Fender Bandmaster hum.

[Oddvar R]

I seem to have some issues with understanding the grounding kingdom. A few years back i built a Fender Bandmaster and never used it. I have recentley been thinking about using it, but noticed there was a slight hum in it. After havin tried different relocations of the grounding points I don't seem to get anywhere. I read somewher that one could try and grounded the grids of the phase inverter with two caps. Is that something to try? And what value should the caps have if that's an idea?

The hum is more or less consistent, no matter what I do, I have taken out all the preamp tubes, and the hum is still there.

[professormudd]

Does pulling all your preamp tubes include your phase inverter? Could be poor filtering. How are the filter caps? How are the tubes? If you pull the power tubes is there still hum? If so then it is the PT interacting with the OT.

[Oddvar R]

The filter caps I have changed, same result. No sound with the power tubes out, but when I take out the PI tube, 12at7, I still get noise, but it's a bit more like 120ish?

[Oddvar R]

It seems like the bias is a tad too high, -70ish. How can I lower that, I wonder.

[Stevem]

That is way too high in fact it seems impossible because that’s what level of AC the transformer winding is providing to the bias diode .

This however is not the cause of the hum you have, at least in terms of the output tubes because with that high a level of bias voltage the output tubes are not idling like they should.

The schematic calls for a bias voltage at the junction of the 220k resistors after the PI of -45 to -48 volts depending on the year of your amp.

If you have new strong testing output tubes I would go with -48 volts.

[professormudd]

Do you have any pics of your build?

[Reeltarded]

Is one tube DOA?

[Oddvar R]

The funny thing is that when I have two tubes in at the same time, the amp loose it's volume to about half or less, when I have either one in in any of the two power sockets, the sound stays loud and at the same level. The bias is set at -46 on both tubes. How can that be? The hum is not very audible now.

EDIT: Ok, so I changed most of the caps and now it seems to be working, the hum is to a minimum, still very low hume, almost not audible. Are thes amps dead silent or is it a tiny hummisj kind of sound? Not irritating, but just to show it's on?

[professormudd]

Some people say things like "there is always a little hum".... But I do believe it is possible to get these amps too quiet for your ear to notice without holding it up against the speaker. It is the sum of all the little things... Your bias, each and every tube, your components, your lead dress, solder joint quality, etc..... Every little thing you do can contribute to hum/noise. I am sure if you keep that thing on the bench and continue to reassess each area, you might be able to continue reduce the noise.

Then again, you might love it to death and hit a point of diminishing returns. I am not an expert by any stretch, but I find persistence to be my friend in this hobby.

[Reeltarded]

is the hum loud enough to feel with your hand on the amp?

[Oddvar R]

is the hum loud enough to feel with your hand on the amp?

No, it isn't. It's very low, but audible.

[Oddvar R]

Some people say things like "there is always a little hum".... But I do believe it is possible to get these amps too quiet for your ear to notice without holding it up against the speaker. It is the sum of all the little things... Your bias, each and every tube, your components, your lead dress, solder joint quality, etc..... Every little thing you do can contribute to hum/noise. I am sure if you keep that thing on the bench and continue to reassess each area, you might be able to continue reduce the noise.

Then again, you might love it to death and hit a point of diminishing returns. I am not an expert by any stretch, but I find persistence to be my friend in this hobby.

I agree to that. I had the same kind of hum with the Tweedle Deluxe, but I just changed the location of one ground wire, and it went away, so it' sounds like that kind of hum.

[Oddvar R]

So now, for some reason, the amp is dead silent whern I remove the 12at7, (I have tried different tubes here, so I guess it isn't the tube?)

I also tested, as suggested by Martin Manning, the AC voltage as in the picture and got 1.2v on the upper and 0.160v aprx on the lower.

It also seems to be a bit higher voltage than suggeste on the layout, as in 440 vs 470?

[pdf64]

But it still hums with the valves removed from V1&2 and the 12AT7 V4 LTP valve in place?

If the HT is higher than what’s noted on the Fender info, maybe it’s due to the mains voltage you’re feeding it being higher than 117V?

[Oddvar R]

Yes it does.

I also tried to cennect a master volume as drawn in the layout underneath, that lowered the noise to almost zero, but that might be cause by the lower volume I guess?