Question about Vibrochamp filament supplies

[SamBooka]

Just a thought.
I know people mention that the filament supplies are noisy for the preamps of vibrochamps. So here is my question.

If you pull all the preamp tubes from a VC and you still hear the hum can, at least temporarily, rule out that the hum is from the filament design and focus on things like the power supply?

 

[mark norwine]

It's a single-ended amp. There's bound to be some hum....

 

[SamBooka]

Thanks Mark. So you are saying there is no point in redoing the filament wiring with a virtual center tap?

 

[pdf64]

If you pull all the preamp tubes from a VC and you still hear the hum can

It may be that the can cap on the B+ is failing.
Pete

 

[mark norwine]

well, how loud is this "hum" we're talking about?

And.....as I often ask in posts like this: is it 120Hz or 60Hz? (measured with a meter, not guessed at).

In a super-simple amp like this,

120 = power supply
60 = filaments.

 

[SamBooka]

Ok.. I guess I am not being clear here.

Yes.. single ended amps hum. But there are quite a few people who say it is worse because of the way the filament supply is set up (pins 4 and 5 jumpered and pin 9 grounded). Then then go on to sometimes suggest replacing the existing wiring with twisted filament wires and two 100R resistors for a virtual center tap. Some even go so far as to suggest raising the voltage up 40v above ground.

My question was simply: if you think the preamp filaments are noisy, instead of doing all that to find that it is a lack of power supply filtering, or a bad filter cap, or a cathode resistor on the 6v6 that is failing, wouldn't it be simpler to pull all the pre tubes? if the noise persists you know it isn't the filaments.

 

[mark norwine]

Well, there's only 1 preamp tube, so pulling it won't be completely informative, since the power tube remains.

A sure-fire way (and old bench tech trick) to determine if your filaments are the source of hum is to disconnect the PT from them, and reconnect them to a 6VDC lantern battery. If filament hum is your issue, the amp will be dead quiet.

Biasing up is never a bad idea, and balancing the string is a good idea too...but it's just as likely that your issue(s) is B+ hum that has no way to be polarity-rejected in a single-ended amp.

All the more reason to measure the hum.....the frequency of the hum will tell you exactly where the hum is coming from.

 

[jaytea4]

It may be that the can cap on the B+ is failing.
Pete

+1 for the B+ filter cap

 

[mark norwine]

It might be the can.

It might not.

So, we have a choice: we can a.) diagnose the issue(s), or we can b.) guess.

Personally, I'm a real big fan of diagnosing things, then fixing them once & right.

But, hey.....who am I?

Sure.....change the can. Why not.

 

[SamBooka]

This is a very odd thread.
Why would I change the can on my amp if there is nothing wrong with the amp?
I am just trying understand why people replace the filament wiring on champs and vibrochamps.

 

[Jeff Gehring]

I think Sam isn't talking about a specific amp here, rather whether one could eliminate filament hum as a potential source of noise in a hypothetical Vibrochamp by the test of removing the preamp tubes and listening.

The VC is single ended so no cancellation of any filament hum will occur in the final (the 6V6) so the possibility is still there. These amps (like some Vox AC-30s and early Fenders) use the chassis steel as a current-carrying path for one side of the filament buss. This offsets the ground reference to one side of the filament winding, which can result in somewhat greater coupling of filament hum into a sensitive amp. Going to discrete wiring for both sides of the filament buss, with the addition of a virtual center tap (100 ohm resistors) can certainly help where this is a problem. The VC is a pretty low gain amp, though, and I wouldn't imagine the coupled hum to be a big factor.

 

[SamBooka]

Thanks Jeff. I appreciate it. I had read something in another thread that reminded me of an old Gerald Weber article. I had done some other work on my VC but thought about changing the filament supply. If it aint broke dont fix it but sometimes you dont know it is broken until you fix it lol.

Anyway.. thanks for taking the time to answer.