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

[Greg_L]

Sorry for confusion. Please see my edited text. Pin 1 and pin 8 are not internally connected

Oooooh, right. Pin 1 truly does nothing.

So.....reboot......only pin 8 should go to ground through the 1 ohm resistor? Pin 1 should be left completely isolated?

If pin 8 is to be grounded, then does it matter if it goes through pin 1 since pin 1 does nothing?? All that really does is force me to use a different bias reading method, correct?

[vintagekiki]

For the 6550, the cathode and screen g3 are connected in the tube itself and executed at pin 8.
To adjust the quiet current connect a 1 Ohm resistor from pin 8 to the chassics.
Pin 1 is neutral (N / C)

[Helmholtz]

it's my method of reading the bias that's misleading me. Is that correct?

Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.

[Greg_L]

For the 6550, the cathode and screen g3 are connected in the tube itself and executed at pin 8.
To adjust the quiet current connect a 1 Ohm resistor from pin 8 to the chassics.
Pin 1 is neutral (N / C)

Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.

Ok, good information. This clears some things up. Thank you very much. I'll implement these ideas later and report back.

[g1]

Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.

I thought it improbable too, especially with the spring type retainers. BUT it is the only thing I can think of that explains the B+ dropping so much with the 6550's compared to the EL34's, even though the measurement suggests they are running colder.

Oooooh, right. Pin 1 truly does nothing.

So.....reboot......only pin 8 should go to ground through the 1 ohm resistor? Pin 1 should be left completely isolated?

If pin 8 is to be grounded, then does it matter if it goes through pin 1 since pin 1 does nothing?? All that really does is force me to use a different bias reading method, correct?

Just to be clear, you are correct that there is no issue leaving the pins this way, just causes problem reading idle current at cathode.

[Greg_L]

I thought it improbable too, especially with the spring type retainers. BUT it is the only thing I can think of that explains the B+ dropping so much with the 6550's compared to the EL34's, even though the measurement suggests they are running colder.


Just to be clear, you are correct that there is no issue leaving the pins this way, just causes problem reading idle current at cathode.

Well friends, I'm not exactly sure what's happened, but with the amp's bias supply back at EL34 specs, these 6550s are working. Working too well actually. They are pulling the juice! I can't get the bias low enough now.

I put the bias supply resistor back to 15k (resistor in parallel with jumper wires) and I can just get them down to around 50-55ma. About -51vdc at the grid. Plate voltage is holding steady at 455. They're glowing blue-ish with no red plating....so far. Grid stoppers are still 220k though.

The tube bases are not shorting to ground anywhere that I can tell, and I am getting solid readings across my 1 ohm resistors with pins 1 and 8 still tied to ground through the resistors.

So what do yall make of this? Any thoughts?

[Leo_Gnardo]

Well friends, I'm not exactly sure what's happened, but with the amp's bias supply back at EL34 specs, these 6550s are working.

Sometimes, to make progress, you have to take a step backward. Which step ... that is the question.

Working too well actually. They are pulling the juice! I can't get the bias low enough now.

I put the bias supply resistor back to 15k (resistor in parallel with jumper wires) and I can just get them down to around 50-55ma. About -51vdc at the grid. Plate voltage is holding steady at 455. They're glowing blue-ish with no red plating....so far.

OK now you have to get that bias voltage up a tad. Vb somewhere in the mid -50's looks like your target area. 40 mA on 6550's plenty enough current. I just had a '74 Super Lead come thru here last week, it had Shuguang 6550's, that's where I set it and it appeared to be quite happy turning out a healthy 111 watts.

It's not the stopper resistors that are 220K I hope, but the bias distribution R's. Stoppers are 5K6 I hope. The ones right on pins 5.

[Greg_L]

Sometimes, to make progress, you have to take a step backward. Which step ... that is the question.

It's entirely possible, and likely, I just did something wrong before. I put the bias circuit to 6550 specs and got nothing. I put it back to EL34 specs and everything works. Who knows.

OK now you have to get that bias voltage up a tad. Vb somewhere in the mid -50's looks like your target area. 40 mA on 6550's plenty enough current. I just had a '74 Super Lead come thru here last week, it had Shuguang 6550's, that's where I set it and it appeared to be quite happy turning out a healthy 111 watts.

It's not the stopper resistors that are 220K I hope, but the bias distribution R's. Stoppers are 5K6 I hope. The ones right on pins 5.

Yes that's what I meant. Sorry, I don't quite know all the proper terminology and get them confused sometimes. The bias feed resistors are still 220k....supposed to be 150k for 6550s.

According to my plate voltage and current readings, I'm dissipating about 25w per tube.. But these Sovteks 6550WEs might be 42w 6550s??

If I'm on the right track, I'd like to just fine tune the bias supply voltage and leave it. How critical is it that the bias feed resistors actually be 150k?

[Helmholtz]

How critical is it that the bias feed resistors actually be 150k?

150k resistors are safe with both 6550s and EL34s. 220k is too high for 6550s.

[Greg_L]

150k resistors are safe with both 6550s and EL34s. 220k is too high for 6550s.

Okay, thank you.

[Chuck H]

It's entirely possible, and likely, I just did something wrong before. I put the bias circuit to 6550 specs and got nothing. I put it back to EL34 specs and everything works. Who knows.

I think this has happened to all of us. When you're sure you've checked and rechecked your work, and "it" (whatever "it" happens to be) still isn't working. Then you change things back and "it" is working. Odds are if you rewired for 6550's again that it WOULD work. You'll never know where an error might have been though. Just take peace in the fact that it hardly matters now. Once you bump the bias voltage a little and install the 150k bias feed resistors (you might want to measure those if you reuse the ones you pulled) it's all the same and good to go. Glad it's worked out.

[Helmholtz]

Okay, thank you.

Actually 6550 manufacturers recommend a max. grid resistance of 50k per tube! But 150k for 2 tubes in parallel (corresponding to 300k per tube or exceeding the limit by 500%) seems to have worked with Marshalls.

[vintagekiki]

What does not connect in series 150 KOhm and 68 KOhm so with mini switch have a choice 150 KOhm or 220 kOhm

[g1]

Actually 6550 manufacturers recommend a max. grid resistance of 50k per tube! But 150k for 2 tubes in parallel (corresponding to 300k per tube or exceeding the limit by 500%) seems to have worked with Marshalls.

The earlier Unicord schematic does show the 6550's have 47K grid leaks for the 6550's. The JCM800's used 150K's.
https://drtube.com/schematics/marshall/1959mk2u.gif

[Leo_Gnardo]

The earlier Unicord schematic does show the 6550's have 47K grid leaks for the 6550's. The JCM800's used 150K's.
https://drtube.com/schematics/marshall/1959mk2u.gif

Interesting! Can't say I ever saw 47K's in that position. Occasionally 82K though, factory installed.