Cathode Follower in Single Ended Amp?

[Steve A.]

Hi Steve,

Thanks for replying, and sorry for adapting your schematic without credit, I actually found it through google image search! But it has been very helpful in working on this project so thanks for creating it. I'm honestly just hacking these schematics together in Paint for now. I'd like to redraw everything once I have it finalized.

Also, I realized that the 40Vdc that I'm seeing is actually the positive bias voltage coming in on Pin 8 which is also tied to pin 7. I'm still trying to understand the point of this, I'm still quite the amateur amp builder but slowly improving haha.

It's funny you mentioned the virtual center tap, I was thinking of adding this to the amp. I think it will fix the issue as the sound I'm getting sounds like a ground buzz (I haven't found any other grounding issues in the amp yet).

I also need to remove the coupling cap before the cathode follower to get that DC reference back.

After all of these years I am tickled pink that so many people are still using the basic format of my TW and ODS drawings and editing them in Paint or whatever to reflect different values that people have found or used. After Dumble himself sent a fax to a supplier about me being a "thieving pirate" I would just as soon not get credit for those drawings...

Steve A.

[J M Fahey]

In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other?

Well, there is a reason to use none of them.
Take signal from the stage driving it and you save a tube, space, and a few parts.
And some heater current.

[ric]

As far as using the plate or cathode of a concertina to drive a SE amp: since one inverts and the other doesn't, how about making it switched after the coupling caps. Then you would have that switch as a phase switch if you wanted to run it in tandem with other amps.

[eschertron]

The traditional CF design in Marshall (& their ilk) is DC coupled following a HOT biased gain stage. This helps with the voltage on the grid and helps set up the kind of distortion that you may be expecting to hear from a CF. I'm using Merlin's page for reference HERE. A cold clipper in front may not have the effect you desire. Try it and see.

[bw1985]

I've been reading that page, but will dig into it further.

I've tried a couple things today, fixed my grounding setup and isolated the B+ filter cap grounds from the input section.

I also tried 2 versions of the artificial center tap, the floated ground and the resistors tied to ground from the heater supply and I'm still experiencing this buzz!

I know it was mentioned that it could be the cathode to heater breakdown. Are there any ways to deal with this? Once again, not something I'm familiar with yet.

[eschertron]

Cathode-heater breakdown is irreversible, buzzing (from heater current making its way to the cathode relatively unimpeded?) is the symptom. Put a different tube in there. If it's immediately quieter, a re-design may be in order (and pull the fresh tube to save it from the horror of that fate!).
To track down the buzzing, ID which part of the circuit it's coming from. Short grids to ground, or pull tubes to see where the buzz stops. If you pull the ground reference to the heaters completely (no CT or virtual CT at all) you should hear a fairly loud buzz. Make that your benchmark. [Ask me how I know! ]

Re: the valve wizard. I have Merlin's preamp book, it goes into more detail about CF. Good read (no, He doesn't buy me a pint for saying this!)

[Steve A.]

The traditional CF design in Marshall (& their ilk) is DC coupled following a HOT biased gain stage. This helps with the voltage on the grid and helps set up the kind of distortion that you may be expecting to hear from a CF. I'm using Merlin's page for reference HERE. A cold clipper in front may not have the effect you desire. Try it and see.

I've been reading that page, but will dig into it further.

I've tried a couple things today, fixed my grounding setup and isolated the B+ filter cap grounds from the input section.

I also tried 2 versions of the artificial center tap, the floated ground and the resistors tied to ground from the heater supply and I'm still experiencing this buzz!

I know it was mentioned that it could be the cathode to heater breakdown. Are there any ways to deal with this? Once again, not something I'm familiar with yet.

I think that most of us here probably had problems with hums or buzzes in our first builds which were usually using tried and tested designs like the tweed champ or deluxe. Building something that has never been built before complicates matters as you don't know if the problem is with the design or the build.

For starters you can assume that the preamp design is okay and look at the PI, the output section and power supply schematic.

The power supply for the heaters is unusual. There were many amps from the 50s that did not use an actual or virtual center tap which made the preamp tubes noisier. Connecting the virtual center tap to the cathode(s) of SE or cathode biased output tubes is a trick I learned from long time contributor Bruce Collins of Mission Amps. But you do not connect the cathode directly to the heater taps, it is connected to the two 100R resistors coming from those taps.

