Univox 2x12 tall tube amp-japanese

[Mars Amp Repair]

Well,
I am overhauling this old Univox-japanese amp from the 60's. It has reverb, too. No model number. It has 3 inputs (2 low gain & one higher gain). It also has tremolo.

The problem is the input sensitivity is verrry low. With a guitar you have to turn the volume all the way up & you still aren't even 1/2 way to 15watts.

I've replaced all the electrolytics as well as all the oil base caps. I'm 99.99% certain everything was replaced correctly. I also double checked.

Unfortunately, it has been too long since I originally checked it out to recall how it originally worked when it came in.

The amp has 3x 12AX7's. 1 & 1/2 of them seem to exclusively work the reverb section.

For power, it has 2ea 6973 power tubes. It will put out about 15watts at clipping. I have no schemo & have drawn a portion of it.

The first stage 12AX7 has a gain of about 15 if I'm calculating that correctly (Av= Vout/Vin?) been too long to really remember that stuff. Basically with about 400MV on the grid, I get 6volts p-p on the plate. I'm thinking that is wayyyy low.

Another unusual item is the plate voltage. It's about 55Vdc. Seems wayyy low to me, but the 220K load resistor is good & goes to a source of about 200V (when loaded). The cathode is bypassed & has about 1.8Vdc on it. that seems normal to me.

The 2nd stage...the tone circuit recovery...has equally low plate voltage (about 75Vdc) through a 100K plate load resistor that is also good. It terminates to a source of about 150V loaded (that sounds low). That stage has a gain of about 15 (.1vp-p in & 1.5v p-p out).

This stage then feeds the PI, which is basically a gain of 1 ckt using just the one triode section. The other half seems to be the tremolo ckt.

What could I be missing here. Dang, I wish I had paid better attention back in tech school in 1973. They were still teaching tube design along with trasistors, too.

Bruce, any ideas? Thanx, glen

[thomasdj]

Just a wild guess..

200v source (220k plate voltage) seems to be a bit too low. I would expect something like 330 voltage to generate 150v at the plate at V1. Check the rectifier tube/diodes. Use a Matchless spitfire 15w power supply schem as reference.. If this not do it then I believe that the power transformer aren't "good" enough or that the cathode bypass caps have a wrong value or are that the cathode resistors are defect. A higher value cathode cap gives more gain..try 20-30uf

Best regards

Thomas

[MWJB]

What value are the cathode resistors for input stage & tone recovery?

Seems odd that the second stage B+ (150v before the plate R) is lower than the input stage (200v before plate R)?

55v on a 12AX7 plate will be no use to man nor beast. If your cathode resistor was less than 820ohms (it would have to be given the plate voltage vs source) then it is dissipating 1.8V = over 2mA per plate...not very likely given source voltage, if you had a huge cathode resistor, voltage accross it would be higher, but then so would the plate voltage?

Shorted/leaky coupling cap, shorted cathode bypass cap?

Check value of power supply dropping resistors.

I'm sure you already have, but sub the 12AX7.

3x 12AX7, of which 1 1/2 run the reverb, one half runs the trem, so that leaves 1x12AX7 for input, tone recovery AND PI (assuming that it's P-P, not parallel SE)? Even if you had input, tone recovery and one triode for the PI it's not going to be a snarling beast.

[Brad1]

Have you tried to identify and find schematics at the following link?

http://www.univox.org/

Brad1

[Mars Amp Repair]

Hello,
Yeah with a fresh outlook today I guess the best place to start as suggested is the power supply, esp when everything else checks out. Isn't it always ;-]

None of those schemos are even close to this amp, but thanx for the link guys. I think they are the same ones on Schemo Heaven.

To address some of your questions;

The cathode resistor for the 1st stage is 2.2K bypassed. the 2nd stage gets the neg feedback, but fundamentally has a Rk of 1.5K & Rp of 100K.

I'll have to draw the power supply line & determine where the problem is. It's a bit squirrely to follow from the board.

I can see that the supply voltage for these initial stages is way too low.

I'll follow up.

glen

[ldervish]

You're probably way past this by now, but it sounds like the U255-R. Check schematic heaven's "bargain bin" and you'll find it.

[jrfrond]

The "Asoorted Amps" link at Univox.org is dead, and that is where a whole sh*tload of Univox and Westbury amp schematics used to reside, as I had supplied them to Tim Patton for scanning.

If you need the schematic, shoot me an e-mail with the model # and a fax# and I can get it to you on Monday.

[Mars Amp Repair]

thanx John,
I did resolve this issue a while ago. I did have to draw a partial schemo of it. But, I didn't make a note of it in my symptom/remedy book...DANG!

To the best of my recollection I found some resistor in the power supply out of value or possible I replaced a cap wrong...something like that. The preamp power supply was wayyy too low. Thanx again, glen