Adding a trim pot in JTM45 --is it as simple as this?

[Chuck H]

It has JJ brand KT66's and a JJ GZ34S rectifier. I'll get it fired up tomorrow with tubes and see where everything is landing.

That should be safe, if not "correct" as far as the bias goes. If anything the range may be too high. Which means the amp would be biased cool. That won't create any dangerous circumstances for the amp and it should probably still be in a useable range, if not ideal.

[mort]

OK I replaced the 56k bias resistor with a 47k and adjust to 55mV across the 1 ohm bias reading cathode resistors. Here's a complete set of voltages. Even though the range is narrow this should be good for at least making sure the amp works, for now. It sounds like it doesn't quite have full volume, sounds too clean like it won't overdrive like it should. I'll continue to check over my work but I'm pretty sure the wiring is vcomplete and all connections are solid. Thoughts?

V1
1 169.1
2 0
3 1.7
6 173.2
7 0
8 1.7


V2
1 153.9
2 0
3 1.0
6 274.9
7 153.9
8 162.2


V3
1 189.0
2 11.4
3 38.9
6 203.6
7 13.3
8 38.9



V4
3 403.8
4 397.1
5 -32.3
55mA

V5
3 402.4
4 395.8
5 -32.3
55mA


V3 isn't supposed to have that much voltage on the grids is it? I tried another tube but no change. Also read the resistors associated with it to make sure they were correct value, etc.

55mA out of a max 85mA is ~65% so maybe slightly cold but should sound normal... hmmm.. stumped a little here...

[Chuck H]

Are you using a 12ay7 or a 12ax7 for the first stage? I don't see your power tube current as the go to problem. That seems fine. There may be something else causing a lower drive level. Have you tested the amp for power in watts? Do you have a scope?

[mort]

all 3 preamp tubes are 12ax7's

Do not have access to a scope.

And as for watts, can't that just be calculated with the data given? for example V4, plate voltage 403.8 x V across the 1 ohm cathode resistor .055 = ~22W.

[Chuck H]

And as for watts, can't that just be calculated with the data given? for example V4, plate voltage 403.8 x V across the 1 ohm cathode resistor .055 = ~22W.

That is the power the tubes are dissipating just sitting there. It's not power the tubes can amplify a signal to. They are different things. 22W idle dissipation is actually very warm. Probably not dangerous. More typical would be 17W or 18W at idle for that amp design. The max current isn't irrespective of plate voltage. The tubes max dissipation is the dominant player here. At lower plate voltages the tube can safely operate at more current. Up to 85mA. At higher plate voltages that current level will over dissipate the tube.

Without a scope it's harder to know what the signal is doing at different stages. So it's harder to locate where a gain fault may be. I suppose it's fair to assume you have the guitar volume turned up full and are using the high gain input?

[mort]

yep guitar is maxed and I've tried all four inputs and the problem is common to all of them. Still looking over wiring for mistakes but not finding any.

I'm seeing several voltage charts in google images that are all showing 0 for grid voltage on v3, but I have over 10vdc on both grids for that tube. I can't conceptualize where that voltage might be coming from if not from inside the tube...


edit: now I'm seeing some voltage charts that show 10-20v on th grids of V3 and some that don't. Not sure what's up with that... but at any rate I did some tube swapping and looks like I had a weak tube on V2. That's the first JJ 12ax7 I've ever got that was bad.. The amp still seems like it should be louder at the lower half of the pot swing(1-5) but on 6-10 sounds ike it should... maybe it's just me though. Gonna come back to it later with a fresh outlook.

[g1]

Consider the PI grid voltages in reference to the cathodes, rather than in reference to ground.

[Chuck H]

yep guitar is maxed and I've tried all four inputs and the problem is common to all of them. Still looking over wiring for mistakes but not finding any.

I'm seeing several voltage charts in google images that are all showing 0 for grid voltage on v3, but I have over 10vdc on both grids for that tube. I can't conceptualize where that voltage might be coming from if not from inside the tube...


edit: now I'm seeing some voltage charts that show 10-20v on th grids of V3 and some that don't. Not sure what's up with that... but at any rate I did some tube swapping and looks like I had a weak tube on V2. That's the first JJ 12ax7 I've ever got that was bad.. The amp still seems like it should be louder at the lower half of the pot swing(1-5) but on 6-10 sounds ike it should... maybe it's just me though. Gonna come back to it later with a fresh outlook.

There absolutely should be 10V or more on the V3 grids. The volume adjustment being normal at the min and max means it's working correctly. The taper of the volume pot is just a little hinky. Most modern "audio taper" (has no fixed definition) pots are "A10" or "A15". Which means that they are only at 10% or 15% of their adjustment when the pot is rotated to 50% of it's travel. Older pots, especially those used in guitar amps, tended more toward 25% or 30%. Making matters worse is that the taper on modern pots isn't usually a smooth, parabolic curve. It's two straight lines. One that ramps to the 10% or 15% value at half rotation and another that, from that point ramps directly to full value at full rotation. So there's a shift in feel where the kink is. These pots often "feel" like there is too much adjustment between 5 and 7 on the control knob (assuming your controls go from 0 to 10). Not a big problem once you know about it and, of course, all the same adjustability is there. Just not as refined feeling and intuitive as it use to be Look into it and you can find pots that are more like the old school designs with a 25% taper and a smooth curve from 0 to 10. They'll also be more expensive. So it becomes a mater of personal priority. I use the plain Jane Alpha pots without getting grumpy.

EDIT: I drew a graphic

[mort]

just for fun here's a few pics of this one. The gut shot was from a while back before it was done.