?

going to a higher bias will reduce the current and at some point you will be running things too lean which means you need more voltage plate to cathode.

you could get a few more volts by replacing the 15K dropping resistors with a choke and of course a plate choke will put you in the other direction of having possibly having too much plate to cathode voltage.

dave

 

20B, posted on January 17, 2017 at 09:11:01
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave, I was looking at the EML website. While pricey, the 20B seems to be a good option. The 20B has 20x amplification which is exactly what the 6C6 has in pentode mode. The recommended voltage is 380V, so not sure if I could run it with the existing power transformer which puts out only 325V. The load impedance is 12..20k ohms.

Here is the link to the specs. thoughts?

 

RE: 20B, posted on January 17, 2017 at 18:35:22
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 You have to realize that that 380V is plate to cathode and the 12K load is assumed to be a transformer (or plate choke). If it were resistively loaded at 12K and drawing 24ma of current the voltage drop across the resistor alone would be 288V meaning you would need a B+ in excess of 650V.

dave

 

RE: 20B, posted on January 18, 2017 at 16:27:59
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Update as of 1/18/17:

I spent the day migrating the amp back to the 6C6 pentode driver. Listening carefully. Then migrating back to the 6C6 triode strapped. I can say with confidence that I prefer the 6C6 triode strapped. The noise floor is lower, which results in a much deeper soundstage. High and mid frequencies have greater texture and a smoother presentation. So, I want to continue on my quest to implement a triode driver for the 2a3 output tubes.

Thank you to those who have posted their input so far.

Here is where I stand and question that need to be answered:

1. The only issue with using the 6C6 triode strapped is that to get near full output, the preamp volume knob needs to be turned all the way up. With the preamp at full throttle, the output is about 90% of max output (just a guess, but close enough for the sake of this conversation). So, is there a way to get the 6C6 triode configuration to get more output from the power amp? If so, how?

2. The 20B driver tube has an amplification factor of 20. The 6C6 data sheet says that the 6C6 has an amplification factor of 20 in Triode mode, and does not list an amplification factor for pentode mode. So, is an amplification factor of 20 enough gain and therefore the 20B might be a suitable driver? Don't know, but your input would be appreciated. I have a near ready schematic of the 20B --> 2A3. Going with this solution requires more $s and modifications. The 20B --> 2A3 requires either a plate choke or a plate transformer that acts as an interstage transformer.

Ideally, we would find a solution to option (1) and if it works nicely, then I could move towards a more permanent solution (2) that leverages the choke and possible interstage transformer.

Any input would be appreciated.

Thanks,
Pat

 

RE: 20B, posted on January 18, 2017 at 18:57:48
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 1. The only issue with using the 6C6 triode strapped is that to get near full output, the preamp volume knob needs to be turned all the way up. With the preamp at full throttle, the output is about 90% of max output (just a guess, but close enough for the sake of this conversation). So, is there a way to get the 6C6 triode configuration to get more output from the power amp? If so, how?

If your preamp puts out 17V like you mentioned and your 6C6 is giving a gain of 15 (fair guess for a resistive loaded tube with a mu of 20) then something is seriously amiss if you can get anywhere near full volume on the linestage since 3V into the amp will run the 6C6 into A2 current and clip the output tube.

Your removal of the cathode bypass cap on the 2A3 may be what is "saving you" since the clipping behaviour looks a lot more polite without it in place. (even though the distortion is still upwards of 10%)
In any case, going from tube A with a µ of 20 to tube b with a similar µ isn't going to change your lack of gain.


dave

 

RE: 20B, posted on January 20, 2017 at 06:06:28
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I took some measurements using a 1k sine wave input at 0.683VAC. FYI: instead of 100k ohms, there is a 220k ohm resistor at the RCA input (I have been playing with different input resistors, FWIW).

Here are the results:

Input signal: 0.683V

6C6
grid 145Vdc
plate 145Vdc (grid and plate are tied together for triode)
filament 4Vdc

2A3
grid 8.77Vac (using signal tone input)
plate 286Vdc
filament 2.6VAC
VDC P-K 237Vdc (target should be 250-265)


The 2A3 plate voltage is low at 286Vdc, typically I see it at 300Vdc.

The 6C6 grid voltage is 145Vdc, I typically see 175-180Vdc when in pentode mode.

