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Thread: My supro is silent... dead silent

  1. #1
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    My supro is silent... dead silent

    This is a 1966 Supro 6497 "combo tremolo" which is a 2X6L6GC amp, 5U4 rectifier, 3X12ax7 in the preamp one of which is used as a trem tube (both sides).

    Originally I thought the OT was bad as I was getting weird readings, however I'm not experienced troubleshooting transformer issues and I may have been making a mistake. I had an extra OT that works perfectly and I stuck that on there, but still... nothing.

    Problem: the amp makes NO sound. No hiss, static, nothing. Power on, and I can hear the PT humming very faintly - AC power in is fine, indicator light works, I have all new caps throughout the amp (filter, signal and otherwise), conversion to 3 prong, all grounds test good, rolled all different tubes, used different speakers, disconnected standby (which is just a PI ground to silence it) - still, nothing. Absolutely no sound. ALL voltages everywhere in the amp are where they ought to be - no funky readings anywhere. I tested all the pots and they all seem fine. This is a two channel/4 input amp, no sound no way no how neither channel. It sounds just like the PI is grounded out, but I've checked it a dozen times and it all seems fine. Tubes are not redplating, transformers not getting hot. To ward off parasitics, I put 2.2K stoppers on the leads from the PI (pin 5 power tubes) and I put a 56pf cap across the PI plates. No difference. This amp is cathode bias, no NFB, no choke. I've repeatedly checked for cold joints, continuity etc. I can't get a pop or crackle or hiss out of this amp to save my life no matter what I touch or jumper etc.

    THIS IS DRIVING ME CRAZY!!!!! Anyone have any ideas?

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  2. #2
    Supporting Member Jazz P Bass's Avatar
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    I did not see anything about a different speaker.
    O/k. The amp passes static Vdc test. Right?
    Try testing it while it is doing something.
    The preamp plates should show a signal on them.
    The grids of the output tubes should show a signal on them.
    No scope?
    Use your meter set on Vac.
    Track it down.

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    The original speaker was garbage (as in, punctures in the cone) so I have been hooking up different speakers - nothing works, and they are all good speakers.

    Static VDC tests all seem absolutely perfect. I can't get it to do *anything* as it will not make a peep. Correct, I am scope-less (hangs head in shame).

    I will go set the meter on AC and check preamp through power amp, see what happens.......

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    Amp on, nothing plugged in and no sound (all inputs are live, however - these are not grounded/switched inputs)

    BTW, this is single gain stage each channel into PI, like a 50s tweed pro.

    V1 12ax7: pin 1(grid leak bias, channel 1) 220VAC, pin 6(cathode bias, channel 2): 556 VAC

    V2 12ax7 paraphase PI: pin 1 385 VAC, pin 6 471 VAC

    Power tubes have 925 VAC on plates (pin 3), @ 914 on pin 4, nothing (0 VAC) on pin 5 input from PI.

    0 VAC on speaker hot from OT.

    Does this mean anything?

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    If I hook my LCR meter into the input of either channel and inject a 120 hz or 1 khz signal, and completely dime the volume knob, I can hear it very faintly coming through the speaker. Very quiet, like it's a mile away. When I do this, the VAC readings on the output tubes drop a bit. Both channels.

    Somehow, I feel that this might be a PI problem. ??

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  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by EFK View Post
    Amp on, nothing plugged in and no sound (all inputs are live, however - these are not grounded/switched inputs)

    BTW, this is single gain stage each channel into PI, like a 50s tweed pro.

    V1 12ax7: pin 1(grid leak bias, channel 1) 220VAC, pin 6(cathode bias, channel 2): 556 VAC

    V2 12ax7 paraphase PI: pin 1 385 VAC, pin 6 471 VAC

    Power tubes have 925 VAC on plates (pin 3), @ 914 on pin 4, nothing (0 VAC) on pin 5 input from PI.

    0 VAC on speaker hot from OT.

    Does this mean anything?
    These measurements are meaningless.
    How can you have 925 Vac on the 6L6 plates?
    What I was hoping for, as you worked your way from the input to the 6L6 grids, is a gradual raising of the signal from input to output.
    It is an amplifier.
    A 50 watt Fender 6L6 will show a signal of around 90Vac, amp dimed, on grid pin #5.
    That is with a 100mv signal in to the amp.
    See what I mean. 100mv to 90V.
    Amplified.

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    Last edited by Jazz P Bass; 12-09-2010 at 11:48 PM.

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    I'm with JPB here. If you can hear the signal from your LCR meter faintly through the speaker, the signal is not getting through somewhere. Bad coupling cap would be my guess.

    What ac voltage do you read coming out of your LCR meter? Follow that voltage through the first stage. What ac reading do you get at the input of the next stage? Keep tracing the signal stage by stage until it dies out, then investigate why.

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    Ooooooohhhhhh! I got you. Thanks very much for explaining that! I'll measure the LCR AC voltage and trace it through. The coupling caps block DC, but should allow AC to pass, correct?

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    Quote Originally Posted by EFK View Post
    The coupling caps block DC, but should allow AC to pass, correct?
    Yes, that's why they are sometimes called blocking caps.

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    You also should check your VDC idle voltages on the plates, cathodes, (and screens) and B+. (VAC readings are meaningless).

    Check coupling caps for leakage which might be affecting the bias of a following stage etc

    You could also check your signal chain for continuity with your Rmeter.

    You could also (carefully) do the 'pop' test on each stage's grid.

