Re: Univox U-1086

Steve L wrote:

Jim,

I picked up the amp today, and have just started to play around with it. My first impression is exactly what you said - not very high gain. In fact, I would describe is as having a fair amount of CLEAN. But I have already seen that if you send your guitar through a booster, it gets nice and nasty. This could be pretty cool.

I have a few questions. Here is the schematic for reference.

http://www.univox.org/pics/schematics/u1086.gif

1. I will be replacing tubes as these are all pretty old.

Old tubes are often the BEST tubes. If they are US, British or German tubes, you may prefer the tone over new Russian, Chinese, Slovak, etc. Even if they are a bit worn.

If they are Russian/Chinese types already, by all means swap them out.



Is there any reason for me to install a tube in position V2? This is the preamp tube for channels 3 nd 4, which I have no need for. I thought I could just leave that spot empty. What do you think?

Sometimes an empty hole can change voltages elsewhere. Just leave an old one in the hole.


Similarly, if I didn't want to use the reverb, could I just leave those two tube sockets empty? The reverb is VERY strong (maybe the adjustment screw on the back will tame it) and is humming a lot at the moment, so its fate is not clear yet.

Same comment. But you may *like* the change, if a there is in fact a change with this circuit.




2. On the other hand, if I wanted to cascade tube stages as you suggested, I could use V2 for that purpose. Perhaps I could use channel 4's half of V2 as the extra gain stage for channel 3. I could take the output of channel 4 at the 470K resistor

the pot?

(removing it from the connection to the other 470K resistors) and connect it to the input of channel 3 at the grid of channel 3's half of V2, which would make channel 4 have extra gain compared to the others. Would I need to do anything else here? Any coupling caps, etc.?

There's a coupling cap before that pot, so that shouldn't be an issue. This is kind of what I've done on my Univox, although being a bass amp, it has some different value coupling caps on the bass side. I still have much tweaking to do. You may want to drop the value of the 1M from input to ground, at your second stage.

You could also install a higher value cap across the cathode resistor to increase gain.



3. What is the purpose of the 1M resistor/200pF capacitor network that is in front of the tone stack? COuld this be killing the gain as well? Perhaps a smaller value resistor would help?

You could experiment there, as well.


4. If I wanted to run the amp at half power, can I just pull two of the 6L6's out? Which ones?

One pair, one on each side of the OPT. But be advised that when I did this on my Univox, I was getting something like 720V on the plates.


If I do this, it will result in a doubling of the impedances at the speaker outs, right? In other words, the 8 ohm tap will now be appropriate for 16 ohm speakers, etc.

Yes.

Or do I have this
backwards? I can never keep this straight.

Thanks,
Steve



Jim wrote:

Steve L wrote:

I may be buying a Univox U1086 PA head from the 70's. I reveiewed the schedmatic and I don't see anything about it that differs significantly from a guitar amp (except for the number of inputs). The guy selling it says he plays guitar through it.

Any info on this piece would be appreciated. Specifically, I wonder about quality of the trannies, caps and hardware. Also, any impressions about the tone when used as a guitar amp instead of a PA.

All comments are welcome.



Gain will likely be lacking. Try cascading two preamp channels at the first gain stage.

I have a U-1061 that I'm playing around with. It's a killer amp.
.

Relevant Pages

  • Re: Mesa F-30 Vs. Express 5/25
    ... the preamp tubes with JJ 12ax7's and it helped a lot. ... also lower the gain structure of the amp but there is plenty of gain ... but this can get expensive if you run the amp several hours a day. ... You might also find that in many cases instead of using the higher gain Lead channel ...
    (alt.guitar.amps)
  • Re: I think I need channel filters - ok fixed it
    ... quite small - starting from those levels you'd need the overall downstream gain to be between -3 and +6 dB to keep to recommended signal levels at the outlets. ... I am driving four outlets via a passive splitter, and have the masthead amp on close to max gain. ... The digiboxes are reporting approx 40% signal strength, but channel quality is at least 85% or better. ...
    (uk.tech.digital-tv)
  • Re: Univox U-1086
    ... I noticed in a Mesa Boogie mark I schematic that I found that the half power switch on that amp simply lifts the ground on a pair of power tubes, ... Perhaps I could use channel 4's half of V2 as the extra gain stage for channel 3. ... This is kind of what I've done on my Univox, although being a bass amp, it has some different value coupling caps on the bass side. ...
    (alt.guitar.amps)
  • Re: mesa express on the 5 watt class A is KILLER!
    ... On Channel 2 (the high gain channel) the 'blues' setting ... move around it's there in front of amp - walk away and it's gone ... when wind down and clean playing through the 5 watts. ...
    (alt.guitar)
  • Re: Parallel stereo = more output?
    ... The output of one channel would see, as a load, the very low output ... lose the advantage you are trying to gain by introducing new losses. ... power doubled then -10%, so no, you get 80% more power. ... amp, it is here that people often get into murky water. ...
    (sci.electronics.design)
Loading