JCM 800- neg voltage too negative

[pontiacpete]

This amp has too negative a bias supply, I can only dial in -60 to -50 volts. Is it best to increase the value of the first resistor off the winding, i.e. the 120K to something like a 220k?
pete

[Gtr_tech]

Its set up for 6550s. The bias supply needs to be changed to correctly handle the bias range for EL34s. Just compare the schematics for each version and that should show you what needs to be changed.

From Plexi Palace:
""European" values for JCM 800s with EL34s are:

22k variable resistor = bias adjust pot
56k divider = resistor in series with bias pot
220k bias splitters = the two resistors connected to the other end of the divider resistor
Purple feedback wire on 8-ohm tap

Amps built for the US market with 6550s had/have:

22k variable resistor = bias adjust pot
47k divider = resistor in series with bias pot
150k bias splitters = the two resistors connected to the other end of the divider resistor
Purple feedback wire on 4-ohm tap

A "complete" conversion would entail changing the components to make the US amp look like the European amp. The main issue is getting the voltage adjustment range into the right area.

A simple procedure that works is (copped from the Voodoo Amps website):

MARSHAL 6550 to EL34
1.) Locate the 47K resistor above the bias pot and solder another 47K in parallel.
2.) Set the bias pot the max (around –38 volts).
3.) Locate the purple feedback wire (that is connected to the 4 ohm speaker tap) and solder it onto the 8-ohm speaker tap.
4.) Install the EL34’s and re-bias the amp.

I did this mod to my JMP 2203, and it works fine. I tried the 220k bias splitters and decided I liked 150k better so I put them back in. YMMV.

Just look inside and see if any of this jives with what you see in your circuit."

[jrfrond]

Or just try 6550's. They sound quite good in those amps.

[pontiacpete]

Thanks guys, I will go with the el34 since the owner says he'd like to keep them in there.

As for your description of the bias circuit, gtr_tech, it is set up exactly like the US specs. I paralleled a 47K and it works like a charm.
As far as the purple feedback wire goes, I think it is hooked up to the 8 ohm lug. But still trying to determine if I'm right about that.

[Gtr_tech]

It wasn't my description, I just did a quick Google on the subject and copy/pasted it rather than typing it all out. There's no voodoo about it tho. Its just a voltage divider and you need to shuffle around a resistor value or 2 to change the range.

I don't care for 6550s in those, but *real* 6ca7s sound nice. Cant speak for the "re-issues"...haven't tried any yet. With the 6550 setup, 6L6s could be interesting with a screen resistor change.....although not "mar$hall purist" approved....