Tube amp question - vintage univox

Discussion in 'Amps and Cabs [BG]' started by njones89, Mar 15, 2018.

  1. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    I had a professional amp tech put my vintage univox under the bench. I got this amp from a friend and played it only for a few hours before the transformer started smoking. The amp tech recapped the amp and replaced some failing diodes and now it's working again. He got me some EHX 6973s, too. The problem is the output. It seems pretty low for this amp (U255R). He told me the speakers are pretty old and if I replace them with 2 Eminence 12" 8ohm speakers I will probably get a lot more out of it. This is a pretty old amp and everything, but I wonder if maybe it is the EHX tubes? They seem to have bad reviews. What could be causing low output? I notice the output signal is reduced further when the tubes get hot and start to break up.
     
  2. Find another tech.
     
  3. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Yeah, that is half a job. If a transformer actually smoked, may well not be in a good state, but plenty else can smoke. If the transformer is damaged, will show up in various live tests. There are a few places that can make transformers to order if you can figure the spec. If it is mostly intact, spec can be figured, but that is bit more of a mission.
     
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  4. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    According to him the transformer is fine, it was just the diodes that had failed. I didn't run it much after it smoked.
     
    TinIndian and Old Garage-Bander like this.
  5. Much? LOL.

    I killed an amp once.
    Took it to get repaired.
    They replaced a diode.
    That was almost 50 years ago.

    Everyone always thinks it's always the capacitors.
    Bunk! I say.
    It's always the diodes.
    You and I have proven that.

    I've leaned to replace my own diodes.
    I have a bunch of them in my parts jar.
    If you ever need a diode-transplant, I'm your guy. :hyper:
     
    Last edited: Mar 16, 2018
  6. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    Hahaha yeah. I turned it on once to verify which tranny was smoking. Didn't try to play it though.
     
    Old Garage-Bander likes this.
  7. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    It's a 4 ohm amp. I have 2 bass cabs at 8 ohms. If I run that into those in parallel, I should be able to verify if it's the speakers, right?
     
    Last edited: Mar 16, 2018
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Rectifier diodes failing will wreck caps, they don't cope to well with unrectified AC.
     
  9. Not sure I follow where you're going.
    Verify if *what*... is the speakers?

    Yes two 8 Ohm cabs connected in parallel with an amp will provide a 4 Ohm load.

    I assume you are verifying whatever it is you are verifying, after the amp has been repaired?

    Once it's fixed, yes go ahead and hook up both cabs and run the amp through its paces.

    Sorry if I didn't pick up on the nature if your question.
     
  10. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    I mean the amp works fine now, it just seems quieter than what it's supposed to. I just plugged it into an 8 ohm GK cab and since it is a 4 ohm amp, it probably should be even quieter, but it was actually signficantly louder. That may also be because it's 4x10s vs 2x12s... I am guessing he was right - the speakers are garbage.
     
    Last edited: Mar 16, 2018
  11. BassmanPaul

    BassmanPaul Inactive

    I think you are using SS thinking as you say you expected the amp to be louder with a lower impedance load. It doesn’t work that way for tubes.

    If the amp has two output tubes the most it will put out is typically 50W. That’s not all that loud for a bass.

    Edit: I’m not familiar with the 6973 so I can’t give you an accurate estimate of it’s capabilities.

    Driver efficiency plays into the equation too. If the GK driver is more efficient than the original driver(s) it’s going to do more with the amount of signal and therefore be louder.
     
  12. BassmanPaul

    BassmanPaul Inactive

    Having looked at the tube specification it’s a miniature Noval/B9A glass tube. I would not expect more than about 15W output.
     
  13. njones89

    njones89

    Mar 27, 2015
    Sioux Falls
    Sorry that I wasn't more clear, you can use this amp for bass, but I am using it for guitar. So it must be 50w then? 15w eh? A tube catalog I found said they can push 12w each if in push-pull config. I don't know if this amp does that or not though. I wonder if the reason for low output has something to do with resistors on the input before the first gain stage. When I play it with an overdrive pedal it gets really loud with just boost... that's what I'd have to do if I want this thing to break up I guess.
     
    Last edited: Mar 16, 2018
  14. BassmanPaul

    BassmanPaul Inactive

    How much power a pair of output tubes put out is entirely dependent on how they're used. With cathode bias maybe 10-15W but with fixed bias maybe 20W is possible. It all depends on what the original designer was going for.