Troubleshooting vibrato circuit

[charlie chitlin]

I admit, this is difficult with virtually no knowledge, but I move onward through the fog.
It's an old Premier 76 and the only schematic I found is not my amp.
1st thing I notice is there is a wire from the jack to a preamp tube; between them is a capacitor (modern orange drop).and a resistor to ground. The resistor is marked 1Mohm with a gold band (+- 5%) and it reads 1.3M. Would that be enough off to undermine my wiggle? I mean...what's the point of a funky ghetto amp with no wiggle?
Every other resistor I find that could be associated with the wiggle is within spec...or real close.

 

[Silverface]

Kinda tough to guess without any detailed description.

What tubes are in it? That may help track down the schematic - or at least one in the Premier/Multivox line with similar circuitry in the trem section.

 

[LudwigvonBirk]

As Wally said/implied, try to hunt down the schematic. That will help you and help your forum-debugger entourage.

That said,

A) a resistor that is off by 30% should be replaced as a general rule of thumb. Those suckers drift, and probably nobody designs for a 30% +/- tolerance. Cheap fix.

But:

B) a resistor feeeding to ground right after the input jack wouldn't affect bias-wiggle or roach-trem in a typical/classic tube trem design. Afaik.



How 'bout backing up a bit: what is the "while-playing-through-the-amp" problem you are trying to solve?

 

[Bendyha]

Is this the schem you found that is wrong?

 

[robrob]

When tremolo goes bad the #1 suspect is a bad tube, then any cathode bypass caps in the tremolo circuit, then tremolo caps and finally the intensity and speed pots. Resistors are the last thing I would look at.

I don't see it in the schematic above but many tremolo circuits use the power tube negative bias voltage as a tremolo "kick start". If that voltage isn't where it should be it can keep the tremolo oscillation from starting.

 

[charlie chitlin]

Is this the schem you found that is wrong?
View attachment 603905

That's the one I found.
Mine only has 4 tubes in the preamp section...2 7N7s and 2 7F7s, and a 5V4GA rectifier.

 

[charlie chitlin]

When tremolo goes bad the #1 suspect is a bad tube, then any cathode bypass caps in the tremolo circuit, then tremolo caps and finally the intensity and speed pots. Resistors are the last thing I would look at.

I don't see it in the schematic above but many tremolo circuits use the power tube negative bias voltage as a tremolo "kick start". If that voltage isn't where it should be it can keep the tremolo oscillation from starting.

I swapped the tubes 1-by-1 (not knowing which is the trem), with no luck.
Is it possible for the trem to use half a tube, like with 12A-7's? Just curious.
Any bypass cap would be electrolytic, right? If so, there are none left...all orange drops.
I'm not sure how to check the negative bias voltage (I'm sure I could learn...I haven't been zapped yet!), and I suppose I would need to know what it's supposed to be before I checked...right?
Aside from the trem, this amp is a killer.
These octal things are greasy!
And it looks like grandpa's radio inside.

 

[corliss1]

How good are you at drawing out circuits? That might be the best plan for something like this if you can't find the right version of a schematic online.

 

[charlie chitlin]

How good are you at drawing out circuits? That might be the best plan for something like this if you can't find the right version of a schematic online.

Not a chance.
I'm figuring out how to work on these things with a layout, but schematics still look like something from the wall of a burial room in a pyramid to me.
It's my next personal challenge...

 

[keithb7]

An awful lot of techs “spray and pray” trem circuits with all new parts...Just sayin.

I don’t recommend you do this. Many good folk here can get you through the trem circuit. Unplug the amp and drain any ecaps.

Locate the trem speed and
Intensity knobs. Trace all the wires from both knobs. Where do they go? They connect to other things. Then again, to other things in the amp. Resistors. Caps, a tube, for example. Use a mutimeter set to “beep” for continuity. This will help confirm where the wires are actually going.

A simple “stickman” equivalent drawing, connecting your parts with lines for wires will get you some help here. Post a drawing. We’ll get ‘er sorted somehow.

We won’t laughg at your drawing... Actually we probably will, but it’ll be harmless and not meant to imply belittling.

 

[Bendyha]

Okay, so we don't have a schematic. I would say try drawing one.....not so hard, and more satisfying than doing a crossword puzzle/sudoko/jigsaw...or watching some crap on the idiotbox.

All loctal tube version...late 40's I would guess. Looks like quite a bit of work has been done in there, so we don't know if it has been done right. But what hasn't been changed, is now pushing 70 years old, so weaknesses are to be expected, and other problems are on there way...so yep, you need a schematic. Some of those resistors look like they have disipated there last watt, and the few remaining original caps in the tremolo circuit (those little oblong brown waffels with the coloured dots) where unlikely great when they where new.

Here is a breakdown of what the parts/areas do, (from what I can see).

A pack of more detailed photo-shots would help us identify parts you maybe unsure of.

 

[charlie chitlin]

Okay, so we don't have a schematic. I would say try drawing one.....not so hard, and more satisfying than doing a crossword puzzle/sudoko/jigsaw...or watching some crap on the idiotbox.

All loctal tube version...late 40's I would guess. Looks like quite a bit of work has been done in there, so we don't know if it has been done right. But what hasn't been changed, is now pushing 70 years old, so weaknesses are to be expected, and other problems are on there way...so yep, you need a schematic. Some of those resistors look like they have disipated there last watt, and the few remaining original caps in the tremolo circuit (those little oblong brown waffels with the coloured dots) where unlikely great when they where new.

Here is a breakdown of what the parts/areas do, (from what I can see).

A pack of more detailed photo-shots would help us identify parts you maybe unsure of.

View attachment 603965

Pretty good!
The un-labeled thing in the middle is a tube socket used as a connector/junction, as the controls are elsewhere.
If it helps, the 2 yellow wires (pins 2&4) originate at the trem pot.
There is only one.
I assume intensity is constant and the knob is for speed.
It's probably no great shakes...it would just be cool to figure it out.

 

[robrob]

Replace those two old tremolo caps. That could be your problem right there. Nice work Bendyha.

 

[charlie chitlin]

Replace those two old tremolo caps. That could be your problem right there. Nice work Bendyha.

The micas?

 

[Bendyha]

As I mentioned, it is hard to see in your photo what is going on, a few more details would be a great help to help you, otherwise we can only guess.
But............ what you seem to have, (not knowing what the values are or how the connectivity is wired) around the one tube, is a chain of foil caps and mica caps,and a few resistors, that looks like an oscillator/phase shift circuit.....values could verify the supposition.
The foil caps

(or what seems to be caps a bit like this) are way over their life expectancy, and likely bad, whereas the mica caps

might be okay, but only measuring would let you know for sure. Why there are both sorts present there, and how your particular circuit works can not be judged from the one photo.

 

[charlie chitlin]

Looks like I'll have to crack it back open!
Can I just check them for DC leakage on the - side?

 

[robrob]

Those old caps in the tremolo circuit are your #1 suspect. They need to be replaced and most likely you'll have tremolo afterward.

 

[Bendyha]

Those mica are based on a really old design.

 

[charlie chitlin]

So...I have to study Sumarian cunieform to figure out the value of these domino caps?

 

[corliss1]

Bottom section - https://technifest.com/technical-do...lor-codes-complete-with-mica-capacitor-codes/