77 twin reverb

[majwild1]

Ok, I have a nightmare going on my 77 twin is freaking out. I keep blowing the slow blow fuse. I first noticed the power tubes starting to red plate and then blew the fuse. checked bias and it was way out of whack, checked grid resistors 2 were out of tolorence change all 4 and tube sockets with new ceramic fired it up checked dc pin 3 525v set bias at 33ma things seemed to be fine but after about 15 minutes bang did it again, I happened to be watching the tubes and I could tell when it started to happen the tubes started glowing blue the starrt to red plate so I shut it down before it blew the fuse any ideas where to start?


thanks in advance

[majwild1]

By the way I just took it in and had it recapped not to long ago.

[Krwkka]

You might try removing the two outer 6L6es, leaving the inner two installed. Turn on the amp and see if the situation improves. If not, remove the inner two and replace the two you removed.

This is the ultralinear 135 watter, yes?

[Enzo]

Have you tried different power tubes?

[majwild1]

yes it is a 135 ultralinear, and yes tried brand new matched quad in it smoked them, now it seem to take like 15 minutes from cold to happen I just had it in to a tech he said my speaker jacks were shorting replaced them and one coupling cap gave it a good bill of health. got it home pluged it in played for about 15-20 min and same thing

[Robert M. Martinelli]

Hi,
Your bias voltage could be drifting from its "normal" value to "less negative" values, causing the tubes to redplate and the fuse to blow.
I would check/test (or have a tech check/test) the bias voltage at one of the tube sockets (control grid - pin 5) to see if your bias voltage drifts over time.
Hope this helps
Best regards
Bob

[pdf64]

In addition to the above,
1. the amps were designed around the STR387, a beefed up variant of 6L6GC. The B+ is very high. Some current brands will cope better than others, consult with your tube vendor for their best option, but none come close to the original.
Once a tube has red plated, consider it suspect - the heat will likely distort and cause mis-alignment of the tube structures.
2. Screen grid resistor value/rating needs to be increased in the these high B+ UL amps, for use with modern tubes. Suggest 2k2 5W

[Krwkka]

Crazy thought, maybe, but you said an amp tech replaced your output jacks because they were shorting.

Check to see if there is a green and yellow wire running from one of the output jacks over to the circuit board. If not grn and yellow, is there any wire at all?

I'm wondering if the tech inadvertently disconnected the negative feedback?? In these amps a wire that goes from the output transformer to the output jack first runs to the main circuit board where the negative feedback is tapped. The wire then continues from the board over to the output jacks.

To the experts here: If the negative feedback is disconnected wouldn't oscillation be a possibility?

FYI, the stock output jacks in these amps are actually designed to short, but that's a different subject...

[Billy R aka DynaFreak]

99% of the time "redplating" is a bias circuit problem. could be a bad connection or bad part. There are not many parts in that circuit. If you are not sure how to trouble shoot it just change them all out BUT WATCH THE POLARITY ON THE DIODE AND CAP(S). Check all your ground connections in the bias circuit. In other words, make it right then check it off your list. Until you have ruled out bias voltage problems dont be distracted by other potential problems. Fortunately, you can verify that the bias voltage problem is fixed with the power tubes pulled out, so you dont have to worry about watching for red plates or keep replacing fuses..

[majwild1]

Crazy thought, maybe, but you said an amp tech replaced your output jacks because they were shorting.

Check to see if there is a green and yellow wire running from one of the output jacks over to the circuit board. If not grn and yellow, is there any wire at all?

I'm wondering if the tech inadvertently disconnected the negative feedback?? In these amps a wire that goes from the output transformer to the output jack first runs to the main circuit board where the negative feedback is tapped. The wire then continues from the board over to the output jacks.

To the experts here: If the negative feedback is disconnected wouldn't oscillation be a possibility?

FYI, the stock output jacks in these amps are actually designed to short, but that's a different subject...

The neg feedback was disconnected as a mod not sure if that's bad.

[majwild1]

When I had my bias probe hooked up the bias did vary when a note was hit.

[melvin]

Depending on where you monitor the bias voltage you may indeed see a normal drift when playing the amp. At what idle current are you biasing this thing? (Oh, I see now: 33ma) If the B+ is pretty high for a 6L6 you may have to bias them cold (25-30ma) so they won't be as likely to go into runaway as they get hot.

[pdf64]

Bias is the current through the tube when there's no signal.
When you put a signal through it, the current through the tubes will increase - that's how class AB amps function.
It is possible, but not likely, that the mod to remove the negative feedback may have made the amp unstable.
Where are you now with this, and what is your plan?
There's no benefit in tinkering with this. If you want to get this issue resolved then there's good advice above.

[Billy R aka DynaFreak]

pdf64 is correct, the bias current will vary with signal applied. Sounds like your bias measurements have all been with a bias probe. You need to measure bias voltage at pin 5 of the power tube sockets and make sure you have a stable negative voltage at that point. Disconnecting the negative feedback actually makes an amp more stable, not less so. An increase in bandwidth and compensation for aging power tubes is the main advantage of negative feedback.

[RWood]

Leaky coupling caps that connect the plates of the PI to the grids of the power tubes will make the bias voltage "less negative" and will cause the symptoms you have described. It is an easy enough job to replace them.