Period correct bias spitter values 82K or 150K for 6550's

[harddriver]

I had a killer 1979 2203 that I am trying to duplicate as much as possible. For this year which bias splitter value was used for 6550 tubes?

I seem to recall 150K and that is what is shown on the Marshall schematic for the 1976 2203 model. I no longer have my original PCB circuit board for reference, but in pics of others it seems 150K was used in the couple I have seen.

I am mainly trying to duplicate the values used from 1978-1980.

I also seen Marshall used 82K bias splitters for 6550's in 1970 for their 1992 Bass Amplifier schematics.

150K's or 82K's? for 1978-80

[axeman]

Q: On a Marshall the bias feeds the two grid resistors 150k for 6550 and 220k for EL34, why do they change this?

A: The 6550 has a lower maximum grid #1 circuit resistance specification than the EL34. Actually, it is spec'd for 50K max in fixed bias, 250K max in cathode bias, but is typically run at 100K in fixed bias. The EL34is spec'd for 700K max in cathode bias and 500K max in fixed bias. Too high a grid resistance can cause the tube to fail because there is a small amount of gas in the tube which can cause grid current to flow. This grid current will cause a voltage drop across the grid resistor that will increase the bias current in the tube and in turn, increase the plate current. This can eventually end up in a runaway condition and the tube will redplate and croak.

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[harddriver]

Thanks for the reply Axeman, I'm going to use the 150K's. I only found one pic of an original Fawn tolexed 2203 with 6550's, the second band on the resistors were hard to read but ended in yellow. THe first band was brown I am assuming the second was green it just didn't look green in the pics.

I just started the build today. A 2203 with an extra gain stage as well. This buildis for my brother so it has to live up to my old 2203 as he remembers it and then get into Cameron territory of gain.

It should be a cool build!

All the other pics of 1976-1982 2203's that I could find of circuit boards were of 2203's that were running 220K's for EL34's.

[axeman]

What type or were did you get the idea for the extra gain stage.

[harddriver]

I am adding an additional preamp tube up front via the Roccaforte/Kevin Oconner design and then feed into the input grid of V1a. The Roccaforte/Oconner layout has been floating around metro for a long while.

Most just convert their cathode follower into an addtional gain stage but I wanted to keep the one input totally stock 2203 then use my other input for the extra gain stage so he can choose regular 2203 or high gain. Maybe one day I will make them footswitchable with a relay.

[joshwilson3]

I am adding an additional preamp tube up front via the Roccaforte/Kevin Oconner design and then feed into the input grid of V1a. The Roccaforte/Oconner layout has been floating around metro for a long while.

Most just convert their cathode follower into an addtional gain stage but I wanted to keep the one input totally stock 2203 then use my other input for the extra gain stage so he can choose regular 2203 or high gain. Maybe one day I will make them footswitchable with a relay.

I haven't heard about that extra gain stage mod. I'm gonna get a Metro 2203 built, and George put in an extra preamp hole. So, I figured I'd put in an extra gain stage via a tube, but wanted to still be able to have the stock 2203. So this looks promising.

Is that extra gain stage like a Soldano or something different?

[harddriver]

It should have as much gain as a Splawn/Roccaforte both of these amps use converted cathode followers in V2 with circuit tweaks. I have my own tweaks that I will be using on this amp but the general way to add a gain stage is public knowledge out there and there are quite a few layouts floating around metro. You could tweak the circuit more towards SLO values if that is the tone you are looking for.

To be honest Josh, with the amount of gain you are looking for sounds like a Splawn would be exactly what you are looking for, I'm not trying to suggest you don't buy a 2203 from George because he makes great products I have purchased many kits from George and love them. By the time you get done modding the 2203 you may have more money into it than if you bought an already modded Splawn and they are very well built as good if not better than a vintage Marshall, used they sell very reasonable.

I had a 2203 rebuilt by Scott Splawn years ago before I got into building my own amps(THANKS GEORGE and the METRO GANG), I love that amp, he has done all the R and D and produced a very nice high gain circuit based off a 2203.

I like Splawn's older mods and early Quickrods, the two button footswitch with two gain levels, pretty much stock 2203 and lead, high gain with 6550's/KT-88's. With the two button footswitch that is essentially what you have a stock 2203 amount of gain and then high gain, as much as any SLO I've heard they are just voiced different. I think Splawn sells this style amp now as a ProStock model you would have to check. GO check out some youtube clips or his site.