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'Turbo' Western Electric 91 amp question...

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Posted on June 18, 2016 at 17:47:56
gilmorneau
Audiophile

Posts: 288
Location: Colorado
Joined: August 10, 2004
 So I'm thinking of building an amp based loosely on the Western Electric 91 circuit, but with a cathode follower driver stage between the pentode input tube and the 300B output tube.

There are a couple of examples of this out there, one being the DIY HiFi Lux, as discussed here:

http://db.audioasylum.com/mhtml/m.html?forum=tubediy&n=178606&highlight=91+thorsten+thorsten&r=&search_url=%2Fcgi%2Fsearch.mpl%3FForumSelect%3DSelected%26searchtext%3Dthreshold%26

And another being Pete Millett's R120 amp as discussed here:

http://www.pmillett.com/r120_se_amp.html

Very similar idea, but look closely and you'll see that one has the CF direct-coupled to the output tube, while the other has the input pentode direct-coupled to the CF. There are certainly other differences, of course, but I'm curious if anyone has any opinions on why putting the direct coupling in one location or the other would be better and why.

If Thorsten or Pete are looking in, I'd be interested to hear their respective takes on the question.

TIA

 

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RE: 'Turbo' Western Electric 91 amp question..., posted on June 18, 2016 at 18:20:45
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 The major technical reason for using a cathode follower to drive an output tube is because it can supply grid current on positive drive swings - maybe for more power, or maybe just for softer clipping. If there is a coupling capacitor, it will get charged on those grid current peaks, and until it discharges the output tube's operating point will be disturbed.

As I recall, the late George Wright dealt with this by using a much larger cap (1.0uF? - I don't remember that detail, and being George he may have changed it from time to time :^) It would keep the shift small as long as the A2 grid current was infrequent.

In any case, if there will be grid current, the operating point should be adjusted from the pure Class A1 ideal.

I believe Thorsten has used a choke load for the CF, with its DCR serving as cathode bias resistor - a very clever way to get direct coupling while still keeping the quiescent output grid voltage close to zero.

Personally, I don't much like what cathode followers do to the sonics. It's not a huge problem, but it's real and audible to many critical listeners. If you must have them, I recommend playing better music which will allow the art to distract you from the sonics.

 

Heathkit as well Mcintosh did this but DC coupled from the CF to the Opts., posted on June 18, 2016 at 23:00:16
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005






"
The only on DCV on G1 of the 6550s we would see is where the cathode of the 12BH7 sits. I'm sure the DC on the 12BH7 cathode is possible when the tube itself is driven into peak clipping..I will have to measure my W6Ms to see what they do beyond the 70 watt rating.I wonder if I can drive it into hard clipping with a 1khz signal and measure the DC on G1 of the 6550s?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

What he said, just one thing..., posted on June 19, 2016 at 08:39:24
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 I don't agree that a CF will necessarily degrade sonics. When the follower is choke loaded, as you mentioned, this issue seems to be remedied. Why that would happen, I don't know, but it strongly implies to me that "normal" followers are lacking in some fundamental way that isn't being addressed by their designers. My SETs use a choke-loaded CF at the grids of the 211s, and the sound couldn't be sweeter. This is one of those areas that deserves more research and testing if I ever find the time.


 

Agree, more of a "how" than "what" problem with cathode followers, posted on June 19, 2016 at 12:04:10
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 I have used MOSFET source followers in this position that sound very good. I believe it is because they can swing more voltage with linearity in the follower than a tube can.

 

RE: Agree, more of a "how" than "what" problem with cathode followers, posted on June 19, 2016 at 23:32:41
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 I have used MOSFET source followers in this position that sound very good.

I have as well..Technically unity gain devices should have no sound tho.I know that isn't always the case but,mosfets so a pretty respectable job as source followers.They are voltage operated,high input impedance devices much like a vacuum tube.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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