Ampeg B15s Woes and bias issues

[ebulakowski]

Hello amazing amp hive mind. I'm struggling through a B15s repair. I am currently experiencing positive voltage in the bias section. At this point I have checked for caps, diodes, and resistors as well as their orientation. I have even lifted the coupling caps to make sure they weren't leaking through. No tubes installed.

As it stands if I bring the amp to 60v on my variac I am getting +7 of bias. I get 140v off of the Bridge rectifier scheme. c26 is dropping me to 45v, then after the bias circuit diode I still get 7v.

Any help or insight is appreciated.

[Latole]

AC on bias ?

What give DC  voltage ; rectifier diode with the help of filter caps

Check and replace D6. You wrote you check it !!!!

[ebulakowski]

Latole, thanks for the response.
Yes, I already replaced D6 and the caps. I have been getting +7vDC with the chassis as my ground reference.

Are you saying I should look for AC?

AC on bias ?

What give DC  voltage ; rectifier diode with the help of filter caps

Check and replace D6. You wrote you check it !!!!

[thetragichero]

when you replaced d6 did you get the polarity right?

[PRR]

> bring the amp to 60v on my variac

Is it wired for 120V or 240V at the wall?

Because the bridge rectifier should be a lot more than 140VDC if this is 60V to a 120V connection. So what else don't we know yet?

[ebulakowski]

when you replaced d6 did you get the polarity right?

Yes, it has been checked many times. Because i'm going nuts with this one.

>
Is it wired for 120V or 240V at the wall?

Because the bridge rectifier should be a lot more than 140VDC if this is 60V to a 120V connection. So what else don't we know yet?

Perhaps I phrased it wrong. That voltage is off of one leg on the high voltage winding. My voltage AFTER the bridge network is 300v (when the variac is set to 60v)

[ebulakowski]

So here is my fresh eyes morning update.
- I started thinking a lot about the CIRCLED X on the schematic and where they tie the hum balance pot in.
- At that point I am getting 23vdc! this of course means lug 2 of the hum balance has the same voltage. So, I looked for DC on the heater line aaaaand, yup, I get as much as 30vdc in some spots.

I try to do my best with clean thought out work. But I will always admit, I am still learning a lot and always will be. That said, what am I supposed to learn here. My gut is saying...

- something is making a connection somewhere it shouldn't?
- is there a common issue i'm not aware of that would result in this?
- Is this a possible transformer issue?

Again, thanks to everyone for input and accountability. I'm grateful to be able to inquire to so many minds that I can continue to learn from.

[sluckey]

Pull the output tubes. Plug the amp straight into the wall. NO VARIAC! Recheck bias voltage directly on the output tube socket pin 5. What have you?

Look at page 5 of this pdf to get an understanding of how that bias circuit works. You may need to replace C26. Best to use a Class X cap.

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[ebulakowski]

Pull the output tubes. Plug the amp straight into the wall. NO VARIAC! Recheck bias voltage directly on the output tube socket pin 5. What have you?

Look at page 5 of this pdf to get an understanding of how that bias circuit works. You may need to replace C26. Best to use a Class X cap.

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Thanks for chiming in. (side note, thanks for all your website work. I was checking out a few of your layouts over the weekend.)
C26 had already been replaced. And all of my previous work was done with the tubes already pulled.

After this mornings post I decided to look into the Voltage I was seeing on the Heaters. I actually discovered an area where it looks like something arced. It was between v2 pins 4/5 and a higher voltage line trace. I cleaned it up and cut out a little of the carbon scorched area and Voila! I have appropriate negative bias voltage. 

Now I presume that unwanted voltage on the heaters fried some tubes. But at least I don't feel like i'm going crazy having already re-woked the bias circuit.

[Willabe]

- I started thinking a lot about the CIRCLED X on the schematic and where they tie the hum balance pot in.
- At that point I am getting 23vdc! this of course means lug 2 of the hum balance has the same voltage. So, I looked for DC on the heater line aaaaand, yup, I get as much as 30vdc in some spots.

After this mornings post I decided to look into the Voltage I was seeing on the Heaters. I actually discovered an area where it looks like something arced. It was between v2 pins 4/5 and a higher voltage line trace. I cleaned it up and cut out a little of the carbon scorched area and Voila! I have appropriate negative bias voltage. 

Now I presume that unwanted voltage on the heaters fried some tubes. But at least I don't feel like i'm going crazy having already re-woked the bias circuit.

