1965 Super Reverb too quiet, suspect OT?

Discussion in 'Amp Tech Center' started by sds1, Sep 21, 2017.

  1. sds1

    sds1 Tele-Afflicted

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    Thanks! So I understand that the DMM and scope show voltage differently, but can you give me a rough guideline with respect to determining whether the amp's measured output power is acceptable?

    I came up with 30W, you got 50W using the scope's p2p values. That's a pretty wide range with 30W being significantly lower than the expected 45W. How should I approach in the future?

    Yes, thank you very much. That was the end goal here. My friend the owner the other day wanted to know if he should go ahead and pick up some new speakers (they were to be replaced eventually anyhow). I can now direct him to do so with confidence that the amp itself is operating normally.
     
  2. clintj

    clintj Friend of Leo's

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    I usually use the scope and do the math, since I need to run it up to the required distortion level. Then, the data is right there in front of me.

    The meter may not be true RMS, or it may have some side effect of the higher frequency going on in its measurement. The scope, if reasonably well calibrated, is accurate. If a schematic doesn't list a rated power, you can either ballpark it based on similar amps with similar OT, voltage, etc or do a heck of a lot of work with reference materials to develop a number.
     
  3. clintj

    clintj Friend of Leo's

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    If he'd like some nice comparisons, if send him that video. I picked a set of the WGS Vet10 for mine after watching that and polling a few folks. Really great speakers for that amp.
     
  4. Cleeve

    Cleeve Tele-Holic

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    Those speakers are hi-fi woofers and might take the power, but will not have nearly the right response for use in an open-backed guitar cabinet. Foam surround speakers like those are designed to use either a sealed or a ported tuned cabinet as a way to help control the speaker movement, and they are best used with highly current damped amplifiers, that is- those that stereo receivers have.
    If you can borrow a cabinet with real guitar speakers in it, or a combo where the speaker has a jack for the chassis and try, you may find that's all it is.
    In other words, no matter what may or may not be wrong with the amp, those speakers are not right.
    Also, in the capacitor house on the chassis, did you check the resistors between the capacitors? Those have a hard life and drift high after some years sometimes.
     
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  5. sds1

    sds1 Tele-Afflicted

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    Thanks man, that's cool that you were able to ID and provide a little more information on those speakers. They will be replaced, that is the next step.

    As part of this process I tried an 8Ohm Jensen reissue and we felt like it didn't help, but that is a bad impedance mismatch and I'm not sure how that affects the ear test.

    Those were all replaced. Nearly everything but the iron was replaced. That's how I kinda ended up looking at the OT (and now speakers), there wasn't anything left to suspect. :)
     
  6. sds1

    sds1 Tele-Afflicted

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    OK thanks! For a 45 watt amp, what in your mind would be an acceptable range of results? Also, you didn't say -- they way I increased volume until onset of compression, you are comfortable with that approach?

    This was a great video, I passed it along. Thank you. I like those Warehouse speakers, they stand out. The consensus on the YouTube comments was favoring the Webers. My buddy has his sights set on the Weber DT (Derek Trucks) series speakers. They have a cool ABB-themed mushroom on the back label, who cares what they sound like am I right!!? :)
     
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  7. clintj

    clintj Friend of Leo's

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    https://www.allaboutcircuits.com/technical-articles/the-importance-of-total-harmonic-distortion/

    It takes me a few menu steps to get there, but many digital scopes can do a quick THD measurement that's "close enough". Analog either takes an added piece of gear, or the willingness to do some math and measurements.

    What's the goal here, though? You're just trying to establish about how much power an amp is making before the onset of distortion. Comparing the input and output with a 2 channel scope, then ramping up volume until the signals no longer match closely in form, should be a reasonably good estimation. Amp makers can be really optimistic on their power ratings at times, too. If the power is around what I expect for a given type (40-60W for a pair of push/pull, fixed bias 6L6GC with a large OT, for example) I'm pretty happy. If I'm not sure, I have no problem asking someone smarter than me for a confirmation. You're not running an engineering lab or doing certification work, you're just checking general health, is my take on it.
     
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  8. sds1

    sds1 Tele-Afflicted

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    Right exactly, I mean in this case I just wanted to know that the amp is ballpark doing what it should be. I just wasn't sure what the margin of error was on that, if you will. I mean a year ago to be honest I never would have guessed that 20% swing on a resistor would be considered acceptable. I think my brain wants this to be less forgiving than it actually is.
     
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  9. Cleeve

    Cleeve Tele-Holic

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    I looked back at some posts where v1a's cathode bypass cap was taking weird amounts of time to discharge, which made me wonder if the cathode resistor is getting a good chassis ground connection.
     
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  10. sds1

    sds1 Tele-Afflicted

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    Yeah I spent some time on that. It was because I was applying a fairly strong test signal to the V1A grid, even when the amp was off and such. This was keeping that cathode cap charged up to about 1.2VDC if I remember right. Whoops. I am now using a properly attenuated test signal.
     
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