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Old 3rd July 2002, 08:12 PM   #11
lgo51 is offline lgo51  United States
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Talking Oh YES

Bypass caps are 140 uF / 60v tantalum, one per side. Found the tantalums in a surplus store, JAN spec, heavy little dudes. Not experienced enought (yet) to critique the sonic affects -- any thoughts?

I've also installed the "AC Cathode Shunt" [don't know what it's really called]. It goes like this: (2) 260 uF / 50v (spec was 6.3v) Non-Polarized electrolytics (T.I.'s) arranged in parallel and strung cathode to cathode.

I have also seen a pair of 6.2v(?) / 1w zeners wired in series (with their cathodes butted together) strung between the tube cathodes to protect the caps in the event of a internal short in the output tubes. Not sure about this one.

I've also noted that in Allen Wright's amp, the entire cathode bias setup (resistors and bypass caps) is 'floating' off of ground via a 68 ohm resisitor.

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Old 3rd July 2002, 08:30 PM   #12
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Good! You halready have the bypass caps. And I see what you mean be cathode shunt, that would act similar to bypass caps
but in a more active way, quite a neat thing if I say so myself...
though I've never seen caps from cathode to cathode, I do see what it would do and I think I'll try it on my next amp...

And in the allen wright, that 68 ohm resistor is probably where
you'd check the bias from for both tubes... In my dynaco MKIII
the cathodes are connected right together and coupled to ground with an 11.2 ohm bias resistor and you're to measure 1.56 volts accross it for proper bias, which is about 70 mA per tube which is right for class A with KT88 tubes the reason the resistor is so small is because the amp will output 60 watts RMS which is moderately high for a lot of tube amps...
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Old 4th July 2002, 01:51 AM   #13
lgo51 is offline lgo51  United States
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Default Hhmmm...

In the A.W. amp, he uses separate bias resistors and bypass caps for each side. But, instead of running to ground, they all run to a single 68 ohm resistor, and then to ground. Bias resistors are 550 ohm if i remember correctly. Tubes are EL34s.

BTW, any thoughts [sonic experiences] on bias current for EL34s? I've seen as low as 65% Pa(max) and as high as 80% Pa(max), about 50 mA and 65 mA, respectively.
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Old 4th July 2002, 01:59 AM   #14
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bias is dependant generally on the class of operation you want and the types of tubes used... Usually between 50% and 80% is
used for class a/ab amplifiers so you really do have some room to play... In general, you tubes will last longer the colder you bias them, that is, less current means longer life. But as you reduce the bias of a puch pull stage, you get out of class A and into AB and if you drop it low enough, you'll get class B which has moderate crossover distortion. BTW, does your amp have any sort of bias adjustments or is it just set by fixed resistors???
If set by resistors and the amp works fine/sounds good with original values and tubes are matched you'd do good to leave it that way since it was designed to work that way...
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Old 4th July 2002, 02:30 AM   #15
lgo51 is offline lgo51  United States
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So, where's the line between A and AB(1,2)??? How can i measure it? Sounds like (ooops, bad pun) comfortably inside "A", but not aggressively is a good target.

My amp is cathode biased. I thought the 'original' setting was a bit cool (about 50 mA, 600 ohms), especially since i've often seen 470 ohms spec'd (albeit by tube books that, ultimately, want to sell you more tubes ;-). Thoughts?
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Old 4th July 2002, 03:35 AM   #16
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Well, the "line" betwee A and AB isn't really a specified amount of current, but is more or less the amount of signal the active devices(tubes) operate for... For example, in Class AB, the devices are biased so that they will work for up to between 51% and 99% of the audio waveform... In class A, any device(s) will work for 100% of the waveform and in class B, the device(s) will work for only 50%of the waveform... So, in class B with two tubes(push pull) one tube conducts for the positive half of the signal while the other conducts for the negative half, but you end up with "crossover" distortion when the signal switches from the high to the low tube and vice versa. In your case, you probably want class AB, where each device will conduct for a portion of the signal positive or negative before turning off, which prevents crossover distortion. So essentially, you should bias so that both tubes have about 50%of Pmax or less... But if you've gone as cold as class B, you'll probably hear distortion at low volumes.
Another factor is grid voltage... If your current is 50ma and your cathode resistor is 600 ohms, then you have a grid voltage of -30 volts. If you fuss with the bias too much, you'll need to redo a lot of the amps original numbers... So, I would really recommend leaving the amp at its 50mA if it sounds fine there. If your amp was designed for class AB and you move it into A, the tubes will go bad quick and you stand the chance of overloading some other components as well.
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