Princeton Reverb build with pot for bias does not have enough negative bias voltage

[salty09]

Hey there.

This is only my second attempt at building an amp and I am thinking I know the answer here, but wanted to verify before I start returning anything.

I built the amp using the Mojotones Black Princeton Reverb small parts kit. I bought ClassicTone transformers from Magnetic Components Inc. Here is the spec for the Power Transformer:
http://www.classictone.net/40-18019.pdf


What I discovered when trying to bias the amp was that my negative voltage for bias being read from the point where the 2 220K resistors meet is -3 volts. My understanding is it should be closer to -30 volts.

I cannot really turn it on for long to try to test much, the tube hits 100 ma with my bias probe. It should be like 18 or so.

The thing that I see that is interesting to me and I am wondering if it is causing my bias problems, the leads from the PT to pins 4 and 6 of the rectifier do not seem to be correct. According to the PDF document, link included above, shows that those two leads should be reading 650 volts between them. I am only reading 630 between the two pins. Would that difference be what is causing my bias voltage problem?

Is there most likely something wrong with the PT?

I have verified that it is for sure being fed 120 volts.

If necessary, I can for sure post pics and anything else needed. I thought I would ask about that voltage first though, because that seems like it may be my problem.

Thanks.

Salty

[g1]

Does the filter capacitor for the bias supply have it's positive lead going to ground?

[salty09]

Hey g1. Thanks for the reply. It does indeed have the positive lead going to ground. Here is a pic in case there is something else wrong. The kit came with a 22K resistor to go in parallel with the cap, I added in a 10K pot as well

[Dave H]

I cannot really turn it on for long to try to test much, the tube hits 100 ma with my bias probe. It should be like 18 or so.

The thing that I see that is interesting to me and I am wondering if it is causing my bias problems, the leads from the PT to pins 4 and 6 of the rectifier do not seem to be correct. According to the PDF document, link included above, shows that those two leads should be reading 650 volts between them. I am only reading 630 between the two pins. Would that difference be what is causing my bias voltage problem?

630V is not the problem. It's the bias supply. It should provide -34V at the junction of the 220k resistors not -3V. Remove the power tubes so you can fix the bias supply. What is the voltage at the bias supply cap -ve terminal?

[Dave H]

The kit came with a 22K resistor to go in parallel with the cap, I added in a 10K pot as well

That could pull the voltage down. I only have the Fender schematic. Could you post a schematic of the circuit as it is now?

[salty09]

Hey Dave,

Thanks for the reply.

I pulled the tubes and measured the bias supply cap's negative lead and show +4.8 Volts

Thanks for letting me know that the low voltage from the PT is not a problem.

[salty09]

Hey Dave. Let me see if I can find the Mojotone one online and edit it to add in the bias pot. They sent a printed one with the small parts kit, but I bet I can find it somewhere.

[Dave H]

I pulled the tubes and measured the bias supply cap's negative lead and show +4.8 Volts

I can't think where a positive voltage could com from. Is the diode reversed?

[salty09]

Ugh! That is it! They have the + and - symbol backwards compared to the symbol on their layout. Their layout has a + next to the Cathode side of the diode and a - next to the Anode side. I should have caught that the actual symbol did not match the + and - symbols they inserted. Let me flip that around.

[salty09]

That was it!!!! Good thought Dave...I feel like an idiot. Lesson learned, compare the layout they give you to the actual schematic. And! Don't just look at the polarity symbols they add! they are totally backwards on their layout diagram.

[salty09]

Thanks a bunch for your help guys. Good call again on the diode Dave. Got it biased at 18ma and 420 volts on the plates. Amp is running quite and everything seems to work! I can see getting hooked on building amps...lol. I do build pedals as well, and why I feel so ashamed I looked at their polarity symbols and not the diode symbol instead.

[salty09]

Ugh, so that led to another problem.... The 1K 2W resistor connected to the OT and rectifier pin 8 just went up in a puff of smoke.

[salty09]

So I am wondering, could have running the tubes with that super high bias voltage damaged them enough to cause them to burn up the 1K Resistor? I had to order some more, I have some but they are 3 Watt and looking at the schematic, it seems that resistor is in a good spot to be a fuse of sorts before the OT and that I probably do not want to increase the wattage handling of that resistor.

[g1]

The lack of bias may have damaged a power tube which is now burning up that resistor that supplies the power tube screens.
I would also suggest you replace that bias cap as it was subjected to reverse voltage and may have been damaged. Being in the bias supply you don't want to take any chances.

[nickb]

Ugh! That is it! They have the + and - symbol backwards compared to the symbol on their layout. Their layout has a + next to the Cathode side of the diode and a - next to the Anode side. I should have caught that the actual symbol did not match the + and - symbols they inserted. Let me flip that around.

Strange as it may seem, the cathode is always marked with a '+' sign. The logic of doing that escapes me now.