Ampeg B15NC bias issue?

[mike_mccue]

Hi,
I've got an old Ampeg B15-NC here to work on. After about 15 minutes warm up one of the power tubes red plates and the power fades.

I'm looking at the schematic and see that the power tubes have a Cathode Bias layout but there is also a Hum Control pot connected to the 6.3vAC heater supply.

It's not clear to me how the hum control effects the bias of the power tubes.

Voltages look normal as I poke around the amp... until the power tube starts red plating. I mainly work on Fenders and was prepared to rebuild the bias supply, but with the Cathode Bias on the power tubes I'm not so sure what to look for.

Any tips?

[Leo_Gnardo]

Hum control should have no effect on the power tubes' bias. I'd look for a leaky capacitor to the power tubes' grid. Pins 5 should measure 0.00V DC, as in ZERO. If it's up a couple volts, that's the baddie. And there's always the possibility of a 6L6 that's just gone bad. How to tell - swap the positions of your 6L6's. If the problem follows the tube it's the tube. If not, it's the circuit and probably the cap that brings signal to pin 5.

[Enzo]

What he said^^^^

The 6vAC hum balance control has absolutely nothing to do with bias.

Certainly the first thing to suspect is the tube itself. So do what Leo said, swap places with the tubes. If the same tube red plates in the other socket then the tube is bad. If the other tube now red plates in the socket where the first tube was, then something about the socket is at fault, a good suspect would be a leaky coupling cap at the grid.


On a Fender, the bias supply affects both tubes, so if just one was red plating, rebuilding the bias supply would not be the choice.

[mike_mccue]

Thank You to both of you.

[g1]

The hum balance is tied to the cathodes to elevate it with some DC, which helps with hum problems, it does not affect the bias.

[mike_mccue]

The hum control is connected to a 6.3vAC source and it seems as if the hum control pot is acting as a voltage divider. I have to admit, I don't really see how connecting the cathode to an AC source helps with hum but I can now recognize how it doesn't effect a DC bias.

I was just starting with some crude assumptions. When I saw the hum control I confused it for something like a Fender bias balance. I didn't expect the cathode biased power tubes until I opened the chassis and saw the schematic.


Enzo said:
"On a Fender, the bias supply affects both tubes, so if just one was red plating, rebuilding the bias supply would not be the choice."

Please correct me if I'm not remembering correctly; Is it not the case that a Fender bias balance pot circuit in need of adjustment or maintenance can bias each, of a pair of power tubes, differently and you can observe one tube red plating without the other joining in?

re:



I now realize that this isn't anything like the Ampeg circuit, but it was what I based the initial idea on.

[Enzo]

Anything is possible, So a bias balance pot that was open on one end would deprive that side's tube of bias. Just as an open grid resistor on one side would. That isn't what I think of when someone says rebuild the bias supply. That makes me think caps and stuff. My point was that the bias supply serves both tubes, and so would not affect just one when faulty. But you are correct, a bad pot could do this.

I don't really see how connecting the cathode to an AC source helps with hum

Well, it is more like connecting the AC to a DC source. Look at the circuit, the cathode coupled AC has no path to ground, there is no circuit for AC through the cathode resistor. The cathode has a bypass cap usually, which means a pretty steady DC voltage. One potential source of hum is electron flow between the heaters and cathodes in the preamp stages, particularly the first one or two. If ever the cathode finds itself more positive than the heater in a tube, then it can attract electrons that boil off the heater, and that can cause a current that makes hum. SO we elevate the preamp tube heaters to a positive DC level. If we use say +20v, now your 6vAC will be alternating 3vAC either side of that or in DC terms 4.45v. So even at the most negative point on the AC cycle, the heaters never are less positive than +15.55v. SO we prevent that heater to cathode current in some early 12AX7.

This elevation doesn;t really do much for the power tubes, but it doesn't get in the way, so instead of only wiring the preamp tubes this way, we just run the whole string elevated.

[mike_mccue]

Thank you for explaining further Enzo.

I think I am following your explanation of the hum circuit but I am having trouble applying the idea to context of this particular amp.

On the Ampeg B15NC I see that the preamp tubes do not have Cathode bypass caps. The driver tube has a Cathode bypass cap connected to the ground bus. The power tubes have a Cathode bypass cap that is connected to ground and that is the only place the hum control is connected to the circuit. When you say the AC has no path to ground are you, perhaps, referring to the fact that this amp does not have a dedicated earth ground?

