Pros and cons of changing V1 cathode resistor values?

[JimmyR]

I recently built another 1987 50W amp. I always link the channels, and was thinking of using a dual 1MA pot to operate both simultaneously so I didn't have to keep adjusting both. But I have noticed that I always have the bright channel a tiny bit lower than the normal channel. My hunch is that it's because the cathodes are biased differently - the bright channel has a 2K7 resistor whereas the normal has an 820.

I have always thought that the normal should have a 1K5 seeing as the cathodes have been split. So what if I used a 1K5 for both channels? (IE one on each channel - a 1K5/.68 for bright, and a 1K5/250uF for normal.) Would that make each channel the same volume (more or less) or would it kill my tone? Has anyone tried this?

TIA!

[neikeel]

http://www.duncanamps.com/tsc/

With what you propose in general:

Channel 1 will have more gain (and may be fizz if you use hot humbuckers, less so with PAFs)

Channel 2 will have less gain but potentially the same muddiness

The calculator will tell you where the frequencies ar that will be most affected.

Have you though about a 1k5/25uF combination for V1a with a small (100pF) cap on vol 2 pot. Then balance that with a 2k7/0.68uF on V1b (with or without cap on the pot)?

the choice of 820R was for a shared cathode with 250uF on a Bassman. To correct for that same bias itis my understanding that 1k5 on each cathode will give the same bias point, but of course the size of byass cap will influence the shelving frequency. A really big cap will pass all the lows - including a lot you cannot hear) and the 0.68 will have an upper mid focus.

[JimmyR]

Yup - that's why I thought using a 1500 on each cathode would equalise the levels.

I think the idea of using a 25uF on the normal channel is a good one - I usually use a 25uF in my Fender tweed style builds as it lets all the guitar frequencies through but not the rumble - that's my understanding, anyway. The only reason I went with the 250 was authenticity. But I guess if I'm going to change the resistor there goes my authenticity!

I'll just have to suck it and see, won't I? Thanks for the response - you have pretty much confirmed what I was thinking. I was hoping someone might have tried it already and could provide concrete evidence.

Cheers!

[neikeel]

I'll just have to suck it and see, won't I? Thanks for the response - you have pretty much confirmed what I was thinking. I was hoping someone might have tried it already and could provide concrete evidence.

Cheers!

My favourite JTM50 has a 25uF/820R on V1 (shared cathode)

It is a silver WIMA low voltage correct for the era and was in it when I bought it, one of the solder joints looks original the socket side retouched. Original spec would have it as 250uF of the same type but they were using that size on the 18watters in '67 so it might have been a 25 in the wrong box? Anyway sounds good with plenty of gain for what it is a nice balanced tone which cleans up with guitar too.

[JimmyR]

That's a beautiful looking amp. I love seeing inside old Marshalls... Yeah I know!

FWIW I use the 25uF/820 in most of my tweed amp builds. I just swapped out the 820/250uF cap arrangement in my 1987 for a 1500/25uF. I left the bright channel as 2K7/.68 as I figure that's a key part of the Marshall sound. We'll see.

[yladrd61]

I like the '68 Spec 820 / 250uf and 820 / .68uf with 100pf bright cap, .022uf coupler no .68uf bypass cap.

[neikeel]

I like the '68 Spec 820 / 250uf and 820 / .68uf with 100pf bright cap, .022uf coupler no .68uf bypass cap.

Cool you like that spec. but it was never a stock '68. Sure the '68 used 820R/250 and 820/0.68uF but then a 0.0022uF coupler on V1b a 5k pot bypass cap and a 0.68 on V2a with other couplers of 0.022uF.

Early shared cathode lead amps use the 100pF on the pot and later organ spec amps were effectively like the early lead amps. In fact these are the sweetest plexi type amps I have (like the one you see above and the 45/100). I did have a 1970 Organ (1989) that was the same. Funnily enough I thought I wanted that amp to be a lead 1987 so went the whole hog but it was harsh and bright, with or without cap on pot so I returned it to original. Quite why I do not know as it had Iskras and mustards all through, maybe the Drake -139OT of that year - don't know.
So in short tweak each individual amp to your ears and style not a 'magic' formula