Mixer Values

[rgorke]

So, I am pretty sure that Marshall went from 270k/500 (or is it 250?) to 470k/500. So, my question is what is the tonal difference? Is it gain or is it a frequency shift?

[neikeel]

Early amps used 270k mixer with 560pF bypass cap (typically a flat red Lemco silver mica or brown RS), in mid/late 67 the 470k resistor was introduced, initially again with flat 560 or 556pF bypass cap, then you see 500pF RS cap in 68. Later the 470pF caps appear, sometimes you see a Murata tubular ceramic in 67 (not sure of value - Tazin probably knows).
The 470pF tubular red Lemco ceramics appear late 67, typically with the metal panels although the flat Micas still cropped up.

As far as effect the 270k/560 is a little cleaner IMO (better with 560 rather than 470pF btw. The 470k gives a little more grind. Of course a lot will depend on the rest of the amp spec (tone stack, PI couplers an NFB etc).

[rgorke]

I am sourcing parts (albeit very slowly) for a JTM 50 build similar to your block end. I was pretty sure you used (Marshall used) 270k. And the 560, 556, and 500 flat Lemco and RS caps are getting harder and harder to find.

[neikeel]

I am sourcing parts (albeit very slowly) for a JTM 50 build similar to your block end. I was pretty sure you used (Marshall used) 270k. And the 560, 556, and 500 flat Lemco and RS caps are getting harder and harder to find.

On that build I initially copied my original (270k/556pF Lemco) but it did not sound the same as the original so I subbed to 470k with the same cap and it just sounded 'right' like that. I hope page 361 agrees?

It is a frequency shift phenomenon and will depend on how you run the amp. I crank the mids, treble, presence and vol 1 pot to around 8, the bass on 2 and the unused input at between 3 and 5. You may find with your settings different values suit you better.

470k/560pF gives you mids and upper frequencies boosted.

[Tazin]

The Murata tubular ceramic caps are 500pF with a 5% tol.

[rgorke]

Ok, so if one would want to have a similar sound of 470k/500pf, using 270k resistors, one would need a larger cap, right?

So, if I am calculating correctly, the 470k/500pf, the cut off frequency is at 677 hz. To get the same cut off, one would need to use a 1000pf or .001uf cap, right?

[neikeel]

Sounds about right but 560pF works well.

[rgorke]

Thanks, I was just confirming the math.

And, did you mean 560pf with 470k or 270k? Or both/either?

[neikeel]

with 270k, but you may need to tweak the resistors to suit your ears!

[maxwedge572]

I know for a fact that going to a larger value cap the amp gets brighter (more low cut) and I'm about 90% sure that going a larger resistor equals less bass cut. I have 270k mixer values and a 595pf cap in my JTM45 clone and I think switching to 470k may improve it because It's a bit bright.

Edit: I was wrong about the resistor part. A higher value resistor in this circuit, with no other changes, will make the channel brighter too.