Tube amp question

gwh

I bought a tube amp that powered on and worked fine the first time I tried it out, but now it will not power on.

It has the following tubes in it:
1-5u4g
1-6br8
1-12ax7
4 - el84/6bq5

Which one of these would keep the unit from powering up if gone bad? I’m sure it could be a lot of other things but I wanted to start with the tubes first.


My knowledge of the inner workings of a tube amp are limited, please type slowly.



Thanks,





G

mhardy6647

Does it have a fuse? Check the fuse.
If the fuse blows on power-up, likely problems are:
shorted rectifier (the 5U4), shorted power supply capacitor, or shorted winding in the power transformer. A bad (shorted) output tube (one of the EL84's) or a shorted output transformer might make it blow fuses too. There are other possibilities, but the power supply's the first suspect in a vintage amp.

Unfortunately, unless you know that a new-to-you vintage tube amp has been in recent, regular use, it's never a good idea to power one up "cold"... far better to use a Variac (variable autoformer) to slowly bring it up to line voltage, reforming the ancient electrolytic capacitors (e.g., specifically the main ones in the power supply).

candyliquor35m

Is it ok to let them sit a month or two without using them?

gwh

Thanks, I appreciate the info.





G

F1nut

The first time you powered it up, did you have speakers connected to the amp?

BTW, what make and model amp is this?

mhardy6647

The first time you powered it up, did you have speakers connected to the amp?

That's a very good question... although, unfortunately, IME if damage (to the output transformers) is to be done by open-circuiting the outputs, it can happen fast and "leave no other marks" :-(

I'm assuming the OP's amp is plumb dead (as in, no filaments lighting).

fredv

candyliquor35m wrote: »

Is it ok to let them sit a month or two without using them?

I think it really depends on how old/new is the amp. Most of the problems is from the surge to the old caps that are somewhat dried out. If the amp is relatively new or totally recapped, then a month or 2 shouldn't be a problem, IMHO. As a matter of fact, I do that all the time. I power on my old (but recapped) tube amps (with speakers connected) every 1-2 month just to pump some juice into the caps to keep them from early death

gwh

Does it have a fuse? Check the fuse.

Yes sir, and it was blown.

The first time you powered it up, did you have speakers connected to the amp?

BTW, what make and model amp is this?

I really do not recall, I think I turned it on then off then hooked a speaker up to it. Does it make a difference? School me on the ways.

It's a Newcomb guitar or whatever mono amp.

Unfortunately, unless you know that a new-to-you vintage tube amp has been in recent, regular use, it's never a good idea to power one up "cold"... far better to use a Variac (variable autoformer) to slowly bring it up to line voltage, reforming the ancient electrolytic capacitors (e.g., specifically the main ones in the power supply).

I'm 100% positive this thing has not seen use in quite some time and has not been in climate controlled enviroment..... I had better read up on that Variac info.

Thanks for the info.




G

mhardy6647

The rule of thumb: Don't run a transformer-coupled tube amplifier without a load (open circuited).

Don't run an "output transformerless" solid state amplifier short circuited.

These two rules of thumb will save one much grief! :-) You can buy a "dummy load" from Parts Express if you don't want to use speakers for some reason.

Tips on bringing up a vintage tube amplifier may be found at:
http://www.audiokarma.org/forums/showthread.php?t=778

If you don't have, and don't wish to invest in a Variac (variable autoformer), you can build a "poor man's Variac" with a light bulb in series with one leg of the AC mains. See: http://www.audiokarma.org/forums/showthread.php?t=41523&highlight=poor+man%27s+variac

Hope this is helpful.

gwh

"poor man's Variac"

Ah yes that's right up my ally, I think I have what I need on hand.

Thank you sir.



G

fredv

If you think you will have chance to work on tubes in the future, this one won't break your bank either - http://www.mpja.com/prodinfo.asp?number=15162+TR