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FDP Forum / Amp Mods, Repairs, and Projects / 1962 BANDMASTER AMP BLEW CAPACITORS HELP



Dec 30th, 2016 07:29 PM   Edit   Profile  

New to the Forum . Saw several threads here and it looked like a very good place for info.

I just bought a 100% all original late 62 bandmaster amp . I plugged it into a variable transformer . I out the transformer on 20 volts and turned the amp power on . I slowly turned the voltage up to 110 . After 2 minutes I turned the standby switch on . After around 2 minutes there was a poof sound and some smoke . The amp was not plugged into a speaker cabinet at the time.

I took the amp out of the cabinet and all the capacitors on the bottom look good . I saw some dark brown fluid leaking out of the 4" x 10" cover over the 6 big capacitors and 2 of them on one end look like they are leaking and the tops are distorted outward slightly.

1. Did I kill the amp by not having it plugged into a speaker cabinet?

2. What section of the amp do these capacitors control?

3. Will using a different type of capacitor in this section change how it will sound?

4. Does anyone know where I can get original capacitors? I realize other ones may be leaking to some degree also but I want to keep the amp 100% original if at all possible even if there is a little degradation in the sound quality.

5. I live in the Riverside area of Southern California . Does anyone know anyone nearby that can properly repair it instead of someone that will simply change all the capacitors just because they are old?

Thanks in advance for any help.

Contributing Member

USA / Virginia

Dec 31st, 2016 05:10 AM   Edit   Profile  

You are going to get different responses on the question of using "new old stock" capacitors for replacement.

But first things first. Powering on a tube amp with no speaker load is a really bad idea. I would not do that again.

The big capacitors in the metal "doghouse" are part of the high voltage power supply for the amp. They do not directly effect the tone of the amp because they are not in the signal path.

I would replace every single electrolytic capacitor inside that amp if you plan to actually gig with it. Electrolytic capacitors have a paste inside that dries out over time. This causes their values to drift way off spec and, as you experienced, to fail over time.

When those big capacitors in the doghouse fail they can, under certain conditions, cause a transformer failure, which is then really expensive to replace.

So find a good tech and have them go thru the amp and replace all the electrolytic caps with high quality new ones.

There is a case to be made for new old stock tubes - that is a completely different story than trying to put 60 year old capacitors into your amp.

I hope this helps.

Contributing Member

Too Near Atlanta GA

Amp Tech Emeritus
Dec 31st, 2016 07:10 AM   Edit   Profile  

There is no such thing as a "good" NOS electrolytic capacitor. You can simulate one by slipping the outer cardboard covering of an old one over a new one, but beyond that, it is not wise or practical to use anything but new, fresh electrolytic caps of the same or slightly higher rated values as the originals.

The primary power supply caps do indeed have effect on tonality inasmuch as they reduce ripple from the rectified DC voltage. Bad or leaking caps can cause ghost notes as well as poor dynamics and, of course, hum.

Totally agree that you should never operate any amp with a tube output circuit without a speaker load on it. Now, if the volume is turned to "0" it won't really hurt it but still, generally a bad idea to do it. Using the amp with degraded vintage caps will most certainly jeopardize your transformers...when they go you have an amp that used to be worth a lot. :) Don't do it...

If you just saw and smelled smoke, you likely just "spewed a cap". If you blew a fuse, you had a total cap failure with a short. Make very sure that the primary mains fuse is the proper value. Many (if not most) vintage amps are often found to have over rated fuses in them...bad idea. Most likely a 3 amp slow blow.

Regardless, don't operate the amp until you get all the power supply caps removed and replaced. In fact you need to have all the primary components checked for value and overall condition. The only time we leave amps in original issue condition is if they will not be used and will simply be a museum piece.


Contributing Member

So. Cal. USA

Dec 31st, 2016 06:59 PM   Edit   Profile  

Send me an email through the forum. My address is in my profile. I do this type of work and can repair your amp.

New modern filter caps will be required. Bias cap and possibly the bypass caps on the main board.

Happy new year!



Jan 1st, 2017 04:29 PM   Edit   Profile  

the volume was on 0.

the fuse is a 3 amp 250 volt and is still good.

Moderator Emeritus
(with many stars)

NW Pennsylvania

Jan 1st, 2017 06:09 PM   Edit   Profile  

The speaker is an integral part of the circuit. Operating the amp without the speaker will cause serious damage regardless of the volume setting.

Hammond is a good guy and only 45 minutes from you. I highly recommend you contact him.

Contributing Member

Too Near Atlanta GA

Amp Tech Emeritus
Jan 1st, 2017 06:19 PM   Edit   Profile  

True enough in theory...but in over 50 years of bench electronics work, I have never seen an amp or any other tube type audio output device damaged when being tested with no speaker load if there is no signal applied and the output gain at nil. The rule of thumb is, however, to never allow a tube type output audio device to be turned on without a load of proper impedance.




Jan 2nd, 2017 04:35 PM   Edit   Profile  

Where did you find your power up procedure?
If the variac had been used correctly, there would have been some chance that the old electrolytic capacitors (ecaps) might have reformed.
As it is, due to the sudden application of the HT when standby was flipped, some at least have now catastrophically failed.
The use of standby isn't needed, has no technical benefit, and in this case, defeats the purpose of a slow ramp up of the mains voltage, ie reforming the ecaps.
Ecap aging may be thought of as similar to tires or sneakers; they will wear out as they are used, they will degrade (deform) as they sit on a shelf, they will degrade if left unused for long periods.
The very best ecaps available currently, eg intended for mil spec applications, even with all the advances in materials technology and 6sigma production over the past 50 years, only have an operational life of maybe 20000 hours, and are several times the price of commercial grade ecaps with an operational life of maybe 2000-5000 hours.
I think it's reasonable to assume that Fender would not pay such a premium for mil grade parts, and that after 55 years, even mil spec ecaps should be regarded as a life expired consumable.

Hopefully the shorted caps haven't damaged your amp's transformers; those are its 'vital organs'.

(This message was last edited by pdf64 at 06:46 AM, Jan 3rd, 2017)

Contributing Member

American Patriot

About as ordinary as you can get.
Jan 2nd, 2017 08:50 PM   Edit   Profile  

No speaker hooked up = trouble.



Jan 3rd, 2017 09:32 AM   Edit   Profile  

Fenders after the tweed era are fitted with shorting speaker jack sockets, which act to mitigate the risk created when there's no speaker load plugged into the amp.

FDP Forum / Amp Mods, Repairs, and Projects / 1962 BANDMASTER AMP BLEW CAPACITORS HELP

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