Fender Twin Reverb massive hum!

[Shannon]

Thanks for the pics. I did have the wrong orientation


1) Retested with all tubes - hum still there and loud

2) Pulled out V3 (12AT7). hum still there and loud

3) Pulled out V4 (12AX7). Hum gone.


Now for the difficult part I guess...how to gain access and replace those possibly faulty caps. At least they seem to be easily available values.

Thanks for all the help thus far.

[bnwitt]

Do you have a known good 12AX7 to put into that V4 position to prove it is not a bad tube? Have you unplugged your reverb pan connections from the back of the amp to see if the hum goes away? If it turns out to be the V4 cathode capacitor, use a 25uf/50 volt cap as a replacement rather than a 25 volt. If you're going to change one electrolytic cap in the amp you might as well do a cap job. How's your soldering technique? Maybe its time to take the amp in for some service.

You will need
6 25uf/50 volt caps
1 5uf/50 volt cap
2 70uf/350 volt caps
3 20uf/525 volt caps (or 600 volt)

Here's the sprague ohm capacitor page at Mouser:

http://www.mouser.com/catalog/629/587.pdf

If you don't feel comfortable doing this you should find a tech.

[Shannon]

Put in new 12AX7's and 12AT7's before this debugging. Swapped out a 12AX7 from my Bassman as well. Same behaviour.

The Reverb pan has me confused. I see the labelled connections but they seem soldered to the pins and not pluggable?

If I need to do an entire cap job, I'm going to have to take it to someone with experience in this field - more for time and experience.

Thanks for all the assistance thus far. Greatly appreciated.

I should be able to direct the technician and not spend too much on the fix.

Getting the right caps will be a slight problem.
Found a supplier for 20mfd 640V and 100mfd 450V filter caps. Will these work?
The smaller ones should be easily available.

[bnwitt]

The Reverb pan has me confused. I see the labelled connections but they seem soldered to the pins and not pluggable?

If it is standard Fender, the shield of each wire is just soldered to the outside of the plug body. you should be able to remove them with a little cc ccw rotational twisting motion while pulling outward.

Getting the right caps will be a slight problem.
Found a supplier for 20mfd 640V and 100mfd 450V filter caps. Will these work?
The smaller ones should be easily available.

All of the values are available at mouser in the link I gave you. You can use 80uf/450volt for the 70uf/350volt caps and the only other one that isn't exact is the 20uf/525volt. The 20uf/500volt should be fine for that amp, of if you want overkill you can choose the 20uf/600volt.

[jonee]

@shannon
Did you find out what was wrong. I have the same problem on my 70's twin

[Tom Phillips]

@shannon
Did you find out what was wrong. I have the same problem on my 70's twin

Jonee,
You stumbled across a really old thread. Based on Shannons posts, he did not have the experience to repair the amp himself and I suspect that he ended up taking it to a tech.

However, there is good troubleshooting advice above and you should follow it if your amp has the same symptoms. Read the info provided by the experienced forum members. Probably best to skip over Shannon's posts and treat your problem as a fresh start project

Regards,
Tom.

[Tom Phillips]

@shannon
Did you find out what was wrong. I have the same problem on my 70's twin

Jonee,
You stumbled across a really old thread. Based on Shannon's posts, he did not have the experience to repair the amp himself and I suspect that he ended up taking it to a tech.

However, there is good troubleshooting advice above and you should follow it if your amp has the same symptoms. Read the info provided by the experienced forum members. Probably best to skip over Shannon's posts and treat your problem as a fresh start project

Regards,
Tom.

[jonee]

Probably best to skip over Shannon's posts and treat your problem as a fresh start project

Regards,
Tom.

Thanks for replying. I do realise it's an old thread. The thing is mine had a "cap job" and worked for a while then the problem returned. I guess the real problem causes the caps to "blow" and I was hoping Shannon had it fixed. And of course I realise my problem could be another than his.