1w jcm type build

[alexburchill]

Hi,
I've built a little 1w jcm style amp with some modifications to the circuit to mimic the #34 mod.
The problem is that it's extremely bright I mean I need to keep the bass on 10 mid/treble on 0 kind of bright!!

I've attached the schematic if anyone could possibly take a look and see what they think would be greatly apprecticated!!

I've already clipped the treble cap on the gain pot and clipped the high pass cap after stages 1&2 but it's still really bright.
Any help would be great thank you.

Here's the link
https://www.dropbox.com/s/iiy7a9sh07ttu ... e.pdf?dl=0

[romberg]

Many resistors on your schematic do not show any values. What are the values of the tone stack pots? What is R15? If R15 is large enough it would more or less take the whole tone stack out of the circuit.

[alexburchill]

Hi thanks for the reply,
The pots are bass 1m log, mids 22k Lin,treb 220k lin
R15 is also 1k

I was told that the bypass caps are too small and it's creating a treble boost through out the whole amp?

[romberg]

The pots are bass 1m log, mids 22k Lin,treb 220k lin
R15 is also 1k

OK. Your tone stack looks pretty much the same as the "big" amp then.

I was told that the bypass caps are too small and it's creating a treble boost through out the whole amp?

The 22nf bypass caps are what is used in the 2203/2204 pre-amps. I would not say they are "too small". They are what was used and part of what gave those circuits their sound. They are brite amps to begin with. Have you ever played a 50w 2204 or a 100w 2203?

Of course the big amps have a totally different power amp section. And it might be worth tweaking things starting there. A general rule you often hear about voicing high gain amps is to take out the bass early in the circuit and then let it back in later.

The cap after the master volume that connects to the power tube would be a good one to try increasing. On a 2204 it is 22nf. But the 2204 drives a phase inverter. So, that is where the major deviations start. You probably don't wanna change all the 22nf coupling caps as then the amp will sound nothing like a big jcm. Which I think is mostly the point of this 1 watter .

Mike

P.S. The cathode bypass cap on the power tube could also be increased to bump up the bass there. I might also play around with the value of the cap on the secondary side of the output transformer.

[romberg]

Also, check out that 47 ohm resistor connected to the switch that says 8 ohm output. On the schematic this resistor does nothing at all as it is totally bypassed by the switch. I have no idea what kind of output transformer you are using. But this resistor may need to be wired differently to balance the impedance seen by the power tube. Right now it is only occupying space.

[alexburchill]

The 47ohm resistor is used in the 0.1w output, I lifted the the schemtic for the 1w/0.1w switch directly from the Marshall 1 watt series.

Would the cathode bypass caps in stages 2 and 3 be giving the treble too much of a gain and leaving the bass behind do you think?

[romberg]

The 47ohm resistor is used in the 0.1w output, I lifted the the schemtic for the 1w/0.1w switch directly from the Marshall 1 watt series.

My mistake. I see how it works now. It is bypassed when not in 0.1w mode.

Would the cathode bypass caps in stages 2 and 3 be giving the treble too much of a gain and leaving the bass behind do you think?

No. In fact the cathode bypass caps are giving you more bass. Some marshall's do not have a bypass cap at all on that third stage. So, they would be even more brite. See:


http://www.valvewizard.co.uk/gainstage.html

Where Merlin discusses the cathode bypass cap.

When you find the amp too brite are you using the 0.1 watt setting? Or is it too brite both ways?

What kind of speaker/cabinet is it using?