Univox U-152

[joshdfrazier]

So, I recently made a post about adding a SS rect to this amp, thinking it was a U-202. Pulled the chassis and found out that it is actually a U-152. I can't find any information on this amp at all, but it looks very similar to the U-202R.

I've doubled the capacitance (is this safe using the SS rect? I think 50uf is max for 6CA4) of filter and what I presume to be bias caps, and replaced the speaker. Sounds great, with a very noticeable increase in volume and headroom.

This amp does not have a phase inverter. The OT is definitely a PP (8k primary, 15w) but I have no idea how it works. I thought perhaps the 6973 tubes were acting as their own phase inverter (cost cutting feature?) but the amp works with one tube pulled, though the other will red plate quite quickly.

Also, I only have the current production EH 6973 tubes and have read that they are horrible and contribute to volume loss and generally "bad" sound. To my ears, the amp sounds good, but I would love more volume out of it to use as my primary gigging amp. I can't justify dropping 100 dollars on a pair of the RCA's only to find that I can't tell a difference.

Power tube voltages, both seemed to be wired in parallel:

B+ 396
Grid 1 = 29mv
Grid 2 = 328vdc
Grid 3/cathode = 24vdc
Plate = 393vdc

[cbass]

what makes you think it aint got no inveter? Most them univoxes had cathodyne PI's

[HeeBGB]

What's the tube compliment? I have a U-305R that I think uses the 12AX7as a PI

[cbass]

What's the tube compliment? I have a U-305R that I think uses the 12AX7as a PI

by the pic it looks like its all ax7's and 6973 with a mullard in v1

[joshdfrazier]

it came with all mullards, but they are a little too gritty for my taste. v1 and v2 are preamp tubes, v3 is for the tremolo circuit. all 12ax7.

then the two 6973, and what would be an ez81.

what's a cathodyne PI?

[cbass]

what's a cathodyne PI?

http://www.valvewizard.co.uk/cathodyne.html

[Stevem]

You've gained some 30 or more volts of B+ with the SS recto so I would check the voltage rating of the first power supply filter and make sure it is not getting slammed with too much voltage before the tubes start to conduct!
Also I would leave the old recto tube in there just of the sake of keeping the filament voltage in line.
Any original .001 oil caps that have not failed yet, likely soon will so I would plan on changing them all out along with all the other oil caps in the amp.
On 95% of the tons of these type univox amps I have worked on, the tremolo was dead due to the 3 .001 caps ink that tube sections plate circuit.
These amps only need a foot switch to turn the trem off.
Also the tube sockets used in these amps are pure crap so when one fails you might as well change out all the others, the heat from the output tubes kills those first.

[joshdfrazier]

Actually, the voltage increase is negligible, really. Only about 15-16vdc increase. The amp appears to have been (almost) totally recapped with orange drops and other polypropylene caps.

Also, I think I located the PI. I suspect it is v2. There are two leads from pin3 (grid 1) of each power tube, which i traced back to this ax7. When the tube is pulled you get absolutely no sound, complete silence. So I guess there is only one preamp tube (v1), leaving v3 to the tremolo circuit.

Also, the 6973 tubes begin to redplate slightly after about an hour of use. I've read that the sovtek el84M is capable of handling screen and plate voltages up to 500vdc, so I may move a few wires around and go that route. Is there any way to squeeze a little more clean volume out of this amp?

[Stevem]

The color coding of the wires in these amps makes it easy to keep track of grids and plates and such without counting pins, it's real nice!

Also I have worked on a bunch of UNivox amps over the years and 6 of them( two of which where mine) had the output tube cathode bypass cap installed backward from the factory( I do not know how they never blew!) so make sure that whoever replaced that cap did not just follow what might have been a wrong factory install.

The Jensen speakers in these amps sound great, but are very low in SPL rating. And a better driver rated up in the 98 to 100 DB range will make for far more of a volume increase than anything you could do circuit wise.

