Gibson GA200 Question

[stereomonostereo]

I'm working on a badly abused 1959 Gibson GA200. After replacing all of the filter caps and a handful of coupling caps, i've got her up and running. My problem now is that instead of getting my -33 on my control grid of the 6550s, i'm getting i'm getting +38. The schematic (i believe) shows a diode coming off of the power transformer before a pair of filter capacitors. The amp, which i don't believe was ever worked on before, has no diode. On top of that, a bunch of the resistor values are different from what's shown on the schematic. I know the design on these changed a bit over the years.

I imagine the amp would have to have the diode in order to get the negative voltage. What i'm wondering is, what value would i use?

[Jazz P Bass]

That +38 volt reading should have one heck of a ripple (AC) on it, if there is in fact, no diode installed.(which seems odd)
Also, the caps, although it is not indicated on the schematic, get wired backwards.
The plus goes to ground.
A 1N4007 diode will do just fine.

[Enzo]

If you have DC of either polarity, I have to believe there is a diode somewhere. Transformers can't make DC.


Is the same +38v on BOTH tube grid pins? Pin 5?

Likewise, are we reading pin 5 instead of pin 4? This amp has a 6V6 tube regulator for the screen B+, and if that is not working right, your screens could be sitting at +38 instead of +300.


As to a diode, it is just a bias supply, so most any diode would work as long as the voltage was enough. I would grab a 1N4007 from the drawer and never look back. But before doing that, I;d be finding out why ther is DC where no diode exists. Or find where they hid the diode. Rear side of the part board maybe?

[stereomonostereo]

Okay, i think i've got it sorted. I replaced the two .05uF bumblebee caps and now i'm getting 45mA on both 6550s. The odd part is that there's no diode. I guess this model is cathode biased?

[Jazz P Bass]

I guess this model is cathode biased?

Is pin 8 at ground?
Kind of silly to cathode bias 6550's, I would think.

[stereomonostereo]

Yep, it's going to a power resistor and then to ground. It's definitely different than the schematic, but i think it's factory. First time i've seen cathode biased 6550s.