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    Old Timer Gtr_tech's Avatar
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    Oh yea....if the original paper/wax coupling caps are still in there....replace them. ALL of them.

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    Well still working on it. I have replaced all of the old "standard" .05 poly caps, however those are not used for coupling in Supros but rather they are actually filter caps hanging off the junction of the 100K/270K series plate resistors, as Merlin explained a month ago give or take. All of the coupling/signal caps in these are ceramic disc, and so far in all the Valco amps I've worked on, probably 20, I haven't found a single leaker yet. Heven't found one in this one yet. The mix resistors for the two channels had drifted pretty far, I replaced them but it made no difference. I'm going to check out the volume pots next, but really, it does seem strange that BOTH channels / all 4 inputs do the exact same thing. If it was a problem in one channel, assuming the mix resistors are good, it shouldn't affect the signal in the other channel I wouldn't think. I get about 8 VAC out of the channels before hitting the mix resistors with .1 VAC going into the inputs. I'm getting about 48 and 55 VAC (these phase inverters are supposed to be notoriously unbalanced) out of the phase inverter to pin 5 of each output tube, with .1 VAC going into the inputs - shouldn't that make more than just a very faint sound w/ volumes at 10?

    I'm drawing out a schematic - badly, but readable.

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    With about 55 VAC on the output tubes (pins 5) that's probably about right for this amp as these 6L6 Supros likely only make about 25 watts. This is driving me nuts!!!!!!! I suspect an oscillation problem, somehow, but don;t know where to look. Given that it is identical on both channels, I feel that it has to be PI through power amp. I rebuilt the PI in entirety a little while ago - no difference. I've jumpered an extra whopping 500 pf across the PI plates - no difference. 2.2 K stoppers on pins 5 - no difference. I'm going to try to clip in much bigger resistors, like 15K, just to see if it makes a difference.

    Anyone have any ideas? Yes this is really how I am spending Saturday night.

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    Just tried 2200 pf snubber caps on the power tube grids ala silverface amps - no difference. I've tried clipping-in 500 pf snubber caps all over the place too - didn't do anything.

    What is siphoning off my output?

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  15. #15
    Supporting Member Jazz P Bass's Avatar
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    I do not see anything about your bias voltage on pin #5 of the 6L6 tubes.
    I did take note that you have a 55Vac signal to the grids.
    If the bias voltage is there, I am leaning towards a bad OPT.

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  16. #16
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    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. #17
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    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. #18
    Supporting Member tubeswell's Avatar
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    To reiterate - it would help to see a schematic and DC (idle) voltages for the plates, screens, grids and cathodes and B+

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    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!!!!!!

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    Last edited by EFK; 12-12-2010 at 02:41 PM.

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    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 Thumbnails Attached Thumbnails Click image for larger version. 

Name:	6497 PI.jpg 
Views:	65 
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    Last edited by EFK; 12-12-2010 at 03:07 PM.

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    Here's the preamp.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	6497 Preamp.jpg 
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  22. #22
    Supporting Member Jazz P Bass's Avatar
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    Thunderbolt schematic

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

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  23. #23
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    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. #24
    Supporting Member tubeswell's Avatar
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    Quote 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.

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    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

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    Last edited by EFK; 12-12-2010 at 06:55 PM.

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    Senior Member bnwitt's Avatar
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    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

    .

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    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.

  27. #27
    Supporting Member tubeswell's Avatar
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    Quote 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

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    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. #29
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    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!

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    Last edited by EFK; 12-14-2010 at 01:01 AM.

  30. #30
    Supporting Member tubeswell's Avatar
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    Quote 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.)

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    No, it's a 5U4 rectifier. I'm going to go through all the grounds first, maybe try to semi-"star" it as best as one can in one of these 2-chassis things.....

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    Improved some grounds to an extent but didn't make any difference. *sigh*

    This is definitely an oscillation issue: if I change the signal that I'm injecting from 120 hz to 1Khz, it doesn't change the problem however at that point I can very noticeably hear a much higher pitch - I don't know how high but definitely up there - coming from the power tubes themselves, pretty loud. I don't need to put my ear right up near the tubes to hear it, it's that noticeable, and it's definitely coming from the power tubes and not through the speaker.

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    I've been worried - after reading a little bit about modular amp sections etc - that when I tried running it through the other amp there may have been some major ground loops present being as I was not lifting ground on one or the other. Anyway, many Supros usually utilize the split amp sections (preamp in top of cab, power amp down in the bottom) and fortunately the cable connections/sockets were somewhat standardized. So, I opening up one of my 1690T amps and tried this again - doing it this way avoided any ground issues, and it worked. The preamp of the "problem" amp made perfect sound into the 1690 power amp, but the 1690 preamp into the problem power amp resulted in the same identical oscillation. So, it definitely appears the problem is in the power amp.

    I have rolled tubes, changed speakers, changed the OT (although the original is apparently good so I'll probably put it back on there eventually) and all of the filter caps. The tube sockets were pretty beat so now it has new power and rectifier sockets too. The cathode resistor and the dropping resistors are new. I have moved wiring around to death.

    The only thing I haven't swapped is the power transformer. I kind of understand what tubeswell is saying above, but since this is a 5U4 rectifier, there are no diodes. Can the power transformer itself be causing this? I guess there's only one way to find out....

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  34. #34
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    Has the amp got global NFB? (If so, try disconnecting it)

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    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

  35. #35
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    Nope, no NFB of any kind.

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