JTM45 and Master Volume Options – Pros and Cons?

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,334
Reaction score
1,435
Location
Staffordshire UK
I joined over there a while back, but apparently said or asked something in a way that got misconstrued by a couple of the more active members and ever since have been treated somewhat as a pariah
Sorry Gene, I didn't notice that.
I suggest to treat it as water under the bridge, forget about it, and use the forum if you think someone there could help. Sluckey and PRR 's knowledge covers most stuff.
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,334
Reaction score
1,435
Location
Staffordshire UK
Regarding the layout https://www.marshallforum.com/attachments/jtm45-layout-alternative-pdf.105874/ I think it looks fine.

I suggest to consider adding heater elevation; it will ease up the stress on the cathode heater insulation of V2, and make V1 less demanding of of valve with perfect h-k insulation, bearing in mind that the input stage cathodes aren't fully bypassed. And my hypothesis is that it makes output valve shorts a little less likely.
And to get rid of standby, or move it to reduce the stress it might cause, eg after the reservoir cap, on the feed to the screen grids etc.

Personally I find the lack of bright cap makes an amp too muddy / woolly., and the 470pF roll off caps on the puts may exaserbate that.
 

BenTobith

Member
Joined
Jan 6, 2021
Messages
59
Reaction score
23
Regarding the layout https://www.marshallforum.com/attachments/jtm45-layout-alternative-pdf.105874/ I think it looks fine.

I suggest to consider adding heater elevation; it will ease up the stress on the cathode heater insulation of V2, and make V1 less demanding of of valve with perfect h-k insulation, bearing in mind that the input stage cathodes aren't fully bypassed. And my hypothesis is that it makes output valve shorts a little less likely.
And to get rid of standby, or move it to reduce the stress it might cause, eg after the reservoir cap, on the feed to the screen grids etc.

Personally I find the lack of bright cap makes an amp too muddy / woolly., and the 470pF roll off caps on the puts may exaserbate that.
Hi Pete, thanks for the feedback.

I’d love to make the standby better. Any layouts or advice you can offer on how to do that, based on my existing layout?

Or, would I be okay just to use a 100k 2w resistor + .1 uF cap across the two lugs of the standby switch?
 
Last edited:

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,334
Reaction score
1,435
Location
Staffordshire UK
I’d love to make the standby better. Any layouts or advice you can offer on how to do that, based on my existing layout?
There’s a yellow wire from the 16+16uF screen grid HT node, to the board turret with the 8k2 shared screen grid resistor. I suggest to have the standby switch break into that wire.
Hope that makes sense?
Ideally bypass the switch with a high value resistor, eg 47k, to keep a bit of anode current flowing whenever valves are at operational temperature.

Also 8k2 there is a very high value, it’ll hobble the power output. I think 1k is more typical, though I suggest 5W, rather than 2W, as it’ll get hot when the smp is cranked.
 
Last edited:

BenTobith

Member
Joined
Jan 6, 2021
Messages
59
Reaction score
23
Question about switches (I wish I better understood this aspect of amp builds):

Because of the transformer I'm using, I have to twist a pair of 18 awg wires (120v) from the PT to the mains switch, along with one of the wires from the amber neon lamp. That's 3 18 awg wires for one tiny lug. It's obviously a bit messy, even with me trying my best to make it neat.

I'm curious, could I just use DPST switches instead, and split the lamp wire and those twisted, 120v PT wires across two lugs on the same side of a DPST switch? Wouldn't that be the same, "electrically," as being on the same lug of a SPST switch?
 

2L man

Active Member
Joined
Sep 30, 2020
Messages
209
Reaction score
139
T
Question about switches (I wish I better understood this aspect of amp builds):

Because of the transformer I'm using, I have to twist a pair of 18 awg wires (120v) from the PT to the mains switch, along with one of the wires from the amber neon lamp. That's 3 18 awg wires for one tiny lug. It's obviously a bit messy, even with me trying my best to make it neat.

I'm curious, could I just use DPST switches instead, and split the lamp wire and those twisted, 120v PT wires across two lugs on the same side of a DPST switch? Wouldn't that be the same, "electrically," as being on the same lug of a SPST switch?
Yes you can and it make the switch "better" when it disconnect both hot and neutral!
 

Latest posts



Top