Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on August 05, 2011, 03:46:52 pm
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Some of you may still remember an attempt I made at a single-ended Plexi circuit. Yes, it's been done before. No, I can't leave well enough alone. Madison's 2204 thread got me thinking about this circuit again.
Two things bothered me about virtually all of the single-ended Plexi-like designs. They don't use negative feedback and the tone stack goes directly into the power tube grid. All Marshall push-pull amps have a phase inverter between the tone stack and the power tubes.
I came up with one approach which is in the attached PTP SE-34 schematic. Basically, there's a voltage divider before the psuedo-PI to knock the signal down, and then the plate resistor is split to create another voltage divider and further reduce the signal hitting the power tube. My thinking was that all of the designs like the AX84 circuits have enough voltage swing in the signal without another gain stage so I would need to reduce the gain of my psuedo-PI. The voltage divider values are just bookmarks at this point - I have NOT built this amp yet.
Footnote: Please ignore everything in the PTP SE-34 before the tone stack or in the power supply for now.
Turns out I'm not the only one who thinks NFB and a PI are an important part of some amps' "sound". Madison's thread got me searching over on the Amp Garage and AX84. A search for "fake pi" turns up some interesting stuff. One guy, Dartanion, builds a production amp called the Absinthe with a full long-tailed pair phase inverter and then just uses one of the outputs to supply signal to two single-ended EL-34s. That seems a bit silly to me, but I'm not selling amps commercially so what do I know? Apparently Ken Fisher built an amp called the Dirty Little Monster which had a "fake phase inverter", but I can't find a schematic of his actual circuit - just guesses based on internet folklore.
Anyway, after looking at a bunch of circuits I see four primary alternatives. They're on the second sheet of the SCH file. I've also posted a PDF format just in case.
#1 really is the same as the PTP SE-34 except for the location of the master volume. What do you all think about putting the MV after the psuedo PI? Is there a possible advantage for the yet-to-be-designed effects loop? Any downside to having the MV inside the NFB loop?
#2 uses a really cold-biased gain stage instead of voltage dividers. I assume that it would introduce asymmetrical clipping. Would that be similar to a LTPI clipping?
I found several variations of #3 but included this one because of the Presence control. I assume that the function of the 22K resistor under the cathode of V3b is to reduce the voltage drop across the triode. Again, that shifts the bias point of the stage but I don't understand the goal. Is it to get a "warmer" tone from the stage similar to what dropping the B+ supply to the plate would do?
#4 is the concertina splitter idea I had the other day. It's a stage with no inherent gain. That's a plus assuming the signal coming out of the tone stack is big enough to drive the power tube. No voltage dividers needed. I think it provides an insertion point for negative feedback but am not sure it would work well. Also, I've read that concertina splitters sound "bad" when driven to clipping so this may not be the best choice for a "pseudo PI".
OK, so I'm out of my depth here. My design goal is to have a single-ended amp with an EL-34 power tube and about 10-12 watts of output that will sound just like a Plexi. Do you all think NFB is an important part of the tone & feel of a Plexi? How about having a distortable gain stage after the tone stack?
One last question for this seemingly endless post: Where would you put the Master Volume? Before or after the pseudo PI?
Just for fun, I'm also attaching a weird circuit I discovered from AX84. There's a pseudo PI stage, but it actually has a negative voltage reference for the cathode resistor. Otherwise it looks kind of like alternative #4.
Thanks in advance for any and all thoughts, comments, etc.
Chip
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Why single ended in the first place?Build a plexi in push-pull class A with two EL34's.To bias it Class A simply use a plate voltage of about 250v and drive the crap out of the EL34's current-wise.100% plate dissipation.
It would sound killer good,be a plexi in spirit and possibly even be better sounding.And above all it would be low output power like an SE amp.
Easier,lighter,all the problems you are talking about would not be problems anymore.Lighter iron and less hassle in the extreme.
I'm going to do one just to prove it can be done.
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M2CW
Alternative 1 is going to give you the most gain out of that lot, and that isn't really a 'pseudo PI" (but rather is just a normal gain stage with an unbypassed Rk and NFB at the cathode). Better still - you can apply the NFB via a AA764 Vibro-Champ-style driver stage, and have the ability for gain-boost (or selective frequency boost) in your driver stage to enhance the signal.
The other 'pseudo PI' options in your plan all result in some form of bigger amount of cathode-current feedback which, would not only lower the stage's output signal, but also clean it up (probably to the point that global NFB isn't necessary), and is more of a HiFi amp solution.
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Why single ended in the first place?Build a plexi in push-pull class A with two EL34's.To bias it Class A simply use a plate voltage of about 250v and drive the crap out of the EL34's current-wise.100% plate dissipation.
It would sound killer good,be a plexi in spirit and possibly even be better sounding.And above all it would be low output power like an SE amp.
Easier,lighter,all the problems you are talking about would not be problems anymore.Lighter iron and less hassle in the extreme.
I'm going to do one just to prove it can be done.
I've just remembered I have this to guitarize :icon_biggrin:
Kagliostro
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Alternative 1 is the only one which makes sense.
But I think what you want to do is build the full 2-triode longtail phase-splitter. Just use one of the two plate outputs. This really is different from any simple 1-triode driver. And no invention needed.
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"Why single ended in the first place?" is a perfectly legitimate question. sluckey's 6V6 powered November makes a lot more sense. Because I bought the transformers for an AX84 SEL a couple of years ago and don't feel like building that amp anymore?
