FINISHED (Just about) - compact JTM45 build.

4114Effects

ICBM said:

4114Effects said:

@ICBM yep, running the bass at about 1-2 out of 10 on the dial. Toyed with the idea of changing the PI and V1 cathode cap to reduce the bass a little but I'm loathe to alter the circuit as it sounds so good. Loves pedals too.

Have you got a bright cap on the Ch.1 volume pot? I can't see in the pic. If not, 500pF (not the daft 5nF they put on later Lead models!) works well. Although obviously not if you turn it up full .

What's this running the bass at 1-2 thing? I don't think I've ever set it anywhere other than 10 on an old Marshall .

100pf mounted on the Bright volume pot, 500pf across the 470k mixer resistor. It's plenty bright, but also insane amounts of bass due to the 220uf cathode bypass cap on V1 and 100nf PI capacitors ...

ThorpyFX

why don't you change the taper on the Bass control. might be a more reasonable option than adjusting the circuit. spread the 1-5 range out over 1-9 etc

4114Effects said:

ICBM said:

4114Effects said:

@ICBM yep, running the bass at about 1-2 out of 10 on the dial. Toyed with the idea of changing the PI and V1 cathode cap to reduce the bass a little but I'm loathe to alter the circuit as it sounds so good. Loves pedals too.

Have you got a bright cap on the Ch.1 volume pot? I can't see in the pic. If not, 500pF (not the daft 5nF they put on later Lead models!) works well. Although obviously not if you turn it up full .

What's this running the bass at 1-2 thing? I don't think I've ever set it anywhere other than 10 on an old Marshall .

100pf mounted on the Bright volume pot, 500pf across the 470k mixer resistor. It's plenty bright, but also insane amounts of bass due to the 220uf cathode bypass cap on V1 and 100nf PI capacitors ...

ICBM

I would definitely go for 500pF - 100pF is the Organ model value.

You could also change the Ch.1 V1 coupling cap from .022 to .0022uF. That will make a lot more difference than the cathode cap or the PI caps.

Modulus_Amps

Try the amp in different rooms before tweaking the bass, it may be excessively boomy in a small room or near a corner and then you can't turn the bass lower than 0. On the other hand you don't want to be left too thin either.

I am in favour of cutting the bass too, but normally use a mixture of smaller cathode caps and smaller coupling caps for Marshall Circuits

RaymondLin

Just seen this, Dan, this is utterly amazing !!!!!!!!!!!!!!!!!!!!

ICBM

And in the mean time, Blackstar are making ID60 solid-state modelling heads which are more than twice the size of this, and of which at least 50% is fresh air...

John_A

Really impressive!  Love my JM-45 (self built), and often wondered if it would work in a small chassis. Can I see a pic on top of a cab to get a better idea of size?

John_A

4114Effects said:

@ICBM yep, running the bass at about 1-2 out of 10 on the dial. Toyed with the idea of changing the PI and V1 cathode cap to reduce the bass a little but I'm loathe to alter the circuit as it sounds so good. Loves pedals too.

I went down that route with my JTM45, there are plenty of cap changes that can reduce bass, but they all make it sound worse, I usually run my bass on zero at gig volume, I would leave well alone and just keep the bass down

martinw

John_A said:

4114Effects said:

@ICBM yep, running the bass at about 1-2 out of 10 on the dial. Toyed with the idea of changing the PI and V1 cathode cap to reduce the bass a little but I'm loathe to alter the circuit as it sounds so good. Loves pedals too.

I went down that route with my JTM45, there are plenty of cap changes that can reduce bass, but they all make it sound worse, I usually run my bass on zero at gig volume, I would leave well alone and just keep the bass down

Wis'd.

John_A

An amp building Wiz from an amp building Wiz

martinw

John_A said:

An amp building Wiz from an amp building Wiz

Ta