Metropoulos Forum

Hi, I've made a couple of mods to my JTM45 already, fender tone stack, 250uf V1 bypass cap, 47k NFB.

I've been comparing the schematic to a few other amps and lots of them have a cathode bypass cap on V2, can anyone tell me how this effects the sound? For reference: For my taste I'm one foot more in the fender sound camp than the plexi.

Thanks,
Ben

chopperferrari wrote:Hi, I've made a couple of mods to my JTM45 already, fender tone stack, 250uf V1 bypass cap, 47k NFB.

I've been comparing the schematic to a few other amps and lots of them have a cathode bypass cap on V2, can anyone tell me how this effects the sound? For reference: For my taste I'm one foot more in the fender sound camp than the plexi.

Thanks,
Ben

Depending on the size of cap you will allow certain frequencies to pass through that stage, so effectively a gain boost. The 330uF gives mush across the tone range (I don't liek it at all but some do) and will be far from the Fender territory you suggest. A 0.68 or 0.47 boost upper mids which you might like, but keep the NFB in the power amp tight (like stock 27k/16ohm).

Hi Neikeel,

Thanks for the help. I've already swaped out the cathode bypass and resitor on V1 to 250uf & 1.5k. What i'm looking at is adding a capcitor across the V2 820ohms resistor- same as in some other amps, any ideas as to what results this would lead to/ if it will even work?

Thanks,
Ben

Hi Ben

Going to 250uF on V1 makes virtually no difference from 330uF. You would be much better going to 25uF or lower.

Yes I was referring to V2a when I replied. The early VH modders used 330uF (AFAIK some still do - but it is not the correct way ). If you want to give the amp a bit more poke and lose some flub decrease the V1a coupler to 0.01uF or lower and add a 0.68 or 0.47 across V2a (a 100v rated cap will be fine)

Neikeel! You've changed my life! I've been trying a 0.68uf and 0.25uf in v1 cathode bypass and it is a total game changer! Thanks!

Is the V1a coupler is that .022uf running off the plate of v1a?

Good job you asked as I meant V1b (ie the second cap along). I have to say if it sounds good now I owuld be rempted to stop there, but in the interests of knowledge give it a go

I've been doing some more experimenting. I'm wondering about the effect of changing the plate bypass caps on v1 (not sure if that's what they are called or coupling caps'). I've got one for V1a and one for V1b 0.022. What would be the effect of increasing or decreasing its value?

Thanks,
Ben