EL34 and 6550 pins 1 and 8

[Randall]

I just retubed a Marshall head that came to me with Sovtek 6550WD in it with the sockets tied pins 1 and 8 together. These tubes will pull as much as 75 - 90 mA with the bias turned full down. I put new JJ EL34s in it, but the most I can get out of them is ~ 18mA/ea. I tried another set, and same thing. I notice on the 6550s pin 1 has no connection, with cathode and supressor grid tied together internally. With the JJs pin 1 is supressor, and pin 8 is cathode, tied together at the socket. Wouldn't this essentially be the same thing electrically?

Why are my EL34s pulling so much less current? The schematic shows EL34s, not 6550s. Also, the schematic shows a 56K bias supply resistor, this amp has a 47K in that spot.

https://www.thetubestore.com/lib/the...tek-6550we.pdf

https://www.jj-electronic.com/images.../el34_e34l.pdf

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Helmholtz]

What is the adjustable range of bias voltage? EL34s may need -38V or lower while 6550 need -44V to -51V in a 50W Marshall.

[Randall]

Very good point. Bias range is -47 to -62v. I find I can get it to -34 to -51v by tacking a 100K resistor across the 47K in series with the bias pot. Got it dialed in now, thanks for the help!

[g1]

There are a few other differences that may have been used in that circuit.
I would double check R24 & R25 (grid leak resistors) and R30 (bias diode feed). They should all be 220K for EL34, but may have been 150K's with 6550.

[Leo_Gnardo]

Very good point. Bias range is -47 to -62v. I find I can get it to -34 to -51v by tacking a 100K resistor across the 47K in series with the bias pot. Got it dialed in now, thanks for the help!

Good solution.

As far as the 6550s, I occasionally find some that need a bias voltage in the high 60's, even in the 70's, to settle them down. Kind of like Leonard Bernstein, those tubes really wanna conduct! In that case, either replace the 47K or put another resistor in series.

[Randall]

"I would double check R24 & R25 (grid leak resistors) and R30 (bias diode feed). They should all be 220K for EL34, but may have been 150K's with 6550."

OK, so this is odd. Schematic calls for 220K, 82K is printed on the board, and 150K is actually there for R24 and 25. I believe they are original. The diode 220R is correct. What to do? I really don't want to lift this board again if I don't have to.

[g1]

You don't have to.
But your parallel resistor may have been compensating (at least partly) for those being 150K instead of 220K.
I have a feeling this amp was originally fitted with 6550's.
Some here have mentioned the 220K grid leaks are fairly important to the EL34 Marshall tone, but the owner is not used to them, so I doubt he would notice.
Should he ever tell you his amp is missing some little something, compared to another of exact same model, it's a swap that might make the difference.

[Randall]

I'm not sure I follow.

"But your parallel resistor may have been compensating (at least partly) for those being 150K instead of 220K.

You are speaking in the past tense. You do understand I just now fitted the parallel resistor in the bias adjustment circuit, right? Is this about bias voltage on the grid? Presently I am at about 32/35.0A @ 475v.

I also recapped it, one can had a dead 50/450v. With fresh tubes and new caps, I think it should be an improvement.

And, what's up with the 82K designation on the board? No one seems to be paying any heed to that.

[vintagekiki]

EL34 is 25W plate dissipation pentode have g3 (pin1) derived at the tube base.
g3 (pin1) and cathode (pin8) must be connected together on the tube socket.
https://drtube.com/datasheets/el34-philips1969.pdf

6550 is 42W plate dissipation beam pentode where is g3 in the balloon connected to the cathode (pin8)
https://drtube.com/datasheets/6550a-ge1972.pdf

EL34 and 6550 for their work have different working conditions, so re bias adjustments should be performed.
If the bias cannot be adjusted by changing the RV1 22k, increase or decrease R26 56k

useful link
https://drtube.com/library/tube-datasheets#PPentodes
https://drtube.com/library/tube-datasheets#BPTetrodes

[nsubulysses]

There are a few other differences that may have been used in that circuit.
I would double check R24 & R25 (grid leak resistors) and R30 (bias diode feed). They should all be 220K for EL34, but may have been 150K's with 6550.

Just curious, but why would EL34 have 220K but 6550 have 150K grid leak? Is it because of pk-pk signal voltage needed to drive either tube to full volume?

[g1]

"But your parallel resistor may have been compensating (at least partly) for those being 150K instead of 220K."

You are speaking in the past tense. You do understand I just now fitted the parallel resistor in the bias adjustment circuit, right? Is this about bias voltage on the grid? Presently I am at about 32/35.0A @ 475v.

Yes, bias voltage at the grid. If the resistor feeding the bias diode (R30) is 150K, you get a more negative bias voltage range than if it is 220K. So you may have been compensating for the "6550" value of R30.
(Your schematic is for a newer version showing EL34 only, I'm using the schematic that shows values for both types of tubes).
As far as the "82K" marking for the grid leak resistors, does this board accommodate the 100W version with 4 power tubes? It could be with 4x 6550 they would use 82K's there.

Just curious, but why would EL34 have 220K but 6550 have 150K grid leak? Is it because of pk-pk signal voltage needed to drive either tube to full volume?

I think it is more about the maximums for grid resistance allowed by the datasheet. Although I believe the 220K value for EL34's is beyond spec. anyway.

[Randall]

Yes, this board accomodates both "Lead" and "100 watt" versions. So, as long as I have my bias voltage where I want it with room to play on either end, I should be good leaving the 150Ks in, correct?

[Leo_Gnardo]

Did we ever find out what model / year of build this Marshall head is? During the years the US importer demanded 6550 be fitted, at first there was confusion at Marshall, resulting in bias being set way too low (voltage) and causing 6550 output tubes to self destruct. Then, after having been "burned" by this experience, Marshall still flip flopped around, trying to set up US-bound amps so they would behave themselves. For instance, 82K bias feed resistors. And varying values of R's in the bias supply. FWIW I've found factory built Marshalls with 82K, 100K, 120K and 150K resistors in the bias feed position, regardless of any markings on the PC board. Granted, 82K's would be harder to drive without obvious distortion, and even more difficult with the stock 12AX7 invert/drive tube. Many accepted this extra distortion as part of the "Marshall tone." Others, especially those who wanted maximum clean volume (believe it or not some Marshall owners really want this!) substituted a 12AT7 and/or swapped the grid feed R's to values generally 150K to 220K.

What to do, to keep Randall's customer happy? Still a question...

[Randall]

It's a 1983 JCM800 Lead, model 2204, 50 watt. The board is Marshall ST1, iss. 202.

[Helmholtz]

If the customer wanted this amp converted to EL34s, he might also expect the "English sound" meaning the traditional 220k grid leaks (EL34s are fine up to 500k).