Halp? Univox tube amp volume loss

Will · Post by **Will** » Sun Jul 26, 2009 4:34 am

Pens wrote:Fuck. I can't find 68uF 450V axial caps.

Go up to 100uf.

Pens · Post by **Pens** » Mon Jul 27, 2009 8:52 pm

DuoSonicBoy wrote:
Pens wrote:Fuck. I can't find 68uF 450V axial caps.
Go up to 100uf.

Yeah I know, but those are expensive as fuck. I found radial 68uF 450V caps, just not axial, and they are 1/3rd the price of the 100uF axials. I'm still shopping.

Will · Post by **Will** » Mon Jul 27, 2009 9:47 pm

The whole industry has started to move away from axial - it's a shame.

I assume yr not buying the fancy German "guitar amp" caps that are like $6, right?

Pens · Post by **Pens** » Mon Jul 27, 2009 10:31 pm

DuoSonicBoy wrote:The whole industry has started to move away from axial - it's a shame.

I assume yr not buying the fancy German "guitar amp" caps that are like $6, right?

No I was looking at Mouser and the Axial 100uF 450V caps were $13 a pop. Compared to $3 per for 68uF 450V radial. FUCK!

Will · Post by **Will** » Mon Jul 27, 2009 11:03 pm

100uf 450v axial caps:

Weber has them, house brand, for $4.25
Antique Electronic Supply (www.tubesandmore.com) has Illinois for $4.23
Tube Depot has Illinois for $4.95 and fancy F&T German for $8.95
Triode Electronics has 80uf for $4.95 and 100uf for &7.95. They also have some of the clamp-mount caps it looks like you might need.

Pens · Post by **Pens** » Sun Aug 02, 2009 6:52 pm

Okay, sorted that shit. Next issue:

Here's the schem for this amp. The problem now is the cap to ground right before the fuse on that scheme, it's listed as 0.005uF but on mine it's 0.003uF 400V. I cannot locate a replacement for this. Now honestly. I'm not really even sure what the fuck it's doing there, so I can't say if increasing it's value is okay. Mouser only has 1uF 450V as the closest thing. Anyone know what the fuck this cap is for here?

Will · Post by **Will** » Sun Aug 02, 2009 8:29 pm

Usually, those are there to decrease radio interference. It should by non-polarized ceramic, which wouldn't go bad.

You only need to change out the polarized electrolytic caps, since they are the only ones that can dry out.

Pens · Post by **Pens** » Mon Aug 03, 2009 12:09 pm

DuoSonicBoy wrote:Usually, those are there to decrease radio interference. It should by non-polarized ceramic, which wouldn't go bad.

You only need to change out the polarized electrolytic caps, since they are the only ones that can dry out.

This is a polarized oil cap. It looks like all the others. And it is definitely polarized. Not worrying about that one then. Thanks.

Sloan · Post by **Sloan** » Fri Aug 07, 2009 3:55 am

FYUCK IS IT FIXED?!!!

Pens · Post by **Pens** » Fri Aug 07, 2009 12:57 pm

Waiting for caps.

Pens · Post by **Pens** » Fri Aug 28, 2009 12:25 pm

FUUUUUUUUU.

Okay, it took me a while to get the damn PCB out to replace those caps, then I stalled for a while with job interviews and shit needing to go to the hardware store for some O-Ring clips since the new "can" cap was a screw-type and not tabs like the old one, but I finished the cap job and put this thing back together.

It is still "quiet".

However, I think the issue is a power tube bias one. I had it half-together when I tested it, and I noticed when playing that the power tubes actually flashed BLUE when I would hit a rawk chord, and it was very saturated. The "quiet" ness appears to be over saturation, as the decay of a chord gets louder.

I think my power tubes are biased hot.

Now, I have to either A) find time to study the damn schematic to determine how to rebias this, install a pot instead of a fixed resistor, and rebias this damn thing when I've never done this before. B) Take it to a fucking professional who has time to do this and I don't, but they'll charge me a fuckload to rebias it.

Pens · Post by **Pens** » Tue Sep 01, 2009 12:17 pm

Mike, I know you've done bias jobs before, any thoughts on which resistor I need to replace with a trimpot so I can work the bias on this thing?

Post by **Mike** » Tue Sep 01, 2009 12:35 pm

Afraid not, I've only biased amps that already have a bias pot in place so I only know the theory and not how to insert one.

Thegearpage.net has some knowledgable people, as does offsetguitars.com - try Brad (his username is lots of theta symbols on offset.com)

Pens · Post by **Pens** » Wed Sep 02, 2009 5:42 pm

Thanks. I'll have to skim some schematics and see where they are on other 6L6 amps, and try to deduce where one goes on this one.

Have a show Friday and I need to write 4 songs by then, so I won't be able to get to this til probably next week.

Pens · Post by **Pens** » Fri Sep 11, 2009 1:51 pm

Okay based on research I need to replace the 6.2k and 33k resistors with a pot and fixed ~15k 2W resistor to ground to accomplish this. However, I'm also looking into installing a pre-amp out jack at this point since I'll be digging around into this. My thought is I can possibly get a tube poweramp kit to do ~100W power amp.

Will keep y'all posted.

Pens · Post by **Pens** » Fri Sep 11, 2009 3:35 pm

Correction, it appears the only thing to do is install a powersink and a "line out box" to do that, so no internal shit necessary. Imma get on the bias pot.

I very badly want to build the power stage of a Bassman 100 as a separate unit now and run the "line out" into it. Actually, I wonder if I could just install a switch-jack inserted between the pre-power stages...wait no, that won't work. I need some way to not hook up a speaker cab but not fry my OT. Wonder if you could lift the output from the power tubes/shunt to ground safely then pull the signal from the wiper of the volume pot?