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Old 17th January 2013, 07:58 AM   #11
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Quote:
Originally Posted by gingertube View Post
Hope this "rave" is of some value to somebody.
Good reading with tons of valuable information, as always.
I definitely appreciate it. Keep it coming!

Cheers,
Martin
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Old 17th January 2013, 09:01 AM   #12
SY is offline SY  United States
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Quote:
Originally Posted by cyclecamper View Post
In the guitar amp I'm fooling around with at the moment, (see attached schematic) with 6 6L6 output tubes, how would such a "grid choke" conversion and conversion to bias pots (instead of "output balance" pot) ideally be done? Replace each of the two 33K resistors with an 8,000H choke and a pot? Or rework the circuit with a choke and pot for each output tube?
In a guitar amp, you're looking for a "sound" and this is one way of tuning it. With RC coupling, the high pass function is a first order filter and thus has no peaking in the frequency response. Now, convert to chokes and you have a second order filter which can have a wide range of response depending on how you choose the values of C, R, and L. A big peak and the resulting whomp (i.e., a high Q at the cutoff frequency) is a disadvantage in a tube amp, but may be what you're looking for in a musical instrument amp. Likewise, a very low Q and some slow LF rolloff may be useful depending on what you're trying to get for a sound.

This is one where I'd spend some time with a simulator juggling values to see the effects on frequency (and hence transient) response.
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Old 17th January 2013, 10:05 AM   #13
DF96 is offline DF96  England
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With a sufficiently beefy driver stage, a grid choke would allow you to stray partway into Class AB2 mode - you could draw some grid current on peaks without upsetting the bias too much and getting blocking due to the coupling cap getting charged up. Think of it as a poor man's transformer drive.
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Old 18th January 2013, 12:58 AM   #14
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Very interesting. Might have some potential for a bass amp. Sounds like one of those things I'd just have to listen to and futz around with.

Back to my question about the parallel tubes on each side...would I need just one grid choke for the push side and one grid choke for the pull side (to ground, instead of the existing 33K resistor) and leave the individual 1500 ohm resistors to each output tube's grid? Do I 'tweek' with resistors for similar but hotter bias current (compared to stock)?
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Old 18th January 2013, 05:11 AM   #15
es345 is offline es345  Germany
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I assume that you refer to the schematic of the Super Twin.
First one remark: the 33K are going to the bias supply not to ground.

To your question: You can use one choke per side as long as you don't want to adjust the bias separately for each tube (pair).

My experience: If you want to go for a bass amp (no guitar overdrive usage) I would insert additional MOSFET follower as driver directly coupled to the power tubes, This has the advantage to enable real AB2 operating mode.
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Old 18th January 2013, 06:02 PM   #16
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You're right, I'm wrong. Yes, the 33K, thru some other series resistors and caps to ground, is from the 60v AC bias secondary winding that's half-wave rectified.

So where would a grid choke go, in the supertwin schematic I posted? Replacing those 33k bias resistors? Will the 'standard' grid chokes handle the bias current of 3 6L6 in parallel? Do I still 'tweek' the circuit with resistors to achieve some bias spec? There's no spec for current or voltage at the grid in the schematic. Can I still use a small pot on each tube for individual bias adjustment?

I'm really not sure I understand AB1 vs AB2; can't I just bias it hot and drive it even harder to achieve the same thing?

And thanks everyone for your comments, so I can learn.

Last edited by cyclecamper; 18th January 2013 at 06:20 PM.
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Old 18th January 2013, 06:48 PM   #17
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now, a slight change of subject inspired by this site:
https://sites.google.com/site/yourtu.../power-scaling

He says he was using the high-voltage regulator "power scaling" kits from London Power. Going for adjustable lower voltage to scale the B++way back, he added a switch to run the B++ from just half the power transformer secondary! That seems like an odd but interesting idea for a more dirty-guitar amp. The regulator would be pretty stiff and hot, but switching one bridge connection from end-leg to center-tap would have more sag. Couldn't he just switch in another dropping rung in the filter ladder (switch only while turned off) and tailor the resistor and cap to his sag tastes? Power-reduction switches to disconnect the cathodes on some output tubes have become popular; how come various B+ switches haven't become as popular? It seems to me that for a low-power mode on a big guitar (not bass) amp I would want one switch that turned off some tubes to reduce current and also lowered the B+ voltage. Then though the volume would not be much less in DB, the power would be lowered so that I could switch in some reasonable-sized power soak resistors at the speaker connection for a 1/4 power switch there. That would get me down to the point where a pair of variable power rheostats becomes practical. And everything would run much cooler than just running a big amp all-out and then trying to burn it off at the speaker output with a huge power soak.

Then he talks about problems with the bias supply when he scales back the B++. I don't understand that. So in a fixed-bias amp, if I want to reduce the B+ don't I want to also reduce the bias?

But I do understand the idea of beefing up the bias supply current and RMS v. via a full-wave bridge on the same transformer taps. That might even be worth doing with an added filter ladder too, just to achieve orignal bias but now with reduced ripple. I understand that the ripple in a push/pull should cancel in the output transformer, but isn't it still worth cleaning up the power? Especially on a grid, where any ripple will get amplified. If that's where the grid chokes go, in the grid bias supply circuit, then not only does the grid choke make it easier for the drivers to push the impedance of the grids for the outputs while keeping a decent DC bias, it also helps filter the ripple out of the grid supply.
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Old 18th January 2013, 06:56 PM   #18
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OK back to fundamentals, if I screw with the bias supply I measure cathode current with no signal...not voltage or current at the grid...

If I solder a low-value shunt resistor in series with the cathode ground for measuring bias current, can I leave the resistor there all the time during operation? It doesn't seem to waste much power if the value is low enough. Does it affect sound at all? Why don't amps come that way, to make it easier to measure the current?
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Old 18th January 2013, 08:34 PM   #19
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Quote:
Originally Posted by es345 View Post
My experience: If you want to go for a bass amp (no guitar overdrive usage) I would insert additional MOSFET follower as driver directly coupled to the power tubes, This has the advantage to enable real AB2 operating mode.
After your comment, I read this:
http://www.geofex.com/Article_Folder...osfetfolly.htm
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Old 18th January 2013, 09:11 PM   #20
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The Super Twin already has cathode followers between the phase inverter stages and the power tube stages, though the cathode followers are not direct-coupled.
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