Old guitar amp question

rntlee · 03:25 AM

Hi,
I've recently come into possession of an old '65 Supro 16T guitar amp. It's completely original and I tried it briefly to see if it still worked. It powers up fine and plays well but has a slight background hum. I see it has an old Mallory FP capacitor in the power supply. Am I courting disaster powering this thing up? I haven't turned it on since I noticed the hum.

Edit: I can't find a Supro schematic but it's apparently the same as this Gretsch.

aardvarkash10 · 5th February 2012, 03:34 AM

yes you are courting disaster. Old amps have a predilection for exploding their electrolytic caps with messy, smelly and destructive results.

Do a quick search in here on resurrecting old amps - there are plenty of threads on the subject

rntlee · 5th February 2012, 12:12 PM

Just one other question,
I can't find the correct value 3 section FP cap (450v 20-10-10), is it ok to use a 525v 20-20-20 in it's place?

eyoung · 5th February 2012, 12:43 PM

you could try it or just use individual caps , don't forget the cap on the 12ax7...

Elwood

GloBug · 5th February 2012, 12:48 PM

Generally you can go up in voltage.
You should be careful raising the capacitance.

In this particular case you are OK. The 20uf in the Gretsch schematic is the first filter cap after the 5Y3 rectifier. You don't really want to raise this much, if at all, it will stress the 5Y3.

The other two should be OK to go from 10uf to 20uf.

You don't have to by a "Can cap" you can buy single smaller capacitors and install them inside the chassis.

GloBug · 5th February 2012, 12:49 PM

Quote:

Originally Posted by eyoung

you could try it or just use individual caps , don't forget the cap on the 12ax7...

Elwood

What cap?

BB62 · 5th February 2012, 01:36 PM

If that cap physically fits it should be OK. The preamp and tremolo are both cathode biased. If anything it should have a little less AC line hum than the original circuit. Be careful in there.

rntlee · 5th February 2012, 05:55 PM

Thanks for the help, guys. I was thinking that an FP cap would be more "original"...though I guess I could leave the old one mounted anyway. It's a kinda desirable amp...the last one on ebay went for $700.
This is the interior: http://i247.photobucket.com/albums/g...e/CIMG4850.jpg
Would I also need to replace the black and the orange cap? Not all the ceramics too I hope?

Sorry about all the dumb q's.

GloBug · 5th February 2012, 05:59 PM

Yes give that orange and the black one a swap too, it also appears to be electrolytic.

Yes for resale value, keep the the old cap and tack the new caps underneath, or "restuff" the cap with three smaller ones.

The ceramics should be OK

costis_n · 6th February 2012, 08:49 AM

Leave the ceramics alone, they are the soul of your tone!

5th February 2012, 03:22 AM	#1
rntlee diyAudio Member Join Date: Jan 2012	Old guitar amp question Hi, I've recently come into possession of an old '65 Supro 16T guitar amp. It's completely original and I tried it briefly to see if it still worked. It powers up fine and plays well but has a slight background hum. I see it has an old Mallory FP capacitor in the power supply. Am I courting disaster powering this thing up? I haven't turned it on since I noticed the hum. Edit: I can't find a Supro schematic but it's apparently the same as this Gretsch. Last edited by rntlee; 5th February 2012 at 03:25 AM.

5th February 2012, 03:34 AM	#2
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	yes you are courting disaster. Old amps have a predilection for exploding their electrolytic caps with messy, smelly and destructive results. Do a quick search in here on resurrecting old amps - there are plenty of threads on the subject __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

5th February 2012, 12:43 PM	#4
eyoung diyAudio Member Join Date: Aug 2007 Location: Sunny SC,USA 15 min south of Charlotte NC	you could try it or just use individual caps , don't forget the cap on the 12ax7... Elwood __________________ "when you open your mind to the imposible, soon you will find the truth...."

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5th February 2012, 12:12 PM	#3
rntlee diyAudio Member Join Date: Jan 2012	Just one other question, I can't find the correct value 3 section FP cap (450v 20-10-10), is it ok to use a 525v 20-20-20 in it's place?

5th February 2012, 12:48 PM	#5
GloBug Banned Join Date: Jun 2011	Generally you can go up in voltage. You should be careful raising the capacitance. In this particular case you are OK. The 20uf in the Gretsch schematic is the first filter cap after the 5Y3 rectifier. You don't really want to raise this much, if at all, it will stress the 5Y3. The other two should be OK to go from 10uf to 20uf. You don't have to by a "Can cap" you can buy single smaller capacitors and install them inside the chassis.

5th February 2012, 01:36 PM	#7
BB62 diyAudio Member Join Date: Jun 2009	If that cap physically fits it should be OK. The preamp and tremolo are both cathode biased. If anything it should have a little less AC line hum than the original circuit. Be careful in there.

5th February 2012, 05:55 PM	#8
rntlee diyAudio Member Join Date: Jan 2012	Thanks for the help, guys. I was thinking that an FP cap would be more "original"...though I guess I could leave the old one mounted anyway. It's a kinda desirable amp...the last one on ebay went for $700. This is the interior: http://i247.photobucket.com/albums/g...e/CIMG4850.jpg Would I also need to replace the black and the orange cap? Not all the ceramics too I hope? Sorry about all the dumb q's.

5th February 2012, 05:59 PM	#9
GloBug Banned Join Date: Jun 2011	Yes give that orange and the black one a swap too, it also appears to be electrolytic. Yes for resale value, keep the the old cap and tack the new caps underneath, or "restuff" the cap with three smaller ones. The ceramics should be OK

6th February 2012, 08:49 AM	#10
costis_n Tinkerer diyAudio Member Join Date: Aug 2008	Leave the ceramics alone, they are the soul of your tone!

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