Preamp Cathode Resistor values, AB763 - 12AX7

[topbrent]

Overall, what are the net tonal changes when tinkering with the cathode resistor values in the preamp?

Specifically, I am curious about the tonal changes in a AB763 Fender Twin Reverb preamp (for simplicity lets use the normal channel-no reverb).

- Stock generic AB763 preamp values .1-.047-100k-250pf - 12AX7
- 100k plates sitting at about 250v +/-
- cathode resistor values are 1.5k(effective) on V1a-V1b.
- 22uf cathode bypass caps

What would happen tonally and theoretically if I jumped that cathode resistor value up to 2.7k or 3.3k.
- note: I would be keeping those bypass caps on both triodes somewhere between 10uf and 22uf

I have seen various boutique blackface esque amps use 2.7k - 3.3k values for the cathodes.

In layman's terms, could we discuss the net effects and tonal differences one could expect?

[Structo]

Well the cathode resistors set the bias point on the tube so you would be changing the center point of that.
In other words you will change the gain of the tube.

This is where the load line graphs come out and I'm still learning how to use those.

But Randall Aiken does a very good job of explaining it.

http://www.aikenamps.com/CommonCathode.htm

[David Root]

Tom is right, that will push your quiescent operating point off center to the right side of the load line compared to 1K5 because you're increasing the bias voltage. So you will get more distortion and easier clipping on that side of the signal.

Nothing wrong with that but it won't sound very Twin-like when cranked.

If you do the same thing on the next triode stage you will clip the other side of the signal. This is standard practice in high gain amps but you don't have to have high gain to do it.

[ChrisM]

If you change the cathode resistor and keep the bypass cap the same you are also affecting the frequency response of the stage.

[Alexo]

I usually opt for 1K or 820, as 1K5 actually sets the bias on the cold side. Arguably, this is part of the Fender sound, but I prefer them biased closer to center for a less "brisk" sound. Then again the TW uses 2K7 and 10K and sounds awesome, so what do I know..

[ChrisM]

I usually opt for 1K or 820, as 1K5 actually sets the bias on the cold side. Arguably, this is part of the Fender sound, but I prefer them biased closer to center for a less "brisk" sound. Then again the TW uses 2K7 and 10K and sounds awesome, so what do I know..

How can you say 1K5 biases kinda cold? Dosen't it all depend on the plate resistor and B+ voltage coming in?

[Alexo]

I usually opt for 1K or 820, as 1K5 actually sets the bias on the cold side. Arguably, this is part of the Fender sound, but I prefer them biased closer to center for a less "brisk" sound. Then again the TW uses 2K7 and 10K and sounds awesome, so what do I know..

How can you say 1K5 biases kinda cold? Dosen't it all depend on the plate resistor and B+ voltage coming in?

Well that was with a 100K plate resistor with your plate voltage at 250 as described in the OP. Usually if you see 250 on the plate, it indicates either a pretty low current draw (cold bias) or a super high B+ (not that unusual in an old Fender) or both.

If you look at the example circuits on the old datasheets, you'll see that the lower your plate voltage, the higher your RK should be to keep the same quiescent operating point relative to available voltage swing. As you can confirm on the load line.

So yes, you are correct that it really depends on your operating point, and in fact many times 1K5 will bias you square in the center of your bias range. ..But if you're seeing 250 volts on the plate, without knowing the B+, my guess would be that you have a high B+ and Rk is set on an optimum spec for a lower B+. I would lower it till your plate voltage equals one half B+ plus 100 volts or so, or even better, till it sounds good.

[Andy Le Blanc]

this where an O-scope is handy....

drive the stage to clip and check for symmetry at the tops of the clipped waveform.

even scab in pots for the Rk, Rp.... tweak away. Ck is larger than needed
to reduce heater hum, it's value can be chosen more critically rather than just
flying with the fender values. The impedance of the stage goes up as you increase
the gain with so the first coupling cap (C) that the stage see's can be reduced.

[David Root]

Or TubeCad. That software is remarkable, I really don't know why more folks don't use it. Yes it's aimed at hi-fi but it is verrry useful, just for grounded cathode stages, and it does a lot more than that.

Of course it can't replace a scope on your amp, but it is a great planning tool.

Oh yes, 1K5 isn't a cold bias, IMO. 250V plate is on the high side unless you're doing a high gain metal amp, and even there it's not essential.

[topbrent]

As an aside, I wouldn't necessarily qualify 250v + on the triodes as excessive.

250v-260v is run of the mill, spec voltage for the larger Fender blackface 6L6 equipped amps, ie. bandmaster, pro, pro-reverb, showman, bassman, super reverb and twin reverb, ect.

Part of the sound of those amps, imo.
I almost always prefer higher preamp voltage in these amps. I have built a few ab763 inspired amps, and have noticed a big change in clean headroom and dynamics when going from 200v to 250v.

Everyone has there own preferences.
- I like my fender style amps to be clean and punchy at louder volumes.
- I can also understand why open jam-night bloooz cats like the lower voltage and lowered headroom, though.

[Gee]

What would happen tonally and theoretically if I jumped that cathode resistor value up to 2.7k or 3.3k.
- note: I would be keeping those bypass caps on both triodes somewhere between 10uf and 22uf

I have seen various boutique blackface esque amps use 2.7k - 3.3k values for the cathodes.

Div13 CJ11?. It always struck me as strange that that amp used 100k/3k3 on the first 2 triodes. I actually built one but with more conventional 100k/1k5 on both triodes. When I get some 3k3's I will try it.

[topbrent]

What would happen tonally and theoretically if I jumped that cathode resistor value up to 2.7k or 3.3k.
- note: I would be keeping those bypass caps on both triodes somewhere between 10uf and 22uf

I have seen various boutique blackface esque amps use 2.7k - 3.3k values for the cathodes.

Div13 CJ11?. It always struck me as strange that that amp used 100k/3k3 on the first 2 triodes. I actually built one but with more conventional 100k/1k5 on both triodes. When I get some 3k3's I will try it.

That is one of them that piqued my interest.

The Diaz/SRV vibroverb is another. It uses a 2.7k/.68uf combo on the v1a-b triodes, though some folks prefer the 1.5k/22uf on the first triode and the 2.7k/.68uf on the second triode arrangement better

[billyz]

I am with Alexo on this one. I find I prefer 820 Cathode resistors with 100k plates on most fenders. But for bass and cleaner tones I go 2.7k or 3.3K and 100k plate. with voltage around 200-250. 1,5k is right in the middle and good all around.

[C Moore]

I am with Alexo on this one. I find I prefer 820 Cathode resistors with 100k plates on most fenders. But for bass and cleaner tones I go 2.7k or 3.3K and 100k plate. with voltage around 200-250. 1,5k is right in the middle and good all around.

If all else stays the same, what do you hear? What do you like more with the 820 that you do not hear/feel with a 1.5k?
Thank You

[billyz]

It is hard to put into words but. I think 820 is a little more compressed and a little gainier sounding. More Tweed like. 2.7K sounds cleaner more headroom. That is to my ears and the way I play.