Series Filament Amp Hum

[bobloblaws]

Scrap the amp. Get him a cheap solid state amp. It would be a lower cost than a transformer. Keep it simple...

You gotta be kidding. We already bought the isolation transformer, they are $25. I think he prefers the sound of a vintage tube amp over solid state, that's the whole point.

[Enzo]

My message was this: hum is not generic. You may well cure a lot of hum already by adding the iso transformer. But there are many other sources - or potential sources anyway - of hum. And each has its own cure.

SO if ther is remaining hum, it may be reduced by the rebuilding the amp most likely needs due to its age.

If the hum now is acceptable, then fine, stop fixing it. If not, then we might have weak filter caps or leaky couplers.

And we need to remember that this is a single ended amp, so it lacks the natural hum cancellation of a push pull stage. It is normal for many single ended amps t hum some.

[Enzo]

My message was this: hum is not generic. You may well cure a lot of hum already by adding the iso transformer. But there are many other sources - or potential sources anyway - of hum. And each has its own cure.

SO if ther is remaining hum, it may be reduced by the rebuilding the amp most likely needs due to its age.

If the hum now is acceptable, then fine, stop fixing it. If not, then we might have weak filter caps or leaky couplers.

And we need to remember that this is a single ended amp, so it lacks the natural hum cancellation of a push pull stage. It is normal for many single ended amps t hum some.

[nevetslab]

After you've physically wired in the isolation transformer, & new power cord all the primary wiring, I'd suggest replacing the old power supply filter. with a 100uF/200V radial lead cap. You could also go with a dual section F & T cap and use the 1-3/8" dia cap clamp. I think the three lugs would fit within the opening where the old cap can is sitting. They have a dual 50uF-50uF/500V part (C-EC50-50-500FT) that could be used in parallel. You could even place a small choke between the first and second cap section for reducing the ripple that much more. With the isolation transformer, you now have the option to use a full wave bridge rectifier, which improves the supply ripple. Antique Electronics Supply carries the F & T buss caps and mounting brackets.

[bobloblaws]

After you've physically wired in the isolation transformer, & new power cord all the primary wiring, I'd suggest replacing the old power supply filter. with a 100uF/200V radial lead cap. You could also go with a dual section F & T cap and use the 1-3/8" dia cap clamp. I think the three lugs would fit within the opening where the old cap can is sitting. They have a dual 50uF-50uF/500V part (C-EC50-50-500FT) that could be used in parallel. You could even place a small choke between the first and second cap section for reducing the ripple that much more. With the isolation transformer, you now have the option to use a full wave bridge rectifier, which improves the supply ripple. Antique Electronics Supply carries the F & T buss caps and mounting brackets.

Thanks, I'll take these options under consideration.

Why was a full wave bridge rectifier not an option previously?

[Enzo]

Can't make a full wave rectifier from a 35W4 or 35Z5 or whatever the rectifier tube was.

If they make a dual rectifier 35v tube I am unaware of it, besides it would require a center tapped power transformer, and since it HAD no power transformer, that is more than we can ask of it.

[Enzo]

Can't make a full wave rectifier from a 35W4 or 35Z5 or whatever the rectifier tube was.

If they make a dual rectifier 35v tube I am unaware of it, besides it would require a center tapped power transformer, and since it HAD no power transformer, that is more than we can ask of it.

[J M Fahey]

My buddy brought me this amp, a Pepco 201 series filament/widomaker amp. An amp tech added a 3 prong plug and bypassed the tube rectifier and simply put a single diode inline for rectification. My buddy claims that when he got it back from the tech a few years ago it had only a low hum which was acceptable to him. When he went to use it recently he found it is making a very loud hum rendering it unplayable. I plan on adding an isolation transformer but I'd like to deal with the hum problem first. If I disconnect the green mains ground wire from the chassis the hum goes away. This seems counter intuitive to me, I always thought that grounding the chassis would reduce hum, not exacerbate it. Is this a case of a ground loop?
On the other hand, if what he told me is correct, that it was working OK and then developed this problem, I suppose it might suggest a failing component that only manifests itself when the chassis is grounded. Does that make any sense? Any insight, suggestions etc. appreciated, thanks.

B

p.s. here's a schematic for an almost identical amp. The power tube is actually 50L6 and the wiring for the for the mains power section is a little different, probably changed around by the tech when the rectifier was replaced with the diode (see attached photo). I'm guessing that big ol' 400 ohm resistor was put in to drop the heater voltage that would have been dropped by the rectifier tube.

[ATTACH=CONFIG]46204[/ATTACH]

Hi bloblolaws, that schematic is VERY wrong and DEADLY, obviously drawn by a no clue Tech.

It shows a 150 ohm 5W resistor joining LIVE supply ground to metallic chassis (directly connected to Musician).
IT will allow DEADLY 250mA to pass (max "Human safe" current is 5 to 10 mA) .
Proper value is 150k (150000 ohms) which will limit worst case current to less than 1 mA .
Tingling but not deadly.

[bobloblaws]

Hi bloblolaws, that schematic is VERY wrong and DEADLY, obviously drawn by a no clue Tech.

It shows a 150 ohm 5W resistor joining LIVE supply ground to metallic chassis (directly connected to Musician).
IT will allow DEADLY 250mA to pass (max "Human safe" current is 5 to 10 mA) .
Proper value is 150k (150000 ohms) which will limit worst case current to less than 1 mA .
Tingling but not deadly.

Good call, Jaun, thank you. Fortunately the actual value is 270K. It looks like the author of that schematic made a copy and paste error whereby the 150/5W annotation belongs to the resistor between the fuse and the heater circuit.

[bobloblaws]

Can't make a full wave rectifier from a 35W4 or 35Z5 or whatever the rectifier tube was.

If they make a dual rectifier 35v tube I am unaware of it, besides it would require a center tapped power transformer, and since it HAD no power transformer, that is more than we can ask of it.

I know very little about it obviously , but I thought a full wave bridge rectifier was always diodes, not tube. Also confused by this from the Valve Wizard: "A bridge rectifier is used to rectify AC from a transformer with a single winding (i.e. no centre tap)" (The Valve Wizard)

[Enzo]

A bridge rectifier as a thing is that four cornered quad of diodes, yes.

But the typical 5U4 or 5Y3 with a center tapped transformer winding is still full wave. With a center tapped transformer, just two diodes will yield a full wave rectification.

This was a tube amp, from the day when everything was tubes. So without that center tapped power transformer, there was no way to make a full wave rectifier. You could use a diode bridge now, but they didn;t have that option. Diode bridges became common in a later era.

[Enzo]

A bridge rectifier as a thing is that four cornered quad of diodes, yes.

But the typical 5U4 or 5Y3 with a center tapped transformer winding is still full wave. With a center tapped transformer, just two diodes will yield a full wave rectification.

This was a tube amp, from the day when everything was tubes. So without that center tapped power transformer, there was no way to make a full wave rectifier. You could use a diode bridge now, but they didn;t have that option. Diode bridges became common in a later era.

[bobloblaws]

So to bring this to something of a close, I put it all together and it sounds quite good, for what it is. I did some troubleshooting in terms of tubes and caps and lead dress but whatever hum remains seems to be inherent. Playing it anywhere short of dimed results in a perfectly manageable noise level. I'm sure my friend will be pleased with the result and he will not be electrocuted. Thanks for all the help!