Ampeg B15 with dreadful hum Help

[freeformfx]

Well maybe I have a clue my ext cab works fine now (I did have an error in my jack wiring) but now my ext amp out sends a gated a bit distorted signal. Doesnt make much sense as the amp sounds awesome, I should say the preamp at least sounds just great aside from the hum. And That signal comes from just after V1/V2 mixing
If also someone could explain to me why the main speaker jack should be isolated since it's ground lug is connected to ground on the eyelet board. (Disconnecting that ground leaves a high pitched oscillation)
I see why the ext. speaker needs isolated but I don't get it on the normal speaker output.

and I thought of one possible place I could have a ground loop(?) I have the cener tap of PT from the standby, grounded on a lug on the cap can and not on the eyelet board grounds. Is that even possible to make a difference?

[g1]

The main speaker jack is probably insulated from the chassis to avoid a ground loop.

[freeformfx]

does anyone have an opinion on whether that PT centertap via standby switch being mounted to the chassis on a bolt rather than on the eyelet board grounds leading to what is supposed to be the one chassis ground at the input jack, whether it constitutes a ground loop? I can easily chop it off and ground it with the power supply caps or near there but don't want to do that if it's not very likely that it's causing the hum. but that IS the only other ground that goes to the chassis instead of the board's ground bus.
i've tried everything to alleviate the hum, after adjusting the hum balance pot by ear for least hum a bit more precisely ( i usually split em down the middle so like 3.3v on one heater side and 3.3v on the other) channel one improved a good bit, channel two is the worse offender now.
and no matter what while using one channel you CAN'T have the opposite channel volume up or it compounds the hum. i tried two NOS Sylvania 6SL7's and got rid of the russian tubes but that didn't help either. And I installed a switching jack for channel 1 input 1 too. The possible ground loop is my only hope right now I guess.

[hylaphone]

Can't tell from the pics, is your hum pot wiper elevated above ground? It should be tied to the 6L6 cathodes

Also you can try a 5R4 to drop B+, I've used it in a couple B15's.

[freeformfx]

Can't tell from the pics, is your hum pot wiper elevated above ground? It should be tied to the 6L6 cathodes

Also you can try a 5R4 to drop B+, I've used it in a couple B15's.

Yeah it's tied to the 6L6's cathode voltage and it seems to be best adjusted nearly all the way CW. I like it when hum balance heater pots give best cancellation closer to center rotation. I thought you meant 5AR4, I really didn't realize those 5R4's were different rectifiers. How much more drop would you be expecting from that tube? Compared to the 5U4 which dropped it like 45v or more I think.
But using any 5U4 curiously caused a crackling in the amp. I tried two known good ones too. The crackling seemed to die down a little but not gone. THough the 5U4 uses 2amps of heater current though which I feel maybe too much, on an amp that has a PT that is hot to the touch to where you can't keep your hand on it. What does the 5R4 say in heater requirements?
I have been desperate to drop voltage on this amp, I tried a 40v zener from weber on it and I think it's bad cause it doesn't drop hardly any voltage
unless I somehow have IT hooked up wrong

[DrGonz78]

So do you have that 40v zener connected in that picture that you posted? I was looking at that bolted item where the center tap wire connects to the standby switch and a wire on the other side of the switch connects through a zener to ground. I see what looks like a brownish wire coming off the standby switch and then connecting to the zener. I also see a black wire that seems to be near the brown wire in that location that is grounded to the chassis. I can't see from the picture where the black wire originates.

[freeformfx]

Firstly I'm sorry guys I had forgot when I took these pics that I'd had the zener disconnected, cause it wasn't seeming to drop enough voltage, so there are no wires going to it in the pics although it's still there
The brown wire is the center tap (connected through standby switch) which is spliced to a black wire and nutted to the chassis. Being the possible ground loop I was asking about.
The all black wire you're seeing there is one of the primaries from the PT on it's way to the main power switch
So as i believe the reason why the brown/black wire was spliced (the Center tap to ground ) I had that wire simply connected to the tip of the zener diode. IS this the proper way to wire the zener for voltage drop to occur? I'd like to know if I had i right, I'd not think it likely that it's a bad diode. And I think I followed Weber's install instructions they supplied and is also the way I've heard installing one described before.
My PT is getting very hot when in play position and unless I use a different rectifier to drastically reduce high voltage the zener was my only other option. which would require me further adjusting that cathode resistor value but that's no biggie.

[DrGonz78]

The way you describe connecting the dropping zener diode seems correct to me. I am new to all this but have had some recent advice on the forum here from more experienced minds. So that part is not a worry as far as I know.

I have read that it is recommended to attach the HT center tap to the negative leg of the first filter cap in the power supply. From what I understand is that attaching the CT to the negative side of the first filter cap helps reduce hum. Before doing this I would wait to have other more experienced minds chime in on the matter.

