microphonic resistor, or end of shielded cable?

[mikepukmel]

From suggestions by Merlin, I put grid stops on the second half of V1 and V2 preamp tubes on a Ab763 type deluxe build (the one Ive been posting too many threads about ). For the grid stops on pin7 that go do the volume control, I soldered one lead to the pin, and the other lead I soldered the shielded wire to and put some heat shrink over.

While debugging the amp, I had the chassis upside down, running, and poked around gently with a chopstick. I was very surprised to find that tapping on that resistor gave an audible sound through the speaker. I didn't get that anywhere else in the amp, even the tubes themselves. Only that component.

Is this common, a microphonic resistor? I don't recall what kind it is only that it is 68k. Should I replace the resistor with a different mfr/type?

[pdf64]

68k is a way higher value than is beneficial for a 12AX7 grid stopper; with early stages it will lead to hiss.
There's a baseline level of noise that's directly related to the equivalent series resistance of the source, and any grid stoppers will obviously add to that.
Ian suggests '8/gm' as a guideline value, so at least 6k8 for a 12AX7, but above 22k will just add noise and treble roll off without benefiting stability (unless the lead dress / layout / design is REALLY bad).

With enough gain, almost anything can be microphonic; maybe there happens to be good mechanical coupling between the grid socket terminal and the actual grid?
Any kind of film resistor should be fine.
Certainly don't use CC for grid stoppers, early stages at least.
They are little tubes of carbon granules.
I remember as a teenager (we didn't get a phone until my brother went to uni in the late '70s!), telephone mics were also made from carbon granules, every so often they would go noisy and you would need to give the mic a bit of a knock to re-settle the granules into a happy place.

[stokes]

Why did you add these resistors between the grid and vol controls?

[ric]

Mike, in your amp pics it looks like you are using some Vishay/Dale rn series metal film resistors. Those are an excellent choice for anything in the signal path.

Similpost with stokes. Yes, if you chose to use grid stops in the pre amp, those little brown ones are low noise.

[pdf64]

Why did you add these resistors between the grid and vol controls?

Small grid stoppers mounted at the tube socket terminals of the input, 2nd and 3rd stages can help to maintain stability if a few tweaks for higher gain are done.
I prefer to avoid screened cable inside an amp, especially the long run from a BF fx channel volume control to tube socket, as I suspect that cable capacitance acts to roll off noticeable treble when the volume control is set around the electrical halfway region and the source impedance goes up around 250k.
Miller capacitance will act on that anyway, but no pint exacerbating the effect unnecessarily.
I think that the Custom 68 DR etc have additional grid stoppers too, but use rather higher values which lead to hiss.

[mikepukmel]

68k is a way higher value than is beneficial for a 12AX7 grid stopper; with early stages it will lead to hiss.
There's a baseline level of noise that's directly related to the equivalent series resistance of the source, and any grid stoppers will obviously add to that.
Ian suggests '8/gm' as a guideline value, so at least 6k8 for a 12AX7, but above 22k will just add noise and treble roll off without benefiting stability (unless the lead dress / layout / design is REALLY bad).

With enough gain, almost anything can be microphonic; maybe there happens to be good mechanical coupling between the grid socket terminal and the actual grid?
Any kind of film resistor should be fine.
Certainly don't use CC for grid stoppers, early stages at least.
They are little tubes of carbon granules.
I remember as a teenager (we didn't get a phone until my brother went to uni in the late '70s!), telephone mics were also made from carbon granules, every so often they would go noisy and you would need to give the mic a bit of a knock to re-settle the granules into a happy place.

Thanks PDF. yes, this amp does have a lot more Johnson noise than I expected it would, must be due to this case. The original suggestion might have been a type'o, sounds like he might have meant 10k? My lead dress isn't spectacular working on that.

Im using metal films everywhere. The reason for the odd mix of resistor types was just availability for each particular value.

[mikepukmel]

Why did you add these resistors between the grid and vol controls?

Hi STokes, well i read as much as I could about what musicians thought about this particular amp. I never actually played through one (did have a BF Princeton back in teh day, and a SUnn). The one complaint that was consistent was blocking distortion (farting out) at higher bass levels and volumes. I read all I could about blocking distortion and the one suggestion for reducing it was to add grid stops after the tone stack, and up the value of the grid stop on the output tubes. I thought to try the additional grid stops on pin7 of the preamp tubes first, see how that worked, then up the stops on the output tubes if necessary.

[mikepukmel]

Mike, in your amp pics it looks like you are using some Vishay/Dale rn series metal film resistors. Those are an excellent choice for anything in the signal path.

Similpost with stokes. Yes, if you chose to use grid stops in the pre amp, those little brown ones are low noise.

Thanks Ric. I will fish around and see what I have in the little brown vishay dale's. I have a really odd mix of brands and types just due to what Digikey and Mouser has available in small quantities.

