Help please! 1973 Marshall Super Lead - EL34 to 6550s bias question

[Greg_L]

You alter the amp to accommodate your tubes, or you alter your tubes to accommodate your amp. Those are your choices. Altering the amp requires soldering parts.

I've soldered parts. I've soldered the proper resistors all over this thing. I'd just rather not change that pot. That requires ordering parts and waiting. And frankly, these tubes are so far off I don't even know what pot I'd need...and it might not even work!

I'm gonna try a different measurement method. I'll be installing the 1 ohm cathode resistors this evening.

[Greg_L]

I've had to increase pots ,or resistors more than once to get the bias where I need it.

Which resistor would I need to change? The 47k? Would I need to decrease it? Can I jumper in a parallel resistor just to see what happens?

[g1]

I'm using the figures off of an old Unicord schematic with figures for USA 6550s.

15K before the diode
15K with the 10uf bias caps
47k in series with the bias pot
150k grid leaks
All checked and verified with an ohmmeter.

Were these parts all there when it was running EL34's? If not, which did you change?

[vintagekiki]

About biasing tube guitar amplifiers

https://music-electronics-forum.com/showthread.php?t=8291

Bias adjusts depending on the + HV and working classes of service (A, AB, B) to the specified anode current value.

http://web.archive.org/web/20120623045707/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Power Tube Bias Charts

http://web.archive.org/web/20110621211343/http://diyguitarist.com/Images/BiasChart-EL34.jpg
EL34 Bias Chart

http://web.archive.org/web/20110621211356/http://diyguitarist.com/Images/BiasChart-KT88.jpg
KT88/6550 Bias Chart

[Greg_L]

Were these parts all there when it was running EL34's? If not, which did you change?

The grid leaks were 220k for EL34s. The bias power resistor was 27k. I piggyback paralleled resistors to get them down to 6550 specs. All readings have been verified.

When I got the amp the conversion was halfass complete. I put it to proper EL34 bias specs and it worked perfectly. Now I'm putting it back to it's original 6550 specs and it isn't.

[Greg_L]

Update -

I soldered in a 1 ohm precision resistor from pins 1&8 to ground on one power tube socket. Took a reading across the resistor. I get the same reading as with the bias probe, so the probe is telling the truth.

Just for kicks, I jumpered a 150k across the 47k that lives in series with the bias pot. That dropped the 47k to around 35k. My bias reading went up at the tube. Not much, but it did go up more in the right direction. My light bulb limiter got brighter though. It went from a very dull dim glow to noticeably brighter, so something is drawing more amps. Maybe these tubes are just bad.

[Enzo]

Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.

[The Dude]

If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

Edit: Enzo beat me!

[Greg_L]

Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.

If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

Edit: Enzo beat me!

Ha, yeah, it just seemed to get brighter than I thought it should. Maybe? I don't know. It's a 300w bulb.

I had a rectifier wired backwards one time. That thing got really bright!

I'm gonna put it back to EL34 configuration and get readings with the EL34s.

[Enzo]

I may be old, but I can still be quick.

[The Dude]

I'm not sure the Mrs. would appreciate that statement.

[mozz]

Taking bias readings with the light bulb or not?

[Greg_L]

Taking bias readings with the light bulb or not?

Both

[The Dude]

Taking bias readings with the light bulb or not?

Good point! Bias should be set without the limiter at full/normal line voltage.

[gtrplayr1976]

Both

Don't use the limiter to bias. You need full power.