Ground Loop/ Hum problem in old tube PA head

[lassoharp]

I'm trying to pin down a small hum problem in an old tube PA head I'm repairing.

http://www.theused.com/manuals/simpson/simpson_ma-8n.pdf

So far it looks to be narrowed down to the output tube - hum doesn't go away with both V1 and V2 out of circuit.  Hum will nearly vanish by grounding the 6L6 grid resistor.

My best two guesses at this point are:

1) grounding loop problem around 6l6 socket - The original grounding scheme in this amp is typical - grounds placed on the tube socket grounding lugs(chassis) and not always in a stage by stage manner.  When replacing the electrolytics I managed to keep the HV CT, 1st cap, and 6L6 cathode/cap grounds tied to a single point.  The 2nd cap right after the choke is tied ~ 5in away on the 6l6 socket lug - possible loop there?  I've tried the 6L6 grid resistor at both places - same result.

2) Slight imbalance in the heater grounding scheme. Though not shown on the schematic, the amp has a pair of 11ohm resistors tying the heaters to ground via the V2 socket lugs.  DCR match is reasonable - AC to ground between each is ~ .25 volts difference.  Stray AC measured on 6l6 grid = .5 V

Cramped space and common ties prevents the above 2 scenarios from being easily tested without some major deconstruction.  Thought I would post here for other suggestions before proceeding.

 

[idylldon]

Since this has a single-ended output section, there will be some hum.  Fender Champs display this a bit, but my guess is that you're thinking it's more than the usual encountered in this setup?

Cheers,
--
Don

 

[lassoharp]

Yes, it does seem to be a bit more than what I would call faint or insignificant.  Comparing this to a homebrew "Champ" I built it's a good deal louder.  You really have to press your head against the speaker to hear the Champ hum

One other thing of note on this amp, when I first got it, it would hum very loudly(far above what it's currently doing) as soon as the power was turned on - well before the tube was pulling any current.  I traced this down to a heater pin(#2) which was bent too closely to the plate (#3) - very nearly touching it.  Bending the pin back into position got rid of the big hum but it seems possible that some is still coming from the adjacent pin. Can't really bend them any further apart.

 

[Butterylicious]

Did you replace the 16uF electrolytic caps in the PS?

>the amp has a pair of 11ohm resistors tying the heaters to ground

That 286mA and almost 2 watts across those resistors.  It's single ended.  Try ripping those 11 ohm resistors out and ground 1 side of the filaments.  Balance virtual center tapped filaments is more of a push pull thang.

 

[lassoharp]

All the lytics brand new + coupling caps.

Good option -I'll give the one side to ground a try - much easier move to make.  I think there may even have been another Simpson amp from that series that used that arrangement.

Thanks

 

[zebra50]

Maybe some pictures would be nice - we always like pictures!

Some of these old amps that some grounds are made to the chassis through the potentiometer metal cans or input/output sockets. As the cans / washers / chassis become oxidised these connections can be troublesome. I'd definitely check those if the amp is wired in that manner.

Just a thought really, based on some recent experience of old amp hum.

 

[lassoharp]

Maybe some pictures would be nice - we always like pictures!

Some of these old amps that some grounds are made to the chassis through the potentiometer metal cans or input/output sockets. As the cans / washers / chassis become oxidised these connections can be troublesome. I'd definitely check those if the amp is wired in that manner.

Just a thought really, based on some recent experience of old amp hum.

That is a good point there.  All the pots and sockets in this amp use mounting contact to secure a ground connection and the tube sockets can certainly become loose as the rivets lose tension. I just fixed a noise problem in an old Fender Deluxe last week that was due to the input jacks not being tight enough - even though they would show continuity with a meter check.  May have to redo all the grounds

No way to do any pics at the moment.

 

[lassoharp]

No difference with 100r - 0 - 100r virtual CTs or grounding one side of the heater string.

