Marshall JMP Biasing 6550 or EL34

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Cap217

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I have another thread about a 74 50w I picked up that had a lot going on. It is biasing to only 22-23ma with EL34s so we assume its made for 6550s. I also have a 1980 JMP 2203 that has been sitting since I bought it so I checked the bias and I get the exact same thing of 22-23ma max with EL34.


I am trying to learn about the 6550 vs el34 resistors and what is needed to work. There is a lot of info out there and it helps but I have some questions that arent clear answers in my research.


6550 = 15k pot resistor, 150k tube grid resistors
EL34 = 27k pot resistor, 220k tube grid resistors


I found that you lower R30 from 47k to 33k to be able to bias in range. This makes sense but my amps are different:

1980 JMP - 47k R30 and 150k grid
1974 - 47k R30 and 220k


So it seems my 1980 is setup for 6550s and the 1974 for el34. I am thinking I lower the 47k on the 74 and run el34s and then I just use 6550s on my 1980.


Questions:

  1. Can someone point out R26? Is this the 15k resistor that is above the 47K R30 resistor?
  2. Does this 15k resistor need to be changed to 27k for el34? Or does lowering R30 47k to 33k do the same thing? This is where I get confused.
  3. My 1980 2203 has 510 plate voltages. What 6550 is best to handle these or are 6550s able to handle more plate voltage than el34s?

Thanks
 
Pics of my 74 and 80. I circled the 15k which I assume is r26.
 

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panhead said:
you can piggy back that resistor to get in range
Click to expand...

The 15k or the 47k? I thought I would piggyback a 120k over the 47k to get to 33k and it would allow me to bias EL34s. Thats the plan on the 74. But the 1980 has 150k grid so I dont know if I need to change those as well?

Also curious about the 15k and how it works
 
Cap217 said:
The 15k or the 47k? I thought I would piggyback a 120k over the 47k to get to 33k and it would allow me to bias EL34s. Thats the plan on the 74. But the 1980 has 150k grid so I dont know if I need to change those as well?

Also curious about the 15k and how it works
Click to expand...
I think that would be fine but I didnt write the book get a second opinion from some who builds on here, I dont
 
Cap217 said:
The 15k or the 47k? I thought I would piggyback a 120k over the 47k to get to 33k and it would allow me to bias EL34s. Thats the plan on the 74. But the 1980 has 150k grid so I dont know if I need to change those as well?

Also curious about the 15k and how it works
Click to expand...
FYI. Redplate has given me a ton of info. Sometimes I have more questions and don’t want to bother him. Like more detail on exactly why vs him just saying swap this for that to get this effect.

Learned a lot from him offline.
 
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