Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Opry Audio on March 08, 2021, 01:22:25 am
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Hey y’all! I went through just about every past thread about reverb circuit humming in the Hoffman PR. Overall the amp sounds amazing, really quiet and after tweaking the trem circuit, I got it sounding exactly how I want.
There’s some hum when turning up the reverb pot. It’s honestly nit picking a bit because it’s not at all loud or overwhelming. I tried a couple of things, ruling out grounding and wrong value of components. Swapped the recovery tube as well hoping a lower gain one would help. Grounding out the wiper of the reverb pot makes the hum go away completely as expected. Removing the reverb tank input the hum persists, but removing the out kills the hum. So it’s in the driver somewhere? Am I on the money about that? I couldn’t seem to find 60hz on the scope at all.. any suggestions for getting this sorted out? I don’t think it’s an issue that would be rectified by shielded cable or anything.. the noise floor without the reverb tank connected is pretty nice.
I didn’t notice it until I turned it up to full voltage on my variac.. even as low as 100v there’s almost no hum. If anyone has some wisdom I would be grateful. I know it’s a common issue. Thanks!
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Some pics of my build.. don’t worry— I went in with my shop vac before flipping it on :laugh:
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Have you tried turning the tank around?
It may be that the pick up transducer is too close to the PT.
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Ground the reverb recovery cathode to same point as the tank output.
That helped in one of my cases.
/Leevi
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Have you tried turning the tank around?
It may be that the pick up transducer is too close to the PT.
Tried that yes
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Ground the reverb recovery cathode to same point as the tank output.
That helped in one of my cases.
/Leevi
Hmm that may help but do you have any suggestions of how to do that with this layout?
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Can you show the complet amp layout ?
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Your tank output is probably grounded on the RCA connector. Check from the reverb recovery tube where the cathode is grounded. Cathode is most probably pin #3 but can be pin #8 as well.
Disconnect the ground end point from the resistor and capacitor and connect them to the RCA ground lug.
/Leevi
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Your tank output is probably grounded on the RCA connector. Check from the reverb recovery tube where the cathode is grounded. Cathode is most probably pin #3 but can be pin #8 as well.
Disconnect the ground end point from the resistor and capacitor and connect them to the RCA ground lug.
/Leevi
I know Doug got quiet reverb using this method of grounding, I will keep your suggestion in mind but I want to try some other things before changing the grounding of the amp / layout.
One thought that I had was that the input of the reverb tank is grounded? Should I try removing that if it’s creating a ground loop?
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Yes, this is only one of many possible causes.
/Leevi
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Just replaced the tank altogether and now it sounds great.. so.. yeah. Thanks y’all! I tried ungrounding the input but it made the tank not work.
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So what type of replacement tank did you use ?
Thanks
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So what type of replacement tank did you use ?
Thanks
I used a belton that I pulled out of an abandoned twin I had lying around. The reverb tank I initially tried was from Mojotone just because it was the cheapest with my wholesale account, but idk.. it’s possible that it’s a different input impedance. I need to check. I checked it out in the twin that was tankless and it was oscillating, so, must be something up with it.
I am working through some other issues as well not having to do with the amp itself exactly? The amp sounds awesome when it’s breaking up but basically every 6V6 I’ve used in it rattles pretty bad. It’s more audible with the more volume. Using JJ’s which I’ve always had great luck with. Maybe I need to get some proper O-rings or look into using tube retainers with rubber padding on them. Not sure if I’m asking too much of these 6V6’s but I thought they should be able to handle it. Could be cabinet vibration as well.