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A 6Y6GT/6922 P-P Amp with MOSFET Concertina Splitter

216.165.190.4
Posted on March 28, 2015 at 10:32:15
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Nearly two years ago, Eli coached me on starting this amp, providing some "hen scratches" for the various sections. I had asked the forum about driving 6y6GT tubes in push-pull, and Eli responded with the idea of using a 6922. He also knew of several transformers and passive components that I already had on hand in my bins. (I'm frugal when possible.) So he sketched out the stages with those components in mind.

Besides using available parts on hand, I added some new parts options in the event that others might be interested in building this. For example, I found a couple of nice Edcor power transformers that are cheaper than purchasing both the Triad N-77U isolation transformer AND a needed booster transformer. These Edcors have sufficient specs at a good price, and they come with end bells if you need the cosmetics. :-)

This amp was also designed to allow experimentation with some other features I had either not yet built or had not experienced by listening. Here are some of those features:


  • CSS on the 6922 anodes
  • MOSFET concertina splitter direct coupled to the 6922 plate
  • AC balance control for the splitter legs, in order to optimize wave symmetry and reduce distortion.
  • Fixed bias, adjustable for each 6Y6GT output tube
  • Regulated screen supply using a VR tube (0C3A)
  • Near-class-B biasing ("deep class AB"?) for efficiency and low current draw as well as allowing higher B+ voltage on the finals

The result is a great-sounding amp that is highly tunable on the bench.

In this thread, I'll report the bench measurements of a working breadboarded amp along with the current schematics. The original thread with Eli's hen scratches is here:

Driving 6Y6GTs with El Cheapo 12AT7 front end?

Thanks also go to Michael Samra, Jim McShane and Don Sachs for their input and help as well. Miles Prower, Tim Robbins and Mark Weiss on diyAudio helped with on-bench 'scope measurement questions, too.

--Jeff Yourison

 

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6Y6GT/6922 Amp Channel, posted on March 28, 2015 at 10:42:05
jyourison
Audiophile

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



Here is the main amp schematic with measurements and good starting values to aim for. I'll post a link to slightly larger images soon.

I set the bias by taking the output tubes into class B cutoff and then backing them out until crossover distortion was gone. The 7 mA current across the 10 Ohm cathode resistors is the sum of plate and screen currents. Output using conservative rms measurements and waveforms is about 7 Watts per channel with 2Vrms since input at 1KHz.

 

6Y6GT/6922 Power Supply Option 1, posted on March 28, 2015 at 10:59:02
jyourison
Audiophile

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



While I have a Triad N-77U and several control transformers to play with, I found this Edcor 170 Volt transformer with a 6.3V winding for about $39. It supplies sufficient current for this kind of biasing, and using the 6.3V winding as a booster brings the total up to the level needed for B+. B+ needs to be "tall" enough to allow the direct coupling of the MOSFET to the 6922.

This power transformer is a good option if you have a separate high current heater transformer on hand. The 6Y6GTs each draw 1.25 amps.

The heaters are elevated, which helps keep this amp silent and hum free.

The 0C3A series resistor gets hot, and could also be set a tad higher. I used a midpoint of [22.5 mA through the OC3] + [4 x screen current per tube (about 2-3 mA per tube, depending some on bias] to determine the series R value. It's fairly flexible, since there is a range of current possible through the 0C3A. You want to keep it safe in case of loss of load.

 

6Y6GT/6922 Power Supply Option 2, posted on March 28, 2015 at 11:09:41
jyourison
Audiophile

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



Here is another Edcor option. It runs about $53 USD. I has a 300 mA secondary, which is overkill for this amp, but if you were to bias "hotter" you'd need the extra capacity. The 180V output means you can use the 6.3V 6A winding for the heaters. I added a 0.05 - 0.1 Ohm resistor to nudge the heater voltage to sit right at 6.3V or a little under.

When my mains voltage went high one week, I added a couple of 10 Ohm WW 25 Watt resistors to the 180V secondary to create a slight drop. This not really needed, unless you're a perfectionist. The amp is fine pls or minus several volts of resultant B+.

A note on the power supply capacitors: these were in the bin. Feel free to use lower values. I would feel comfortable using 330 muF. The inrush current limiter was a suggestion from Eli, knowing I had these large ones (actually, even larger ones) on hand.

 

6Y6GT/6922 Bias supply and anode load CCS, posted on March 28, 2015 at 11:31:35
jyourison
Audiophile

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



Here are the bias supply and anode load CCS as built. I used twelve turn cermet pots from Mouser (1/2 watt is fine), and they give just enough control when using a screwdriver to adjust to the third digit on my DMMs.

