Fix the mystery amp

[jhaneyzz]

I'm waiting on the mail to come to complete the 4 pedals currently in the queue, leaving me wondering what to spend the 3 day weekend on...

This tube amp was given to me my my local guitar shop several years ago. It was built by a former employee of the shop and I can't be 100% sure it has ever worked. I've never done anything with it because, until recently, I have adhered to a strict vow of low voltage... I've weaned myself on a few tube pedals now and have a good sense of how not to get zapped so I want to take this on. Definitely going to bin the butt-ugly enclosure so no worries there.


So... Where does one begin diagnosing problems with a tube amp? looking at you @RetiredUnit1 ...

Dirty secret... I've never actually played a guitar through a tube amp, so this is definitely a life goal checklist item.

 

[Chuck D. Bones]

You understand high-voltage safety, right?

First, you need a schematic. Do you have one?

 

[RetiredUnit1]

Well it's a class A single ended 6v6 circuit with a solid state rectifier and a choke. Can't really read the numbers on the output transformer (OT), but the power transformer (PT) is a general use Hammond. I'd have to say it's probably a custom champ.

The first thing you want to do is plug it in and touch all the bare wires. Ok, that's a joke.

The OT appears to be a fender part number.

I'd start by pulling the chassis after taking a load of pictures of all the wired connections, and see how the electrolytic power capacitors look. Are they symetrical or bulging, tight or leaking. If they looked good I'd pull the tubes, and with it upside down on the bench hook it up to a variac at zero volts. Turn the switches on, and then slowly turn the variac up to about 10vac and leave it there for an hour. Then slowly every hour turn it up another 10vac, provided you don't hear any popping or see smoke until I got it up to 110v.

I'm guessing this was built when voltage was 110v not 125.

What this process does is to "reform" the electrolytic capacitors. If you just plug it in after 30 years of non use they're likely to just blow. Sometimes spewing their contents in all directions. lol.

Then I'd use the multimeter to check the voltage going to the first cap and see if that's what the transformer is rated at. You can find that on the Hammond site.

Then I'd check the voltage at the last cap to see if it's within reason, and if so turn it off, put it back in the box, plug the speaker and tubes back in and try it....

 

[RetiredUnit1]

Here's the champ I've s*l*o*w*l*y been putting together. Ready to solder the second half of the board, then it's time to start drilling holes in the 10x6x2 aluminum box I have for it. btw, If you KNOW that the amp has been played recently, you can just go ahead and give it a try. By recently I mean within the last five years.

However, that appears to be a 400w stereo speaker, and this is at most a 5w amp. Not going to hurt anything but guitar amp speakers and stereo speakers are completely different animals. Oh, and it does have a three prong cord. Hmmm.....

That's a good thing. It either means this was built in the not too distant past, or someone upgraded the cord. All I know is my coffee cup is empty and I need to go fix that!!!!

 

[jhaneyzz]

You understand high-voltage safety, right?

First, you need a schematic. Do you have one?

I fully realize that I never actually provided any useful information in the original post.

I went down this morning and identified that I was gonna work on it. took the initial pictures, sat down with a coffee and made that post.

Then I went down to shop to clean up enough to put it on the bench, unbox it, take some pics so we could ID the circuit and get a schematic.

Then I saw a squirrel...

spent the rest of the morning making a new guitar stand to replace the one that died in an unfortunate pedestrian related collision..

I'll post some meaningful photos and relay the salient point defining what I mean by "It doesn't work."

P.S. I have in fact done my pre-requisite refresh on high voltage safety this week.

More later...

 

[RetiredUnit1]

It's an odd build. Whoever made it did not orient the transformers in different directions, that can induce humming. The speaker is crazy large for the watts, the rectifier looks like it was made for something drawing 125 amps instead of 2. And after putting the OT and PT on opposite sides (good) put the choke right next to the OT (not great). I wouldn't be surprised to see it has no circuit board and is just PTP resistors and caps attached to components (bad).

On a completely different note, I got my butt kicked by a microbe last week. There is a **NASTY** head cold going around. My nose looked like Rudolf yesterday, but is getting better today thanks to judicious use of hydrocortisone creme.... I had covid back on Christmas day last year, no where near as gnarly as this bug was.

 

[jhaneyzz]

I am suddenly now glad that I am not your neighbor...

 

[jhaneyzz]

The plot thickens.

Here are some gut shots and shots of the status lights.

Interestingly, right after I flipped the standby switch and it lit up. I hear a sputtering electrical discharge near the 6V6 and the P-TF22905 transformer. The 6v6 was starting to glow a bit at the top but I was distracted by the wisps of smoke coming from somewhere beneath the transformer as near as I can tell..

Unpulgged. went to write this email..

Looks like a very strange build Not like any amp I've ever seen...

 

[Gordo]

Wait. You've never played thru a tube amp. Dude, since you're likely local we need to get together worse than I thought... Plus I still have my shop set up.

 

[RetiredUnit1]

Tell me you didn't turn this on with the speaker disconnected? That's the kiss of death for the output transformer. They melt.

 

[jhaneyzz]

I did not. however... I can't be 100% sure I had the polarity of the speaker wires correct.

 

[RetiredUnit1]

I did not. however... I can't be 100% sure I had the polarity of the speaker wires correct.

Yay!

Polarity only matters when there is more than one speaker - It's an A/C circuit.... If you have two speakers and the polarity on one of them is reversed, then the sound waves cancel each other....

There's a 9v battery test. Hook one wire up to the negative terminal, and quickly touch the other to the positive terminal. If the speaker moves outwards, that's the positive side. If the speaker moves inwards, it's the negative side.

Smoke huh? Wow....

Any testing should proceed with the power tube removed, that way you cannot cause damage to the OT....

 

[PJS]

I am not an expert on tube amps. Not even a little bit. I do know that speaker polarity won't matter though. Main thing with polarity is that if you wire up 2 with different polarity then they will cancel rather than reinforce

 

[jhaneyzz]

Smoke = bad

 

[jhaneyzz]

I might rather start from scratch with a nice little champ kit rather than dig into this little death trap…

 

[jhaneyzz]

Or… bizarrely enough, it turns out Gordo lives 2 blocks from me so I can just walk over and get a nice tube fix from the neighbor…

 

[TheHammer1982]

Just a heads up but that Pioneer ts-w301c is an old school car audio subwoofer.

If it were me I would pull any salvageable components off of it and start over.

ETA: This is coming from a guy who has been working on amp (that’s worth about $300 on a good day( off and on for the last year lol.

 

[HamishR]

I'm with Hammer82 - I don't think it's worth trying to "fix" this amp. I think it's only going to hurt you.

 

[jhaneyzz]

I'm with Hammer82 - I don't think it's worth trying to "fix" this amp. I think it's only going to hurt you.

Yeah. Kinda took me 30 milliseconds to come to the same conclusion...

 

[vigilante398]

Yeah. Kinda took me 30 milliseconds to come to the same conclusion...

Why hello there, I hear you have a tube amp that needs fixing I'm not 2 blocks away like Gordo, but I'm not far...