Fender SF Twin Reverb Hum

[JAelec]

I don't like the solder joints of the screen resistors.
The wire insulation should never get into the solder.
Some insulation types like PVC release acid when heated, giving rise to corrosion.
Also the big solder blobs extend to the body of the resistor.
Means that the resistor was exposed to ecessive heat during soldering.
This in turn might have increased resistor value and/or might have caused internal cracks resulting in unstable resistance.

Should I replace the screen resistors because of the solder blob issue? Or just measure them and keep them if they are within tolerance?

When going through an amp like this... even though the main problem (faulty tube) has been found... are there other stuff that I should check? Like a routine checkup list with common failure points? Or typical parts that often need replacing that you would just replace by routine?

I know that the tube sockets are such a checkup point... the octal sockets were quite loose, but I tightened them by bending the pins with a small metal tool. Or is it better to replace them?

I also checked if the board was conducting through the wax at various random points on the fiberboard. Did not find anything suspicious.

If you think I should check anything else, don't hesitate to say so... I'm eager to learn from more experienced folks!

[Helmholtz]

Generally I don't recommend "fixing things that aren't broken".
But it's not clear yet why your idle currents were jumping.
Are current readings now stable and reproducible?

[Tom Phillips]

... the octal sockets were quite loose, but I tightened them by bending the pins with a small metal tool. Or is it better to replace them?...!

Let's be sure that you are not confusing the normal feeling of "loosness" due to the design of the octal tube socket. This is because each individual female socket pin floats in the tube socket structure to allow each socket to align with each pin of the tube base. In a good tube socket, each pin of the tube is held tightly but the whole tube ends us with a little play that makes it feel loose.

[Helmholtz]

Does it make any noises when you gently the power tubes in their sockets?

[JAelec]

Does it make any noises when you gently the power tubes in their sockets?

gently tap or gently wiggle? i think you missed a word…

[JAelec]

Let's be sure that you are not confusing the normal feeling of "loosness" due to the design of the octal tube socket. This is because each individual female socket pin floats in the tube socket structure to allow each socket to align with each pin of the tube base. In a good tube socket, each pin of the tube is held tightly but the whole tube ends us with a little play that makes it feel loose.

They quite stiff now after tightening the pins…

[Helmholtz]

gently tap or gently wiggle? i think you missed a word…

Sorry, should read gently wiggle.

[g1]

But it's not clear yet why your idle currents were jumping.
Are current readings now stable and reproducible?

Still wondering whether this issue has been resolved.

[JAelec]

Does it make any noises when you gently the power tubes in their sockets?

I tested wiggling the tubes and, they seem to be a good fit, nothing happens when I wiggle them.

From V7 to V10 the control grid resistors (1.5k) measure within tolerance; 1.5k, 1.49k, 1.57k and 1.46k

From V7 to V10 the screen resistors (470 Ohms) measure within tolerance; 513, 492, 487, 504 Ohms

Generally I don't recommend "fixing things that aren't broken".
But it's not clear yet why your idle currents were jumping.
Are current readings now stable and reproducible?

Still wondering whether this issue has been resolved.

Bias V7 + V8 side:

Resistance: 37.6 Ohms
Vdrop OT center to plate: 2.72 V
Plate voltage: 461.2 V

Plate current V7+V8 = 2.72/37.6 = 0,072 A
Divided by 2 for average each tube = 0,036 A
Plate dissipation = 0,036 * 461.2 = 16.7 W
% of max = 16.7/30 = 0,56 = 56%

Bias V9 + V10 side:

Resistance: 35.3 Ohms
Vdrop OT center to plate: 2.20 V
Plate voltage: 462 V

Plate current V9+V10 = 2.20/35.3 = 0,062 A
Divided by 2 for average each tube = 0,031 A
Plate dissipation = 0,031 * 461.2 = 14,4 W
% of max = 14.4/30 = 0,48 = 48%




If I put in a matched quartet I get:

V7+V8 side:
Vdrop: 2.8
Plate voltage: 459.3
Plate current avarage per tube: 0.037 A
Plate dissipation: 17,1 W
% of max: 57%

V9+V10 side:
Vdrop: 2.6
Plate voltage: 460.1
Plate current: 0.037 A
Plate dissipation: 16,9 W
% of max: 56%

In both cases results are after matching with the bias balance pot so the amp is dead quiet.


...So I guess the main issue Is resolved and I get reproducible measurements. Thanks you for you assistance!

But I still have some thoughts and questions if you don't mind;


So if I just replace the x1 faulty tube and keep x3 old tubes I get 5mA difference between sides. With a full matched quartet I get <1mA difference.
Is it ok with a 5mA mismatch between either side?

Is it ok to go cheap and just replace the faulty one or is it good practice to replace all four with a matched quartet?

I would rather go cheap but I don't know if there are any negative aspects of this that I haven't thought about... perhaps the other tubes are more likely to fail then a new matched set, but who knows they could be going strong for quite some time, and then it would be a wast of money to throw them out...

[g1]

5mA difference between sides is not bad. If it were my own amp I would just replace the 1 tube, but I am not gigging with it so it will depend on your circumstances.
Are you gigging? Do you need the 100W?
If not, another option would be running only 1 pair, and disconnecting 1 speaker.

[JAelec]

5mA difference between sides is not bad. If it were my own amp I would just replace the 1 tube, but I am not gigging with it so it will depend on your circumstances.
Are you gigging? Do you need the 100W?
If not, another option would be running only 1 pair, and disconnecting 1 speaker.

This amp is in a rehearsal space at a music school. I’d rather not mess with pulling tubes since it is not my amp.

I run my private Dual Showman with only two power tubes, but with a speaker impedance mismatch as I’ve read the Fender transformers can handle a halved or doubled impedance mismatch.