Volume drop after redplating tubes.

[92lx50]

Hi everyone! New guy here but have been lurking and reading for a while now. I have a Randall RGT 100 combo. This amp was working perfectly until two of the outside tubes started to redplate. I noticed this because I was running it into and extension cabinet and had the back of the combo facing me when I noticed a slight drop in the volume and saw them glowing. I quickly shut down. I did move the two tubes that were red plating to the other sockets just to rule out the sockets and the red plated there also.

Fast forward to new tubes, all new electrolytic caps on the power board and new wire wound screen grid resistors and I also cleaned and re-tensioned the preamp and power tube sockets. The amp powers up fine and biased but the volume/output is really low.

I tested the diodes on the power board and one (circled in red in the picture) tested bad. Could this possibly be the cause or is it something else?

Any help or suggestions would be greatly appreciated.

[Tom Phillips]

...I tested the diodes on the power board and one (circled in red in the picture) tested bad. Could this possibly be the cause or is it something else?...

That circled diode looks like it could be the bias supply rectifier diode. If so it is probably the cause of your original problem and is definitely a serious problem now if it is "Bad". However, there is another single diode and what looks like burned resistors on the same PC board. That could also be the bias supply too. Do not power the amp on with the output tubes installed until this bias supply is verified good / repaired.

Tell us what you mean by "Bad" with respect to that diode. Is it open, sorted or other?

With the power tubes removed I recommend that you measure the voltage at each power tube pin 5. When the bias supply is working properly you will find a negative voltage with respect to system ground at each tube. It will be in the order of -40 to -55V. Please tell us what you find and post a schematic in this thread for reference.

[92lx50]

Hi Tom, thanks for the reply. I tested the diodes with my DMM using this video https://www.youtube.com/watch?v=qSDRsz5-t7I this was the only one that was "0" both sides.

[92lx50]

Tom, do you know what type or value this diode is?

[Tom Phillips]

Probably any standard 1N400X type diode will work. I just use 1N4007s in these cases. Higher voltage rating than needed but not expensive so I stock them in bulk.

The attached photo shows the other suspect parts circled in green. (Reference my edited first post)

[92lx50]

That diode tested fine and so did those resistors. I'm just going to replace them anyway. I've already replaced the electrolytics as in the attached picture including the bigger "Sonicor" ones just not at time of this photo.

Do you happen to know the value of the 2 burned looking resistors?

[g1]

Here's the schematic:

[Tom Phillips]

Attached is a copy of the schematic for the RGT-100. I see that there are several power rails derived from the low voltage secondary of the power transformer which explains all the rectifier diodes that can be seen on the PCB. I also uploaded a copy of the owners manual since I had it handy.
randall rgt-100 - schematic v2.x (clearer).pdf
RGT100-RGT100ES-RGT100HT-OwnersManual.pdf

[Tom Phillips]

That diode tested fine and so did those resistors. I'm just going to replace them anyway....Do you happen to know the value of the 2 burned looking resistors?

Nope. I don't know the resistor values. You will need to trace out the circuit and look them up in the schematic. Also, I recommend that you only replace parts if they test bad or look damaged. That would be best practice and reduce the likelihood of rework damage. The exception would be old electrolytics.

Welcome to the forum by the way.
You posted good photos and that really helped.

Tom

[Audiotexan]

My hunch is that those are the two 2k2 resistors near the 4v zeners that feed the green and red LED's (with the 2w 100ohm resistors lead into the green wires) and then to the footswitch area. Hard to say with any certainty though, because I can't make out the values on the two small blue Nichicon's right there by them. So again, just a hunch. Still doesn't have anything to do with red-plating tubes (unless that first diode is the one you mentioned that is shorted, as the bias is derived further down that same path), but does affect a secondary section of the PSU.

OP: Can you read resistor color codes without a chart or follow schematics ok?

[92lx50]

My hunch is that those are the two 2k2 resistors near the 4v zeners that feed the green and red LED's (with the 2w 100ohm resistors lead into the green wires) and then to the footswitch area. Hard to say with any certainty though, because I can't make out the values on the two small blue Nichicon's right there by them. So again, just a hunch. Still doesn't have anything to do with red-plating tubes (unless that first diode is the one you mentioned that is shorted, as the bias is derived further down that same path), but does affect a secondary section of the PSU.

OP: Can you read resistor color codes without a chart or follow schematics ok?

Unfortunately I can't read schematics very well. As far as resistors, I can look up the color bands plus I know a tech that I use when it's something I can't do myself and he probably has them and the diodes.

I would just like to fix this myself if I can. I've been a guitar tech and builder for 30 years and am proficient in soldering and wiring. I've been biasing, re-capping and swapping out potentiometers in my own amps for years and am VERY aware of the voltages and dangers. I'm not a cheapskate, I just enjoy learning and the satisfaction of fixing things myself.

[Chuck H]

Looking at guitar wiring diagrams seems like reasonable prep for tracing a schematic to me. You just need to see what's connected to what with some recognizable arrangement as a starting point. You can do it. I believe in you

Almost certainly a bias issue and I hope no permanent damage has been done to the amp. Don't fire it up again with the power tubes in place until that diode and any associated bad components are replaced. Then, with the tubes out, take voltage readings taken from the socket pins and post the results.

[Jazz P Bass]

Unfortunately I can't read schematics very well.

The key item that is missing on most 'schematics' is the board layout.

Schematics are fine but without the board layout diagram you are pretty much flying blind.

I find it helpfull to hand draw the relevant part of the schematic in question.

[g1]

I've circled the 5 bias components in red:


The "browned" resistors are as described by Audiotexan in post #10 above. As you have found, they are ok, but they will get brown as they run hot.

You may just have a problem with a couple tubes (the ones that redplate no matter which socket they are in).
But best to verify you have good bias before installing new ones, as per Tom's pin5 measurements in post #2.

[Tom Phillips]

I agree with g1 and, upon reviewing the pics again, I not longer think that the resistors I circled in green in post #5 are burnt to the point that they look damaged.
Tom