As for driving the output tube from a cathode follower I would have tried wiring the last two stages as a Marshall or tweed bassman DC coupled cathode follower only driving the output tube rather than the tone stack. But only after running my design through a forum like this for feedback on how to wire the EL84 to the cathode follower.

BTW I had thought that |A| on the cathode of the EL84 was a test point and not connected to the heater supply.

Good luck!

Steve A.

[bw1985]

I think that most of us here probably had problems with hums or buzzes in our first builds which were usually using tried and tested designs like the tweed champ or deluxe. Building something that has never been built before complicates matters as you don't know if the problem is with the design or the build.

For starters you can assume that the preamp design is okay and look at the PI, the output section and power supply schematic.

The power supply for the heaters is unusual. There were many amps from the 50s that did not use an actual or virtual center tap which made the preamp tubes noisier. Connecting the virtual center tap to the cathode(s) of SE or cathode biased output tubes is a trick I learned from long time contributor Bruce Collins of Mission Amps. But you do not connect the cathode directly to the heater taps, it is connected to the two 100R resistors coming from those taps.

As for driving the output tube from a cathode follower I would have tried wiring the last two stages as a Marshall or tweed bassman DC coupled cathode follower only driving the output tube rather than the tone stack. But only after running my design through a forum like this for feedback on how to wire the EL84 to the cathode follower.

BTW I had thought that A on the cathode of the EL84 was a test point and not connected to the heater supply.

Good luck!

Steve A.

I'm going to reconnect the 100R resistors and see if it helps further kill the buzz. I've added some additional shielded wire in the preamp section and it seems to have improved...but only slightly.

Looking at the AX84 SEL, they are using a cathode follower in an SE amp (my amp was originally an AX84 P1 extreme), but there are a few additional resistors and a diode before the CF...would those make a difference in my design and should I implement them?

http://www.ax84.com/static/sel/AX84_SEL_101004.pdf

[eschertron]

If you're talking about R13 and D3, I think you'll find them mentioned, along with the design rationale - on the valve wizard page linked in post #19. Plus, there's a jungle of shielded connections called out in the drawing you linked. Lead dress is important in a high gain design like this. There's easily 70-80dB of gain in the preamp. Any amount of noise introduced early will be too much.

[Steve A.]

The advantages of the DC-coupled cathode follower in this design...

Although I had earlier recommended a traditional design basically like half of a cathodyne phase inverter I imagine that using a DC-coupled cathode follower would push the output tube a lot harder. Is that the case here?

That sounds pretty cool because simple SE amps often lack the gain and distortion of their larger brothers. One exception to that is the JCA PicoValve as designed by Andy Marshall of THD.

In any case for a different take on using a DC-coupled cathode follower in a 3 tube SE amp here is Andy's design which unlike the modded Trainwreck SE design discussed here puts the tone stack after the CF like a Marshall rather than after the initial gain stage like a BF/SF Fender. The PDF file has a more detailed drawing than the JPG.



jetcity%20pico%20valve_1.pdf


Steve A.

P.S. I modded my PicoValve to be more like a TW Rocket which was Ken Fischer's take on the AC-30 which the tone stack driven by the CF. If I can find the schematic I drew up I will post it.

[Steve A.]

In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other?

Well, there is a reason to use none of them.
Take signal from the stage driving it and you save a tube, space, and a few parts.
And some heater current.

My bad! Looking at the revised drawing from the OP what I saw looked like half of a cathode phase inverter. As you suggest why waste a tube stage for a phase inverter for a single-ended amp.
My most recent post points out the error of my ways. I repent, Lord Jesus... forgive me for my sins!


Steve A.

[Steve A.]

As far as using the plate or cathode of a concertina to drive a SE amp: since one inverts and the other doesn't, how about making it switched after the coupling caps. Then you would have that switch as a phase switch if you wanted to run it in tandem with other amps.

As Juan pointed out my suggestion would be a waste of a gain stage. Besides it would be much simpler to add a DPDT switch between the OT and the speaker jack(s).

Steve A.

[ric]

As Juan pointed out my suggestion would be a waste of a gain stage. Besides it would be much simpler to add a DPDT switch between the OT and the speaker jack(s).

Steve A.

Oh, I don't think a gain stage is ever wasted!
$-)

It also dawns on me that set up with the anti-pop resistors could be used as a boost.

I agree with Juan that there's no need. I was offering what-if.

[tpaairman]

What would be the reason for adding a phase inverter to a singled ended amp?