Multivox Premier 120 combo w/tremolo

[davohilts]

Hello,
I've been rebuilding a Premier 120 with 6v6's, 3-12AX7's, and a 12AT7. In the tremolo circuit there were 2 really strange capacitors, one was a round cylinder like a resistor with color bands brown, black, orange, black and finally blue, the other was about the size of a piece of hershey's chocolate, Micamold Type 345 with the numbers CN43AE503M. Since the tremolo circuit doesn't work I pulled them and replaced the Micamold with a .05 orange drop and the round cylinder type with a .01 orange drop. The tremolo now works but is noisy, it makes a hissing sound as it pulses and I can turn the tremolo beat control and quiet it down but it also kills most of the effect. Any ideas as to what to try, I've resoldered all the connections in the circuit, tested resistors for compliance...

Secondly, the output transformer 4 wires, a common black, an orange, a yellow and a green. I measured the dc resistance on the Jensen alnico speaker and it is 14.6 ohms (16 Ohm). Is there a way to tell which wire coming off the OT is the 16 Ohm lead? Are the colors some kind of standard?

Thanks in advance for any help.

Dave

[soundguruman]

The 16 ohm tap is the one with the most turns, the most ohms to common.
The hiss is common in amps that use a pulsed negative voltage to shut down the output tubes. This pulsing voltage shuts off the grid of the tube to create the tremolo. However optimizing the values will produce less hiss. BUT it will still hiss some, no matter what.
The rectangular cap is a silver mica cap, which has very low noise and leakage. Well, you will never find a silver mica cap with that high of a value, .05. The largest one ever made was probably .02 and usually the value is much much lower than .02
One of your caps is still good, you can measure it.
One of the caps is probably bad.
But there is hope for looking up capacitor color codes, especially in old spec sheets and textbooks.

[davohilts]

thanks for the help!
so if I measure the resistance from the common to each of the three leads the one with the highest resistance will be the 16 ohm tap?
you are right about the silver mica cap, it measures spot on .05 while the other round one should measure .01 and it measures .039 so it is out of tolerance unless it was put in on purpose that way. I'll do a web search for info on reading the stripes to see what it is.
it is too bad that the tremolo sounds so bad, but I guess old doesn't always mean good.
Dave

[Rhodesplyr]

Dave,

I've worked on one of these amps before, the older version with 6SL7s, and I've got one in the pipeline in the shop that's the same version you have.

IMHO, the tremolo in this amp is one of the nicest I've ever heard, and, overall, it's a great little amp when properly rebuilt. If yours has a metal back, try putting some damping material on it to keep it from ringing at certain pitches, and you might want to think about shielding under the amp chassis for a full Faraday cage.

It's a trivial point, but mica capacitors were made in some larger values. There's an online surplus place that has them as high as 0.2uF, but those were probably made in the days before reliable plastic film capacitors. Mica caps generally don't need replacing.

Your other bumblebee paper capacitor has aged and drifted upwards in value predictably and should be tossed. I can read the stripes (Brown, Red, Orange), and I'm pretty sure it was supposed to be a 0.01uF capacitor (10,000pF). I think the blue stripe is the voltage rating.

If the tremolo is noisy, you might have either a noisy carbon composition resistor or a noisy tube. Look for larger value resistors with voltage drops across them like tube plate resistors. Any of the resistors in the tremolo oscillator circuit could be replaced with modern metal film types for better stability and lower noise. It won't mess up your tone.

Try swapping some known-good tubes.

Does hiss increase/decrease if you change the position of the instrument channel volume control?

At the very least, I can tell you that the tremolo on this amp, when working properly, isn't particularly noisy, so long as you aren't judging it by modern high-end audio standards.

[Enzo]

Apply a small signal to the amp, don;t change any settings, measure the output voltage at each speaker tap. The higher the voltage, the higher the impedance. Use them in order of voltage to order them in impedance.


Those flat caps were molded mica - plain old mica - and they also made molded paper caps in that format. The cylinder one was a molded paper cap. The color code reads like a resistor except in picofarads instead of ohms.

Caps had voltage stripes in addition to capacitance and tolerance

[davohilts]

Thanks for the reply! I changed out the 470K resistors and changed the cathode resistor to a 270ohm and this has helped a lot! I have 2 schematics and one calls for a 270 ohm cathode resistor and the other for a 250 ohm. Measuring the mA current on pin 3 of the power tubes yeilded 40mA with the 250 ohm resistor and that seemed high, it dropped to 35mA with the 270 ohm resistor. Mine has a wood back panel with a metal screen panel opening towards the bottom. That's a great idea about adding shielding under the chassis! I need to put the amp back together and play it in the cab but the hissing and drop in volume isn't happening now. There is some kind of tremolo variable resistor adjustment the seems to effect intensity but I can't seem to slow the tremolo down very much so maybe I need to install a larger (1 meg.) speed control?
Anyway it is sounding a whole lot better now and I'm excited again.
I got the speaker impedance lead figured out, its the green lead for 16 ohms to match my speaker.
Thanks again!
Dave

[davohilts]

I got the amp back together, it is a real job putting the chassis and now that I've played thru it I think I'm gonna have to pull it out again. The tremolo is way overdriven and sounds really bad with the 12AT7 as V2. I have another schematic which shows the 12AT7 as V3 and when I switch it out is helps with the overdriven sound and I can see where when properly done this is a good sounding tremolo. The other schematic his this one and they are similar but different. I'm not sure which one to use to attack the resolution of the tremolo issue. Does anyone have any experience with either of both of these schematics??
Thanks for any help.
Dave

model120schematic.pdf

[davohilts]

Okay, I got the amp chassis out and found my problem. I forgot to solder one pin of V4 and when I installed the chassis the resistor came loose from the pin and was hanging in space. I had thought the amp was sounding great on the test bench but when I put it all together it sounded horrible. I quick solder job and then I modified the chassis so the metal chord caddy box next to the chassis doesn't have to be removed to pull out the chassis, this makes the removal and installation much easier. My original schematic I was working from is definitely the correct schematic, but wow, there were several versions of this amp with slightly differing layouts! Thanks to everyone who helped me get thru this one!
Dave

[lowell]

Hi can you please tell me what the stock preamp tubes are in this amp and in what positions? I have one in for repair and the tubes are too old to read. Thanks in advance!

[davohilts]

Hi Lowell,
The first preamp tube, far left closest to the input jacks is a 12AX7, directly behind it (towards the front of the amp) is a 12AT7, then moving to the right is a 12AX7, then a 12AU7. The speaker is 16 ohm. Good luck.
Dave

[lowell]

Thanks Dave. Not sure we have the same amp. All the preamp tubes are in a row... and there is a 6AQ5 and a 6EU7 in here. It's a 120R. Guess the R makes it different.

[davohilts]

Wow, that is definitely not the same amp. Its amazing what a single letter can to to a amp schematic.
Dave

[baddog]

All of those bumblebee caps should be checked for DC leakage. I have a bunch removed from old organs and everyone leaks dc voltage to some existent. Just an FYI

[davohilts]

All of those bumblebee caps should be checked for DC leakage. I have a bunch removed from old organs and everyone leaks dc voltage to some existent. Just an FYI

I agree,everyone I have come across leaks or has drifted badly from spec value

[baddog]

I like them as tone caps in a guitar but don't use them in amps