Taking an RMS control out of a circuit

[WORMDIRT]

Sundown Rebel has an RMS control that lowers or raises the bias current on the front panel. I believe it just grounds current from the screen grids of the power tubes. Anyway this thing is superfluous at best and has been giving me problems recently and I want to take it out of the circuit and use the front panel space for some mods.

I'm assuming, from looking at other 6l6 based amp schematics that I just leave pin 4 connected together from all the tubes. Is this correct?


[IMG1145]http://imageshack.com/a/img538/3594/PukOrj.gif[/img]

[d95err]

On the schematic, the RMS control is a variable cathode resistor on the power tubes. I.e. the amp uses combined fixed and cathode bias.

To get rid of it, set the RMS control to whatever setting you like and then measure the resistance. Replace it with a 5W fixed resistor with that value.

[pdf64]

Yes, the rheostat just adds resistance between the power tube cathodes and ground, creating a mixed bias arrangement and reducing the maximum power output, eg so that the power amp will overdrive at lower volume levels.
Most simply, just replace the rheostat with a wire.
It's nothing to do with the screen grids.
A 1 ohm resistor in series with each power tube cathode is useful for checking that its bias is suitable.

[Stevem]

When I first worked on one of these amps years ago I could not believe that the designer Dennis Kager basically put in a circuit that nearly doomed Fenders early silverface amps in the 70`s!

[Leo_Gnardo]

When I first worked on one of these amps years ago I could not believe that the designer Dennis Kager basically put in a circuit that nearly doomed Fenders early silverface amps in the 70`s!

HA! You got it - the power amp "laming" circuit, made adjustable. Good ol' Dennis K - he made lemonade out of that lemon didn't he?

[teemuk]

Well... both Sundown and Fender Silverface amps are known to really divide opinions.


For example, keep in mind that OP is obviously tearing the circuit away...

[matt h]

(deleted)

[Leo_Gnardo]

Well... both Sundown and Fender Silverface amps are known to really divide opinions.


For example, keep in mind that OP is obviously tearing the circuit away...

He has no need for it and that's 100% OK with me. Unless it's a real "collector item" I say modify all you want, get the sound you or your clients are after & go for better reliability too. AFAIK Sundown and Seymour Duncan Convertibles are the only amps with this type of control. Since the 80's better methods have been developed, power scaling for instance.

[teemuk]

AFAIK Sundown and Seymour Duncan Convertibles are the only amps with this type of control. Since the 80's better methods have been developed, power scaling for instance.

If you research, you will find that this is not quite true. But it's another story and not the topic of this thread.

I have no hard feelings about anyone disliking certain amps or certain features of them - even less modifying them. Just made a remark that these amps - like the Silverface Fenders which were also brought to discussion - seem to divide a lot of opinions. So it's probably not as much about Fender failing to incorporate this particular "mixed bias" circuitry and Sundown succeeding, than it is about individual opinions and preferences of persons you happen to ask about it.

[WORMDIRT]

On the schematic, the RMS control is a variable cathode resistor on the power tubes. I.e. the amp uses combined fixed and cathode bias.

To get rid of it, set the RMS control to whatever setting you like and then measure the resistance. Replace it with a 5W fixed resistor with that value.

That is the other option I should have thought about. Word.
What about the cap that is attached to the pot? I'm assuming it isn't needed once the pot goes away.

[WORMDIRT]

Well... both Sundown and Fender Silverface amps are known to really divide opinions.


For example, keep in mind that OP is obviously tearing the circuit away...

He has no need for it and that's 100% OK with me. Unless it's a real "collector item" I say modify all you want, get the sound you or your clients are after & go for better reliability too. AFAIK Sundown and Seymour Duncan Convertibles are the only amps with this type of control. Since the 80's better methods have been developed, power scaling for instance.

Yeah I have no love for this thing, as a "collectors item". The control takes the bias from, wayyyy too cold and ugly sounding, to exactly where I set it at when I biased up the amp in the first place, and who needs that??

[pdf64]

I could not believe that the designer Dennis Kager basically put in a circuit that nearly doomed Fenders early silverface amps in the 70`s

The Vox AC50 (MkII) used a mixed bias arrangement.
If, having tried one of those, you think that it's a tone sucker, then ... well, we'd have to agree to differ!

[Stevem]

Don't get me wrong, in reality I think it was more the increase in power supply voltage for greater wattage and the change in the PI for less distortion that cut deeply into the sales of those era Fender amps, as that made them too stiff sounding!
Then latter the UL output stage really did it for me guitar amp wise, it was WTF time again!
And then that makes me wonder why the hell with all the effort that Fender put into the 400ps Bass amp that they did not go UL with that output stage?
There where some really wacky times back there with Fender!!!

[d95err]

On the schematic, the RMS control is a variable cathode resistor on the power tubes. I.e. the amp uses combined fixed and cathode bias.

To get rid of it, set the RMS control to whatever setting you like and then measure the resistance. Replace it with a 5W fixed resistor with that value.

That is the other option I should have thought about. Word.
What about the cap that is attached to the pot? I'm assuming it isn't needed once the pot goes away.

If you replace the pot with a resistor, keep the cap. If you replace it with a wire, you can remove the cap.

[WORMDIRT]

Done and done. Thanks guys.