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811A - A2 amplifier

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Posted on September 13, 2014 at 14:25:45
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005



Hi All
Finally finished 811A - class A2 monoblocks, using mosfet follower. Attached is the circuit, and went over board on 811A filament supply and mosfet supplies. There are in a separate chassis, and the filament supply for 811A is LCLCRCL using lundahl and electra-print chokes I had on hand. The mosfet and 811A bias supplies are choke input supplies

The driver is a 6L5G. The finished picture is attached below, followed by the circuit pictures.

Initial sound is nice and punchy with good bass. Needs to break in for some time before doing any critical listening.

thanks
ganti

 

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RE: 811A - A2 amplifier, posted on September 13, 2014 at 14:26:49
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005



Here is the amp circuit

 

RE: 811A - A2 amplifier, posted on September 13, 2014 at 14:32:37
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005



Here is the filament supply and mosfet supply circuit

 

RE: 811A - A2 amplifier, posted on September 13, 2014 at 17:19:09
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005
 overlooked to acknowledge that this amp was inspired by

- Article on the web "MOSFET Follies - letting the Solid State Demon into your tube amp"

- Power Drive concept from Tubelab

thanks
ganti

 

RE: 811A - A2 amplifier, posted on September 13, 2014 at 18:40:28
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 Wow, what great project.

The MOSFET source follower opens up a whole new set of high power triodes that sound great. I love my A2 amp. Very nice.

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 06:40:36
Cleantimestream
Audiophile

Posts: 7550
Location: Kentucky
Joined: June 30, 2005
 I must say, the 811A sounds superior {to my ears} in every way to a 300B. One cannot ignore George Anderson's powerdrive... using SS for current is the right use for chips as that is one area where valves just fall flat on their face. That circuit you have built has left nothing to chance.

Most excellent.


The Mind has No Firewall~ U.S. Army War College.

 

Why Not Just Build a MOSFET Amp?, posted on September 14, 2014 at 07:57:18
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Seriously, if you're going to interject SS sonics into the signal chain, why bother with the 811A and all its messy support circuitry? It's a lot of trouble for little or no benefit.

 

RE: Why Not Just Build a MOSFET Amp?, posted on September 14, 2014 at 08:21:43
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 With all the feedback involved in a source follower how much SS sonics will there be?

TK, I don't know the answer to this question.

I'm using the "mu" outputs of the SS CCS's in the plate circuits of the stages in the phono pre.

Unless I completely redesign around totally different tubes I have no choice, I need the lower output impedance they provide.

I don't "hear" anything I would call SS sonics but 'what do I know'?

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

 

RE: Why Not Just Build a MOSFET Amp?, posted on September 14, 2014 at 08:22:30
Cleantimestream
Audiophile

Posts: 7550
Location: Kentucky
Joined: June 30, 2005
 Hmm, I would not castigate another for using SS for the bottom end below 100 hz even though that is not my cup of tea. Live and let live say I. Perhaps I practice more tolerance in certain areas than others... I do not know. I DO know tubes swing voltage effortlessly... which explains their use even today in broadcast transmitters. They do NOT like current, the obvious domain of SS. The proper tool for the proper job say I. If you have not utilized 'power drive'... in other words, IF you are practicing "contempt prior to investigation" you MAY remain...
in everlasting ignorance.


The Mind has No Firewall~ U.S. Army War College.

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 10:34:47
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 I think it is fascinating that you use the 6L5 as your driver. This tube is the missing link between the 6P5 and the 6J5 and yet nobody uses them. I am just as guilty as I have a nice collection of 6L5s but have never gotten around to using them; instead I have been seduced by the sexy 'new' high transconductance pentodes and triodes released in the 1950s and 1960s.

I spent years (OK, decades) listening to the 76 and 5687 families of tubes and in the last few years I have tried branching out and trying new types. Having played with the new Triode Load Line Simulator for several days now, I am reminded that the 5687 family (7119, 7044, E182CC) are very linear tubes with extraordinary low distortion, at least on paper. I may end up going full circle back to the same old families of tubes but trying the 6L5 this time as the voltage amp instead of the 76 or 6J5.

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 11:10:44
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005
 I felt with a preamp in the chain, and with 811A biased at +20V, 6L5 had the gain I needed where I can turn the volume to little over half and the sound is loud. My speakers are Zu Def IV's and are 100db efficient.

I do like the sound of the 6L5, and had built couple of linestages in the past with a stepdown o/p transformer.

I will be experimenting with a lundahl plate choke for the 6L5 at some point.

Right now, just letting the amp play and break in..

ganti

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 11:20:27
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 How's the damping factor with that design?

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 11:31:58
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005
 Seems fine with the Def's

The bass is tight and not bloated.

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 13:04:18
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 I'm no expert but I think CB's question was a technical one.

I don't see any voltage feedback that would lower the plate resistance of the 811a but I do see current feedback that will increase the plate resistance.

What is the in circuit plate resistance of the 811a and what is the primary impedance of the output transformer?

Thanks.

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

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 15:51:21
sganti
Audiophile

Posts: 202
Location: San Francisco - Bay Area
Joined: December 11, 2005
 There is a 16 ohm winding from the o/p for local cathode feedback, which I use, that lowers the Rp of 811A

I really don't know Rp of 811A in A2 mode, but I have seen 811A amps using anywhere from 3.5K to 7K, so I settled for a James 5K that came with the 16 Ohm feedback winding.

ganti

 

RE: 811A - A2 amplifier, posted on September 14, 2014 at 16:55:41
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 I stand corrected.

The KNFB does lower the plate resistance.

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

 

RE: Why Not Just Build a MOSFET Amp?, posted on September 15, 2014 at 09:20:22
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 "With all the feedback involved in a source follower how much SS sonics will there be?"

A better question might be whether we can be certain the chacteristics of SS aren't audible in such a circuit. Like you, I'm not sure, but why take the chance? There have been many tube-based circuits designed through the years that are capable of driving an output stage into grid current with low distortion. I suspect using a MOSFET this way has more to do with parts availability and cost than with sonic integrity.



 

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