PRR - I mentioned that the Absinthe circuit does exactly what you describe. However, I didn't explain that I plan to use the triode that appears uncommitted in the PTP SE-34 schematic for the effects loop recovery stage.
Thanks for reading my lengthy post and giving me good, constructive responses.
Cheers,
Chip
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Found one on Amp Garage.
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Not to get off subject but do you need a resistor between the 50K negative feedback pot and the transformer(first schematic)?
With the pot fully rotated the OT is connected directally to the gain stages cathode.
I get that the 50K and the 1K2 form a voltage divider that will adjust the amount of NFB, but do you ever want to dump the whole signal from the OT to the tubes cathode? Is that OK to do? Is there such a thing as too much NFB?
(I don't want to hijack the thread and turn it into a discussion of NFB instead of about adding a psudo PI so let's not get too deep into this subject here, but it bears pointing out. OK/ Not OK?)
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Found one on Amp Garage.
Can you point me to where you found that schematic? It's obviously a variation of Alternative #3 and I don't understand what the objective of lifting the cathode like that is.
Thanks,
Chip
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Not to get off subject but do you need a resistor between the 50K negative feedback pot and the transformer(first schematic)?
With the pot fully rotated the OT is connected directally to the gain stages cathode.
I get that the 50K and the 1K2 form a voltage divider that will adjust the amount of NFB, but do you ever want to dump the whole signal from the OT to the tubes cathode? Is that OK to do? Is there such a thing as too much NFB?
(I don't want to hijack the thread and turn it into a discussion of NFB instead of about adding a psudo PI so let's not get too deep into this subject here, but it bears pointing out. OK/ Not OK?)
Jeff - thanks for your observation, and you are correct. My intention is to use the pot in testing to find a good value for the NFB resistor and then fix it, using the hole in the back of the chassis for a different switch. This circuit is a collection of hare brained ideas from all over the place, and I expect that a LOT of things will change during testing.
Cheers,
Chip
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Can you point me to where you found that schematic? It's obviously a variation of Alternative #3 and I don't understand what the objective of lifting the cathode like that is.
Thanks,
Chip
http://ampgarage.com/forum/viewtopic.php?p=151129&highlight=#151129 (http://ampgarage.com/forum/viewtopic.php?p=151129&highlight=#151129)
Hope that helps.
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I don't understand what the objective of lifting the cathode like that is.
Are you talking about R11? If so, that's the tail resistor for a typical LTP PI. This circuit uses a Marshall Plexi LTP as a basis for it's pseudo PI. It's a direct copy of the Plexi circuit but with one triode (and associated components) removed. I think the idea was to simulate the muju of a PI?
Compare the above V2b circuit to the attached Plexi V4 circuit. Look familiar? I shaded the area that has been removed. Heck, it's even drawn the same.
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You can't just yank one section and have the other section act the same as if it were there. There's an interaction going on between these two sections. Seeing as the cathdoes are tied together the current for both tubes will be going through the 470 ohm and the 10K resistors. If you simply remove one tube section the voltage across the 470 and across the 10K resistors won't be the same as when the other tube section is used.
Maybe doubling those values would be a closer rep?????? V=(I/2)*(2R)
I don't know, I don't have the answer but thought that should be pointed out.
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You can't just yank one section and have the other section act the same as if it were there.
I agree. But that's exactly what the above circuit did. Someone thinks it probably maintains some of the vibe of the LTP PI, or maybe not. It may just be another quest for the holy grail of tone. Really seems silly to me. Trying to simulate the sound of a big AB1 pushpull amp with a single ended amp may not be possible.
If you want to nail the sound of a Plexi 1987, buils a Plexi 1987.
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Maybe doubling those values would be a closer rep?????? V=(I/2)*(2R)
I don't know, I don't have the answer but thought that should be pointed out.
Good point. And don't forget that the presence pot and feedback resistor have also been eliminated. Maybe a better idea would be to build the entire LTP PI complete with NFB and presence pot, but only use one output.
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Maybe doubling those values would be a closer rep?????? V=(I/2)*(2R)
I don't know, I don't have the answer but thought that should be pointed out.
Good point. And don't forget that the presence pot and feedback resistor have also been eliminated. Maybe a better idea would be to build the entire LTP PI complete with NFB and presence pot, but only use one output.
Jeff & sluckey - thanks so much for your thoughts!
By a freak accident, Alternative #3 in the first post doubles the cathode resistor values and includes the Presence pot.
Honestly, I'm beginning to think that my design goal is kind of stupid. It's not going to sound like a 50 watt push-pull amp no matter how it's designed. The Absinthe amp which includes a full LTPPI and uses only one of the two outputs doesn't sound great to me.
I'm just going to build the darned thing with a point-to-point layout concept that should allow maximum tinkering. Hopefully it will sound good, if not Plexi, sooner or later.
Thanks again!
Chip
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Hello the world! Sorry dredging this up.
Have you made any progress with this idea? I'm busy dwelling on it and I'd like to know if you have any recent insights.
Thanks!
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Bump for 12 year old thread!
I too am curious about the Fake PI stage for a single ended parallel design I am integrating into an AX84 SEL amp.
Has anyone successfully gotten the #3 to work with a presence control? Having that critical presence control I think would really allow me to open up the amp and cut through which is what it struggles with currently.