Bleeder resistors - better on any particular filter cap?

[tubeswell]

On the 6G15 I finished last month I put 235K bleeder resistors (Two 1W 470k resistors in parallel) on the second filter cap (the one that takes out the ripple), and it drains the caps nicely at switch off.

But when I use the 6G15 with other amps there is a hum. Not a big hum, but enough to be annoying when the thing is idle. The hum is there when the guitar vol is cut right off, but is gone with the amp vol off but it gets louder as you turn the amp volume up.

So I am pretty sure the hum is coming in from the unit in front. The amps are quiet by themselves (i.e. without the unit in front).

I think the hum is related to some extent to the unit's output cable, because when I put a shorter, good quality cable in, the hum is much reduced.

I have potted around with the grounding to no further avail. The grounding is split-ground type (filter caps and output tube cathode and grid load R and Reverb tranny primary grounded together at a PT bolt, and the pre-amp grounds and pre-amp filter cap ground together at the input socket ground - and the heater CT is 'elevated' at the 6V6 cathode pin.

The output socket is not insulated from the chassis, but only the tip is connected so there can't be any loop happening there.

I wonder whether having the bleeders at the 2nd filter cap is better or worse in terms of ripple removal. (Does a drain here limit the effectiveness of ripple reduction?)

Or would installing a Mains phase-reverse switch help at all?? (it is a 3-prong mains wired system - but I wonder whether, in using the amp in conjunction with other amps, there could be some out-of-phase thing causing it to hum, which could be killed by swapping the primaries around?)

Any pi-deas?

[Enzo]

I have never heard the second filter cap described as the one that removes the ripple. The first cap does most of that, the second one just cleans up what is left. I would put my bleeders on the first cap, as it is the highest voltage in the system. No, I don;t think they contribute in any way to your hum.

The output socket is not insulated from the chassis, but only the tip is connected so there can't be any loop happening there.

What do you mean there can't be a loop? Of course there can. A ground loop occurs when two pieces of gear have dissimilar chassis potentials. When you connect them together with a signal cord, that potential difference flows through the signal cable shield. That 60Hz - or 50 - is then impressed upon your signal.

If the amps are silent, and the hum is not diminished by turning the guitar volume to zero with the revedrb connected, then it sounds likw a loop to me. Connect your reverb to the amp input, but connect it to the mains either through a ground lifting adaptor, or use clip leads to connect mains to the hot leads. In other words break the ground connection. If this reduces hum, then we broke a loop.

I suppose you could unground the amp instead. or if you want total isolation, find a battery operated amp to plug the reverb into.

A shorter signal cable being less hummy is consitent with my theory. The lower the ground resistance in the cable, the less chasis potential difference there will be.

[tubeswell]

Thanks for those thoughts Enzo - That was my suspicion. And so today I switched the reverb unit off for a while (out of curiosity) and left the amp on, and the hum remained. It only stopped when I unplugged the reverb unit from the mains or when I unplugged the connecting lead, so its got to be coming from a ground loop. I am a bit reluctant to lift the mains ground of either the unit of the amp, but I think maybe if I can find a 1:1 audio transformer I can make an isolator to go between the unit's output and the amp's input.

[Enzo]

Just lift it for testing to verify if it kills the hum. CErtainly that would be no more dangerous than working in live tube circuits.

[Diablo]

If I understood Enzo's last tutorial on this subject, if you break the ground of the reverb unit's power cord, you still have a good safe ground on the reverb unit as long as you keep the instrument cable connected between the reverb unit and your guitar amp, and as long as your guitar amp has a grounded 3-prong plug. Just remember that the ground connection for the reverb unit is the instrument cable ground.

[Enzo]

I don't think that would be my preferred solution, but it does describe electrically what would happen. I would much prefer the isolation take place in the signal connection rather than relying on the signal cord as the safety ground.

[soundmasterg]

3 steps to Nirvana...


1. Get one of these: http://www.robertkeeley.com/product.php?id=48

2. Plug the amp and reverb unit in. Set the device for both at the same time.

3. Problem is gone. Play with wild abandon.

Greg

[Diablo]

3 steps to Nirvana...


1. Get one of these: http://www.robertkeeley.com/product.php?id=48

2. Plug the amp and reverb unit in. Set the device for both at the same time.

3. Problem is gone. Play with wild abandon.

Greg

Geez......$289.
I think I have more important things to spend my money on.

[Enzo]

You can make a box with a little 1:1 transformer in it by yourself. Make the box metal for shielding. Use insulated jacks so the sleeve doesn;t ground to the box. I use the CLiff jacks - "Marshall jacks." - with the large plastic nut. Ground the box to ONLY ONE of the jack's sleeve.

IN fact, I usually make them with a short cable and plug out one end with a jack on the other. I plug it into the receiving device input jack and let it hang. PLug the cord from the signal source into the jack on the box.

[tubeswell]

Hi Enzo - Do you think this tranny work ok for this application?

http://www.jaycar.co.nz/productView....=&SUBCATID=378

[Enzo]

For five dollars it is worth a try, but I'd prefer something a little higher in impedance. I don't recall where I last bought them, but I think the ones I used were 15k:15k. On the other hand, they are just transformers, so other tham womdomg resistance...

try it.

[R.G.]

Geez......$289.
I think I have more important things to spend my money on.

Well, if you can solder, you could use the Hum Free ABY switcher at GEOFEX.

It uses $2.50 Mouser/Xicon transformers and hot rods them to get frequency response needed for clear guitar. The input is an opamp, so there is no input loading.

A bare transformer is always a compromise, especially on a guitar pickup with its high impedance for treble.

I've never seen inside a Framtone switcher, but I'm pretty sure it's a buffer driving a transformer or two, just from the description.

[tubeswell]

For five dollars it is worth a try, but I'd prefer something a little higher in impedance. I don't recall where I last bought them, but I think the ones I used were 15k:15k. On the other hand, they are just transformers, so other tham womdomg resistance...

try it.

Okay I might get one made up

[tubeswell]

Well, if you can solder, you could use the Hum Free ABY switcher at GEOFEX.

It uses $2.50 Mouser/Xicon transformers and hot rods them to get frequency response needed for clear guitar. The input is an opamp, so there is no input loading.

Hi R.G.

I tried searhing for it just now on your site, but couldn't find it for some reason? Have you got a link there please?

(Ed. Note) Never Mind - I found it thanks.

[tubeswell]

1st result (neither startling nor successful)

Okay so I tried a cheap isolating tranny (the model described in this pamphlet, but without the secondary CT wired up) in a metal box with two sockets (one of which was non-grounding - see schematic attached). It stopped the hum fine (which I guess it proves that it was a ground-loop hum), but it also attenuated the signal too much (I trust the guy in the shop gave me the correct 1:1 thing I asked for and not something with a different ratio). It also changed the frequency of the signal (taking out most all of the bass and leaving a nasally mid band). So don't bother trying this at home.

The only useful purpose of the box as it is, is to test for suspected ground loops. (I guess that is the cheap tranny - it is only a tiny thimble of a thing). I suppose one of RG's op-amp buffered circuits might make the tranny sing better, but I might resort to a higher spec tranny first if can find one.