[mike_mccue]

It occurs to me that I should post the schematic that came with the amp I have here. I have realized that there are many variants of this circuit and that some of them, such as the Ampeg B-15 NF, have no cathode bypass caps on the power tubes.

medium size schematic of Ampeg B-15 NC:


Extra Large Size schematic of Ampeg B-15 NC is available here: http://harmoniccycle.com/hc/images/g...NC_1964-XL.jpg

[Enzo]

The smaller tubes, may or may not have cathode bypass caps, depending upon where they are and so on. But even so, they rarely have a lot of voltage across them. The power tube though will just naturally have a substantial voltage on its cathode. We are not connecting the heater circuit to that cathode because we want it involved with the power tubes, the only reason we hooked it there was to make use of the handy DC voltage just sitting there. Kinda like sitting your small child on a phone book to raise him to the table. It never mattered that it was a PHONE book, just that it had the size we needed.

SO by connecting the heater string to that +25v or whatever it is, we now have made all the heaters in the amp sit at +25vDC with respect to ground. The power tubes don't care either way, but the triode stages at the inputs will no longer be able to add any heater-cathode hum.

When I say "ground" I am referring to the circuit common - the chassis. I am not referring to the earth connection of a three wire power cord. We do want a three wire power cord, but that is a separate issue from the heater string.

The only cathode bypass cap that matters in this regard is the one on the output stage, it smoothes out the DC voltage there. That the other stages do or do not have bypas caps doesn't change that they can have heater-cathode hum. And remember, in terms of design, we are talking potential sources of hum. Not all preamp tubes will make the heater-cathode hum, but if a simple connection makes sure NONE of them will, then why not do it? It works even without a bypass cap so long as the voltage there never falls below the cathode voltages of the rest of the tubes.

I mentioned AC path to ground for one reason, to try to make clear we were not applying AC to the cathode, but instead connecting the cathode DC to that AC. If I just have the 6vAC sitting there, with no connection other than the tube heaters, it is only 6v in itself, not 6v to ground/chassis. So if I connect that to the tube cathode voltage, there still is not a complete circuit to ground for that AC. So I am not really putting any AC on the cathode.

[mike_mccue]

Hi Enzo,
Thank you very much for such a thorough and thoughtful reply. It has been very helpful and now I think I actually get what you are explaining where as I had thought I had but probably did not.

As it happens, I have just determined that the red plating is particular to the tube and not the socket, so I think I'll get some fresh tubes and move forward with optimism that it may just be a bad tube.

Thanks again for the help. This amp is a family "heirloom" of a musician friend of mine who has just received it from his dad who had gigged with it back in the 1960's and early 1970's. I want to convey to you that they will be thankful as well.

[mike_mccue]

Update: I tried some old, used but ok condition, tubes in the amp in anticipation of getting a fresh pair and the red plating showed up again. I guess one of the old tubes was more sensitive to the issue and so it appeared that one tube was red plating and the other was not. Now I see that all the tubes slowly trend to red plating and that some just do it faster than others.

I checked for DC on pin 5 and found it was leaking through the coupling caps so I'll work on replacing those caps before I do anything else.

[Leo_Gnardo]

I checked for DC on pin 5 and found it was leaking through the coupling caps so I'll work on replacing those caps before I do anything else.

Doubtless replacing those coupling caps will improve the situation. Just for the record, what voltage did you read on pins 5?

[mike_mccue]

Hi Leo,
It never really settled down. When the amp wasn't fully warmed up it was stable at about +3vDC, but as the amp really warmed up I had to pull the tubes that couldn't take it and I used the one tube that was the most resistant to red plating. One of the sockets had voltage climbing to over +30vDC before I stopped and starting looking through my stuff for fresh caps.

I don't have any 0.022uf 400vDC values, just 0.01 and 0.047 so I'll order some and get back to this later in the week.

I'm thinking of buying some Mallory 150s and some Orange Drop 715s and flipping a coin just before it's time to solder them in. I think I'll also get a pair of JJ 6L6 for the musician to use as a known good pair for getting this amp running after years of storage and I'll encourage him to consider a pair of NOS tubes once we know the problems are sussed out. If you have any opinions to share about that I'm open to suggestions.

Thank you for your help and advice.

[loudthud]

Check the 270K grid leak resistors for the power tubes. Replace if they have drifted out of tolerance. Too high a value can cause tubes to red plate or thermally run away as they heat up. This can look like you have a leaky cap because a positive voltage appears on the grid.

Does the amp have the Dad's name engraved on the Ampeg logo that lights up?