Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on August 06, 2016, 06:36:14 pm
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Hello,
I have a cool design on my mind, but wondering if it would work in the field.
Basically the amp would be a 5E3 Tweed Deluxe type amp with the following topology:
gain 1 > volume and tone > gain 2 > concertina PI > 6V6 PP cathode biased.
Now the cool design feature would involve a switchable cathode bypass cap that could be connected to a SPDT switch that would send it either to the gain stage 2 cathode or to the 6V6 PP cathode.
This way, one could switch from a low preamp gain / high power gain to a high preamp gain / low power amp gain depending on whether the cathode bypass cap is on the 12AX7 or the 6V6s.
On paper it sure sounds good (as this simple SPDT switch would totally change the amp character, like 2 amps in 1 with the flip of a switch).
But would it presents reliability issues? would the switching produce popping sounds?
I'm mostly concerned about the cap switching charges from one cathode to the other.
Thanks for the tips, best.
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Cap on push-pull cathode bias has almost no effect on gain.
Get clip leads, wait for a very clear-head day, and try it.
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Cap on push-pull cathode bias has almost no effect on gain.
OK, I don't doubt, but in this case can you explain to me how this works :w2: :w2:
(http://i.imgur.com/yZrnG1j.gif)
from this thread
http://el34world.com/Forum/index.php?topic=4401.0 (http://el34world.com/Forum/index.php?topic=4401.0)
Thanks
Franco
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I guess the term "gain" applied to power amp is a misnomer. But I was using it as a general term to express the concept of "loudness".
I toyed around switchable cathode bypass caps in SE amps and it does make world of difference. The lack of one attenuate the loudness but also changes the character of the amp a little bit (hard to explain). Never toyed around it w/ PP amps though.
Regarding my switchable cathode cap idea (switching the cap from a preamp stage cathode to the power amp stage cathode), my concerns are two folds:
1. Would it pop when switched?
I guess no since the cap would always keep a charge (be connected to a DC source) EXCEPT for the few milliseconds when the switch is in the "in between position". But then, this time lapse would be too short for the cap to totally discharge (I'm planning on using a fairly high capacitance unit, like 100uF).
2. Would it present issues for the tube life?
Here I'm mostly concerned about the switch sending the cap from 6V6s to 12AX7, thus sending a bit of the remanant 6V6s bias voltage to the 12AX7. Would this constitute a severe voltage surge? I guess not really since the 6V6 bias voltage would be around 25V, which is not all that much.... Don't know really.
Could someone confirm / infirm my reasoning?
Would love to understand those circuits more in details.
Thanks.
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The cap in these cathode bias situations is mainly there to keep the bias and operating point of the PA "stable"-ish. If you want to experiment with gain in the power amp section Versus gain in the preamp either a simple preamp gain/master volume setup or a tweakable negative feedback arrangement around the power amp (sometimes referred to as a "soul control") may give you better results.
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> how this works
I could have said "common cathode bias". Nearly the only way it is done.
In effect, each side bypasses the other.
A cap is useful only for extreme dynamic overdrive side-effects.
The plan you point at *splits* the two sides. Now bypasses (or not) have strong influence on gain.
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> Would it present issues for the tube life?
No.
> Would it pop when switched?
Yes. Preamp cathode is say 1V. 6V6 cathode is say 20V. When cap at one voltage is transferred to the other, bias will change and recover (pop).
Cap at 20V, thrown into a ~1V cathode, will _POP_ as it throws the poor 12AX7 into hard cut-off for some tenths of a second. Cap at 1V thrown into 20V node will pull the 6V6es on HARD, but they will pull-up quick, and you don't have huge gain to the speaker because little gain cathode-plate and because of push-pull action.
Is two caps too much money? Or is it simple switching?
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Many Thanks PRR
Franco
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The ol' switched cathode bypass cap on a preamp tube or CF stage has been done a bazillion times, nothing novel here. It's been labeled a "fat" switch, "boost" and many others. There's several ways to wire it so it will not loudly "pop" when you switch it in the first time. I say first time because the cap charges then keeps it's charge for a while so then no popping thereafter. But to never hear a pop - put a 1M 1/8 or 1/4 watt resistor across the switch. That is all.
Messing with the 6V6 bypass cap is not recommended. You lose high end clarity & performance without it. Experiment without one, it feels and sounds like way old school lo-fidelity & seems like a blanket is on your speaker. You'll want to put it back and if left on a switch that switch won't get used very often after the novelty wears away.
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Thanks PRR for the amazingly precise yet easy to grasp answer. Exactly what I'm looking for.
As you say a 22uF cap is cheap, a DPDT switch is too... but I was trying to find a simple and elegant solution for the build.
In the end I think I'll experiment it but will stick to a more versatile approach implying a 3P4T switch toying with the preamp tube cathode bypass cap / power tube 1 cap / power tube 2 cap.
(based on the schemo posted by Kagliostro - which is off a Kevin O Connor book if I'm not mistaken).
Best.
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I'm not sure about, but think the schem I posted is by Geezer but draw by Tubenit, members of this forum
May be you are also interested on the K resistor switch used by Vox on the AC30CC2
(http://i.imgur.com/hrkeCTp.jpg)
Franco