Tube Amp Archive

Here's my voltages in red.

So I tried disconnecting the NFB wire, the hum remained, as I touched the wire back on the 16 ohm terminal I would say it very slightly reduced in its intensity.

I also tried switching the PI tube, same result. And I tried using a shielded cable coming from pin 7, same result.

Ok I got to spend some time with the amp finally. I started by just checking voltages in the preamp, the value of the dropping resistors was just an estimation by me so I wanted to see how much it was dropping from the PI to v1. So I've got 298vdc on the plates of the 6k6's (which are NOS RCA's). Th...

I figured out the bias thanks Martin. I'm going to poke around this weekend, and see if I can track down this hum issue, and also maybe plug a guitar in for awhile and see what it sounds like even with the hum. Thanks Again

I think there is something going on with one side of the bias adjustment, I believe maybe the 25k pot is bad, but I think this not the main issue. I am temporarily moving to a single bias adjustment (shared on what I believe is the good pot), to eliminate variables. The squeal is gone and I did not ...

I took it off standby, said my little prayer "...please no troubleshooting" and of course ...the man upstairs decided I have more lessons to learn :) First thing, very loud hum, too loud to be normal, especially when all the volumes are down and no guitar is plugged in. I wanted to see what the volt...

IMG_1030.JPG IMG_1029.JPG IMG_1028.JPG IMG_1027.JPG IMG_1026.JPG Thought I would include some pics. It took my a couple more days than I had hoped but its mostly done. I probably won't leave the FWB as shown, but I just wanted to get it working. So as tested I have -39vdc after the diode, thank god...

Found an old 0.18uf 400v, should be good enough to use until the 220nf arrives, at least to see if its working. I'll get started on it and hopefully have it done today, or tomorrow at the latest. Once I get it working I'll be curious to see what affect 47nf vs. 220nf has on the bias voltage.

Looked everywhere and I don't have a 220n 400v, only 160v which is not enough. I will order this and build it as shown. Thanks again.

Whoops, of course. In that case 228vac. It will be easier to just redo the power supply circuit then to check all the diodes and move the standby switch location etc. I've got some still in the package 1N5408 that I know are good, more heavy duty then is necessary but then I don't have to worry. I'l...

This can't be right?

I just unhooked the secondary sides from the FWB and tested them individually (against chassis ground) and got these numbers.

This is way too low though, how was I ever getting 225 vac on one side of the rectifier circuit?

ok, I have a lot to redo and verify.

First, yes I definitely missed that the PT cannot be 215-0-215 because there is no CT.

I'm going to dismantle the power supply and start by measuring voltages of the PT with 120v applied.

I changed the cap to 47nf, the resistor to 56k, and I swapped in a new diode. The negative voltage after the diode changed only slightly to -0.177. With the voltage information that I have obtained so far can we eliminate the PT and the FWB as both working correctly? Can the bias circuit after the d...

Did some voltage checking, this is what I found.

@sluckey @johnnyreece, I will check on both these tomorrow morning and report back, gotta take my kid to his baseball game.