The bias supply is a tripler using a handy, cheap Radio Shack transformer. The tripler topology is slightly sensitive to the value of the capacitance -- more capacitance will keep the voltage losses down to some degree. The 4K and 10K Ohm fixed resistors put the pots in the middle of the desired bias range. Watch the polarity of the caps, and be sure to ground the supply loop.

The idea is to attain a good bias point for each 6Y6GT tube that also provides a current match across the cathode resistors. Those resistors should be matched per tube pair and be spot on 10 Ohms for ease of math conversion from Volts to mA. (E.g., a 0.07 Volt reading means 7 mA, since I = E/R, and 7 mA = .07/10)

Of the four 6Y6GTs I started with, the final bias point for three of them in nearly the same, while one needs almost an extra 0.5V. It's nice to have a granular level of adjustment. I'm not sure one would hear that level of mismatch, but I haven't played with that at all on the bench.

The CCS is modeled after the LTP in Eli's El Cheapo, but built for anode use.

At least the "top" MOSFET in the CCS should have a small heatsink attached. Same for the phase splitter IRFBC20s. I have a small sink on all the MOSFETs, mainly to keep it all cool in the final casing up.

 

6Y6GT/6922 Link to larger schematics and PDF file, posted on March 28, 2015 at 11:52:01
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Here is a link to larger schematics (.gif format) and a .pdf that contains all the schematics at full resolution.

The gifs are large, and if viewed in a browser, clicking on the image once may allow it to zoom to full size. You can also download to view or print in other programs.

 

RE: A 6Y6GT/6922 P-P Amp with MOSFET Concertina Splitter, posted on March 28, 2015 at 22:57:04
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Hi Jeff,

Wild amp !! Nice.

Are you still using the Bob Fulton TEK scope these days ??

Jeff Medwin

 

RE: A 6Y6GT/6922 P-P Amp with MOSFET Concertina Splitter, posted on March 29, 2015 at 06:49:48
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Thanks, Jeff. At the moment, the 'scope is being used by my brother, who does electronic repair. We're crunched for space at the moment, and when I get more room, I'll ask for it back.

 

RE: A 6Y6GT/6922 P-P Amp with MOSFET Concertina Splitter, posted on March 29, 2015 at 08:08:14
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "Wild amp !"

The characteristics of the 6Y6 pretty well dictated the "route" taken.

The B+ rail was going to be "short". I wanted, circumstances permitting, to follow the KISS precept. The CCS loaded 6922 section DC coupled to "concertina" phase splitter seemed obvious. However, triodes can't swing close to the B+ rail, but FETs can. Enter the SS phase splitter.

Elements that work in a modest B+ rail environment were selected. An added bonus is that all the small signal devices exhibit high transconductance, which provides resistance to slew limiting induced by the GNFB HF error correction signal.


Eli D.

 

6Y6GT, posted on March 30, 2015 at 06:55:02
FenderLover
 I really like this tube. What brand/sub-type do you use? I like the coke bottle GE 6Y6G.

 

RE: 6Y6GT, posted on March 30, 2015 at 10:55:28
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Well, since I wasn't sure I would really like the 6Y6 type, I purchased some GE JAN 6Y6GTs on eBay. These are the straight bottle.

So now I find that I do really like them, I might like to hear the -Gs. There may be some gray-glass version, too.

What do you like about the -Gs?

 

RE: 6Y6GT/6922 Link to larger schematics and PDF file, posted on March 30, 2015 at 11:00:33
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 My first thought is that you can't get much voltage swing from the split-load inverter - if there's 97 on the gate, about 92 on the source, and the drain resistor will have the same drop, so the drain is at 220-92V, leaving about 36V across the transistor. Might JUST make twice the bias voltage peak-to peak. A little lower on the 6DJ8 plate would be better. The source and drain loads SHOULD be equal - they're not, so it's probably canceling 2F distortion from a non-ideal operating point. Also there would be an advantage to running the phase inverter at the same current as the gain stage: net current drawn by the two stages is constant, since the AC components are equal and opposite. I think more decoupling would be better too, given the large swings in supply current in deep AB.

All minor tweaks to a very good start. Have fun!

 

RE: 6Y6GT/6922 Link to larger schematics and PDF file, posted on March 30, 2015 at 13:05:43
Eli Duttman
Audiophile

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

The FET source resistance value is adjustable. That allows for "exact" mirroring of the 2 phases. Sometimes, a little asymmetry sounds better than "dead nuts". A lot of folks like a bit of 2nd order HD. The builder/owner gets to chose. :>D

"A little lower on the 6DJ8 plate would be better."

Do you think changing the CCS load to 7 mA. would be enough to get that tweak done?

"I think more decoupling would be better too, given the large swings in supply current in deep AB."

