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My supro is silent... dead silent

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  • #16
    This is a cathode bias amp. The original bias resistor was 200 ohms, which at 412 VDC on pin 3 had the 6L6 tubes idling at 28 watts apiece. This was a little warm for my tastes. I replaced it with a 250 ohm for starters (still on there at the moment) and currently it is at about 420 VDC and dissipating about 24 watts at idle. Still warm AB1 as I understand it, but we'll see. I may eventually go to 270 or 300 ohms.

    I took off the original OT and popped on a brand spankin' new replacement I happened to have (not identical but perfectly suited to this amp) - you guessed it, NO DIFFERENCE!!!!!!

    Something very funky is going on here. I almost feel like I am overlooking something very simple, but if so, I can't see what it is. I feel like I have checked everything, even things which technically should have no bearing on the issue.

    A little while ago, just to see if something wrong in one channel could be somehow bleeding into the other channel and causing both to act in the same manner, I lifted - one at a time - the signal from each preamp stage feeding the mix resistors. Did not make any difference.

    BTW. the tremolo is typical Supro trem as it was somewhat standardized by the early 60s - the only connection to the rest of the amp is via the shared cathode on channel two (one triode of trem tube shares cathode resistor with channel two triode) - I have yanked the trem tube and checked the trem circuit - no change at all.

    While having the amp on and signal going in, volumes full up, I chopsticked the thing to death both preamp and power amp - could not get the sound to vary one iota.

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    • #17
      FWIW, right now, w/ signal going in and volume on "10," (but not much sound at all) on pins 5 I have (V4) 58 VAC and -14.5 DC, and (V5) 56.5 VAC and -8.4 DC.

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      • #18
        To reiterate - it would help to see a schematic and DC (idle) voltages for the plates, screens, grids and cathodes and B+
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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        • #19
          I'll try to finish out the (bad) schematic of this shortly.

          426 VDC right off the rectifier (5U4) - from there, 20uF filter, 1K power resistor, 20uF/10uf filters in parallel to screens (all bigger Supros did this to compensate for the low 10uF in the main can cap), 10K dropper and 10uF to PI and preamp.

          IDLE (volumes both 0):

          pin 3: 423 VDC
          pin 4: 415 VDC
          pin 5: @ 17mV DC, 0 VAC
          pin 8: 34.1 VDC

          VOLUMES ON 10 (little sound coming through speaker):

          pin 3: 377 VDC
          pin 4: 364 VDC
          pins 5: -8.7 VDC and -14 VDC, 56.5 VAC and 58.5 VAC
          pin 8: 60.2 VDC

          I REALLY appreciate the help!!!!!!
          Last edited by EFK; 12-12-2010, 01:41 PM.

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          • #20
            Here is the preamp and PI. Sorry it's not very professional looking!

            (still trying to get the preamp schem to load - the uploader jams on me a lot)
            Attached Files
            Last edited by EFK; 12-12-2010, 02:07 PM.

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            • #21
              Here's the preamp.
              Attached Files

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              • #22
                Thunderbolt schematic

                Here is the Thunderbolt schematic.
                The PI looks identical, other than that 470K to ground off the one leg.
                Attached Files

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                • #23
                  Yep, most of the early through mid 60s Supros used a lot of the same layout with minor cap and resistor changes here and there. Those are some big-a** grid stoppers on the T-bolt, but then being a bass amp, they probably weren't concerned about top end losses.

                  I have never been so aggravated by a problem as this - no matter what I do, nothing changes!

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                  • #24
                    Originally posted by EFK View Post
                    IDLE (volumes both 0):

                    pin 3: 423 VDC
                    pin 4: 415 VDC
                    pin 5: @ 17mV DC, 0 VAC
                    pin 8: 34.1 VDC
                    Looking at those - Pin 5 shouldn't have any DC volts on it. Maybe test the coupling cap going to that grid for leakage. (A leaky cap will throw the bias off in the tube, which will affect the volume)

                    Also get the DC voltages for the pre-amp plates, grids and cathodes.
                    Building a better world (one tube amp at a time)

                    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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                    • #25
                      Honestly the mV DC at idle on pins 5 was fluccuating, and frankly I can usually pick up mV readings just waving the test probe around any amp in the air! The entire PI is now rebuilt, so the coupling caps are new.

