Getting JMP 2204 bias in range

[RussBert]

Any considerations when switching to EL34's from the 6550/KT88? Can I just pop them in and bias accordingly? thanks.

Yes. The bias circuit must be modified for amps that came stock with 6550's to run EL-34's

 

[Mxzx]

Hello, I am fighting the same issue on my '79 2204.

I have plate voltage of 405 with EL-34's. I can only bias up to about 29ma. My calcs say about 43.2 (25 divided by 405= .0617 x .7= .0432).

I have a 47k bias resistor that appears to be stock, I'm guessing I would have to lower that resistor to bias up? Any suggestions? I think I have 27k and 33k's here.

Thanks

 

[jgab]

Some tubes just draw more current for some reason. I had a pair of EH EL34 tubes that I just couldn't get up to where I wanted them biased. I changed to a different brand of EL34 and everything fell right into place.

 

[Mxzx]

I just gave that a try, no difference. Mine looks as it came with EL-34's, but who knows for sure, is there a way to ID that?

Thanks

 

[Mxzx]

Also, any idea what is going on here with the added cable to V1?

 

[mickeydg5]

Hello, I am fighting the same issue on my '79 2204.

I have plate voltage of 405 with EL-34's. I can only bias up to about 29ma. My calcs say about 43.2 (25 divided by 405= .0617 x .7= .0432).

I have a 47k bias resistor that appears to be stock, I'm guessing I would have to lower that resistor to bias up? Any suggestions? I think I have 27k and 33k's here.

Thanks

The amplifier may be setup more for 6550.
Can you post a good picture of the insides around the bias circuit?

 

[Mxzx]

Sure here are a couple, let me know if you need more.

 

[mickeydg5]

The gray cable is sheilded and is used to cut noise feeding guitar signal to V1.

The amplifier is setup for 6550 power tubes. Did you get it loaded with EL34 tubes?
I guess the decision that needs to be made is whether you would rather 6550 or EL34 power tubes.

 

[paul-e-mann]

Did you see the cost of 6550 tubes? I'd go EL34 just for the savings and easy availability.

 

[mickeydg5]

I figure he may say the same. But those who like 6550 sound stick with them.

 

[Mxzx]

Yes, I bought it as you see it. It has the grid resistors, I think they are called, across the power tube pins, does that make it set up for EL-34's?

What do I need to do to set up for EL-34's, change the bias resistor?


Thanks

 

[mickeydg5]

The big black resistor located on the power tube socket is for the screen. Those are fine as is.

You will have to change things on the board to cater to the EL34 tubes.

Are you familiar with tube amplifiers, the components, discharging power supply capacitors and dealing with high voltages in there?

If not, it is best to take it to a technician and have it converted for EL34 use.

 

[MarshallDog]

I have a 1980 JMP 2204 and it came all stock with no mods and 6550 tubes. I took my amp to my local Marshall Authorized Amp Tech so he could convert it over to EL34's as I had never done this before and couldn't get the bias current up within the 70% range and didn't want to chance screwing it up. After I got it back I opened her up to check his work and see what he did and all he did was piggy back a 47K ohm resistor onto the 47K bias resistor...that's it. He also said that sometimes depending on the amp and the condition of the bias pot, you may need to tweak this value up or down a bit so you can get your bias current as needed.

I have had the amp for two years now and it plays and sounds great with no issues. I have changed tubes many times and bias the tubes to 70% using the Weber BiasRite Meter and the Weber Bias calculator which simply uses a 70% max.

Hope this helps...

 

[Mxzx]

Yes, I know how to work on them, solder, discharge, etc.

MarshallDog, I figured this is all I would have to do. Just didn't know what value of resistor to piggyback. I have a 50w Metro Plexi clone that I was having the same issue with. I posted on the Metroamp forum same question and got the correct value and fixed it, but the Metro forum is currently down.

Figured there would be some good knowledge here as well!!

I think I will solder a 33k on it and see how that does. I think I put a 27k on the Metro but not sure, guess I could pull it apart.

Thanks

 

[mickeydg5]

Yes that is the easiest way to do it, but not standard. Hopefully the 33k will not drop the resistance too much.

 

[Mxzx]

Yeah, I guess we will see.

Another theory question, if his tech piggy backed on another 47k, does that in effect make it 23.5k?

 

[mickeydg5]

19.39k

 

[Mxzx]

Just found the resistor value calculator!

Looks like his is in fact 23.5k, my scenario would be 19.4 as you point out above, way to little.

i think I have a spare 47k that I will put in series and try 23.5k.

If no good, then I guess remove 47k and start with 33k then 27k?

 

[mickeydg5]

Unless you have a 220k to swap out with the 120k bias supply dropping resistor. That would be the best place to start if changing out components.

 

[Mxzx]

I meant to say parallel or piggyback the 47k on the existing, not series, for 23.5k resistance.

What do you mean about swapping a 220k for 120k? Would it hurt anything to try the piggyback first?