Diode check valves are already part of the decoupling scheme. Increasing the decoupling cap. value to say 22 muF. would provide extra energy storage for those times when the check valve operates. What say you about this?

Do you have any thoughts about lowering the end to end impedance of the O/P "iron" primary, in order to increase power O/P?


Eli D.

 

Could you expand on this statement?, posted on March 30, 2015 at 14:04:40
jyourison
Audiophile

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

You wrote:
"Also there would be an advantage to running the phase inverter at the same current as the gain stage."

1. Is the advantage you wrote of contained in your following statement ("net current drawn by the two stages is constant") or is it something else?

2. Also, would the IRFBC20 function well at 7 or 8 mA (the gain stage current)? It's running in the low 20s now.

Please also see my reply to Eli below, which is also addressed to you.

Thanks,
Jeff

 

Effects of Source to Ground adjustment, posted on March 30, 2015 at 14:32:43
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Eli (and Tom),

Here's what I experienced on the scope in adjusting the AC balance pot. I was using dual inputs to show both sides of the splitter. The images are for 1 KHz at 2Vrms with 10 V per division I believe (please pardon the dust!):

1. At equal resistances, one side of the sinus form had a flat spot. That disappeared upon raising the overall resistance of the Source to Ground leg slightly.



2. Further increase in the lower leg "evened out" the top and bottom of the sinus form, so that their amplitudes matched (well, as much as I could get it to match).




So I thought that I was reducing second order HD by making the form symmetrical. Is that the correct idea? It did require slightly mismatching the resistances.

There is still a minor mismatch in amplitude of top and bottom, but note that the scope had drifted slightly in vertical position, so it's a little better than the reticle grid shows.

What is causing the flat spot in the first image with equal resistances on both sides of the splitter? Is bias insufficient in the 6922? Peak voltage at 2Vrms probably exceeds my 6922 measured bias at idle. Or is this a cutoff at the MOSFET? Or a sag? If this flat spot symptom could be corrected at the cause, would I be able to match the resistances and not have much distortion?

Thanks,

Jeff

 

RE: Could you expand on this statement?, posted on March 30, 2015 at 17:09:31
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The FET will be just fine at the lower current. However, transconductance (gfs) will decline. Win some, lose some. Compromises will forever be with us.


Eli D.

 

RE: Effects of Source to Ground adjustment, posted on March 30, 2015 at 17:15:10
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Tom pointed out that we are on the razor's edge in the FET's voltage swing dept. Perhaps what you see is related to that.

While we are waiting for Mr. Bavis to post again, would it be possible to try the 6922 CCS at 7 or even 6.5 mA. Let's see what happens.


Eli D.

 

Reduced 6922 Current (6.5 mA) results are good!, posted on March 30, 2015 at 21:05:11
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 The results are positive. No flat top at 2Vrms input with slightly less current through the 6922. The splitter legs are symmetrical, too. See the first image below. This is 1KHz at 2Vrms,10V per division. The resistances are equal for each leg.




NOTE: With both sides equal in resistance, the waveforms for top and bottom legs looked good.

One glitch was a Source to Ground leg measurement, which showed some oscillation. I have had this before, and it may be a probe issue, although it does show up on both channels at times, and with both of my probes. It sometimes disappears. It could be the lanky breadboard layout too. Any tips for checking into that would be helpful. One positive note is that it disappears at 1.86Vrms. So it would only appear at full volume with the volume pot turned all the way up.

See the next image and the "bubble" of possible oscillation. Anyone know what this represents and why it might be occasional?



Here is the same wave form from Source to Ground, with the input slightly reduced by 0.14V, or at 1.86V. (The blurred curve is my fault, not the trace):



Over all I am more than happy with the shape this is taking.

--Jeff

 

Results with lower current in the 6922 (6.5mA), posted on March 30, 2015 at 21:17:46
jyourison
Audiophile

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

I set the current for 6.5 mA through the 6922, reduced from 8 mA.


  • The plate is now at 90VDC.
  • The splitter rails are at 87VDC each.
  • The B2+ (the splitter HV rail) has now "sunk" some to 215VDC.
  • B+ has dropped as well, so this may represent an extra pull of current though the supply by the CCS.
  • Sine waves look much better at equal resistances.


Do these numbers sound better to you?

Thanks,
Jeff

 

6Y6G coke bottles, posted on March 31, 2015 at 06:48:44
FenderLover
 The tone of the 6Y6G tubes which I heard seems a little "bigger" or more open than the "GT." But, that maybe simply due to the bigger coke bottle shape. A psychosomatic thing. Coke bottle shape just looks more sexy! 8^)

I think the same way in amps I own, which have the coke bottle shaped 6V6G or 6L6G. Versus 6V6GT or 6L6WGB.

