Preamp designs using a pentode stage

[PeanutNore]

For my next undertaking, I'm planning on gutting the Crate BV120 head that I have laying around and constructing an entirely new circuit using the chassis and transformers (which seem actually pretty decent, the PT especially which has a separate winding for bias supply, a 15-0-15 winding, and a center tapped 6v heater winding.)

I've got 4 noval preamp slots to work with and was planning on dedicating one to the Svetlana EF86 I have laying around. Rather than use it as the first stage à la Vox, I'm planning to throw it later in the preamp, like the pentode stage in Sunn amps like the 100S and the Sentura ii. I really want to find more examples of preamps that use a pentode stage and see if it gives me any ideas. Otherwise, my first impulse is to just replace the last triode gain stage in a standard Marshall / Bassman style preamp (or JCM800) with the EF86 DC coupled to a cathode follower. Anyone got examples of a better way to do it?

[Justin Thomas]

I don't remember which, but one of the larger Sampson Matchlesses used a pentode later on, I think right before the FX loop? I wanna say it was the Clubman. It's not the DC-30 or Cheiftan... it didn't use an EF86 but a different kind, but I'd imagine you can work around it.

Justin

[eschertron]

This is exciting! I've been playing with a design using a cascode as the third stage, had no idea that the Sunn 100s had a similar design. From Merlin's book, I decided on putting the 'pentode-y' (cascode) function late in the preamp to take advantage of its distinctive clipping character.

I'll definitely be following this thread closely. Thanks for pointing toward a variety of amps with this idea in a preamp

[loudthud]

The Sunn design really uses the pentode as part of the power amp with global feedback to the cathode and the plate drives a concertina phase inverter. The 120W and 60W amps used a 6AN8 and the 40W versions used a 7199. Ampeg has some similar designs using a 7199 in their power amp.

[eschertron]

The Sunn design really uses the pentode as part of the power amp with global feedback to the cathode and the plate drives a concertina phase inverter. The 120W and 60W amps used a 6AN8 and the 40W versions used a 7199. Ampeg has some similar designs using a 7199 in their power amp.

OK. Since you brought it up, what's the reason for connecting the NFB to both the recovery stage AND on of the UL taps?

[PeanutNore]

The way I've thought of the Sunn design is that the pentode is for gain recovery after the tone controls and wouldn't start to distort until the amp is running pretty loud. I'm mostly interested in being able to get the pentode breaking up early, so I'm placing the tone stack after the pentode. I could either use a triode after the tone stack for recovery or a cathode follower before it. I'm not set on the tone stack yet but I plan to keep it pretty minimal - possibly just a FAC for bass and a variable low pass filter for treble, plus master volume.

[tubeswell]

I've done this before with a pentode and a CF. Works good. The CF buffers the AC load presented by the triode-pentode morph control (as well as presenting a low-impedance output for any following load)

[PeanutNore]

I've done this before with a pentode and a CF. Works good. The CF buffers the AC load presented by the triode-pentode morph control (as well as presenting a low-impedance output for any following load)

Did you use the pentode + follower as the whole preamp, or did you have additional stages before or after it?

[loudthud]

OK. Since you brought it up, what's the reason for connecting the NFB to both the recovery stage AND on of the UL taps?

The feedback from the UL tap is probably just to iron out a little wrinkle in the small signal square wave response. It was carried over from the Dynaco design. IIRC it was 75pF, or was it 39pF?

[uneumann]

I've got 4 noval preamp slots to work with and was planning on dedicating one to the Svetlana EF86 I have laying around. Rather than use it as the first stage à la Vox, I'm planning to throw it later in the preamp, like the pentode stage in Sunn amps like the 100S and the Sentura ii. I really want to find more examples of preamps that use a pentode stage and see if it gives me any ideas. Otherwise, my first impulse is to just replace the last triode gain stage in a standard Marshall / Bassman style preamp (or JCM800) with the EF86 DC coupled to a cathode follower. Anyone got examples of a better way to do it?

Here is an example - I've been using this (or earlier versions) as my "pick up and go" amp for over a year. It's in a Princeton cab. The EF86 is driven hard for OD. The screen is modulated (follow the PS signal) for variable compression. The pentode output drives a (SS or cathode) follower that feeds into a 4 pot tone stack adapted from Merlin's book example. A cascode mixer follows that for another OD stage. If you count the prior LIO stage (V1b), there are 3 OD stages with varied drives so many OD qualities are possible. I like the EF86 for its different clipping tone and potential for screen modulation. It's just a different component to add to the design mix.



More info and pics about this project are here... https://sites.google.com/site/string.../princeton-aot

[nickb]

Here is an example - I've been using this (or earlier versions) as my "pick up and go" amp for over a year. It's in a Princeton cab. The EF86 is driven hard for OD. The screen is modulated (follow the PS signal) for variable compression. The pentode output drives a (SS or cathode) follower that feeds into a 4 pot tone stack adapted from Merlin's book example. A cascode mixer follows that for another OD stage. If you count the prior LIO stage (V1b), there are 3 OD stages with varied drives so many OD qualities are possible. I like the EF86 for its different clipping tone and potential for screen modulation. It's just a different component to add to the design mix.

[ATTACH=CONFIG]41361[/ATTACH]

More info and pics about this project are here... https://sites.google.com/site/string.../princeton-aot

It's very refreshing to see so much originality

[tubeswell]

Did you use the pentode + follower as the whole preamp, or did you have additional stages before or after it?

Tried it several ways.

As a single pre-amp stage (in an AC4ish type build), and before and after other stages (in a bigger amp). I noticed the most differences in the AC4ish circuit.

[PeanutNore]

More info and pics about this project are here... https://sites.google.com/site/string.../princeton-aot

This is so different from anything I have ever seen before and so far beyond me, I'm in awe. I'm going to have to show this to my old bandmate who's doing his EE degree and see if he can help me understand the power amp. He's got a much better handle on solid state circuits than I do. To the extent that I can understand the preamp, it does seem like I'm planning something similar to what you'd get with SW1 on "Aggr" and S8 on "CF" - 2 triode stages then EF86 into a cathode follower driving the tone stack.

[eschertron]

More info and pics about this project are here... https://sites.google.com/site/string.../princeton-aot

Mind blown. And I appreciate the detailed journaling. Thanks!

[eschertron]

The feedback from the UL tap is probably just to iron out a little wrinkle in the small signal square wave response. It was carried over from the Dynaco design. IIRC it was 75pF, or was it 39pF?

With the smallish cap (390p on the schem I saw) maybe the goal is to feed back frequencies from the primary that are higher than those present in the signal on the secondary side?