Vibro Champ with minor issues

[Potato Cakes]

I was using the stock values from the schematic I had, so I don't know what would be throwing off the voltage if that is indeed the issue. Also of note, I wound putting ECC83s/ECC803s in the preamp and oscillator stages as that is as close as I had to a 12AX7. I have a number nice 12AT7 and 12AU7s but when I put them in either the preamp stage and/or the oscillator stage the tremolo effect was barely audible, even with the settings maxed out. They also caused a lower than acceptable drop in the plate voltage. I know these require a different bias resistor but at the time I was just trying to see if the distortion issue would go away.

I'm very certain that the bypass cap on the second stage is correct. Also, when I remove that cap from the circuit, the issue is still there. Again, the client has the amp so I can't know for certain at this point, but I know that is something I looked at during troubleshooting.

The issue is there with or without the foot switch.

I decided I'm going to have to build another one of these because I can't stand not solving electronic mysteries that I somehow create for myself. I will be doing the 6L6 version and testing with 4 and 8 ohm speakers. Probably going to be a couple months before I can start though.

Thanks!

Paul

 

[AnalogPackrat]

Maybe you've got a bum 1k5 cathode resistor or other part out of spec. Did you use carbon comp? Measured before soldering in place?

 

[CJ]

Those ecc83s tubes have a lot of extra gain , people use them in the first stage of the Marshall,s for Xtra crunch

A normal ecc83 measures about 8 on my orange tube tester and the ecc83s measures about 12 so that extra gain could be causing the distortion.

 

[AnalogPackrat]

Maybe try a plain jane 12ax7 in the oscillator. No need to waste the fancy stuff there anyway. If the gain is too high that second booster triode after the oscillator might be exceeding the design range of signal applied at the driver's cathode.

 

[Potato Cakes]

That makes perfect sense. I just didn't have any proper 12AX7s at the time and pulled from another piece of gear. I'll find some proper tubes and test again. I did not realize that the ECC83s had so much gain.

Thanks!

Paul

 

[Potato Cakes]

Maybe you've got a bum 1k5 cathode resistor or other part out of spec. Did you use carbon comp? Measured before soldering in place?

All carbon film and not carbon comp. Resistors were measured as far as I can remember.

Thanks!

Paul

 

[MHProd]

Have you tried a a 470r on the 6V6 screens? And/or something like 5K6 on pin 5. Sometimes, the tremolo just pushes the amp into HF oscillation. You could also try a small cap across the NFB resistor. Using a 5751 might be an option but from experience, these types of amps can be prone to such issues.

 

[CJ]

XFMR has been disassembled the hard way

There was a bar b que at the pri due to inadequate pri-sec insl, about 20 turns were melted into 1 big turn, results: dcr looks normal as there is still continuity but inductance was a paltry 7.5 H at 20 Hz due to the big ring of shorted turn sec wires stealing all the current from magnetic induction, freq response normal turns ratio test normal, but no H.
Pictures at 11.

 

[Potato Cakes]

Thanks, CJ! Explains the voltage issue I had with that installed. You can imagine my frustration when I took the amp most of the way apart but still couldn't find the problem. With a long history of making easy, simple mistakes an actual bad transformer was the last thing I suspected.

Thanks!

Paul

 

[CJ]

ok,

 

[CJ]

i thought this thing had epoxy end-capping over a layered paper core,
not usually seen in audio transformers, usually reserved for the HV stuff to preven tarcing from coil to core,

 

[CJ]

we done gotta shiny new heat gun, all those take-a-parts did a number on the old one,

 

[CJ]

ok, so what the crap, there is a nylon bobbin revealing itself, that ain't no epoxy end cap at all,

SE amps use a gapped core stack, no gap on the tongue, only the legs,
probably about 2 or 3 mil, to messed up to measure,

Looks like we got short stacked. That core should measure 0.625" thick but it is only 0.53". The last time i got short stacked was at IHOP, i ordered 3 Swedish pancakes and only got 2,

 

[CJ]

all spooled up and ready to go,

 

[CJ]

so that is what that epoxy end cap turned out to be, a mylar tape decoy,

 

[CJ]

pri anchors

 

[CJ]

secondary wire measures about 0.021", 83 turns in two layers,

layer insl is a little thin for this ga. wire,

 

[CJ]

everything looks ok from this side,

 

[CJ]

holy guacamole, what happened?

 

[CJ]

rare so see a partial layer at the finish,