JMP 6550 to EL34 Conversion

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UsrName

Member
Good evening gents. I just picked up a 78 2203 with matching cab (G12-65 s). I've had other EL34 loaded Marshalls and after playing this one with 6550s for a bit I realize I prefer the more familiar sound of EL34s.

Anyway, I believe I have the correct info regarding resistor values to convert the bias circuit but not sure exactly what type parts to get, ie. brand, metal film, carbon film resistors, etc. Does anyone know for sure? Or does anyone have a link to a preferred parts site?

Thanks!

BTW, I'm really digging this cab compared to my blackbacks!
 
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Saw your post about this on TGP. Good places for parts:

Mouser
Digikey
Parts Connexion

For bias section, go with high-quality, modern metal film. 1 watt, 1% tolerance, 500v rating. Vishay/Dale CCF series are good. PRP are good. Beyschlag MBE0414 are good. Just make sure whatever you use fits the board and mounting holes.

For signal resistors in the PI and grids of power tubes, metal film also work great. I recommend PRP or Beyschlag. RN65D or RN70 are also good. So are those CCF.
 
^^ is spot on, also this might help. I’ve noticed allot of people just change the dropping resistor and call it a day. It’s a great visual representation and written by metro George so you know it’s accurate.
 

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So I've seen some people mention that leaving the bias splitters at 150 will not change anything and would lower the perceived output at bit when switching to EL34. I wouldn't mind this, as this amp has a lot of headroom anyway. Also, there's only two 6550s in at the moment and I'd like to have all 4 tubes in it.
 
I've also seen people mention only piggybacking the bias range resistor, which seems like the simplest way to get this done. What do you guys think?
 
Let's focus just on modifying the amp for EL34. Please list everything you initially thought you were going to do for that one goal.
 
Ok, here's what I initially planned:

swap 150k bias splitters for 220k
swap 15k bias drop resistor for 27k
change the negative feedback wire from 4 to 8 ohm tap
 
If simply piggybacking another 47k on the bias range resistor, and leaving everything else as is without affecting the tone, I would much prefer to not lift the board if I don't have to.
 
UsrName said:
If simply piggybacking another 47k on the bias range resistor, and leaving everything else as is without affecting the tone, I would much prefer to not lift the board if I don't have to.
Click to expand...
You can do this. It’s fine. It’s cleaner changing the 15k to 27k but not necessary to make it work.

Try a 220k or 150k over the 47k. That should get you in range for EL34’s.

I much prefer 220k splitters over 150’s. 150’s sound softer, or more brown, to me.
 
UsrName said:
Ok, here's what I initially planned:

swap 150k bias splitters for 220k
swap 15k bias drop resistor for 27k
change the negative feedback wire from 4 to 8 ohm tap
Click to expand...
Those changes should work. What value are the power tube grid resistors in your amp?
And yes, you can keep the 150K bias splitters/grid leak resistors if you want.
The NFB change isn't necessary to run EL34s. That's just a voicing change... personal preference.

UsrName said:
If simply piggybacking another 47k on the bias range resistor, and leaving everything else as is without affecting the tone, I would much prefer to not lift the board if I don't have to.
Click to expand...
What is your stock bias drop resistor? Is it 15K or 27K?
If the original is 15K, piggybacking (paralleling) a 47K on top = 11K
If the original is 47K, piggybacking another 47K on top = 24K
If the original is 47K and you want to get to 27K, you'd want to piggyback a 65K or something close to that.
If the original is 15K, you can't get to 27K with another resistor.
 
RedPlated said:
I much prefer 220k splitters over 150’s. 150’s sound softer, or more brown, to me.
Click to expand...

Yeah I think 220K is the way to go, but 150K can sound good depending on the rest of the circuit. It's not a huge difference either way.

Most people don't realize that lowering the value of the stock 220K is effectively like using a PPIMV. That's all a PPIMV is... a dual-pot version of these resistors. Lowering the value, like lowering the PPIMV knob, lowers the output of the amp, reduces the effectiveness of the NFB circuit (presence, depth as well), and reduces the effectiveness of the EQ controls.
 
RedPlated said:
You can do this. It’s fine. It’s cleaner changing the 15k to 27k but not necessary to make it work.

Try a 220k or 150k over the 47k. That should get you in range for EL34’s.

I much prefer 220k splitters over 150’s. 150’s sound softer, or more brown, to me.
Click to expand...
I like it a little browner so this should work for me, (there's a pun in there somewhere I'm sure, lol).

Just to be clear, the 47k resistor I'm referring to is the one right next to the bias pot. So I'd be piggybacking to this one.
 
FourT6and2 said:
What is your stock bias drop resistor? Is it 15K or 27K?
If the original is 15K, piggybacking (paralleling) a 47K on top = 11K
If the original is 47K, piggybacking another 47K on top = 24K
If the original is 47K and you want to get to 27K, you'd want to piggyback a 65K or something close to that.
If the original is 15K, you can't get to 27K with another resistor.
Click to expand...
The bias drop resistor is 15k. The 47k resistor I'm referring to is the one right next to the bias pot. So I'd be piggybacking to this one. Correct?
 
UsrName said:
The bias drop resistor is 15k. The 47k resistor I'm referring to is the one right next to the bias pot. So I'd be piggybacking to this one. Correct?
Click to expand...

Ah. Yes, the 47K by the bias trimmer adjusts the range of the pot. You can piggyback another resistor over that one to change the range of the bias trimmer.
 
UsrName said:
I like it a little browner so this should work for me, (there's a pun in there somewhere I'm sure, lol).

Just to be clear, the 47k resistor I'm referring to is the one right next to the bias pot. So I'd be piggybacking to this one.
Click to expand...
For EL34’s, that 47k should be changed to somewhere between 36-39k to get them to bias in range. So use a 220k or 150k in parallel with it to get those values. Just alligator clip each value in and check the bias. Use whichever one gets you to your bias point with room to go up or down.

And yes, it’s the 47k next to the bias pot.
 
FourT6and2 said:
Yeah I think 220K is the way to go, but 150K can sound good depending on the rest of the circuit. It's not a huge difference either way.

Most people don't realize that lowering the value of the stock 220K is effectively like using a PPIMV. That's all a PPIMV is... a dual-pot version of these resistors. Lowering the value, like lowering the PPIMV knob, lowers the output of the amp, reduces the effectiveness of the NFB circuit (presence, depth as well), and reduces the effectiveness of the EQ controls.
Click to expand...
Interesting. This might actually be desirable to tame the power and allow me to turn the master up more.
 
RedPlated said:
For EL34’s, that 47k should be changed to somewhere between 36-39k to get them to bias in range. So use a 220k or 150k in parallel with it to get those values. Just alligator clip each value in and check the bias. Use whichever one gets you to your bias point with room to go up or down.

And yes, it’s the 47k next to the bias pot.
Click to expand...
Perfect. This makes sense, I'll give it a shot and report back.
 
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