Univox U1000 repair/modification help

[Lucaswaggle]

Hi i have a late 60s Univox U1000 (schematic below). It uses 6l6gc tubes but i want to swap them for 4 kt88's. I know i need to replace the transformers for this which is ok because the power transformer is already blown. Just looking far advice on how to do this mod and what transformers would work.

[PRR]

> EL34 ..... I know i need to replace the transformers for this

Should work fine with stock iron. (If it were healthy.) Check that pin 1 goes to cathode, at smoke-test quickly read cathode current and if over-high shut down and change bias. 35-40mA (0.35V-0.40V in the 10 ohm resistors) is a fair goal.

"80W" iron (Marshall or Fender) would be a fair replacement for burnt iron.

[Lucaswaggle]

Im going to be perfectly honest... i dont know what most of that means. Ive been lookig all over the internet for a way to learm more about tube amps but everythjng ive found is either too advanced or is really basic. Whats a good place to start to understand everyrhing that youre talking about?

[sluckey]

Im going to be perfectly honest... i dont know what most of that means. Ive been lookig all over the internet for a way to learm more about tube amps but everythjng ive found is either too advanced or is really basic. Whats a good place to start to understand everyrhing that youre talking about?

Start here...

     https://robrobinette.com/Generic_Tube_Amp_Mods.htm#EL34

Then scroll to the top of the page for links to a whole lot more info.

[Lucaswaggle]

Thank you very much. Also im considering going with quad kt88 tubes. I know that id need to change a lot for those but im leaning more towards it. Also any mod suggestions for improved sound are appreciated. And id like to put bias knobs on the outside of the amp to make biasing easier.

[Williamblake]

Speaking for myself, you already came to rhe right place. And there is Uncle Doug on youtube. And the valve wizard. And Aiken amps. Also check _everything_ (DC voltages) and write it down so you can compare and learn from your mistakes. Never estimate something will work just because, because with tubes it even might. Have the circuit you are working on drawn (expressSCH), with Spreadsheats you can enter you voltages into and print as PDFs so you can see what has changed and to not do the same calculations again and again. Might seem like a lot of work but saves time and lets you see what is right and whats not before something smokes. To me Open Office is a great tool, dont write voltages on paper you dont understand a week later.

[sluckey]

And id like to put bias knobs on the outside of the amp to make biasing easier.

That's kinda like praying for a tornado at your front door!

What do you hope to gain by changing tube types? From the discussion thus far I get the idea that you're really not up to the task. So, soak up all the knowledge you can before jumping in half-ass prepared. That includes knowing what to do if things go badly. You only get to burn down the house once. 

[Lucaswaggle]

And id like to put bias knobs on the outside of the amp to make biasing easier.

That's kinda like praying for a tornado at your front door!

im sorry but how is that like praying for a tornado? the amp needs major repairs already so i might as well take the extra time to make it as good as i possibly can. trim pots and external knobs for biasing are fairly common these days so im afraid i dont see the issue. and yes my experience is limited but thats why im here.

[sluckey]

external knobs for biasing are fairly common these days

I may have fallen behind these days. Can you give an example of an amp with external knobs for biasing?

[nandrewjackson]

How is it built from factory? PCB? Turret/eyelet board?
How do you plan on rebuilding? Turret/eyelet?




What is your experience level? Would this be your first big amplifier modification?


That appears to be a fairly complicated schematic, in the scheme of things, wordplay intended.


You said power transformer is no good, that's $$ a big piece of the puzzle right there.

[thetragichero]

with a new power transformer you'll have to figure out where the tremolo is going to get it's b+ from as it likely won't have your violet wire
what are your goals with changing power tube type? not saying it can't be done, just that maybe it shouldn't be done (without a compelling reason). keep your first repair simple

[Lucaswaggle]

How is it built from factory? PCB? Turret/eyelet board?
How do you plan on rebuilding? Turret/eyelet?




What is your experience level? Would this be your first big amplifier modification?


That appears to be a fairly complicated schematic, in the scheme of things, wordplay intended.


You said power transformer is no good, that's $$ a big piece of the puzzle right there.

yes, this will be my first major amp modification. but im up to challenge and im eager to learn how to do it. also im not overly attatched to the amps current circuit (ive never even heard what it sounds like and videos of it are scarce) so perhaps a complete rebuild would be more efficient?? again all of your advice is greatly appreciated.

and my reasoning for the kt88s is that i adore their sound clean and dirty, which i cant say for any other tube type. and i know that quad kt88s is overkill but i want the extra volume on tap.

if it is in fact easier to rebuild the amp to the specs i want, here are my requirements in case anyone has a good suggestion. i need bright and normal channels, 3 knob eq, presence control, spring reverb and trim pots for biasing. And for any other modification or new circuit suggestions my favorite tones/amps are David Gilmour's Hiwatt DR103, Mick Ronson's Marshall Major "The Pig" and tweed fender bassman's. i want good cleans and lots of bark and midrange.

hopefully thats all coherent, and thank you for all the help

[Lucaswaggle]

How is it built from factory? PCB? Turret/eyelet board?
How do you plan on rebuilding? Turret/eyelet?

this is how i purchased the amplifier, im not sure if its been modded or repaired previously.

[nandrewjackson]

I don't know the value of that amp as it sits, needing repaired, but, I'm thinking if you shop it around, you could sell it how it sits, and use the money to start a scratch build of what you want to have.

[Lucaswaggle]

I don't know the value of that amp as it sits, needing repaired, but, I'm thinking if you shop it around, you could sell it how it sits, and use the money to start a scratch build of what you want to have.

im beginning to feel the same way. any recommendations on circuits/amps to copy? perhaps a part list if anyone is feeling generous?

[dwinstonwood]

Please have a qualified tech check over the amp before powering it up.

The installation of the 3-prong cord was not finished.

There may be other connections that were left dangling, too.

[thetragichero]

I don't know the value of that amp as it sits, needing repaired, but, I'm thinking if you shop it around, you could sell it how it sits, and use the money to start a scratch build of what you want to have.

im beginning to feel the same way. any recommendations on circuits/amps to copy? perhaps a part list if anyone is feeling generous?

if you're in the us i might be interested in purchasing a broken amp from you. i've got a friend who is a fan of these old univox tube amps (the cabinets certainly looked cool)

[Lucaswaggle]

id love to talk with you about selling it. is there a private message feature on the forum here?

also i have a black univox u1226 head id sell

[PRR]

I had a six 6L6 version. That WAS a very good, albeit tame, amplifier. This example has work done. The OIL caps are original but I have doubts about the small resistors (which were fine parts). The main filter caps are new, to be expected, but not a fan of bathtub caulk mounting. The 5W resistors on the octals must be new (the Xircon brand was not invented yet)-- why were these replaced? Typically a total melt-down of the power stage. Has that issue been found and fixed? And YES that BARE wall-ground wire hanging near 490V lines is begging for real trouble (lethal voltage on chassis).

[PRR]

...is there a private message feature on the forum here?...

Click the "word balloon" under the name of the member you want to contact.

Very new members may not have this power (it can be used to spam in secret). You may have ask the other member to PM you.