Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on September 27, 2024, 05:07:17 pm
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Hi all. Hope all is well!
I am hoping I can get a few more eyes on this amp. It has me stumped.
So, I have been working on removing a lot of the hum inducing issues along with changing out the original can caps.
Amp is very quiet now and strong.
Moved on to fixing the reverb and I cannot find the issue for the life of me.
I have done the following:
1- Checked all voltages on V3. Nothing is unusual.
2- Sub'd in a known good V3. No change
3- Checked the reverb pan spec and they are spot on for a Gibbs type pan
4- Sub'd in another pan. No change
5 - Measured the reverb tranny, Nothing was open. Primary was 620 ohms. Secondary was 280 ohms
6- Checked every single resistor in the send/recovery circuit. Nothing was out of the ordinary
I get a splash when I gently move the pan so the recovery seems fine. I don't think I am getting an input signal into the reverb circuit.
The reverb cable that comes with the amp originally is not obtainable. So i just hard wired the reverb tranny secondary to the terminals and hardwired the pan output to ground/grid of V3...essentially just bypassing the 3 pin plug.
I even found a pic of how the reverb tranny was connected at the terminal so my secondary leads were correct. Of course..I am taking the assumption that amp was correct lol.
Any suggestions would be much appreciated.
Thanks!
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If you own a scope then signal trace from the 500pF through to the input of the tank.
If you don't, put an AC signal into the amp and use the AC volts function on your meter to measure for AC voltage at the 500pF and at the grid and plate of V3a. This will establish at least that signal is getting into your transformer. If you can measure signal into the transformer and replacing the tank did nothing to fix the problem, then that leaves just the transformer as the problem.
I've seen some recommend connecting a speaker to the reverb send to test for signal present at the tank input. I'm unsure what impedance is best for the transformer, so I don't know if that's a good idea in this particular circuit.
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If you own a scope then signal trace from the 500pF through to the input of the tank.
If you don't, put an AC signal into the amp and use the AC volts function on your meter to measure for AC voltage at the 500pF and at the grid and plate of V3a. This will establish at least that signal is getting into your transformer. If you can measure signal into the transformer and replacing the tank did nothing to fix the problem, then that leaves just the transformer as the problem.
I've seen some recommend connecting a speaker to the reverb send to test for signal present at the tank input. I'm unsure what impedance is best for the transformer, so I don't know if that's a good idea in this particular circuit.
Thanks Strato.
No scope where I am at currently and the AC signal testing is it a good idea...just need to create one with what I have in this location/outside the shop. Looks like I can get an app for the signal and just buy a USB-C to 1/4" for the amp. Wasn't aware they had apps like that!
But seeing AC at the grid of 3A and stopping there removes the possibility of it not going out the plate no? In other words, why not test AC at the 3A plate?
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If you have a variac, you can check the reverb transformer with it (sometimes when you check them with a meter, they seem good but are shorted anyway). Hook up the variac to the primaries and set it to zero volts, as you turn up the voltage slowly, the secondaries of the reverb transformer should come up too at some fraction of the input voltage... say 1/10th or 1/20th of the input voltage. When you get to one volt on the secondaries, what you see on the primaries will be the turns ratio. 10/1 or 20/1 or 24/1 or whatever. I don't know what the turns ratio is on that transformer.
If the variac grunts, there is a closed short.
If the voltage doesn't track, there is an open short.
Dave
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If you have a variac, you can check the reverb transformer with it (sometimes when you check them with a meter, they seem good but are shorted anyway). Hook up the variac to the primaries and set it to zero volts, as you turn up the voltage slowly, the secondaries of the reverb transformer should come up too at some fraction of the input voltage... say 1/10th or 1/20th of the input voltage. When you get to one volt on the secondaries, what you see on the primaries will be the turns ratio. 10/1 or 20/1 or 24/1 or whatever. I don't know what the turns ratio is on that transformer.
If the variac grunts, there is a closed short.
If the voltage doesn't track, there is an open short.
Dave
Thanks Dave.
No variac here. I am on vacation and trying to do things outside my shop :)
I have a meter and some basic supplies.
I am not suspecting the tranny because I get a splash when I hit the tank. That tells me that at least half the tranny is working i.e. output is good. Unless I am off in that logic. I suppose the primary could be bad though but I see voltage on the primary and I would think I would not if there's a short/issue?
I don't think the dry signal is getting to the tranny at all OR it is not getting from the tank back into Loudness pot.
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Ok ...solved!
Cleaned the jacks on the tank and reverb came back!!
Appreciate all the help fellows.