[mikepukmel]

Small grid stoppers mounted at the tube socket terminals of the input, 2nd and 3rd stages can help to maintain stability if a few tweaks for higher gain are done.
I prefer to avoid screened cable inside an amp, especially the long run from a BF fx channel volume control to tube socket, as I suspect that cable capacitance acts to roll off noticeable treble when the volume control is set around the electrical halfway region and the source impedance goes up around 250k.
Miller capacitance will act on that anyway, but no pint exacerbating the effect unnecessarily.
I think that the Custom 68 DR etc have additional grid stoppers too, but use rather higher values which lead to hiss.

Thanks PDF. As mentioned the amp does have more hiss but the hiss only increases to an untolerable level when I put the volume way up (near max), and Ive never played at that level. I didn't have the knobs on when I was fiddling so I don't know the levels. If the size of the stop in ohms should be more like 10k, and that will do the job, reduce blocking distortion and help with stability, I will switch out the stops I have in there now.

So far, however it turned out, this amp is *bright*, maybe a little too bright.

I did notice, maybe this is to be expected, that the sound quality is good at very low volume. But it really picks up (sounds better) a little higher. I suspect it was less than 1/2 vol. I didn't have the vol up enough to get blocking distortion or clipping yet, though.

[Jazz P Bass]

Hi STokes, well i read as much as I could about what musicians thought about this particular amp. I never actually played through one (did have a BF Princeton back in teh day, and a SUnn). The one complaint that was consistent was blocking distortion (farting out) at higher bass levels and volumes. I read all I could about blocking distortion and the one suggestion for reducing it was to add grid stops after the tone stack, and up the value of the grid stop on the output tubes. I thought to try the additional grid stops on pin7 of the preamp tubes first, see how that worked, then up the stops on the output tubes if necessary.

From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
25uf will pass way too much low frequencies.
Try a 15, a 10 or even a 4uf.

Have you ever encountered that symptom on a Marshall amp?
No?
Look at there circuits. They use a 0.1uf cap.

[Chuck H]

From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
25uf will pass way too much low frequencies.
Try a 15, a 10 or even a 4uf.

Have you ever encountered that symptom on a Marshall amp?
No?
Look at there circuits. They use a 0.1uf cap.

Since hum abatement was already an issue for this build...

I did the partial on the first stage cathode with BF topology preamps for a long time and it's a good way to mitigate farting bass. There's just so much LF through the BF topology preamp. But lately I've been moving the partial bypass to the second (tone stack/volume recovery stage) on the principal that a fully bypassed first stage is more ideal for minimizing filament circuit hum injection on the signal path. I've never done any actual testing to see what/if the real world advantage to this is. Since there's no clipping of the second preamp stage at any volume setting with that preamp design partial bypass on the second stage works just fine. You do need to go to a lower value cap to get similar results. More like 1uf or 2.2uf.

JM2C

[mikepukmel]

Thanks everyone!

Boils down to 3 issues then?

Grid stops help fix:

1) keeping unwanted RF noise out of amp (e.g. radio stations etc)
2) stability problems, e.g. HF oscillation
3) changing time constants to reduce blocking distortion.

Fender put a 68k near the input jacks but would this only help 1 since its not on the tube socket?

After some more reading, still not sure where the blocking distortion occurs, which tube stage(s).

Not sure about my wiring, don't know enough to judge which parts of the lead dress result in bad oscillations.

[pdf64]

From what I have seen on Fender amps of this era, the "farting out" is usually caused by the first tube cathode bypass capacitor value being too high.
25uf will pass way too much low frequencies.
Try a 15, a 10 or even a 4uf.

Have you ever encountered that symptom on a Marshall amp?
No?
Look at there circuits. They use a 0.1uf cap.

Dunno about 0.1uF cathode bypass? Rather 0.68uF comes to mind.
But for best heater hum rejection, early, especially input stage cathodes should be fully bypassed.
For BF Fenders, the cathodes of 2nd / 3rd stages seem better places go 'go partial', both in terms of hum rejection and blocking distortion mitigation.

[eschertron]

JM2C
Blocking shouldn't be an issue for the input stage, unless you're slamming the amp with a herzog-like pedal with 10vpp output or so. It's the stages that do get signals that exceed the input HR that need blocking mitigated.
Likewise, with the typical small-signal at the input there will be no distortion of the LF content, so partial bypass can be reserved for a later stage. I modified a champion 600 reissue at the power tube (functionally the third gain stage) with a small coupling cap and a partial cathode bypass, and got good results. The signal simply wasn't large enough to be a problem in the earlier stages.

[stokes]

The original design had no blocking distortion problems,so I fail to see the need for them in the places you added them.You say they are microphonic,so I would suggest removing them.If they are microphonic you are adding noise that shouldnt be there.These amps are not hi-gain circuits so there shouldnt be any blocking distortion problem.