cap job?

[mike_mccue]

Hi,
I have a late 70's Fender Deluxe Reverb. I have owned it for 12 years and it's been very reliable. Recently it has begun the habit of suddenly oscillating with a low hum.
I have built a few hobby amps etc but have no experience troubleshooting older amps.

It seems the problem only occurs after the amp has warmed up... so I am assuming it's thermal related.

I've pulled tubes and done some swaps but returned all the existing ones because nothing seemed to stop the trouble. I tried a Groove Tubes solid state rectifier "tube"replacement as well.

I just removed and replaced the 5 big B+ caps in the tray and 3 - 25mf-25vDc electrolytics on the preamp cathodes.

The problem seems to have become much less frequent... the caps were all 30ish years old... but I haven't eliminated it.

What would you recomend next?

Are coupling caps (orange drops) considered electrolytic? Should I start replacing those?

Thanks for any suggestions,
mike

[Bruce / Mission Amps]

There are more then three 25uF-25v cathode bypass caps in that amp... don't waste your time with partial electrolytic cap jobs!
Replace all of them at the same time. Also replace the bias supply filter cap with a new 50uF-100v to 100uF-100v cap.
Off the top of my head, it sounds like a bad or going bad power tube... or possibly the filter cap for the bias supply is leaky causing your power tubes to run under biased and too much idle current.

Modern made Plastic or epoxy caps like Mallory, O'Drops, MPP Xicon... etc, will probably last 100 years and are not electrolytic. I doubt there is any reason to mess with those.
The electrolytic caps are the ones that have a polarizing mark on them... + or -.
The coupling caps are not polarized and for the most part can be installed in any direction.

That amp came with a 5U4GB rectifier which passes much less B+ voltage then a solid state one!!! Don't randomly shove a solid state rectifier in that amp unless you know how to re-bias the power tubes for the extra B+ voltage the SS rectifier will create.
That could lead to power tube damage if the bias supply is not set up right or has a bad filter cap.

[mike_mccue]

ok then,

I see 3 more electrolytics... I'll replace them.

thanks,
mike

[tubetonez]

You may find this helpful for the cap locations:

http://www.el34world.com/charts/fenderservice1.htm

[mike_mccue]

Thanks,

I am about to install the remaining electrolytics.

I read somewhere that many Fenders with a bias balance pot have broken ground leads... they were solid core wire.



It appears my amp has a stranded wire that might have been the same ground connection and it's loose as well.

My question; where do I reconnect it? I can't find an obvious broken solder in the vicinity.

thanks,
mike

[Tom Phillips]

quote=mike_mccue;29383] …I read somewhere that many Fenders with a bias balance pot have broken ground leads... they were solid core wire….mike[/quote]
I never heard that. Doesn’t make sense to me. By the time Fender was using the bias balance circuit they had already switched to stranded wire. Also, the solid core wire, as used in the black face and earlier amps was not particularly fragile unless it was abused.

quote=mike_mccue;29383]…It appears my amp has a stranded wire that might have been the same ground connection and it's loose as well. My question; where do I reconnect it? I can't find an obvious broken solder in the vicinity…[/quote]
It appears that your amp has been converted to the BF bias adjust circuit. The left side of the 10k Ohm resistor as shown in your photo would normally be grounded to the back of the pot. That point is obscured by the white wire in the foreground. Is it soldered there? If not then solder it to the back of the pot and adjust the bias. If the 10k resistor is not grounded then your bias voltage will go to the full negative bias voltage available and your amp will be over biased (i.e. low bias current) The loose wire shown looks like one of the ground jumpers that Fender used in the SF amps. Is it soldered to the back of the pot? It looks like a leftover from a previous mod and you could just remove it. Also make sure that the bias adjustment pot nut is tight because it provides a mechanical ground path.

Regards,
Tom

[mike_mccue]

Thanks Tom,
I studied up a bit and know I see what you mean... I guess the amp was modded for a bias adjust rather than the bias balance before I bought it.


I finished the cap job by installing a 80mF 150vDC electrolytic in the bias supply.
After this the amp seemed to launch into oscilation almost immediately after power up. The symptoms actually got worse. before the cap job I could play for 5-10 minutes before the oscilation ran wild.

I pulled the driver stage 12AT7 to try to isolate the problem and powered back up. The amp was silent.

I replaced the driver with a NOS RCA JAN 12AT7 and everything seems fine now.

I was wondering... is there any particular reason a cap job may have exasperated or pushed the old 12AT7 "over the edge"? or should you just chalk it up to a coincidence?

I'm not trying to nitpick but if there's an opurtunity to learn something I like to do so

thanks very much,
mike

[Tom Phillips]

Originally Posted by mike_mccue

Thanks Tom,
...I replaced the driver with a NOS RCA JAN 12AT7 and everything seems fine now.

I was wondering... is there any particular reason a cap job may have exasperated or pushed the old 12AT7 "over the edge"? or should you just chalk it up to a coincidence?...

Mike,

Some people will say that the voltage went up a little after the cap job and stressed something over the edge. I say..."Stuff just happens" and "everything is working OK up until just before it not working OK". Anytime you mess with something old you run into this stuff. Coincidence is as good an explanation as any.

I'm glad you got it fixed. You did good troubleshooting.

Regards,
Tom

[mike_mccue]

Tom,
Thanks for the reply and your insights.


best regards,
mike