                      Preamp channel 1 (single triode) idle/vol. 10 (grid leak bias):

                      pin 1: 102 vdc / 128.6 vdc
                      pin 2: -.9 vdc / -1.31 vdc
                      pin 3: .1 mv / .1 mv

                      Preamp channel 2 (single triode) idle/vol. 10:

                      pin 6: 245 vdc / 211 vdc
                      pin 7: 2.7 mv / 2.7 mv
                      pin 8: 2.94 vdc / 2.53 vdc

                      PI input idle/vol. 10:

                      pin 1: 173 vdc / 142 vdc
                      pin 2: .1 mv / -.9 V
                      pin 3: 1.47 vdc / 1.3 vdc

                      PI second stage idle/vol. 10:

                      pin 6: 202 vdc / 184 vdc
                      pin 7: .1 mv / -.6 V
                      pin 8: 2.07 vdc / 1.71 vdc
                      Last edited by EFK; 12-12-2010, 05:55 PM.

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                      • #26
                        Do you have a signal tracer you can use to verify signal at the output of each triode's plate from front end to PI? If not, you could build Doug Hoffmans listening amp shown on this page:

                        Amp Tools

                        .
                        Warning! Some Electronics devices contain lethal voltages that can kill you. If you do not feel qualified to work with dangerous voltages, refer your repairs to a qualified technician. By giving you online advice, I am assuming no liability for any injury or damages you might incur through your own actions.

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                        • #27
                          Originally posted by EFK View Post

                          Preamp channel 1 (single triode) idle/vol. 10 (grid leak bias):

                          pin 1: 102 vdc / 128.6 vdc
                          pin 2: -.9 vdc / -1.31 vdc
                          pin 3: .1 mv / .1 mv
                          Pin 3 should be showing around 1-2V. Recheck the wiring and the cathode resistor there
                          Building a better world (one tube amp at a time)

                          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                          Comment


                          • #28
                            That channel is a throwback to the 1940s - it's grid leak bias, so the cathode is tied directly to ground (see schematic drawing above). I'll have to check my other Supro that's a grid leak but I'm pretty sure there shouldn't be any appreciable voltage there.

                            This is insane!

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                            • #29
                              Well folks, if you're still with me, today I broke out the big gun. I grabbed another 6L6 amp - another cathode bias amp with no NFB - and split the preamp off from the PI. Then I did the same to this Supro. I've never done this before so I hope I'm doing it right. First I fed the preamp signal from the Supro to the PI/power amp of the good amp: you guessed it, it did the same thing. Because the good amp is a ltpi and makes more power, the 120 hz I was feeding it was amplified a little louder, and one thing I noticed was that on top of the signal was a very high pitched "whine," kind of like the weird buzzing noise a fluorescent light makes, or like a rabid mosquito. It did this through *both* Supro preamp channels. Next, I fed the preamp signal from the good amp into the PI input of the Supro. This is where it gets really wierd: not only did it do the same thing, but there was a bit of the 120 hz signal as well as the screechy mosquito noise *coming through the good amp's speaker as well!!!!!!!!!!* And a weird ticking noise about every second. How can this be happening? There was no signal going to that amp's output! If I "dimed" the preamp volume, the Supro stayed the same but the good amp screeched at me like when the plate leads are reversed. Again - I WAS NOT using that amp's output!

                              What the hell does this mean? I'm only guessing at this point, but I think two things: I think I have a severe case of oscillation occurring somewhere in the Supro, and somehow this oscillation is either occurring in the power supply or the oscillation is occurring somewhere in the signal chain and it is infecting my grounds and thus traveling all throughout the amp. In some way it was infecting the good amp's output section even when it was not connected to anything, so I'm thinking the grounds might be contaminated? Is it possible for a PT or a power supply to oscillate?

                              Could a ground loop cause a problem this severe? Supros are generally grounded willy-nilly all over the place, and the the preamp and power amp are two separate chassis. It's never seemed to cause any problem previously in any of my others, however. While I've now done a lot of work inside, it was untouched when I got it (didn't work then either, plus the speaker had a hole in it...) and I have not deviated from the original layout/schematic.

                              I sure hope someone has some ideas on this because at this point I have hit a wall. I've almost completely rebuilt this Supro at this point, which will be great if I ever actually get it working!
                              Last edited by EFK; 12-14-2010, 12:01 AM.

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                              • #30
                                Originally posted by EFK View Post
                                Is it possible for a PT or a power supply to oscillate?
                                Yes it is - Merlin covers this in Ch 2 of his 2nd book. Is your power supply SS rectified? (The slower switching time of ordinary diodes can interact with the leakage inductance of the PT and cause HF ringing.)
                                Building a better world (one tube amp at a time)

                                "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                                Comment

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