Edit: I did this exact wiring on a Fender champ that I built. I am pretty sure this is correct for this amp too, but I am not 100%.

[freeformfx]

Thanks yeah I think that's the right way to connect it too. CenterTap wire connected to standby switch, the other lug on the switch with a wire going to the zener diode's tip (anode). Or probably correctly grounded to the filter cap ground if not using a zener. But I didn't get the 40v drop when I connected it that way.
Yeah I'm waiting for someone else to chime in on using the zener in this amp. I think our logic is right though.

[DrGonz78]

Yeah I was not all too worried about how to connect the zener diode. Actually I was more concerned with hooking the HT CT to the negative side of the first filter cap. Thinking more on it now I would just try it personally. The amp I am building now I plan on connecting the HT CT to that same location. I have read too many times now that this is correct. However, if anyone knows otherwise then that would be great information to know too.

Edit: But I am confused as to why you did not get any voltage drop across that zener diode.

[freeformfx]

I know I may try hooking up the zener again before I decide to move that ground off the chassis and to the first filer ground in hopes of eliiminating some hum at least
Any other votes for a 5R4 in a B15, it doesn't use more current? I know it drops alot more voltage though.

[nevetslab]

While I've never seen a HV Power Supply with a zener in the C/T winding going to ground, it sounds wrong. That's your return current path of the windings, and it needs to go directly to your power supply filter ground, as it's high charging current. And normally, in using zeners, they need to be current limited so you don't dump too much current into them (the Zener current to get them to regulate). You're better off dropping voltage with a 5AR4 or 5U4GB (which will drop more DC voltage than the 5AR4, since you have more than you want to begin with.

With regards to built-in ground loops, or what may have been wired in during the re-build, you DON'T want the power supply charging current in the path of your low noise input ground. I've never been a big fan of 'RF' Grounding technique as so many of the tube amps are built with, where everything is tied to the chassis all over the place. Fact that some are quiet and others, same model, are full of hum still mystifies me, but establishing your high current ground (1st two stages of power supply filtering for P/T primary and screen Supply filters, along with the C/T lead from the HV Secondary), isolated from the chassis, and then taking that center node of the high current ground return to chassis at one point, you have a chance of reducing the induced power supply ripple (120Hz).

60Hz coupling is field-induced...that can be a lot trickier to control. loop area with gain makes a great way to induce that magnetic field into the circuit, so you're looking at the build factor and all the effective circuit loops that can serve to couple that 'hum field' into. Break the loop, or reduce it to a very small area are two effective ways of controlling it. Can that mean changing the layout of how the eyelet board or tag board was constructed. YUP..

[DrGonz78]

I have been reading up on the subject of zener diodes connected from HV CT to ground. Some have reported that the fixed bias of their amp had changed by the difference of the voltage of the zener diode connected. Hence, fixed bias voltage supply supposed to be -50v and after switching the amp on from standby the voltage changes to -99v. In that case they had a 50v diode connected and it seems that it changed the 0v reference of ground, causing incorrect readings of the bias supply line. Since this Ampeg is using cathode bias it will not have a fixed bias supply like the example I gave above. However, I am curious what other's have to say about the 0v ground reference changes in relation to a cathode biased amp. Won't it still have similar problems because of this very issue?

So perhaps the best straight forward approach is to just wire the zener in series with the B+ supply to acquire the voltage drop. This is the approach I am using on my current build, because I was told here on MEF that the zener to ground method can make biasing the amp problematic. If you do go the route of wiring that zener diode in series with the B+ then you will need to isolate the zener from the chassis and that task might seem like a pain. First thing first to me on this amp is see if the hum gets better with the CT grounded to the negative leg of the first filter stage.

I would definitely try to lower the B+ with a different rectifier tube if you have a 5U4GB around give it a try.

[Enzo]

It depends on whether your bias supply comes from a tap on the B+ winding or not.

[freeformfx]

It depends on whether your bias supply comes from a tap on the B+ winding or not.

this one's cathode biased'so I disconnected the ct ground (from standby switch) from the chassis ground and put it in on the board buss with the first filter cap. The hum may have been slightly improved I just can't say but all of a sudden I have trouble with high some maybe ultra high pitch oscillations with my volume and treble controls on both channels. That definitely wasn't here before.
So after having a go at the lead dress a bit had no luck and went back to chassis ground and though it was still there I was able to work it out by manipulating lead dress. sometimes randomly. So I'm back where I was I think even after my biasing and all. I hope so cause it was sure sounding good coming out of that portaflex cab before I started on it. But that hum, and the PT gets so hot I had to put it on the bench.
Is it sensible for that chassis ground for CT to behave that way?