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Hi, I just recently replaced a friends power transformer on a 1974 JMP Lead & Bass 50. The B+ voltage is 448VDC. I set the bias with the cathode method to 38mV. With the guitar cord inputted to the #1 jack, the two volumes down, you can still barely hear a strummed chord. With volume turned up to 1 there is sound and all controls work. As soon as you turn the volume up past 1 and hit a chord the amp will get louder and the speaker will start distorting/oscillating and jumping up and down in the basket, even after I mute the strings. Nothing will stop it until I put it in stand-by. I swapped out the tubes, no difference. I cleaned the contacts on the input jacks, no difference. Any thoughts would be appreciated. Thanks.
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If the cathode resistor is shared on both tubes, 38mV is too low.
About the low frequency oscillation, whatever took out the old transformer may still be in there. Check the reservoir cap and power supply resistors, if they're really hot then you still have a problem on the PS. -
Microphonic tube? Is the amp physically separated from the speaker cab?
If it's definitely electrical rather than microphonics, as a sanity check, try removing the negative feedback resistor from the OT secondary.
If that doesn't help then look to the B+ caps; try paralleling up a known good one with each in turn.My band:- http://www.youtube.com/user/RedwingBandComment
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Thank you for the input. I measured the output tubes with a 1 ohm resistor from pin 1&8 to ground. The tubes are not matched very well, V4 is 30mV and V5 is 39Mv. I replaced all the 50uf-50uf-500VDC filter caps and the electrolytics on the board. I suspect the signal Mustard caps on the board, but I really didn't want to change them unless absolutely necessary. I started to do a pin voltage test on the preamp tubes, V1 pin 1 is 158Vdc, as soon as I touched V1 pin 6 the speaker started screaming and vibrating up and down rapidly. This is with nothing connected to the input jacks and the volumes down. Yes, the speaker is separated from the amp. I'll disconnect the feedback circuit, thanks. Any other thoughts?Comment
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Touching pin 6 with a multimeter and getting noise and even oscillation is normal in a hi gain stage. Do you have grid stoppers on the output tubes? (A resistor in series with the signal going into pin 5)?Originally posted by BStringThumper View Post
Do you have a cap between the two plates of the phase inverter? Sometimes it'll stabilize the amp. Are the tube sockets all tight and socket connectors clean?Comment
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Hi. I don't have any grid stoppers on the output tubes pin #5. No cap on the P.I.. I did retighten and clean the sockets, but they are old. The speaker oscillation is quite violent, nothing I've ever seen before. I will pull the coupling capacitors and test them. Thanks for the help.Comment
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Grid stoppers may help. The cap on the PI plates will only stabilize a higher frequency oscillation. Adding grid stoppers will change the tone so slightly you may not even notice, and it may solve your problem, give it a tryOriginally posted by BStringThumper View Post
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Here are some voltages. This is with all controls set at zero.
Pin# 1 2 3 4 5 6 7 8
V1 180 - 1.6 - - 160 - 1.6
155.2 1.25 - - 163.4 1.19
V2 140 - 1 - - 250 140 140
147.2 - .994 - - 261.1 147.3 148.7
V3 220 - 40 - - 210 - 40
208 - 35.1 - - 205.8 - 35.5
V4 - - 380 390 -30 - - -
- - 450 445 -39.9 - - -
V5 - - 380 390 -30 - - -
- - 450 444 -39.9 - - -
At volumes below 1 the amp works, if set the volume at 2 and hit a chord the speaker will start oscillating wildy. Muting the strings will not stop it. If I turn all the controls to zero, or put it in standby it will stop. Strange.Comment
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Hi JMAF. I tried to import my Excel spreadsheet and this site did not like it. I had to input all that data one line and space at a time. I had to delete the first couple of tries before I gave up on the spreadsheet. That's probably what you saw. Thanks again for all your help. I've never seen a problem like this.Comment
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OK. For a moment I thought I'd been at work for too long today, seeing things...or maybe I have beenOriginally posted by BStringThumper View Post...
Don't sweat, your problem is not strange at all, it's common in fact. I hear it going into oscillation in my head after an open E chord right now. I think the last time I saw that was the negative feedback was inverted.
Those Marshall's are tuned to be stable at the limit, if you change a single thing, you've got an oscillator. Yours seems to be classic case of this....you increased the plate and G2 voltages and the amp went unstable.
First of all, did you try what pdf64 suggested? Removing the NFB? Before I sign off, here is another simple test you can do: remove V1. Turn it on, does it still oscillate? IF it does, it's actually good news, you've isolated the problem farther ahead. Take off V2, does it oscillate? And so on. IF it keeps oscillating up until the inverter, it's probably the negative feedback problem.
If it stops oscillating after removing V1, then it's a global problem and you've got to mod the amp to compensate for the higher power tube plate voltage : reduce some gain elsewhere, add caps, etc...
PS. Your voltages look fine to me, only thing I noticed is you increased +B and G2 voltages, which will change the tone and the response of that output stage. Unless I'm missing something.Comment
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Hi JMAF, Thanks for the suggestions. I pulled the NFB wire from the presence pot. The amp lost volume/power. But, I had to turn the volume up to 4 before it would start shaking the speaker when I hit a chord. But it wasn't loud at all. Just to clarify, when the guitar is plugged in, I'm muting the strings, I can turn the volume up without the speaker starting to oscillate. If I turn the volume to 1 and hit a chord it rings out normally, but if I turn the volume up to 2 and hit a chord the speaker will start violently jumping in and out. When I mute the strings it does not stop, I have to turn the volume back to zero, once that didn't work and I had to flip it to standby. The presence knob does make a difference to when it freaks out. The higher it is the quicker it freaks out. I'm thinking I better recheck the grounds since the NFB made a difference. I'm confused by your suggestion to remove V1, there is no signal without V1. I pulled it anyways. The amp is quiet, no sound from a chord either. The amp is pretty quiet with everything hooked up. Does this still sound like oscillation??? Maybe this is not the problem? Thanks again for the help.Comment
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Sounds exactly like what amps do with no grid stoppers on the output tube control grids. Toss some 5.6k's in series with the grids and I bet the problem ain't a problem anymore....The farmer takes a wife, the barber takes a pole....Comment
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The farmer takes a wife, the barber takes a pole....Comment
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Hi JMAF, GTR_TECH, PDF64:
Thank you all for the help. The amp is now working!!!! YAY!!! Here is what I did. JMAF, when you asked about grid stoppers................I should have put them on right then. I was going by the factory wiring schematic. I'm rebuilding/restoring this amp. I didn't want to modify it. Then GTR_TECH said to put them on. I luckily found some kicking around. I put them on. What a difference. I've never heard this amp before, it's really clean, not much gain. So again, thank you everyone. Regarding the 5.6k resistors. I have two watt metal oxide resistors there now. Do you have any recommendations? I want to replace them with new ones. Right now I have two different brands installed. Would 5.9k resistors work without changing the sound much? You guys saved the day.Comment
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