Ampeg B15 Build -- Bias supply question

[hylaphone]

Hey gang, just fired this build up for the first time, no tubes, and noticed there isn't quite enough negative bias.
Schematic suggests -50v, I'm getting -35.



I have tried swapping the 100k, the 56k, and the diode, then tried a few different values for those first couple resistors. Didn't change all that much..

Transformer is 380-0-380, voltages look good elsewhere..

The negative bias should read correct without tubes installed, right?

Or am I missing something colossally obvious? Wouldn't be the first time...

[The Dude]

What is the actual measured plate voltage you have? You're right that the bias voltage should read correct without the tubes installed, but correct is a relative term in this case. I'm just wondering if all is low or only the bias voltage.

[loudthud]

That 100K needs to be atleast 1W, 2W would be safer. If you used a 1/2W it could have gotten too hot and drifted upward in value lowering the bias voltage.

[Jazz P Bass]

Here is an Ampeg schematic.
2 Watts 100K it is.
(Interesting bias fed hum control)

[bob p]

What's up with that "K" on the Piazza drawing? I couldn't find a second point labelled K on the schematic.

[loudthud]

Between the Hum balance pot and the bias voltage. For some reason old Ampegs used a negative voltage standoff on the heater supply instead of positive voltage.

[The Dude]

What's up with that "K" on the Piazza drawing? I couldn't find a second point labelled K on the schematic.

I believe it to be simply a test point.

[g1]

What is the actual measured plate voltage you have? You're right that the bias voltage should read correct without the tubes installed, but correct is a relative term in this case. I'm just wondering if all is low or only the bias voltage.

Yep. The ampeg schematic shows -56V bias with a plate voltage of 450V. Is your plate voltage near 450V?

What's up with that "K" on the Piazza drawing? I couldn't find a second point labelled K on the schematic.

Between the Hum balance pot and the bias voltage. For some reason old Ampegs used a negative voltage standoff on the heater supply instead of positive voltage.

Interesting. So if the heater winding got close to ground (bad tube etc.) you would have the 100R hum pot in parallel with the 47K (R34) in the bias circuit and pull down the bias?

[hylaphone]

OK, fired up on the variac to 110v (don't want to exceed the filter caps rating with unloaded B+), 5AR4 only, I'm getting -36v bias and 485v to the plates.

This PT has no filament CT, instead of the hum balance pot I'm running 100ohm resistors to ground. They are NOT tied to the negative bias. Would that effect the readings?

That 100k resistor in the bias circuit is 3w, the others are 1w.

[Jazz P Bass]

So walk it through.
What is the voltage at the transformer side of the 100K?
What is the voltage on the other side?
Until it hits the diode, it will be Vac.

[hylaphone]

Oh man, I had the cap backwards in the bias circuit. Switched it around, now we got -50v on the dot!

Thanks for the help!

Now, regarding negative voltage to the heaters -- is there a way to implement it into the 100r artificial ct? Or is it even necessary?

[Jazz P Bass]

If it was me, I would throw that cap away & install a brand new one.
E Caps do NOT like to be reversed.
Just a thought.

[hylaphone]

OK will do. I was using 10uf from the Piazza schematic, the one you posted uses 100uf -- I should be ok with 33uf? All I have on hand..

[g1]

Oh man, I had the cap backwards in the bias circuit. Switched it around, now we got -50v on the dot!

Funny, I was going to ask that but thought if that were true it would have blown up by now .

Now, regarding negative voltage to the heaters -- is there a way to implement it into the 100r artificial ct? Or is it even necessary?

You would connect the 100R's to the neg.voltage rather than to ground. I wouldn't suggest doing it unless you have a hum problem you can't get rid of any other way.

OK will do. I was using 10uf from the Piazza schematic, the one you posted uses 100uf -- I should be ok with 33uf? All I have on hand..

That is an error on the Piazza schematic. I appreciate his efforts but I always try to avoid his schematics unless factory versions are not available.
The 33uf will work, you may get a bit more hum than stock. I would suggest you slate it for replacement with proper value at some time in the future.

[The Dude]

I, too, thought about the bias filter being backwards. But, like g-one, I didn't ask about it because I thought it would have blown up if it were reversed.
33UF should be just fine as long as the voltage is rated high enough. Your bias supply will just be better filtered.