I am unsure why the 2a3 plate voltage is so low and is the 6C6 grid 145V voltage OK since this is triode strapped?

Pat

 

RE: 20B, posted on January 20, 2017 at 16:57:08
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 see you upped the bias to 4V and the driver is giving you a gain of 12.8. This puts you at just over 50Vrms (70V peak) into the grid of a tube biased at 50V so everything looks to be in line and I don't think you will get much more out of this situation. (unbypassed 2A3 inot quad esl63's)

This doesn't explain why a preamp that puts out an alleged 17V needs to be turned up all the way since that should cause some pretty severe clipping to an input stage biased at 4V with a gain of 13

dave

 

what is the suppressor grid (pin 4) tied to?..-nt-, posted on January 20, 2017 at 08:16:47
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 9

 

RE: what is the suppressor grid (pin 4) tied to?..-nt-, posted on January 20, 2017 at 08:32:29
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Pin 4 and Pin 2 (plate) are tied together.

 

RE: what is the suppressor grid (pin 4) tied to?..-nt-, posted on January 20, 2017 at 08:53:05
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 where are you measuring 4volts on the 6c6?

 

RE: what is the suppressor grid (pin 4) tied to?..-nt-, posted on January 20, 2017 at 09:34:37
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 The filament supply is 4V. I am using 2 LEDs in series to produce the 4V.

 

RE: what is the suppressor grid (pin 4) tied to?..-nt-, posted on January 25, 2017 at 10:36:09
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 The heater of the 6c6 should have 6.3 volts across it.

I assume the two LEDs are between the cathode and ground.

The 4 volts is your bias voltage (cathode) not you heater (filament) voltage.


Tre'


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

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 06:15:32
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 My pre-amp has 27 db / 17.4V RMS output. The pre-amp also has an internal setting whereby I can reduce the output by 6db if necessary. The pre-amp's manual suggests running the output at the lower output level.

I am open to changing the tube sockets. I desire a single triode (not dual) which implies a 4 pin socket.

I have been using a battery bias on my 6c6 tubes. If possible, I would like to continue to use the battery bias on the new driver tube.

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 18, 2017 at 20:42:07
tube wrangler
Manufacturer

Posts: 2484
Location: USA
Joined: January 29, 2007
 Hi Pat.

I've been viewing the posts on this subject,
and can see that this is just wrangling in the mud....

The reason that people use a high-gain (mu of 85-to-100)tube at an amp's input is to conserve energy at the input-signal level. What happens with all this gain at the input stage is that the tube responds with speed, and
superb detail and powerful dynamics to small changes in input signal amplitude.

Usually the standing plate current is much lower also. What this
delivers is FAR greater response-to-signal "jump factor", and also to
signal content because the input signal doesn't have to drive into as
much standing plate current. The low plate current in the 100mu tube allows a far greater percentage of the input signal to be processed into
music.

Such a setup delivers superior performance on High-Efficiency speakers ONLY.

This effect diminishes rapidly as speaker sensitivity decreases, and becomes a liability after the input voltage level has to be pushed hard enough that the input tube saturates-- it cannot provide enough power to overcome the Miller Capacitance of the tube being driven at that power level.

When speaker sensitivity decreases below about 97db/watt level, amplifiers of the 45, 2A3 and even 300B types require Higher Power
driver stages with lower amp-factors because the amp must be driven harder by the input signal in order to drive the less efficient speaker.

You are using a Quad ESL-63. I would use a Solid-State amplifier of
over 200 watts on this speaker, and for tube power, I would use a Push-Pull amplifier of at least 75 watts per channel.

Trying to drive this speaker with a 2A3 is a complete waste of your time.....

-Dennis-


 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 03:05:34
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I am not sure how you would implement it, but if you can find out how to use it you might try a 26. I have been very happy with using them with both 45's and big transmitter tubes.

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 17, 2017 at 02:40:55
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 I see a battery biasing your input/driver tube... I don't see a battery supplying your heaters.

You're making a lot of changes. I'm curious to know how you went about the changes - making changes and listening - and what you have learned.

Do you have a photo of your layout?

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Opinions please: need recommendations for a Triode driver for a 2a3, posted on January 16, 2017 at 17:43:08
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 6J5G round plate if it has enough gain.

 

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