Hoffman Princeton Troubleshooting

[Timmly]

Hi, figured I'd bring this here since both me and my amp tech are stumped. Tried to build a hoffman princeton reverb kit, but ended up with no output. Everything lights up, theres just no output. The output transformer is a deluxe reverb tranny from hammond, we tested it it for continuity across both primary and secondary, and there doesnt seem to be any shorts. The oscilloscope shows the signal (we just plugged a phone into the input and played a tone through it) is fine until it gets to the power tubes, so we tested the power tubes and they are in full working order. Bias current shows between 0 and 1 milliamps despite there being a plate voltage of 458 volts. Adjusting the bias pot doesnt do anything to the bias current. What's bizarre is that the voltage on the 47uf cap in the bias circuit shows almost 150 volts on it, despite being rated for 100 volts. We used multiple multi meters to check this but got the same thing, and he tested the capacitor and it seems to be functioning.

We checked all of the connections underneath the board and they are all fine, and there doesnt seem to be any shorts along the circuit at all.

A couple of notes on mods: -diodes on rectifier as a safety measure (disconnected in the picture I took because we were trying to eliminate some things that might be causing issues)
-extra capacitor on rectifier as an extra measure against noise
-grid stopper on phase inverter, not sure if its significant but figured I'd mention it
-deluxe reverb output transformer

Pictures since they're too big to post:

Output section: https://imgur.com/a/G5Csl9L

Rectifier: https://imgur.com/a/Tgpj0QP

V5: https://imgur.com/a/GVArYMW

V6: https://imgur.com/a/LW6XDkS

[sluckey]

Wrong value resistor. See pic and change. Should make the bias happy.

BTW having 150V on a cap rated for only 100V is stressful to the cap. Since some expensive stuff depends on the reliability of that cap I suggest replacing it.

[Timmly]

Thanks Sluckey!

[purpletele]

I enjoyed that post!

[EL34]

Hi, figured I'd bring this here since both me and my amp tech are stumped.

A wrong resistor value is easily diagnosed

It's one of the sticky topics at the top of this forum
http://el34world.com/Forum/index.php?topic=17701.0

[Timmly]

Hi, figured I'd bring this here since both me and my amp tech are stumped.

A wrong resistor value is easily diagnosed

It's one of the sticky topics at the top of this forum
http://el34world.com/Forum/index.php?topic=17701.0

Ah, yeah I think we just got caught up thinking it was something more complicated, I should have gone over my board more carefully. At least I didn't blow any tubes or fry a transformer haha

[sluckey]

So, is the amp singing a happy tune yet?

[Timmly]

So, is the amp singing a happy tune yet?

Not quite, accidentally broke the 22k trying to get it out of the awkward spot it was in (the reverb footswitch jack). Going to put 2 10k resistors in the bias circuit to get close to where it needs to be, set the bias, and see if she works.

[Timmly]

Looking rough but I think this will work until I order an axial cap and a proper value resistor for it (no where in town to buy components, unfortunately)

https://imgur.com/a/pno4fvv

[Timmly]

Okay so I set the bias (a little cool cuz iirc fender does that too), but now I have quite a bit of noise. When I turn the volume up slightly theres a rhythmic buzzing sound that goes faster when I turn up the speed on the tremolo, and it seems to get quieter as I turn up the volume. Also when I touch the chassis theres a very light change in the sound, so I'm guessing theres something majorly wrong with my grounding? I used the same star point for the preamp as the resistors on the heater, could that be it?

[shooter]

could that be it

move it n see 

[Timmly]

ok well i switched my grounding scheme to a single star ground, except for the heater resistors, and that didn't do anything about the noise. also tried a different tube in V4 and that didn't do anything either. going to print out a copy of the layout tomorrow and do that highlighter tracing thing to see where i made another mistake.

[Timmly]

Ok so I went through it properly and the amp works for the most part but now, but when i engage the reverb it starts buzzing very loudly. I checked both the send and return tubes, and they are both fine. Reverb driver transformer doesn't have any shorts. Could it be my reverb tank? It was brand new when I got it but it got moved around quite a bit since I also moved to a new recently, could something have gotten damaged in there? I checked inside and nothing looks bad but I could be wrong.

[tubeswell]

Ok so I went through it properly and the amp works for the most part but now, but when i engage the reverb it starts buzzing very loudly. I checked both the send and return tubes, and they are both fine. Reverb driver transformer doesn't have any shorts. Could it be my reverb tank? It was brand new when I got it but it got moved around quite a bit since I also moved to a new recently, could something have gotten damaged in there? I checked inside and nothing looks bad but I could be wrong.

make sure the pan's sensitive output transducer is away from the PT-end of the amp.


Also where is the 12AT7 reverb driver grounded? on the screen supply node ground return?


And where is the 1/2 12AX7 reverb recovery stage grounded? with the pre-amp filter cap ground?

[Timmly]

make sure the pan's sensitive output transducer is away from the PT-end of the amp.


Also where is the 12AT7 reverb driver grounded? on the screen supply node ground return?


And where is the 1/2 12AX7 reverb recovery stage grounded? with the pre-amp filter cap ground?

both the 12at7 and the 1/2 12ax7 are grounded with the preamp

[Timmly]

Took a pic just now: https://imgur.com/a/jIv2j2d

The buzzing is still there when I just connect the reverb input and output together with a cable, so it's not my reverb tank. Is it how I routed my wires?

[Timmly]

bumping because I've looked at it a bunch more and can't seem to figure out what's wrong (troubleshooting is clearly not my strong suit lol). only thing I've changed from the pic above is that the black wire from the reverb transformer is under the heater wires now