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6AQ5 PP amps

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Posted on April 8, 2014 at 14:01:52
ecline6
Audiophile

Posts: 58
Location: Ohio
Joined: January 13, 2014
I wanted to ask anyone that has built a 6/12AQ5 PP amp what front end tubes you used. I know the 6AQ5 is similar to a 6V6 and most 6V6 amps I am familiar with used 6SL7's x 2 or 6DE7'S X 2 for the front end tubes. I was thinking of a three tube front end build with a high gain input tube and two lower mu inverters.

I have all sorts of choices. I have a hundred or so TS 6SU7GTY's, an abundance of locktal 14F7 and 7/14N7's, 6CG7/6GU7's, 12AV7/7062's and 12AU7's. Any suggestions???

I am expecting about 10 watts per channel PP and 20+ if I use four tubes per side PP ... is that your experience???

What did you use for your rectifier???? I am thinking 6AX3 damper diodes x 2 or a damper diode /SS hybrid rectifier. I have quite a few 5AR4's and 5U4G's as well.

I appreciate any comments!

 

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RE: 6AQ5 PP amps, posted on April 8, 2014 at 16:05:29
Gingertube
Audiophile

Posts: 545
Location: South Oz
Joined: October 8, 2004
Do a search for "El-Cheapo". Push Pull 6AQ5 with 12AT7 front end.
The designer (Eli) is a knowledgable and it is highly regarded by those who have built one.
Cheers,
Ian

 

RE: 6AQ5 PP amps, posted on April 8, 2014 at 22:45:34
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
I saved the full size schematic.
I couldn't find Eli's homepage anymore.
So if you want it - email me.

DanL



 

6AQ5 is a good front end tube, posted on April 9, 2014 at 03:31:35
PakProtector
Audiophile

Posts: 12356
Joined: May 14, 2002
so for that matter is an EL84/6BQ5. Pentode-rigged, LTP for one. bypass the g2 to the cathode with 1-2 uf.
cheers,
Douglas

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

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 08:30:16
drlowmu
Manufacturer

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

Depending upon how efficient your speakers are, I may be able to assist.

I like 6AQ5s P-P triode connected, with no negative feedback loop in the amp at all. Maybe good for 5 kick - a** Watts. Just a simple two stage amp. I'd use a differential front end, a high mu dual triode, and I LOVE to simply direct couple into the 6AQ5 grids. Three stages are not necessary, should be avoided, is inferior.

Contact me privately, and IF your speakers will be efficient enough, I can work up a schematic that fits your power transformer on hand. Gratis.

Joe Rosen up here on this Forum, used to say something like, " the best 6BQ5 will get eaten by the worst 6AQ5 " and that makes me chuckle. The 6AQ5 tube is a little firecracker !!

Jeff Medwin

 

You can use almost any driver tube and rectifier with that tube., posted on April 9, 2014 at 10:43:29
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
Depending on the circuit you can use:
12AX7, 12AU7, 12AT7, 12AY7 etc.

or 6SN7, 6SL7, etc.

drlowmu has some good suggestions- keep it simple as you can.

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 14:29:22
ecline6
Audiophile

Posts: 58
Location: Ohio
Joined: January 13, 2014
No negative feedback in triode sounds very tempting but I don't think 5 watts is enough for my speakers.

I have a KT88SE build that does 5 watts triode/10 watts UL /12 watt pentode. All my speakers sound dull and bland on only 5 watts triode. 8 watts pentode(when I use s 6L6) is about as low as I can go.

Has anyone tried 4 x 6AQ5 per side PP triode???

I am reading most think a simple two stage amp with a high mu triode is best. I have lots of TS 6SU7GTY's that are perfect for that task.

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 18:54:33
RC Daniel
Audiophile

Posts: 1922
Location: Brisbane
Joined: November 3, 2002
It may not be only the wattage limitations you are hearing.

It is unlikely that an amp configured to run a tube three different ways will run the tube optimally each way. Moreover, there is also a point that tubes should be run in their native state, that is, a pentode will sound better run as a pentode.

Cheers.

"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 23:12:55
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
"Moreover, there is also a point that tubes should be run in their native state, that is, a pentode will sound better run as a pentode."

In the specific case of the 6V6 family, which includes the 'AQ5, the generalization definitely does not hold. 6V6 family tubes make WONDERFUL triodes. FWIW, it has been said that short of a #45 DHT, you will not get better sound than that produced by trioded 6V6 family tubes, in this power range.


Eli D.

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 23:15:37
amnesiac
Audiophile

Posts: 717
Joined: August 21, 2002
Agree with Jeff pentode and feedback is not much good and also you should consider triode 2 stage no feedback is a much much easier amp to engineer. Even easier if you use a centre tapped grid choke.

 

RE: 6AQ5 PP amps, posted on April 9, 2014 at 23:42:02
Eli Duttman
Audiophile

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



Not my home page. The "El Cheapo" project started on the Audiophile Talk site, which was hosted by Steve Deckert. Unfortunately, the site was "hacked", totally corrupted, and taken down.