It may also be due to the era which these tubes were made. 1930-WW2. Many are mil spec (VT designation), gray-glass, black plate, with deep colored silvery-brown getter flash. Construction is outstanding.

 

RE: 6Y6G coke bottles, posted on March 31, 2015 at 07:34:41
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Thanks for the heads up. When I stuff this all into a chassis, I'll leave some room between the output tubes to handle the larger size Gs. It's good to hear of others' experiences, especially with those tube types that are not discussed often.

 

RE: Reduced 6922 Current (6.5 mA) results are good!, posted on March 31, 2015 at 08:12:53
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 A tot of rum for Mr. Bavis! The splitter was clipping, but not any more. :>) Even with the MOSFET "concertina", things are darned tight. No way would a triode "concertina" have worked.


Eli D.

 

I plumb forgot the larger decoupling capacitors on the schematic, posted on March 31, 2015 at 14:21:58
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Oops! The 5 muF film capacitors shown on the power supply schematics are the bypass caps for a larger capacitor that should also be shown. In my case, there was a F&T 32 + 32 muF dual electrolytic on hand, à la El Cheapo's B2+ supply. This larger cap was left out of the schematic!





The 22 muF film caps recommended above might be better than the F&T with film bypass. I have used metalized film Solen caps with success.





For motor run capacitor lovers, 22-24 mF "oilers" should work well, but space will probably be a factor in casing up this amp, given the CCSs, bias supply, adjustment pots, etc.





--Jeff

 

RE: Could you expand on this statement?, posted on April 1, 2015 at 17:37:13
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Two questions:

1. What is the advantage in having the currents through the voltage amplifier and the splitter be the same?

2. Given that the voltage across the MOSFET itself seemed to increase slightly (and current decrease) when I lowered the anode CCSs, how do I figure the change in resistor vales on the legs? With the drop in CCS current, the voltage drop across the leg resistors went from about 94 and 94 to 87 and 87. The B2+ voltage rail dropped some too (220 to 215). But that doesn't account for all of the shift in the leg measurements. So there seem to be some moving targets here.

Would anyone ever put a CCS on the splitter? That might force a matched current, but I have no idea if that would work or just be overbuilding.

Thanks for any insights.

--Jeff

 

RE: Could you expand on this statement?, posted on April 1, 2015 at 19:02:16
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Don't put a CCS on the splitter. Remember, FETs and pentodes are "constant" current devices. Do you want "head butting? I think not. Also, the volts needed to operate a CCS are not present.

Go back and reread Tom's post. He gave you a rationale.

Altering the total resistance the FET's drain current flows through is how you control that current's value. DC coupling the FET to the 6922 anode turns it on "hard". The internal resistance of the turned an FET is so small that it can be ignored.


Eli D.

 

RE: Could you expand on this statement?, posted on April 1, 2015 at 20:16:46
jyourison
Audiophile

Posts: 279
Location: Wisconsin
Joined: June 20, 2006
 Okay, then, 6.5mA across 215 VDC B2+ gives a total of about 33 KOhms, or 16.5K per splitter leg. The1.5 KOhm pot can still have some effect on the total for fine tuning. Maybe 16.5 KOhm fixed on the drain leg and 16 KOhm fixed plus the pot on the source leg for starters. Tweak the leg values on the bench using the actual B2+ rail voltage to attain 6.5 mA, matching the CCS current.

Does this seem good?

Thanks,

Jeff

 

RE: 6Y6GT/6922 Link to larger schematics and PDF file, posted on May 14, 2015 at 12:06:26
jyourison
Audiophile

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

I have added the tweaks discussed in the thread below, and am attaching a new PDF set of schematics.


  1. Added the missing 'lytic cap on the end of the power supply, after the B+ rail is split into channels. I use a F&T 32 muf x 32 muF double cap. The 10 muF film caps shown in the original are maintained, and could be reduces slightly if needed (4.7 muF should be fine).


  2. Reduced the current through the 6922 from 8 mA to 6.5 mA via the CCS.


  3. Changed the resistance on the MOSFET splittler legs (a) to be equal on the upper and lower legs and (b) to pass 6.5 mA through the MOSFET to match the current through the 6922.



The 'scope now shows things to be nicely balanced and the full 2Vrms sine waveform is no longer clipped at the MOSFET.

Due to the added capacitance and the reduced current draw through the splitter, B2+ and B+ voltage readings have risen some (about 2-3VDC). But the voltages in the splitter legs are much better, as the CCS also "eats" the excess plate voltage on the 6922 at the slightly lower current.

The amp sounds great!

--Jeff

 

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