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Author Topic: Univox hybrid PA head  (Read 4489 times)

GEMullard71

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Univox hybrid PA head
« on: March 10, 2010, 08:25:38 PM »
I bought a Univox U-50 PA head a while back (at a steal price) and learned that it was a hybrid; solid state pre with a tube power section.  The amp has two 6L6's plus a 12AX7, which I recently replaced, and has some solid state rectifiers as well.  I was wondering if it was possible to change out the solid state rectifiers in favor of some 5U4-G's.  I've since hooked up the amp, which is 50 watts RMS (more like 40 or so) to a cab, added a Big Muff, and got a really great tone out of it.

GEMullard71

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Re: Univox hybrid PA head
« Reply #1 on: March 10, 2010, 08:35:40 PM »
The one thing that I've been hearing from a few tech friends of mine is that I can substitute the overly clean sound with a tube based pedal that will act, somehow, as a preamp of sorts.  The amp itself is amazingly loud, considering that it's a 1971 PA head, and I have been able to get a really great tone from the Big Muff that I've put in line.  My question is that considering that Univox was a bit sloppy in their amp construction, as far as leads and circuit soldering and attachment, is that should I invest in rebuilding this amp for use as a guitar enabled and tube driven goliath, or should I sell it and buy something that everyone has?

GEMullard71

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Re: Univox hybrid PA head
« Reply #2 on: March 11, 2010, 01:09:08 AM »
I'll say that my amp is nothing compared to a late 50's Plexi or a Metropolis remake but, it still cranks out the tones of a great tube amp when connected to a decent tube preamp pedal, not that the religious ones would bless this. 

phatt

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Re: Univox hybrid PA head
« Reply #3 on: March 11, 2010, 06:18:46 AM »
Hi GEM,
          Not sure I understand what it is you are chasing but you have the best part covered as it uses a tube poweramp section.
Depending what sound you require a lot can be done with the power amp to make it compress earlier.
(remember nearly all tube power stages compress but it's dependant on a varity of factors)
Contry to what you have been told it is very easy to make a SS rectifier section *Sag* like a glass one.
(also far cheaper to do that than rebuild the PSU to run glass)

A Compressing power tube stage is regarded by some as the holy grail of tube tone but unless you play at full volume or close to it then most tube power setions don't get worked hard enough to hear much power stage compression.
The modern tube amps tend to be multi cascaded triode stage distortion units with *master volumes* and rarely do they get pushed loud enough to attain power amp compression.
(IMO Master volume tube amps kill tube amps unless you play meatal thrash ,,in which case none of it matters much as it becomes impossible to tell the diff)

Remember that all that is needed for power tube dist/compression is to just make the in-coming signal *Bigger*.
A dedicated valve preamp might be nice but can be expensive and you may not need it.
Adding a pedal that creates much of its own distortion but hardly increases the signal swing voltage will do little to evoke power stage compresion.

Fet boosters running from 18 volts or so will work wonders for such a situation as they often make the signal much bigger.
With some work you maybe able to evoke a bigger signal from the SS preamp that already exists in the circuit.
Hope it helps you out.
Have fun, Phil.