For the "umpteenth" time, I'm uploading the schematic graphic.


Eli D.

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 05:05:16
ecline6
Audiophile

Posts: 58
Location: Ohio
Joined: January 13, 2014
Thank you for all the input. I never considered triode and I was thinking a three stage amp would be better .... which by consensus seems to be unnecessary and at worst a step back in sound quality.

A special thanks for the schematic.

 

What Eli said... absolutely agree best sounding push pull I have*, posted on April 10, 2014 at 09:40:30
Cleantimestream
Audiophile

Posts: 7542
Location: Kentucky
Joined: June 30, 2005
!
The Mind has No Firewall~ U.S. Army War College.

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 11:41:26
BofService
Audiophile

Posts: 741
Location: Atlanta
Joined: February 28, 2003
Thanks so much Eli ! We appreciate your efforts on behalf of the community.

May I make a small suggestion/request? If you have the recommended voltage values for B+, B2+, and B- perhaps you could consider adding them to the drawing. If I recall from a couple years ago those values used to be covered in the thread, but of course no longer. Those who may be considering a build with different iron will need to know.

Gary

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 15:08:04
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
You're likely hearing an amp optimized for the higher power operating modes, and not so much for triode mode. If you went through it and made permanent changes consistent with troide-only operation, you'd probably change your mind.

I agree with Jeff about sticking with class A and a LTP in front of it. You won't need (or want) much gain with the tubes triode connected, a 12AU7 will work if you want low-ish sensitivity, otherwise a 5670 might be a nice choice to look at.

Also, for a LTP, you want the highest possible impedance under the cathode for best performance. I would recommend investigating tube and solid state current sources for this application.

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 18:29:59
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
The needs of the 12AT7 small signal circuitry rule the roost. The 'T7 triode sounds GOOD with 200 to 225 V. on the plate and an IB of 3 mA. A B+ rail of 355 V. takes care of "everything", if you keep "check valve" decoupling. The drop in the decoupling network shown is under 2 V.

The B- rail voltage is pretty flexible. It has to be "tall" enough to power the CCS and provide "compliance" with the drive signal. The B- setup shown on the graphic is very economical, along with starting "softly", which keeps the 'T7 grids from becoming positive relative to the cathodes. Builders in "240" VAC power zones will have to work out another B- supply scheme.


Eli D.

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 22:01:17
RC Daniel
Audiophile

Posts: 1922
Location: Brisbane
Joined: November 3, 2002
Re: pentodes run in their native state, just passing on what others have claimed. Good to know the 6V6 can sound nice when correctly implemented when wired as a triode.

Still, I don't think an amp designed to run three ways will run each optimally... and that the triode mode is likely to be a compromise.

Cheers.

"In the beginner's mind there are many possibilities, in the expert's mind there are few." Shunryo Suzuki

 

RE: 6AQ5 PP amps, posted on April 10, 2014 at 22:52:07
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
3 ways will be tough. OTOH, triode/UL switching usually works out OK. In the case of 'AQ5s, with their fragile screen grids, a 1 KOhm resistor to connect g2 to either the plate or the UL tap ensures that dissipation is not excessive. The fragility of g2 is the key difference between a 6AQ5 and a 6V6. Don't expect miracles of heat handling from the little 7 pin mini package.


Eli D.

 

FYI ..., posted on April 11, 2014 at 00:32:57
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
Also in the family are 6L6 and KT66.
These are really nice in triode.
Totally different but nice.
KT66 is more 45-ish.
6L6 is more 300B-ish.

DanL



 

+1, posted on April 12, 2014 at 08:52:09
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
A take-off of Art Loesch's amp from the 1990s which I owned.

Devils in the details.

Jeff Medwin

 

A question becomes...., posted on April 13, 2014 at 00:44:12
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
Yes, a choke or a driver transformer is a viable and nice way to split phase, and have a low AC return path for the Final's grids.

The question is, would a direct couple, which requires a CCS on the common cathode tail of the driver tube, be better or worse than a (a) added series coupling cap into the choke and / or (b) if choke or driver transformer, any of the anomalies of a wound transformer, limited bandwidth, phase shifting, insertion losses.

It may be hard to beat 3 inches of Siltech wire or Kimber AGSS in a direct couple.

 

RE: 6AQ5 PP amps, posted on May 1, 2014 at 20:45:05
drlowmu
Manufacturer

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

I try to design my 6AQ5s to operate at 210 VDC P-K and about 27 mA, give or take a small amount, triode connected.

That is ( 210 times .027A. ) or 5.67 Watts plate dissipation, on a 9 Watt ( triode connected ) rated tube, which is 63 percent of rated plate dissipation.

Notice how close this 63% is .....to the Golden Ratio ? Tube should sound relaxed, unstressed thermally, and last for 10,000 to 50,000 hours !!

Jeff Medwin

 

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