Appears to be filtering related.  I'd already upped the original values from 16uF to 22uF during the initial recap.  Clipping in an extra 20uF at the first two nodes reduced hum but it's still above what I hear in my homemade "Champ" which uses a little less filtering and no choke.

Any possibility a poorly performing choke could contribute in ways beyond simply acting as a 300 ohm resistor?  In other words - aggravate the problem.

 

[zebra50]

You probably know this, but the choke, power tranny can induce hum in the output tranny - that's why you sometimes see them at 90 degrees to each other. But these are often arranged well on old amps.

Do you have a scope so you can trace the ripple from the rectifier onwards?

 

[Butterylicious]

+1 on rivet grounds often being a problem.  Have you tried a different 6SJ7?  Fussy tubes, often very microphonic and hummy.  Is pin1 grounded..... shoot.... is the whole unit truly earthed with a 3 prong cord?  The schematic doesn't even show line iso caps on the primary.

 

[lassoharp]

Haven't tried with the scope yet.  The hum seems pretty well isolated to the output tube - with V1 and V2 removed it still hums so I'm assuming the 6SJ7 isn't a problem.

That's a good idea on the choke/OT - they're less than 1/4in side by side beneath the chassis. Should be easy enough to pull the choke away and see if that helps. Open frame on each.

I've got a good 3 prong system in place - been shocked too many times over the years to trust 2 prong on anything over a watt- tube or SS. Yes, the schemo was right - no 'death cap' was on the pri.

 

[Gus]

Did you try a different 6l6?  Did you check the 6l6 socket?

 

[PRR]

> Balance virtual center tapped filaments is more of a push pull thang.

No.

The drawing implies that the heater winding may be internally grounded. Read the AC Volts off each lead. If 3V and 3V, your resistor CT is effective. If 0V and 6V, the internal ground over-rides the external CT.



Replace the 0.1uFd at the 6SJ7 G2. These go bad a lot.

The C-L-C filter should reduce B+ hum to negligible. Therefore it is grounding or layout.

The first filter cap must return directly to the PT HV CT lead. This path must not run around here and there through the rest of the circuit.

For gitar-only use, disconnect one end of the 1Meg from 6L6 plate to 6SJ7 plate. This is NFB, to mellow-out the harsh flavor of naked 6L6. It also brings B+ ripple into the amp. For gitar, the raw flavor of naked 6L6 is acceptable and often prefered. Your choice.

 

[lassoharp]

> Balance virtual center tapped filaments is more of a push pull thang.

No.

The drawing implies that the heater winding may be internally grounded. Read the AC Volts off each lead. If 3V and 3V, your resistor CT is effective. If 0V and 6V, the internal ground over-rides the external CT.



Replace the 0.1uFd at the 6SJ7 G2. These go bad a lot.

The C-L-C filter should reduce B+ hum to negligible. Therefore it is grounding or layout.

The first filter cap must return directly to the PT HV CT lead. This path must not run around here and there through the rest of the circuit.

For gitar-only use, disconnect one end of the 1Meg from 6L6 plate to 6SJ7 plate. This is NFB, to mellow-out the harsh flavor of naked 6L6. It also brings B+ ripple into the amp. For gitar, the raw flavor of naked 6L6 is acceptable and often prefered. Your choice.

PRR nailed it. 1 meg gone - hum gone!  That's a new one for me.  Hiss rose considerably but hiss is better than hum at the moment.

Looks like no internal grounding on this - it's actually a different(later) model than schematic - I rebuilt to the schematic version.  They'd originally used 2 11ohms way upstream from the HV CT for heater CT.

Thanks to everyone for chipping in.  With the level of discussion on this forum hum/buzz can seem a mundane topic but it's so often a bottom line issue esp with tube gear.  Those worrisome hours sure can drag out.  Thinking about cannikin's four(?) DIY Federal limiters with apparently still unsolved hum issues gives me an antsy feeling.  But sometimes things like that can be the reality.