1990 fender red knob twin crucial problems

[Jquijano]

I have one Fender Red Knob twin amp under repair at the moment with several problems.

1. There seems to be some ground issue that diminishes when you touch your guitar bridge or any other metal part of your guitar with your hands.

2. If you dial the reverb pot over 4 it start producing some loud hum and intermittent crackle

3. There is some light to medium frying bacon like sound that comes and goes randomly.

Any solution to these issues?

[Jon Snell]

What happens if you earth (ground) the amplifier chassis?

[DrGonz78]

Run a ground wire from grounded chassis to reverb tank body. Just a thought for a quick test, nothing permanent.

[Jquijano]

Well the tech finally fixed the amp. Apparently one 12ax7 preamp tube was damaged and affected the others including the reverb noise when dialing over 5. The issue with the ground loop was also solved by biasing the amp to -40VDC and balance at 0. This are the measurements taken with my taiwanese digital multimeter. I also have a weber bias probe. Should i check this with the bias probe directly instead of using the inputs terminals at the rear of the red knob twin?

[g1]

I think you mean you measured 0.04VDC at the rear panel bias jacks (40mV). That is the factory setting and there is no need to double check with bias probe. The hum was probably due to the balance not being at zero.

[Jquijano]

Now that all the problems were solved on my 1990 Fender Red Knob Twin, i did noticed that the tube sockets are flimsy. I do have 6 brand new 12ax7 tube sockets that could be used as replacement. Have you had any trouble attempting this swap?

I bought 6 of these model:

https://www.amazon.com/Fender-Vintag...s%2C224&sr=8-3

https://www.amprepairparts.com/ts9700.gif

[Tom Phillips]

Now that all the problems were solved on my 1990 Fender Red Knob Twin, i did noticed that the tube sockets are flimsy...

I have never had tube socket trouble with the 1990 Twins that I maintain. The big issues have been heat from the 10W board mounted resistors and the non strain relived 7 strand flying wires from the PC boards. The strands are brittle and start breaking one by one when you need to flip a board for service. I replace them with good quality 19 strand hookup wire for customers who want to make the amp a keeper. I also re-rout them through the strain relief holes that Fender ignored on some early models and add holes for the later amps like yours because Fender "simplified' the board in later years. I guess some bean counter said "why pay to drill a hole if we don't use it anyway."

Since you said "...all the problems were solved..." I suggest you consider leaving the original tube sockets in place.

[Jquijano]

Well the only thing that is still affecting is constant hum which is widely reduced if you touch the guitar strings or the bridge. Quite strange as i tried differeng gibson les paul, sgs, strats, jackson and the problem persists. I opened the amp to verify any cables not fastened correctly and could not find anything. I even turned off the lighta and move away 15 feet from the red knob twin and hum is still there. Could this loose prong be the cause?

[Jquijano]

Really strange as none of my other tube amps have this ground hum issue

[Helmholtz]

If the ground pin is loose it might no longer provide safety grounding of the amp chassis.
This means a safety risk and is likely to cause hum.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Tom Phillips]

Did you measure the ground pin to chassis resistance? What is that measurement?

Another test you can do is the temporarily attach a lead from the chassis to earth ground and then test with the guitar again to determine if the hum you hear goes away. The approach is to troubleshoot to isolate the issue so you don't spend time "fixing" things that are not the problem.

In any case, If the line cord earth ground pin is physically loose in the plug, then I would replace the plug or the line cord assembly.

[Jquijano]

Ok. Just to let you know that the ground pin in the original power plug was broken. I replaced it with a new 3 prong plug and no more problems. I would still like to verify that the bias resistors are within value. Do i really need to desolder one of the legs to verify this?

[g1]

I would still like to verify that the bias resistors are within value. Do i really need to desolder one of the legs to verify this?

Do you mean the 1 ohm resistors for the power tube cathodes? You can measure those without desoldering.

[Jquijano]

I believe the ones in questions are R216 and R215 for test point 201 and 203. I want to be sure their value has not drifted and im getting the correct bias reading. Please confirm if these are the only resistors i need to double check.