Sudden low bias on Deluxe Reverb

[Joeriz]

Hi all,

Earlier today I did the tremolo "LED mod" to my Hoffman Deluxe Reverb. I tested it out before I closed it up and it worked great. Put it back together and played for a while...still no problems. Then after 15 minutes or so I had a sudden loss of volume, the signal became distorted, and a buzz started coming from the speaker. I opened it back up and took a look around and saw no apparent problems. While open, I plugged it in again and started playing...same issues. No apparent issues with the LED - it lit up just fine and pulsed when I switched the tremolo on. Then I checked the bias on the power tubes: very low...about 2-3ma. I tried adjusting it and the buzz got worse. I also measured the plate voltage on the power tubes and it is low: 300v. However, the voltages coming off of the power transformer and rectifier seem just fine.

At this point I've checked every component in the bias circuit and they all test fine. I changed the rectifier tube and power tubes for known good ones to no avail. I pulled both the tremolo tube and the phase inverter but the issues remained so whatever is wrong seems to be isolated in the output or power supply section.

The one thing I haven't tried is removing the LED and putting the cap and resistor it replaced back in. However, that doesn't seem like it should be the culprit.

Any thoughts?

Thanks,
Joe

[sluckey]

The one thing I haven't tried is removing the LED and putting the cap and resistor it replaced back in. However, that doesn't seem like it should be the culprit.

That should be the first thing you did. Always suspect the last thing you did to the amp.

However, if all you did was replace that resistor/cap with a LED, there's no way that could cause your failure. May you inadvertently flubbed something totally unrelated to the mod, such as broke a wire, moved some critical component, etc.

Pull the power tubes and measure voltages on all pins of the 6V6 sockets.

[Joeriz]

Here are the voltages with the power tubes pulled:

Pin        V7               V8
1           0                 0
2           0                 0
3           341             341
4           339             339
5           -50             -50
6           -50             -50
7            0                 0
8            0                 0

On the rectifier I get:

335 AC at pins 4 & 6
341 at pins 2 & 8
5.4 AC across 2 & 8

341 at 2 & 8 is low and I get similar readings with a different rectifier tube.

[Joeriz]

Seems that it was a bad filter cap. I paralleled a known good cap across the filter caps (one at a time). When I did the second one, the buzz went away and I got about 450v on pin 8 of the rectifier tube (without power tubes in the amp).

Odd since the caps are only a few months old but it does happen.