Getting rid of hum in a dual showman

Snarl 12/8 · Post by **Snarl 12/8** » Mon Jul 06, 2009 5:24 pm

I've got this old amp that a guy custom made for me over ten years ago. The builder said that it was now, essentially, a fender Dual Showman. Yesterday I jammed with a guy and he played through it and it sounded awesome, except for the goddamn buzz that the thing has always had, which is the main reason I almost never play with it and I never, ever record it. After last night, I'm motivated now to fix the hum.

Before I take it to someone, is there any chance this thing is fixable by me? Are there any adjustments inside or things to try shielding or whatnot? I'm pretty into DIY. I can solder, read a schemo at a rudimentary level and I'm not too afraid of electricity. Speaking of which, what's the procedure for draining those big caps?

A pointer to a web page about dealing with hum and draining caps would be awesome.

Thanks,

Mudcloth · Post by **Mudcloth** » Mon Jul 06, 2009 7:16 pm

It could be the filter caps. If one or more is failing they won't filter out the a.c. properly. Just checking them for age will tell you whether or not they need to be replaced. If they're over 15 years old they should be replaced.
With the amp unplugged, make sure you attach a jumper from pin 1 of V1 [preamp tube] and flip the standby to the on position before you go pranking around in there. This will drain excess voltage in the caps. Be sure they're the facing the right direction [polarity] when you re-install them. Do one at a time and mark the chassis with a sharpie as to where the positive and negative sides are.
If you can, post a picture of them or post their values.
Good luck.

Snarl 12/8 · Post by **Snarl 12/8** » Mon Jul 06, 2009 7:37 pm

Mudcloth wrote:It could be the filter caps. If one or more is failing they won't filter out the a.c. properly. Just checking them for age will tell you whether or not they need to be replaced. If they're over 15 years old they should be replaced.
With the amp unplugged, make sure you attach a jumper from pin 1 of V1 [preamp tube] and flip the standby to the on position before you go pranking around in there. This will drain excess voltage in the caps. Be sure they're the facing the right direction [polarity] when you re-install them. Do one at a time and mark the chassis with a sharpie as to where the positive and negative sides are.
If you can, post a picture of them or post their values.
Good luck.

Thanks for the reply. They are definitely at least 15 years old. I got this amp made before my daughter was born, and she's 14 now. The thing is, this thing has always hummed like a motherfucker. The guy could've given me lame caps, but I'm skeptical that's the [only] cause.

Edit: I don't mean that to read like I'm not going to follow your advice. I should definitely check the obvious first.

Mudcloth · Post by **Mudcloth** » Mon Jul 06, 2009 9:12 pm

Snarl 12/8 wrote:Edit: I don't mean that to read like I'm not going to follow your advice. I should definitely check the obvious first.

No prob. There might be more to it than the filter caps. It might be rectifier related as well or instead of.

Snarl 12/8 · Post by **Snarl 12/8** » Mon Jul 06, 2009 11:48 pm

After five minutes of poking around in there I've decided to punt. Point to point hack jobs are beyond my pay grade at this time.

sound for sandwiches · Post by **sound for sandwiches** » Tue Jul 07, 2009 9:15 am

are things grounded willy-nilly to the chassis all over the place or is there a star ground point where a bunch of wires all make a chassis connection? if it's point wired and the grounding is messy you'll wind up with hum. sometimes it takes trial and error with grounding to get things right- I recently built a fender-type amp where the minimum hum came from star grounding everything except the input jack, which has a chassis connection and a ground wire to the preamp filter cap. took me a few hours to find the right arrangement.

Snarl 12/8 · Post by **Snarl 12/8** » Tue Jul 07, 2009 1:03 pm

sound for sandwiches wrote:are things grounded willy-nilly to the chassis all over the place or is there a star ground point where a bunch of wires all make a chassis connection? if it's point wired and the grounding is messy you'll wind up with hum. sometimes it takes trial and error with grounding to get things right- I recently built a fender-type amp where the minimum hum came from star grounding everything except the input jack, which has a chassis connection and a ground wire to the preamp filter cap. took me a few hours to find the right arrangement.

Honestly? I can't tell for sure what's going on in there. I think the guy that built this thing for me took me for a ride. From what I can tell the filter caps are shorted to themselves. One of them has a huge blob of solder across the bottom of it. The thing sounds awesome, aside from the hum though.

roygbiv · Post by **roygbiv** » Tue Jul 07, 2009 8:54 pm

Hi Snarl

Here's a couple of places you could have it checked out -

http://www.leightonaudio.com/
(Jeff Leighton)

or

http://www.soursound.com/
(Brian Sours)

I've had work done at both places. Neither is cheap, but both seem to be worth it, as they know what they are doing, run professional shops, and have a backlog of work (i.e., they must be doing something right).

Either would probably give you an idea of what might be the problem, and an estimate for how much it would cost to fix.

Leighton Audio is over in North Portland, SourSound is in the inner SE.

Snarl 12/8 · Post by **Snarl 12/8** » Tue Jul 07, 2009 10:17 pm

Thanks for the recommendations! That was going to be my next question.

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Getting rid of hum in a dual showman

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