Mixed auto/fixed bias in amp

[GainFreak]

I was looking at this Leslie 147RV reverb amplifier and noticed that it has simultaneously a fixed and kind of cathode bias or something.
So what's the purpose of that RC combo in the cathode?

[Leo_Gnardo]

I was looking at this Leslie 147RV reverb amplifier and noticed that it has simultaneously a fixed and kind of cathode bias or something.
So what's the purpose of that RC combo in the cathode?

I don't see the "fixed" bias, no negative voltage supplied to the 6550 control grids.

What is notable, the OT secondary is part of the self-bias network, providing a feedback loop around the output tubes & transformer. AFAIK this is unique to Leslie amps.

Aside from the involvement of the OT secondary, the RC combo works as in all other self biased amps.

[Justin Thomas]

I don't see the "fixed" bias, no negative voltage supplied to the 6550 control grids.

Top left quadrant, by the B+, is another feed - -11V, going to the 270ks.

Justin

[octal]

Well, from a manufacturing perspective, the addition of the cathode bias components allows tubes with a wider range of characteristics to be "plug and play" due to the self adjusting nature of cathode bias compared to straight fixed bias. You probably get more power output this way than just 100% cathode bias. Also a bit of protection should the bias supply fail.

Some silverface Fenders used a similar arrangement, as did some versions of the Vox AC 50.

[octal]

Leo must be looking at the bottom amp, not the top amp.

[Leo_Gnardo]

Leo must be looking at the bottom amp, not the top amp.

Right, the schemo came up magnified on my screen, all I saw was the usual 147 amp.

Now that I see the separate reverb amp, I concur with your #4. 7189 is a beefed up EL84. If you have one of these and can't easily find 7189's, you could put a trim pot in series with R28 & dial down the bias current to a level EL84 would get along with. Might be a thought to replace C23 with a higher voltage rated cap if the necessary bias voltage rises beyond -15V.

[GainFreak]

...you could put a trim pot in series with R28 & dial down the bias current to a level EL84 would get along with...

According to some Mesa schematics like DC3, F30 etc they are running EL84s at approx the same conditions unless they use some beefed up version.

[Merlinb]

I was looking at this Leslie 147RV reverb amplifier and noticed that it has simultaneously a fixed and kind of cathode bias or something.
So what's the purpose of that RC combo in the cathode?

[ATTACH=CONFIG]42845[/ATTACH]

The cathode resistor is only 33 ohms, so it makes very little contribution to biasing (1.4V). Mainly it just provides a convenient way to monitor the cathode current. If it were a low-wattage type it would also serve as a fuse. Ordinarily the cap is used to reducing distortion in the (class AB) output stage, but here it doesn't make a lot of difference since the resistor is so small.

[GainFreak]

Mainly it just provides a convenient way to monitor the cathode current.

I was suspecting something like that however you'll be able to read only the combined current.
I don't understand why even in modern amps most manufacturers are reluctant to put 1 or 10 Ohm resistors in each tube's cathode like couple of more cents will destroy their budget.

[Enzo]

They don't bother because such a very tiny percentage of people who buy their amps will even know what it was for, let alone how to use it.

[GainFreak]

Here's a funny thing. Recently a friend brought an ENGL for retubing. They bothered to use zero Ohm resistors (jumpers) to connect all power tubes' cathodes to a common ground bus but didn't bother to put 1 Ohm resistor (maybe worth the same couple of cents).

[Enzo]

Yes, there will always be examples to the contrary of anything.

[Leo_Gnardo]

According to some Mesa schematics like DC3, F30 etc they are running EL84s at approx the same conditions unless they use some beefed up version.

I get DC3 & F30 etc with the complaint "help my output tubes are melting" ! In goes a trimpot, often the bias voltage winds up around -17V. One sure thing about Mesa, especially their EL34 amps: just because Mesa is doing it, doesn't mean it's the right thing to do. Sure you can run EL84 at high voltage, the specs tell you 800V max ( ! ! ! ). But so what the spec sheet, experience shows us EL84 can last a long time and sound great in the 250 - 325V range. I hope experience still counts for something.

[GainFreak]

From what I know about EL84 they can take higher plate voltage but that doesn't apply for the screens. The specs show up to 400V if dissipation doesn't exceed 8 Watts so max plate voltage and less than max screen voltage looks like a survivable situation although it doesn't add much to the resulting power.

[g1]

Sure you can run EL84 at high voltage, the specs tell you 800V max ( ! ! ! ).

You mean EL 34 ?