Farty bass on '66 Fender Pro

[jaysg]

One of the things I'm learning thru this is that, while the web is full of amp-builders, it is woefully short of amp-fixers, or at least ones who will share their knowledge.

Sorry this hasn't been simple, but I think you've got about all the help you can get this way. You need a scope and a function generator to isolate your problem area. You need similar working amps to compare to.

[rfgordon]

Most of the time, guys bring me very simple things to fix: a cap job, burnt plate resistors, bias adjustments, tranny replacements, stuff like that. That stuff is easy and pretty straightforward. This vexing problem with this amp, however, may require a real test bench, which I don't have and can't afford. I may have to give up after replacing a few more resistors and reflowing the grounds. Just don't know what to do, short of completely rebuild the thing. I just have this nagging feeling that it's something ridiculously simple.......

I might have to start turning repair jobs away, since you never quite know what ya got till ya get in there. Trouble is, the "real" electronics tech in Charlottesville has about a 3-4 week backlog of stuff. Ya want a bias ajustment? Drop it off and come back in a month. I mean, that's silly--I can uncab the amp, install 1 ohm resistors, set the bias and recab the thing before the guy's even bored with my guitars! What's a guy to do?

Of course, it goes without saying that I'd turn away any vintage Vox amps...only madmen venture inside those things!

[mlp-mx6]

Do you have another output xfmr that you could jumper in there? (carefullly, of course!) Seems you've eliminated every cap and resistor that could have been the cause. I also recall you eliminated the power xfmr and tubes.

I empathize, this MUST be frustrating!

[Johnhenry]

I'm a retired amp fixer, i would tell you to put your scope on it but that's not alway's possible, I started out with the scope, but after you've fixed a bunch, it's a waste of time for most repairmen, but on amp's that are difficult it is helpful to use the scope,
you do have a bass responce problem, here's where i would look first, Filter cap's, and voltage dropping resistor's, ohm out the choke, meggar tranny's and choke for bleeding to ground, cathode bypass Capacitor's and resistor's, and you may have a bad Pot in the EQ section, plus i think you have tried this through other speaker's ?, and i have found bad coupling cap's between stage's that have gone bad due to age, also make sure the right value coupling cap for this amp is installed at the input of the phase inverter, these are just some simple things that come to mind, I had a problem like this on a Super reverb and it turned out to be a bad Pot, drove me crazy for a short til i put it on the scope and found it. but i think i read where it's doing this on both cha's,if it is, then your problem is after the EQ and could start at the input of the phase inverter, i would look very close at the correct Schematic for the amp your working on and see if somebody might have lifted a few of the small cap's that Fender installed at various place's like across the phase inverter and on the tube socket's,some of the Silverface amp's had these cap's and they were installed to correct problem's just like were talkin about ! Remember, it's not Rocket Ship Science, It's just a danged old tube amp.
I just reread all the replacement part's you've tried, sound's like your amp has maybe developed a lead dress problem, and to see if that's the problem you can try adding a 49pf 1k cap across the phase inverter at the 100k & 82k resistor's, i just fixed a marshall that was having note degeneration problem's, had like a raspy second harmonic behind the main note, Fender used this trick also on the Silverface amp's, you can try any cap size from 17 pf to 220 pf to see if it get's rid of the raspy note's your hearing.
John

[rfgordon]

Johnhenry,
thanks for the encouraging words.

I talked to Gerald Weber (Kendrick Amps) about this, and he thinks its a parasitic, and I've come to agree. Blackfaces can be problematic in this regard anyway, and I discovered last night this one is more heavily mofidied than I had realized. For example, the Normal channel signal mixes with the Verb channel at the reverb entrance, instead of at the PI entrance. Hmmm, what other weirdness will me and my meter find? I suspect the instability lies in an evil synergy between the icky Fender lead dress and the mods that were done by some other guy...I'm gonna have to lift and separate the board to make sure there aren't any hidden signal paths to deal with.

We shall see.

[jelle]

I have worked a lot on these AB763 style amps...

I also suspected lead dress issues....please see above.

Can you please post a pic of the amp so we can try to help you.

Jelle

[Bob-I]

Johnhenry,
thanks for the encouraging words.

I talked to Gerald Weber (Kendrick Amps) about this, and he thinks its a parasitic, and I've come to agree. Blackfaces can be problematic in this regard anyway, and I discovered last night this one is more heavily mofidied than I had realized. For example, the Normal channel signal mixes with the Verb channel at the reverb entrance, instead of at the PI entrance. Hmmm, what other weirdness will me and my meter find? I suspect the instability lies in an evil synergy between the icky Fender lead dress and the mods that were done by some other guy...I'm gonna have to lift and separate the board to make sure there aren't any hidden signal paths to deal with.

We shall see.

I should've thought about that too, sorry. I just had a similar situation with a homebuild, I'd describe it a "wooly" in the mids. I found the issue by puting the scope tuned in at fairly high frequencies, there was a clean sine wave showing at the PI so I focused on lead dress in that area. Cleaned it up easily.

Good luck, PO is a bitch

[rfgordon]

Fixed it!! Woohoo!!

Gerald was correct, it seems. I did his recommeded Blackface lead dress mods, only on the verb channel so far, and it fixed the problem.

Verb channel input wire shielded, as is now V2B grid wire off volume pot. I'm not a Blackface expert by any means, so all this was very educational.

I also pulled the reverb wires out from under all the pots and redressed them up in the air, so to speak.

I moved the tone stack caps and their 100k R to the backs of their respective pots, then connected them with one wire to the plate R of V2A. That eliminated a LOT of grid wire.

I wanted to keep his Normal-into-reverb mod, so that channel's feeder wire got a shielded wire.

And now, with grid wires shorter and/or shielded, it's much quieter, too.

I reckon I'll mod the Normal channel tomorrow, since I gotta get ready for a gig now.

To all y'all, thanks for your ideas and encouragement. If that guy with the 70 Silverface Twin who called the other day ever brings it to me ("It starts out fine, but after a while just kind of loses it."), I may have another adventure to share.

Have a groovy weekend.

[Sonny ReVerb]

Great work Rich! Thanks for sharing your trouble-shooting adventure. It was very educational. (btw - Stop by Big Jim's and have a BBQ for me in celebration. I still miss that place.)

Sonny

[mlp-mx6]

Congratulations! Plus - you not only fixed it, you made it better.

[rfgordon]

Sonny,
Yeah, Big Jim's is pretty good, though with Pig Daddy and the Pit Stop this town's got better Q than it used to.

We played a party out at Lake Monticello and had "Pennsylvania Chicken." I don't know what makes it Pennsylvania, but it sure was good. Good food, good times, good music and great tubes made for a nice Saturday nite!

[Johnhenry]

Way to "GO" Rich, Glad you got it ! Even with all the test Equipment I had on my bench i can still remember some Hateful Amp's, LOL !
You just have to be hardheaded sometime's !
enjoyed your post,
johnhenry

[dehughes]

Yeah man, good job! Way to stick with it. Troubleshooting can be a real pain sometimes....and other times the problem is perfectly obvious....