ShortScale.org

I know I've seen this answered on here before but I can't remember or find it right now.

Basically, my Univox tube amp head suddenly sounds about half volume as it used to, and sounds a bit out of phase or something as well.

I tried swapping in new preamp tubes but that all checked out.

I'm pretty sure I know the answer here but I'm hoping it isn't. I don't want to rebias this thing. It has carbon comp resistors FFS. I don't want to replace those.

Halp?

You shouldn't necessarily have to replace any resistors to rebias it. Is there a trimpot inside it?

This could be a power tube issue, do you have any spares you could swap in?

Nope, no trimpots. This thing is from the 60s before they had thought of fancy things like trimpots for biasing or grounding.

I don't have any spares for power tubes, just pre tubes. I'm searching now to see if there's some way to just test a power tube with a MM.

Any noise or extra distortion? Those would point to a circuit problem.

In all likelihood it's just power tubes.

1. AMP PROBLEM
2. CHECK TUBES

I can't find any info on how to test if a tube is good or not. I'm going to have to buy some new power tubes.


Dammit. I don't want to rebias this thing.

Can you borrow a pair from a friend?

Either way I hope you can find some reasonably priced ones and that that is all it is.

Mike wrote:Can you borrow a pair from a friend?

Either way I hope you can find some reasonably priced ones and that that is all it is.

Actually, that's a great idea. I might be able to borrow a pair from a friend. I'll see about that first.

If the power tubes are cathode-biased, you can measure the voltage drop across the cathode resistors and calculate the current flowing through each power tube that way. You might need to interwebz some info to see if what you're getting is in the right range for the amp... or check a tube spec sheet to see if it's at least in the usual operating range for that type of tube.

If the power tubes are fixed-bias and you have a schematic that shows what the voltages are supposed to be at various places in the amp, you can test the voltage at the power tube plates and/or nearby places in the power supply to see if they're somewhere close to what's printed on the diagram. If the voltage is unusually high, then one or both power tubes may not be passing as much current as they should. You can also check your grid voltage, to make sure that your bias supply circuit is functioning OK.

In both cases, the amp's switched on, the standby switch is off (meaning it's in ROCK position), and you're naturally being careful not to mega-zap yourself from the high voltage you're working with (especially at the power tube plates)! I don't know how definitively this will tell you whether your tubes are good or not, but it's a way to check that they're holding the right bias. And that should be most of the story.

If swapping tubes doesn't clear things up and/or the voltages look OK (if you tried the above options), I'd guess it's time to replace the filter caps (which may be responsible for phasey tones) and all the cathode bypass caps (which may be responsible for the loss of power). Basically, if the amp hasn't had its electrolytic caps changed in the last 10 years or so, it's probably time to do 'em.

DUDE

u should post lots of pics of the inside of the amp. because it would RULE.

I'm honestly scared to open up the amp. I'm looking for a pair of 6L6's to test with. All my friends appear to be using EL34 's.

Update on this. I just got a set of 6L6s from a friend to test. It's still losing volume, and I A/B'd it to my roommates Marshall 50W head, definitely quieter.

Odd thing, I was testing with just hitting an E chord and letting it ring, and the volume seems to go up and back down at various points like a tremolo effect, but not steady. Just sort of randomly.

Someone told me it could be the power transformer, or the output transformer. I'd prefer it to be the caps, I'd have no problems swapping in new ones.

Any thoughts? Caps or transformers?

Volume pulsing suggests caps going bad.

Looks like I'm recapping this bitch then. Will update after that's complete.

This is the amp we are dealing with here.



This is the guts.




OKAY. So, I checked the voltages on the caps there, they all had about 1V in them still which started to drain as I had my multimeter across them, EXCEPT FOR ONE. The left Blue electro cap had 0V in it. Every other cap had around 1V. Is this the culprit here?! Should I replace them all? The gray ones are goddamn oil caps, I'd rather not replace those if I can avoid it. Personally, I am thinking I should replace both blue electros and leave the oil caps alone.

OPINIONS?!

Replace all the polarized caps with normal electrolytics. The amp is over 30 years old - it needs it.

DuoSonicBoy wrote:Replace all the polarized caps with normal electrolytics. The amp is over 30 years old - it needs it.

Shit, I've been out of the building electronics thing for too long, anyone have a decent and not overpriced amp cap supplier lank handy? I lost all of my old bookmarks...

I had good experience with Parts Express - huge selection and very low prices.
http://www.parts-express.com/home.cfm

Mouser is also great, though a but difficult to navigate. The advantage is they stock higher-temperature caps, which last longer in tube amps.

Weber (www.tedweber.com) has taken to stocking 600v caps, which can be good for the main power filter caps.

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