Any way to replicate this tone?

[daz]

In a past post i had mentioned that with my amp using separate cathode resistors for each of the two EL34's, i found bypassing them with caps sounds very different than lifting those caps out of the circuit. It also is much lower in volume w/o the caps. But the big difference is the tone is much richer w/o them. The following was an explanation someone here replied with, and i saved it.....

If we compare, say, independent 500 ohm cathode resistors to a shared 250 ohm resistor, we get exactly the same DC conditions: the tubes bias up to the same idle current.

However, if the cathode resistors are unbypassed, the AC conditions are different. The independent resistors give negative feedback that lowers the gain, power output and damping factor: and all the interactions with the NFB loop that implies.

A shared resistor doesn't give negative feedback, because the AC components of the two tubes' cathode currents cancel each other. One tube is turning on as the other turns off, so the total current through the resistor stays the same.

In Class-A at least. If you drive it into Class-B on signal peaks, then you get a similar effect to independent resistors, but with twice the NFB.

i understood that to some degree, but i still don't understand it to the degree i would need to answer the one big question i have, and that would be, is there a way to replicate that tone in my PA w/o the huge volume drop when i lift the caps?

[loudthud]

Do you mean gain, volume or both? What is the power output at clipping with the bypass caps in and out? How would you compare the tone between bypassed and just connected together? 500 ohms is really going to knock the effective transconductance of the tubes down. Your feedback loop probably doesn't do much with the caps out.

[daz]

I just want to replicate the tone i get with the caps out with the volume as loud as when they are in. And yes, the NFB does less with the caps out, tho it still does plenty.

So to clarify...

caps in: low volume, spongier richer tone

caps in: louder less rich and spongy

I want the sound to be like it is now with caps out, but i want the output loudness i get with them in. I don't know if thats possible because i don't fully understand whats going on.

[Don Moose]

Would this be a case where adding sag at the B+ end of the power amp would get you there?

I didn't see brand/model mentioned, but if you have a tube rectifier, swapping for a saggier one would be relatively easy. Ted Weber has some CopperCap models that could replace silicon diodes with a saggy circuit.

Hope this helps!

[daz]

I already have a sag resistor at the start of the B+. The amp is a home build that i basically just designed from several ideas, so it's not really exactly like any common amp. But it's closest to a JCM master volume amp but with a much different power supply and cathode biased EL34's and SS rectified.

[octal]

A few thoughts- I have been playing around with a similar thing.

You can partially bypass your cathode resistors- the cathode of one tube goes to two resistors in series (the total value adding up to your original value cathode resistor.) Put a bypass cap across the bottom resistor to ground. Playing with the ratio of the two resistors will allow you to trade off the tone you like versus output power & gain. You may find a good compromise.

You could also convert the amp to fixed bias, but also include a smaller value unbypassed cathode resistor in the cathode. (Kind of like those dreaded ultralinear Fenders.) I did this with pretty good results. I like local NFB over global NFB and this arrangement provides a bit of it. Some Magnatone amps used this arrangement too.


Nathan

[daz]

Nathan, thank you !!! Thats a great idea, and you wanna know why....because i already have 2 resistors on each side ! So i can try it in seconds and will do so tomorrow. Fantastic idea, and i think i did think about doing that once a long while back and forgot before i did it. I have to start writing my ideas down because i always do that.

anyways, i have 450R on each tube. But when i determined the bias R i needed i couldn't find 450R in a 20 or 30 watt aluminum heat sink package locally. But i DID find 225R's, so i used two of those per side. At the time i was bummed i couldn't find 450R's, but now i'm glad i didn't ! anyways, i will give it a try first thing tomorrow and if i feel it works good but needs to be less or more i'll simply try different values like a 150R and 300R in series and try the cap across each. (of course reversing them when i do so the one the cap is across is the one that has the grounded end)

I'll let you know how it works out. Thanks !

EDIT: i forgot to mention i DID have fixed bias at one time, but it just didn't sound nearly as good as CB. Even with the caps in circuit it sounds a lot better than it did fixed. Now whether it would be the same if i tried fixed the way you're saying i don't know. But to be honest i'n not exactly sure what you mean.

[daz]

Thinking about this now i'm not sure if this will be any different than what i already tried. I used a pot to do a similar thing. But when i tried this i realized something. i tried using a pot to put resistance between the caps and ground but it did nothing. then i figured out why.....it's not thier relation to ground that was causing the tonal and volume change, it was that the cap negitive ends when gtounded were together. so in other words, all i have to do is tie the negitive leads of the two caps together and i get that lower volume and spongy rich tone i described. So i then tried simply putting the negitive cap leads to the pot so i could gradually add resistance between them, and that did what i was trying to accomplish at the time, which was to be able to dial in a bit more volume while not losing too much of that tone. Basically to get a mixture of the two sound. So i'm not sure how doing what you described will do anything more than what i did with that pot. in any case i'll give it a go. i wonder if i should try putting the + side of the caps to the middle between the 2 225R's on each side, then soldering the cap negitive leads together but not necassarily to ground. Sorta like the opposite of what i did with the pot. instead of seperating the caps at the negitive end with resistance, seperate them at the hot end. I suppose i should us a pot to do that, then if i find a spot where i'm retaining the tone i like and with the volume i get with the caps full on, i can get cathode resistors, two per side, to replicate that. Hmmmm....brain is starting to hurt....i'lll have to think about this in the morning.

[Steve Conner]

Maybe what you like is the lack of NFB loop gain caused by the unbypassed cathode resistors. Less NFB means less damping of the speaker bass resonance, which might explain the subjective feeling of "richer". Also, I noticed in another thread, you said you liked the effect of an oversized presence cap, and that's what that does: bleed off much of the NFB.

Have you tried leaving the caps in but disconnecting your NFB? This would give high power, but no NFB loop gain at all.

Or maybe what you want isn't possible: it could be that the sweet spot that gives the tone you want is always going to be 9.5 watts or whatever.

[daz]

Have you tried leaving the caps in but disconnecting your NFB? This would give high power, but no NFB loop gain at all

i have a NFB on/off switch, so yes. But i don't use it because with the NFB disconnected it sounds great but the high end is overbearing to the point i have to turn the tones all way way down.

[Steve Conner]

Ah ha, we're getting warm! Put a small capacitor across the NFB off switch, that should tame the highs. Not sure quite what value, probably a few nF.

[daz]

I wouldn't know where to put it becaise it's a DPDT switch where one side puts the PI tail to ground on way and to the NFB the other, then the other side connects or disconnects the OT tap from the 100k R. But no matter because i removed the switch just now and i'm going with NFB only.