Confession of a rank amateur tube amp repairman

[mac dillard]

Had a friend call and ask if I would take a look at his Fender AB165 Bassman. It was distorting really bad if the vol. was turned up past 2. He had replaced all of the tubes. We went all through it checking voltages, components, grounds, tubes, ect. Could not find any thing wrong. He said, I think it is the OT. I said naugh, it is never the OT. We put the scope on it and the signal began getting bad at the PI. Replaced PI tube again and checked all components again. Could find nothing wrong. He said, I think it is the OT. I said, naugh, It is never the OT. We checked resistances of the OT and could find nothing wrong. He ended up sending it to a repair shop in Chicago. Guess what, it was the OT. Egg on face.

[mhuss]

The unlikely causes will bite you every time.

To quote Holmes/Doyle, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".

[xtian]

Yes, bad OTs--especially PARTIALLY bad OTs, the ones that still work, but badly--are rare. Yet I've had two of them in 6 years, in about 200 repairs. So that's about 1 in 100.

[Leo_Gnardo]

The unlikely causes will bite you every time.

To quote Holmes/Doyle, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".

That pretty much describes my approach to concluding "bad transformer." Considering the replacement cost, I want to eliminate any other possibility. Then it doesn't take a whole lot of time to clip in a known working transformer - if the amp improves at that point you know what you have to do.

[nsubulysses]

If the signal started getting bad at the phase inverter then how was the OT bad? Just out of curiousity, care to share what shop it went to ??

I just worked on a V4B for the "premier" amp repair place in Chicago (will not accept ANY solid state repairs ) that was given up on becasue it would only put out 1/2 power. They tore up the phase inverter looking for the problem, replaced almost every part. One of the coupling caps (C12) had been removed and never replaced. So, yeah, it puts out 1/2 power because only 1/2 the phase inverter is getting a signal at the grid. It took under 10 minutes to realize this with a scope. Felt good to sort this out quickly since this shop is $100/hr and will only work on easy vintage tube amps. I believe Enzo calls places like these "skimmers."

[mac dillard]

The shop was Chicago Amp Works. They have an internet site at Chicagoampworks.com.....I know nothing about them. My friend has contacts in the Chicago area so I would assume that is how he knew to contact them...Can say this, turnaround was quick. Cost was reasonable...The tech did approach friend about modding the AB165 to Blackface. That was kinda costly so he declined. We live in a rural area in SouthWest Arkansas and reliable amp repair here is not readily available. Have tried an instate repair shop but the turnaround was very long. Nice to find a shop that was reliable and fast. $40 to ship there for us. We do what we can as far as repair but that is not what we do all the time so some things get over our head.

Can't answer your question about the signal getting bad at the phase inverter. That is what we saw and it threw us off. Might have been a different problem.

[Leo_Gnardo]

The tech did approach friend about modding the AB165 to Blackface. That was kinda costly so he declined.

Just as well. Some techs make "blackfacing" out to be hoodoo voodoo that's costly to do, you know those mojo bones and graveyard dirt don't come cheap. In actuality you're never more than a handful of components away from "blackface." Depending which model of Fender amp, from 3 to 9. Figure $50-60 at most including labor. Besides, does your Bassman sound good? If it sounds good it IS good, that's what matters most.

[mac dillard]

Just as well. Some techs make "blackfacing" out to be hoodoo voodoo that's costly to do, you know those mojo bones and graveyard dirt don't come cheap. In actuality you're never more than a handful of components away from "blackface." Depending which model of Fender amp, from 3 to 9. Figure $50-60 at most including labor. Besides, does your Bassman sound good? If it sounds good it IS good, that's what matters most.

Full agreement there. The amp does sound good. As My buddy said, if I wanted a Black face I would buy a black face. I have built several amps from known schematics so I get a lot of request in this area to help with a problem amp. I do the best I can but the skill set to repair an amp is different from building an amp. I find that I get a lot of help from this board.

[nsubulysses]

The shop was Chicago Amp Works.

That place seems to have a good rep

[Justin Thomas]

My AB165 <IS> a Blackface! And it sounds DAMN good, with only two mods: removed 220K on the 6L6 plates (local feedback), and apparently at some point, mine had a bad OT too. It now has a Twin Reverb OT in it. The extra nice part is, I've got spares of both trannies laying about from a customer who was moving & unloaded all his parts on me, if I agreed to sort them out. Well, I added a bias adjust to go with the balance, too, but that's not exactly a "mod."

Nice amp! I use it for bass and guitar.

Justin

[mac dillard]

My buddy had removed the 220K resistors but put them back in. He said he couldn't tell any difference. I think the AB165 is the only model that they show up in. Must have been some reason for having them.

[Justin Thomas]

Then maybe I'll try putting them in mine... it came without them, though the little terminal strip is still there...
I do know, these are really picky about wire layout, especially in the controls area... every time I have to futz around there, I have to make sure the placement is exactly as it was before...

Justin

[olddawg]

Did they give you the bad parts back. Back in the day I had to send gear back to the manufacturer for "evaluation" occasionally if it was a problem unit. I got into the habit of marking boards and major components when I did so they wouldn't cause me of "missing the obvious" and just swap out a board or something. A major shop may have a transformer in first if one was on the shelf.... Then diagnosed the real problem but still charged you for the OT and didn't mention it. Happens more than you think.

[Justin Thomas]

The changes as Fenders evolved were always for more clean power... whether these experimental ideas succeeded or not is up for debate. Hence the wacky things in this model - I don't think these changes were all the Evil CBS's ideas. Leo's whole progression even with MM was toward loud and clean, and Leo was still very much involved at this stage - stayed til 1970, when stuff REALLY went wacky.

Where I find this amp really shining is when I use it for my bucker-equipped lap steel - the sustain & compression are amazing.

If you haven't read it yet, Machine Gun Amps does a stage-by-stage breakdown of this model, and suggests only very Minot changes. Some of it I agree with, some I don't, but there's no drilling, so I think it's worth checking out.

Justin

[xtian]

If the signal started getting bad at the phase inverter then how was the OT bad?

The amps I saw with bad OTs had the same symptom! Because of the "unhealthy" load of the bad OT, it was causing the signal after the PI to look really ugly, making me think it was a PI issue.