Having a hard time debugging this one...

[mort]

ok I tacked in a 5M and a 1.1M for temporary and I now have guitar sound coming through the speaker! Also hooked up the tank and the reverb does work.

The output is low and has a little noise in it but I have to break for dinner.

You guys kick ass. Be back in a while...

Oh and I'm accustomed to having to swap the OT leads on a fresh build sometimes. Usually because it goes BBBBBRRRRRRMMMMMMPPPPPP on start up. Could that phase reversal(?) problem also just cause low output like I'm experiencing now?

[nickb]

ok I tacked in a 5M and a 1.1M for temporary and I now have guitar sound coming through the speaker!

Well, so long as it's your guitar then that's good

Also hooked up the tank and the reverb does work.

The output is low and has a little noise in it but I have to break for dinner.

You guys kick ass. Be back in a while...

Oh and I'm accustomed to having to swap the OT leads on a fresh build sometimes. Usually because it goes BBBBBRRRRRRMMMMMMPPPPPP on start up. Could that phase reversal(?) problem also just cause low output like I'm experiencing now?

If the phase were wrong it would tend to oscillate or at least be louder. Noisy suggests to me that you are having to crank the volume up to hear anything and that suggests low gain early on around V1a - check that area. Do the hi & lo inputs work as they should? Also turn down the reverb just in case the noise is coming from there.

[mort]

ok, done with dinner and back at it. It is making some varying high frequency noise that seems to be associated with V3 and V4. When I get my small needle nose pliers near the lead that goes to V2 P2, it starts making ALOT of noise and moving that lead around has ALOT of effect over the noise(just realized this is with the presence switch closed). With the presence switch open the guitar sound is much better, seemingly full volume, but still has some unwanted distortion riding on it.

Here are some fresh readings after the above circuit corrections. I unplugged the tank and left out the 3 triodes associated with the reverb.




V1
p1 122.5
p2 .00
p3 1.02
p6 124.7
p7 .00
p8 .97

V2
p1
p2
p3
p6
p7
p8

V3
p1 190.3
p2 .35
p3 2.13
p6
p7
p8

V4
p1 185.1
p2 -.93
p3 1.9
p6 276.9
p7 63.89
p8 76.1

POWER TUBES
p3 458.4 & 456.5
p4 379.5 & 379.5
p5 -57.4 & -57.6
p8 0 & 0

[mort]

Ah... I never installed the 100k & F.B.

doing that now...

edit: scratch that. I DID install the F.B. but might have found a mistake on my layout.. will report back in a few.

[mort]

ok so I did find a minor wiring error and corrected it. With the presence switch open it's got good full sound but there's still a small amount of unwanted noise riding on it. And when the presence switch is closed there are noises that sound kinda like oscillations and also the guitar sound distorts in a sudden and unpleasant way. Those leads associated with V3 P2 are MUCH less sensitive than they were earlier though.

[nickb]

When someone says 'noise' I take it to mean hiss and that is why I said what I did in #17. What you just described sounds like it's borderline unstable.

Could be the feedback phase. Remove the feedback and see if it's any better. You can try swapping the phase but my guess it is correct as is.

Instability is due to output signal getting back to an earlier stage. It could be poor location of wires, keep big swing signal wires like output transformer primaries away from anything. Keep wires going from plate to grids close to he chassis to help reduce capacitive coupling of unwanted signals to them. Unfortunately there are many way for this to happen and most of them are to do with layout.

[mort]

With a little more prodding around I found that the wire from V3 P2 to the diodes still did in fact have a strong effect on the stability of the amp, so it got removed and replaced with a shielded one, grounding the shield, and this seems to have stabilized the amp while in presence mode, but now there is a general hum regardless of where the volume is set or if anything is plugged in or not. Did I introduce a ground loop via the shield? Doesn't seem like it should have... I'm gonna put a shielded cable on the input to V1 just for precaution and maybe work the ground scheme a little different and see what happens.

If there are any more thoughts please share. I'll check back frequently.

[mort]

I've tried several different combinations of grounding and nothing seems to change the hum. The hum was not present at all before the shielded lead got put in. If I change it back to a regular wire then it will go back to being unstable again on the presence mode. Hmmmm....


Here's a photo to show the grounding. I split the grounding buss to get the phase inverter and bias grounds off of the preamp grounds. The power section grounds, bias and PI all ground together toward the left. And the preamp section all gets grounded back to the chassis on the right. That's how I generally do grounding schemes and don't run into to many problems. Should I ground the shields somewhere else? I tried putting a 500pf cap from the shielded lead to ground and it reduced the hum but also dampened some of the glassiness of the top end so I removed it.

Any other ideas?

[Jazz P Bass]

I've tried several different combinations of grounding and nothing seems to change the hum. The hum was not present at all before the shielded lead got put in. If I change it back to a regular wire then it will go back to being unstable again on the presence mode. ]

Did you ground both ends of the shield?

Most times, you do not.

Sometimes, you do.

Sometimes, neither.

[mort]

Did you ground both ends of the shield?

Most times, you do not.

Sometimes, you do.

Sometimes, neither.

I tried neither and one. Did not try both. Will though.

[nickb]

Ground one end only, preferably at the driving end.

What are the blue/brown wires that go across the board? They look like they might come from the output tubes and go to the output transformer. Also they go right past the screened leads presumably with sensitive signals. If this is the case they need to be re-routed.

Diagnose your hum/buzz. Turn down the volume controls and see what happens. Is this 60Hz heater hum or 120hz power supply buzz?

[mort]

The blue and brown wires are the OT primary. I've moved all of those wires where they sort of intersect in the middle and they don't have any effect on the hum. And they were all situated as they are now when there was no ground hum at all. Not really sure if it's 60 or 120 hz buzz.

The noise does increase if I put a metal probe close to where the two shielded leads connect on the board. But nothing (so far) makes the hum get any quieter. It's consistent whether anything is plugged in or not and no matter the volume setting on the amp or guitar. Totally wasn't there when the amp was unstable with oscillations.

[nickb]

The blue and brown wires are the OT primary. I've moved all of those wires where they sort of intersect in the middle and they don't have any effect on the hum. And they were all situated as they are now when there was no ground hum at all. Not really sure if it's 60 or 120 hz buzz.

The noise does increase if I put a metal probe close to where the two shielded leads connect on the board. But nothing (so far) makes the hum get any quieter. It's consistent whether anything is plugged in or not and no matter the volume setting on the amp or guitar. Totally wasn't there when the amp was unstable with oscillations.

The reason for not putting them ( blue/brown) there is instability, not hum. Keep the big sigs away from the li'l' 'uns!

I don't why a properly grounded screened lead should make things worse unless it's connected to a hummy ground. You always take that out and see if it is repeatable. With the blue/brown moved to a sensible location i.e. away from everything you might not have instability anyway. Also consider that the instability could have been masking the hum.

60 or 120Hz: the famous test attributable to the equally famous Enzo. Unplug your lead from your guitar and touch the end - that is 60Hz hum. Now go compare.

[mort]

Well this is interesting. Just stopped by the house and figured I would do the Enzo test. I had moved the amp to the other side of the shop to a staging area and just plugged it in where it sits and there was no hum (was probably 60Hz) but instead now I'm getting a washy sound that sounds similar to snow on and old TV set.

[nickb]

Well this is interesting. Just stopped by the house and figured I would do the Enzo test. I had moved the amp to the other side of the shop to a staging area and just plugged it in where it sits and there was no hum (was probably 60Hz) but instead now I'm getting a washy sound that sounds similar to snow on and old TV set.

I suspect it's still unstable.