That amp has dc elevated heaters. Look at the schematic, the -bias dcv is applied to the heaters hum-dinger pot's wiper.

You will always have whatever the -dcv bias is on the heaters.

Read this on tube heaters, from Merlin's web site;

http://www.valvewizard.co.uk/heater.html

[ebulakowski]

Thanks for that link! I'll have to read through it a few more times for sure. Lots of great stuff there.

- I see what you're saying that my negative bias supply would be applied. But what I was actually seeing was a  positive voltage. And that was showing Both on the heaters and the bias supply.
- When i went to correct the trace/arc issue I found it resolved the problem. I now see the appropriate negative voltages in those areas.

That amp has dc elevated heaters. Look at the schematic, the -bias dcv is applied to the heaters hum-dinger pot's wiper.

You will always have whatever the -dcv bias is on the heaters.

Read this on tube heaters, from Merlin's web site;

http://www.valvewizard.co.uk/heater.html

[sluckey]

That amp has dc elevated heaters. Look at the schematic, the -bias dcv is applied to the heaters hum-dinger pot's wiper.

Hmmm, I wonder. Seems like the heaters may be dc sunken. Now there's even more voltage difference between cathode and filament. I thought the whole idea was to reduce the potential between cathode and heater.      

[Willabe]

Yeah for a CF, but I think they did it for noise?

But like you say now the heater/K difference is even greater on that CF. Not sure new production tubes will be able to take it.     

[HotBluePlates]

Hmmm, I wonder. Seems like the heaters may be dc sunken. Now there's even more voltage difference between cathode and filament. ...

Yikes!!  That seems like a truly-bad idea, since there's no tube cathode running down to a negative rail.

... I think they did it for noise? ...

Can't be.

Heater-to-cathode leakage can induce hum via two mechanisms.  One is to envision the heater-cathode as a diode that is forward-biased when the cathode is positive of the heater, thereby passing a 60Hz pulse of half-wave rectified hum to the cathode and into the audio.

That's why we usually have a positive voltage for elevating the heater: we're reverse-biasing the "heater-cathode diode" so that the cathode is never positive (relative to the heater) at any part of the heater's AC voltage cycle.

Tektronix (possibly others) would have a negative DC reference for heaters of some tubes.  However, that was only for some tubes, typically used as differential amplifiers whose cathode connected through 100-150kΩ resistors down to a -150v supply rail (the same tube might have 100-200kΩ up to a +350v supply).  And there, it was mostly about reducing heater-to-cathode voltage (I'd have to look, but I think the reference was the -150v rail itself).

[ebulakowski]

I won't lie, this topic has moved into an area that is a bit beyond me at the moment. But what I gather is, as designed, the B15s has a few bad ideas implemented. Anything I should be wary of or address?

Hmmm, I wonder. Seems like the heaters may be dc sunken. Now there's even more voltage difference between cathode and filament. ...

Yikes!!  That seems like a truly-bad idea, since there's no tube cathode running down to a negative rail.

.

[sluckey]

Ampeg referenced the hum balance pot wiper to the output tube cathodes in their cathode biased B series amps. But in their fixed bias amp the hum balance pot wipers are referenced to the negative bias or ground. They seem to have just flipped a coin to see what the wiper would be connected to. No pattern that I can see.

I wonder if the filament connection to the negative bias voltage was a factor in your weird bias voltage issue?

[PRR]

Negative can work also. Assuming nothing is already strained. Any difference can be better than no difference.

Anyway the 90 day warranty did not cover tubes.

[HotBluePlates]

That amp has dc elevated heaters. Look at the schematic, the -bias dcv is applied to the heaters hum-dinger pot's wiper.

Hmmm, I wonder. Seems like the heaters may be dc sunken. ...

Negative can work also. Assuming nothing is already strained. ...

Turns out, negative works.  The guy in this thread was struggling with hum in a vintage B-15-N, which was cured when he connected the hum balance pot to the negative bias supply.

[sluckey]

I never had any doubts about hum reduction by referencing heater voltage to the negative bias voltage which is at AC ground potential. My comments were aimed at the fact that in doing so there is now a greater voltage potential between filaments and cathode. Ampeg amps are solid and are built upon good tech. Maybe the heater/cathode voltage is not as critical as being touted today. Heck, every other tube chart characteristic has been abused by the guitar amp industry. Perhaps the new millennium perfectionist techs ain't as smart as the old '60s engineers. Too much time wasted crossing Ts and dotting Is. Time to get back to the breadboards!