Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: proaudioguy on January 17, 2024, 11:22:45 pm
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So I got the new switched pot in for the intensity. The switch did NOTHING. Turned out it was doing nothing. I already emailed the store in Germany, then I was sitting there about to put the original 50KRA back in and I saw another alpha pot I had with the same switch on the back. I took it apart and move the switch over and voila, it worked. The increase in volume is slight, nothing to write home about. I have a 1M resister between one of the switch contacts and ground to make sure it wouldn’t pop. Whether on or off, there is a 1Meg to ground there. It worked. I had .02, .01 .02 caps on the trem oscillator and it wasn’t slow enough so I pulled the .01 and put in a .02. Now its slow enough but won’t go fast enough so I pulled the 100K resister off the SPEED pot and swapped to a 50K resister which I had pulled off the bias vari board during the conversion. Now the trem doesn’t work at all. So I stuck an 82K in there, but actually, the resister should only affect the maximum speed. I can’t get the trem to come on now no matter what I do. I have confirmed with the speed pot disconnected there is 1Meg from that terminal where the speed pot connects to the tremolo circuit to ground. When I pull out the tremolo shorting plug I am using for testing (because the pedal wires are screwed to the inside of the cabinet), the 1meg goes to open. The 1 Meg combined with the 3Meg plus 100K resister drops that point from ground to speed pot terminal to around 800K ish. Turning the speed pot all the way up takes it to whatever the speed limiting resister is, at the moment that is 82K. So everything appears to be connected still. I am at a loss as to what the issue is. It was working, it was great, now its not working and all I did was try to change the speed limiting resister. Is there a good way without a scope to test the oscilator is still working? What about the roach? Seems like either the oscillator OR the roach is no longer working, but why? What next? I was about to be DONE with this amp and going to play it for a few months (until the normal channel mods commence).
EDIT: when I plug in the shorting plug into the back of the amp the tremolo starts and just gradually goes away. Any ideas?
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I swapped the tube with no joy. Tremolo does start when I hit the footswitch then goes away within a few seconds. Is there an issue having 3 .02 Caps? It was working fine before I swapped the 100K speed limiting resister out.
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Is there an issue having 3 .02 Caps?
Yes. You can get by with two .02s but with three the circuit will not start reliably. If you want a slower trem then replace the LFO cathode resistor/cap with a red LED. Then try the third .02.
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So I went back to .02, .01, .02 and it was working so I tagged a .01 on top of the other .01 and that is also working and the speed it good so its a bit messier but it works. Both sets of legs from the 2 .01s are in the turret holes. Its so hard to fit everything. All the more reason I hate turrets. Every future boards will be eyelets (except the one I just ordered, LOL).
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So I come out this morning and the trem is dead again. I checked everything I could. I changed some oc the caps. FInally I swapped the tube and boom its back. Is this a tube killer? Voltages on the plates are over 400 on V5A and between 300 and 400 on V5B.
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The circuit is not a tube killer. But with three .02 the operation will be iffy. Remove one of those parallel .01s and the circuit will likely be happy. Or do the red LED thing.
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Video
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The circuit is not a tube killer. But with three .02 the operation will be iffy. Remove one of those parallel .01s and the circuit will likely be happy. Or do the red LED thing.
This is what it did to the tube.....
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This is what it did to the tube.....
Very doubtful. That tube has a crack in the glass and allowed air inside.
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Well I got it out of my stash of tubes and put it in the amp, all worked for at least 30 minutes. Went to bed. Got up the next day, it didn’t work. Really wierd. This after 2 other tubes would not work. They would start when I clicked the FS, then stop oscilating. One of those was the tube that was in there doing the bias wiggle all these years.
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One of those was the tube that was in there doing the bias wiggle all these years.
Well, that was before you started mucking around with it.
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The circuit is not a tube killer. ...
This is what it did to the tube.....
There are some JJ tubes whose glass is so thin they break super-easy, perhaps even from heat-cool stress.
However that nice Tungsram tube has stout glass & should have been fine. Unless the glass gets cracked at the pins because of over-enthusiastic effort to insert the tube before all pins are properly aligned with the socket.
The Fender sockets have a little "pip" that align with the gap of the 12A_7 pins. Could help with aligning future tubes.