Looking for some help with an old rats nest amp

[dude]

A friend picked up an old late sixties "Ray Mullin Music" amp. It's a rat's nest but pretty easy to figure out. Just wondering if anyone can lead me to a schematic that's close to this SE ended amp?


It's SE,  power tube 9571,  preamp 12AX7 and tremolo 6AV6 tube, diode rectifier.  Just a volume pot that has on/off switch and tremolo pot on/off regulates speed.   I wanted to use the mic input for a tone knob.  The 9571 cathode bias R is 100ohm (2watt) with no e cap...?


al

[HotBluePlates]

It's SE,  power tube 9571,  ...

Did you mean "7591"? It's about the size of a 6V6?

[dude]

It's SE,  power tube 9571,  ...

Did you mean "7591"? It's about the size of a 6V6?

I'm out to lunch today, yes HBP, 7591. Just trying to figure if an 50/100uf e-cap across the cathode bias R would be an improvement.  Seems to be no e-caps across the cathode R's on the preamp tubes either, just the resistors. I could take a picture of the guts. The only small e-cap is in the tremolo circuit 10uf/50.


Also, want to change the mic input to a tone pot.  The amp sounds ok, all resistors check out ok, the cap can is old but no hum (40,40,10).


The volume pot works ok but the last eighth of a turn goes from loud bright (not as much volume output as my BF Vibro Champ) with break-up just starting to a lot of fuzz, and distortion, although not a bad distortion. Was hoping to get a more gradual increase on the volume to distortion and add a tone control.


al
   

[HotBluePlates]

... Just trying to figure if an 50/100uf e-cap across the cathode bias R would be an improvement.  ...

Not necessarily. It might only allow the output tube to distort sooner, which might not be an issue to solve with a 7591.

... I could take a picture of the guts. ...

That's probably a good idea.

... The volume pot works ok but the last eighth of a turn goes from loud bright (not as much volume output as my BF Vibro Champ) with break-up just starting to a lot of fuzz, and distortion, although not a bad distortion. Was hoping to get a more gradual increase on the volume to distortion and add a tone control.

You may just be hearing the 7591 go from amplifying to overloading.

[dude]

Trying to add a tone knob in the mic input, any help appreciated.


Something like a 5F2 Princeton?

[dude]

I'm wiring the tone pot as in a 5F2-A Princeton.


Just have a few questions: (V1) pin2 grid and pin 7 grid, pin 2 has a 2.2meg to ground (I think I should remove it ?) and run that grid to a .01 cap and to volume pot. Wire the volume and tone pots as in a 5F2A. Run the middle lug of the volume to pin 7. done?


See pic 2, that 2.2meg resistor from pin 2 to pin 3 (the cathode). Just remove it?


http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_5F2A.pdf


al

[sluckey]

See pic 2, that 2.2meg resistor from pin 2 to pin 3 (the cathode). Just remove it?

No. That half of the tube is set up for grid leak bias--- cathode grounded, high value grid resistor to ground, series blocking cap on grid. That 2.2m resistor is very important.

[dude]

So, do I just leave it, from the grid to cathode and run pin 2 to volume and also cathode?


Or do I have to change the way the the tube is biased?


Is it possible to add the tone knob without messing with the basic circuit

[PRR]

> 2.2meg .... remove it?

Why are you even messing with the input of the first stage??

All you need is on the Volume control.

[sluckey]

I don't think adding a tone control that affects only the mic input is a good idea.

It would be very helpful if you would take some time to draw a schematic of that simple amp. Then it would be very easy to make the mods that you want.

I would not modify anything on pin 2 of that tube since that goes thru a coupling cap directly to the input jacks. It would be better to have the vol/tone controls between pin 1 and pin 7 like the 5F2A. It will likely be very simple to add a tone control once we know what the circuit looks like now.

[dude]

I understand, I'll try to draw a schematic, basically to learn.


I wasn't looking to put a tone control on just the mic input but taking the mic input out completely. And using that hole for the tone pot. Wiring like a 5F2, exactly like PRR did. Just that the 2.2meg on the grid was odd to me, I never came across a "grid leak bias" for a 12AX7 and that resistor to the cathode and ground was confusing to me.




Also, the coupling cap from the inputs 1 and 2 (the mic would be disconnected and a tone pot there) is odd, haven't seen that either. But I assume (?) because this amp has no tone control that coupling cap (.022) is for the tone circuit .


PRR, I was only messing with the first stage grid because I've never seen an amp with a resistor on the input grid to the cathode ground...?
I'll have to read up on grid leak bias...?


Reading is one thing, but having someone guide you when you don't quite understand what you just read, is very helpful. For me, I have no one to ask a question when I read an article and don't quite understand, I have to keep reading and figure it out.   


Thanks,
al


I just reread what I posted about messing with the grid, I'm out to lunch again. The 5F2A tone circuit has nothing to do with the input grid. It's the plate that goes to the tone control, not the grid. Don't know what I was thinking.






   

[PRR]

I would REALLY heed Sluckey's advice to trace the circuit to see what you have. Compare to well-known 12AX7+6V6 schematics and mark what is different. Could save a LOT of time and false guesswork.


> assume (?) because this amp has no tone control that coupling cap (.022) is for the tone circuit .

No.

We want to get a part-Volt DC between grid and cathode. The "usual" way is a 1K resistor , bypassed, in the cathode circuit to ground, with a <1meg resistor grid to ground. The "old" way is to ground the cathode, use a >2Meg resistor grid to ground, and block low DC resistance in pickup with a series capacitor.

Both forms are widely used.

The cathode resistor bypass cap was problematic when electrolytic caps failed even faster than they do now. The film cap of the grid-leak almost never fails.

Fender and most others transitioned to cathode bias around the time that extra hot guitar pickups appeared in the 1950s (also e-caps got less bad). Gridleak consistently gives maximum gain but not a consistently high overload level. Cathode bias is almost as much gain with a well defined overload level.

In the context of a "small" amp, both forms are still valid. Often part of the charm of an "old cheap design" IS the "poor" input overload action. Something you don't readily get in "good" (bland) designs. Anyway the world is full of cheap chip amps with excellent clean inputs. The only reason to keep this "old junk" is its imperfect amplification.

Tone control goes between stages, often near the Volume control point. Putting it at the input loads-down the guitar in unpredictable ways (and ways already exploited by the Tone knob on most guitars). Between stages it is consistent and we can predict what it will do (but we already know from 60+ years of one-tone-knob Champs).

[dude]

Thanks for the info and help, I'm able to add a tone pot.


I gave the schematic my best shot, but trying to convert a rat's nest to a schematic is beyond my limits at this time. There's no schematic even close to this one to use as a guide, I've gone as far as I could without help. I'll post what it did to show I tried. I probably could draw a layout much easier.


This circuit was probably made in the early to mid sixties. A guy in RI who owned a music store had these made in the USA to sell in his music store, no info anywhere.


I was thinking of converting it to a 5F2A as this amp only sounds decent when dimed otherwise it's has no sparkle and volume. I've tried different known good tubes, no difference. Checked all the R, all read ok per their bands. I tried a different spk too. I think a 7591A, 5F2A with tremolo might sound decent with plenty of volume.


I'll change the cap can, see if that helps.


al

[PRR]

Compare with 5E1. What is different?

Yes, the mish-mosh between the input jacks and the first grid. Ignore that for the moment.

The interstage between V1a and V1b is likely very-very-similar.

The trem oscillator (other small tube) will have three ~~0.01uFd caps from plate to grid. Something off the stage will inject somewhere at V1.