70's Vibrolux Ugly distortion

[abelljo]

Hello, I'm new here. Stumbled across this awesome site while searching for a solution!

I had a friend sell me a 70S Vibrolux Chassis. It was all there except it had a replacement output transformer. It is an older probably bassman OT. Since there were other minor issues I could not really test before moding the amp. I did the BF mod, cap job and eliminated the pull boost. I also changed the bias section so that I could adjust the bias. The problem is that I'm getting an ugly distortion overtone at any volume and on both channel it is not very noticeable when playing single noted, but when playing chords or two notes at once it is UGLY! (Speakers are new C10Q's) It also has like a canceling effect as the notes die quicker than usual with the nasty distortion. Almost like some kind of bad oscillation or something. It's been a while since I have been a tech and just play music and have a different day job now :/ So your help is appreciated.

P.S. all voltages are correct and bias is set at about 28ma or so Output transformer reads fine on primaries and secondaries. I replaced all electrolitics, but non of the coupling caps. I also replaced the plate resistors with new 100ohm 1 watt metal film as well as the power supply resistors. I also modified the power supply to be like to more powerful amps with the 2 80mfd caps in series and the 2 220k ohm resistors paralleled etc. Long P.S. just thought worth mentioning.

[cbass]

Did you mean to say 100k ohm ofor the plate resistors?

[ampgeek]

Can you plug it into another speaker cab to eliminate the new speakers from the equation?

I have experienced odd, new speaker coil rub conditions that sounded similar to your description.

Good luck,
Dave O.

[M Fowler]

Well first off welcome to TAG.

when playing chords or two notes at once it is UGLY!

Sounds like either out of phase at speaker jack or PS caps. But you changed out the E-caps and left the coupling caps so I'm scratching my head right now.

Mark

[abelljo]

Did you mean to say 100k ohm ofor the plate resistors?

Yes the 100K plate load resistors.

Thanks!

[abelljo]

Well first off welcome to TAG.

when playing chords or two notes at once it is UGLY!

Sounds like either out of phase at speaker jack or PS caps. But you changed out the E-caps and left the coupling caps so I'm scratching my head right now.

Mark

I considered reversing the OT secondaries, but I have dne that when I get squeals from it being out of phase before and this is a different kind of problem. I don't have my old B&K anymore......Ugh.

[abelljo]

Can you plug it into another speaker cab to eliminate the new speakers from the equation?

I have experienced odd, new speaker coil rub conditions that sounded similar to your description.

Good luck,
Dave O.

I could try that just to get it out of the way, but it sounds like something else. It's not the first time I have experienced this, but I have overhauled the amp and somehow eliminated the problem. I just don't have the time to do a complete overhaul this time. Especially with work, and a 3 year old hanging around. lol

In the past it was less obvious, but a faint distortion as he note would fade.On those other amps it could have been anything. Bad tubes, bad resistors or caps along the chain. Who knows. lol

I'm should have known better than to buy amps from my friend in Puerto Rico, he keeps them in his garage with, dust, spiders, cob webs, roach eggs, you name it. Lol Not to mention he shipped the amp in box with no real protection just a few balled up news paper pages and a piece of plywood. Don't know why the plywood was there, perhaps to sturdy up the box. Needless to say the amp got banged up on it's way to Miami and the chassis was bent and dented in several places. The fuse holder was destroyed as well. I straighten up the chassis and got it running, but not perfectly. YET.

[Firestorm]

Late '70s Fenders are the poster children for bad lead dress. You can try putting the parasitic suppression caps back in (on the 6L6 grids), but there are also issues with the wiring of the tone caps that usually won't address. You really do have to clean up the spaghetti wire to make the sound right.

Is the problem the same on both channels?

If it's more on the Vibrato Channel, one possible issue with removing the pull-boost involves the resistor (usually 470K IIRC) that goes between the boost switch and the 470K coming off the reverb pot wiper. (There should also have been a 1K coming off the boost switch going to ground). When you remove the pull-boost, you have to leave the 470K and the 1K in place (or better, add a 220K to ground from the junction of the 470K/3M3/10pF like on the blackface models). This is part of the voltage divider network that sets the dry signal level going into the reverb mix amp. Without it, the signal is too hot and parasitic problems are, err, amplified.

[M Fowler]

I was thinking more a phase issue when people add gain stages etc. but if you did not mod the amp then it should be correctly phased.

Mark

[abelljo]

Thanks Firestorm, I'll try that, but the problem is with both channels, although I didn't play the normal channel enough to see if it is less noticeable there. I can clean up the lead dress coming from the power supply if that's what you mean, but I'm not sure if you are suggesting particular placement for the grounds. I know it is probably the same location as with BF amps. The first stage of filtering is by the PT ground and the other stages closer to the reverb vibrato pots. I don't mind adding the suppressor caps. My ear is not that sharp to notice much difference in tone, besides I play jazz and I don't need as the amp to have as much access to highs or great overdrive tones! I should have left the SF circuit alone!

[Firestorm]

Thanks Firestorm, I'll try that, but the problem is with both channels, although I didn't play the normal channel enough to see if it is less noticeable there. I can clean up the lead dress coming from the power supply if that's what you mean, but I'm not sure if you are suggesting particular placement for the grounds. I know it is probably the same location as with BF amps. The first stage of filtering is by the PT ground and the other stages closer to the reverb vibrato pots. I don't mind adding the suppressor caps. My ear is not that sharp to notice much difference in tone, besides I play jazz and I don't need as the amp to have as much access to highs or great overdrive tones! I should have left the SF circuit alone!

I wasn't thinking of the PS grounds, per se (although Fender reverbs have the screen supply grounded with the preamps, which it sort of shouldn't be.) The tone caps, though, have massive lengths of unshielded wire connecting them and parasitic coupling tends to make the amp sound like it has a blanket over it. Also generates a pronounced fizz trailing the notes. Gerald Weber says you should move the caps to the pots, but that's overkill. I've had luck replacing the tone cap wiring with 3-conductor double shielded wire (all the signal leads inside a foil shield inside a braided shield) which seems to kill all the badness. If you want the amp cleaner, btw, change the 220K grid load/grid return/bias feed resistors to 150K. Surprising how much difference this usually makes.

[billyz]

I have had an Output Transformer behave exactly like you are describing. It measured fine. try subbing in a know good one .

[abelljo]

I have had an Output Transformer behave exactly like you are describing. It measured fine. try subbing in a know good one .

I'll try that this Friday, I have a spare as well. Hopefully I can figure it out. I'll post the results.

Thank for all the advise!

[abelljo]

I think I'm close to finding the problem. I think I was running only one side or operating in class a. It looks like pin 3 of the old socket was busted, but when I put a socket I found in a drawer it started catching fire This socket had burn marks, but am not sure what could be the problem. with it. It is something with pin 3 and no it is not shorting anywhere. If I remove the secondary wire it stops doing the crazy sparks fire thing. It's as if the allow is bad and the wire doesn't want to be connected there. Very strange. Anyway, I had changed this socket from another amp for the same reason before. It was a new Belton socket. Very strange. anyhow I ordered a new socket just to eliminate any possible issue with that. BTW the amp exhibits the same sound/distortion and volume issue if I disconnect one of the primaries.

[billyz]

A bad socket would do it. If it is burned from arcing it should be replaced. Pretin your joints before soldering. And do not use lead free solder. Try sanding off the joint if difficult to solder too, and use extra flux.