Converting the amp to fixed bias would provide a bit more clean head room and over all power.
Two speakers will give you twice as much volume and twice as much clean head room!
6973 tubes are far cleaner sounding than any EL84 ever will be, as that is what RCA made them to do.
The. EH tubes sound brash to me when pushed into clipping and they have 1/3 the life of the N.O.S stuff and fall prey to going microphonic a lot.
If I had the extra green in my wallet at some point I would buy a set of N.O.S tubes before they dry up forever!
Does the red plating follow that tube if swapped, or stay with the socket, if so it's time for atleast cleaning / polishing that socket by passing a big enough round wound string in and out of each female, and then applying a light coating of WD40 or contact cleaner with lube by means of a Q tip to keep corrosion at bay.
Be sure to drain the amp before you do the string cleaning thing!

[joshdfrazier]

Fortunately, my cathode bypass cap was not installed incorrectly, so no worries there. The speaker had to be replaced, as it was producing a really nasty distorted/rattling sound as if the voice coil was rubbing on something it should not have been. Then speaker I have in there now is much more efficient, but I feel as though I could do better. Thinking about the Red Fang (might be too large) at 103spl. That seems to be the "loudest" speaker available that I can find!

Fixed bias... Now you've got my attention. currently the amp uses a 300ohm wirewound and a 100uf cap in parallel with the cathode/grid to ground to get the 24vdc. How would I go about converting it to fixed bias?

[JazzGuitarGimp]

+1 on the fixed bias mod suggestion - it will also help with the red-plating issue, I imagine.

[joshdfrazier]

Is it as simple as grounding the cathode and creating a negative voltage source? And would I apply that negative voltage to grid 2? Am I doing too much guessing?

[Stevem]

For fixed bias you need to pull B+ off at the OT center tap, pass that thru a rectifier, ( and 1n4004 is plenty) filter it( a 100 uf @ 100 is good) and then bleed that off to a range of about -21 volts through a resistor to ground.
From what I can make out from the tube spec chart at the Duncan amps site that - 21 volts looks about right.
Ground those resistors like you posted and make sure you wire up the diode and filter for negative voltage.
If you start off with About - 26 volts coming out of your bleed resistor and then adding a 10k or so pot you may have just about the right range to trim that - 26 ( real cold, but safe bias) down to - 21.
So you can measure the actual idle current, place a 1 ohm, 1% 1 watt or better resistor in series. With each tubes cathode, this will let you place a meter set for dc across that resistor and directly read the ma current the tube is pulling.
There is tons of bias circuits you can copy on web to implement for this.

Yes the red fang is a great sounding and loud speaker as I have two of them in my 66 silvertone 1484, so much so that I had to change out it's for crap 1/4 inch Masonite baffle for. 1/2 plywood braced reinforced one!
Sounds killer though!

[joshdfrazier]

Hey thanks for all of your help steve. I made my usual ax84 style bias supply on a breadboard, grounded my cathodes with 1ohm resistors, and yeah, huge difference. I also managed to snag a pair of RCA 6973 (REAL 6973's, not the weird subs) on ebay for 44 bucks. I believe that made a difference as well. I'm wondering if I should put a larger OPT in there now? I don't have a way to accurately calculate wattage, but it seems like 15w is a little underrated for the amount of volume I'm getting now.

Also, each tube is biased around 18/19mA which is about 60% plate dissipation for a standard EL84, I'm not sure how close the 6973 tubes are to el84's in design

Edit: actually, I intended to build a 50watt 6l6 amp in a similar chassis and pop it in this amp to get the clean volume I wanted. I'm wondering if I could just pop in some octal sockets, adjust the bias circuit and buy a new opt to get the volume I want and use el34/6l6. I know I probably won't get close to 50 watts with the current power transformer, but it would save $$. I got a 1.5 amp 6.3v tap on the transformer, I know this is probably pushing it. Got a 2 amp 9v thrift store tranny I could mod as well. Thoughts?

[Stevem]

It would be cheap enough to step up to a Hammond 125 series OT, and even their 15 watter is over built when compared to the amps original OT!

Yea, if you sub in a additional PT just for the filaments than the original PT would put out a smidge more current for the secondary ( MORE CLEAN HEAD ROOM) and run cooler while you are driving it harder.