Hi,
I've got some monster 845 SE amps based on the Grover Gardner circuit. (Parallel 6N7 to triode EL34 to 845, SS rectified CLCLCRC. This isn't my troublesome two chassis stereo version of the same circuit I've posted about in other threads.)
When I got them, one had a low level 120hz hum. After using them for two years, I took them to the bench to replace an intermittent power switch. I got overly excited and replaced all of the power supply capacitors (many, given the series caps), changed the filament supplies changed to Coleman regs with transformers to match, swapped in newer and fancier switches and jacks, tidied up some wiring, etc.
After deploying the parts cannon and bringing them back to life, one amp is still very quiet and the other STILL has a low level 120 hz hum.
It could just be unlucky layout on the one side. They are huge, but maybe not huge enough. Maybe the big ground buss arrangement that is fine for one isn't loved by the other. I'm not going to explore major layout changes or a complete rebuild. I could turn the PS chokes 180 degrees, but I can't imagine that would be much help.
I'll keep staring at it until at least next weekend and check all the solder joints again. I'll review some threads about hum, but I've read a lot after battling many humming tube amps. Other random thoughts?
Paul
I've got some monster 845 SE amps based on the Grover Gardner circuit. (Parallel 6N7 to triode EL34 to 845, SS rectified CLCLCRC. This isn't my troublesome two chassis stereo version of the same circuit I've posted about in other threads.)
When I got them, one had a low level 120hz hum. After using them for two years, I took them to the bench to replace an intermittent power switch. I got overly excited and replaced all of the power supply capacitors (many, given the series caps), changed the filament supplies changed to Coleman regs with transformers to match, swapped in newer and fancier switches and jacks, tidied up some wiring, etc.
After deploying the parts cannon and bringing them back to life, one amp is still very quiet and the other STILL has a low level 120 hz hum.
It could just be unlucky layout on the one side. They are huge, but maybe not huge enough. Maybe the big ground buss arrangement that is fine for one isn't loved by the other. I'm not going to explore major layout changes or a complete rebuild. I could turn the PS chokes 180 degrees, but I can't imagine that would be much help.
I'll keep staring at it until at least next weekend and check all the solder joints again. I'll review some threads about hum, but I've read a lot after battling many humming tube amps. Other random thoughts?
Paul
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For starters, a picture of the troublesome build, a current schematic and any wiring layout diagrams you may have done might be useful for the readers of this thread.
You can also let us know what you mean by ”low level”…i.e. how many millivolts in AC rms (with inputs shorted)? Do you have scope pictures or have you measured the noise/hum at idle (with inputs shorted) on an audio analyzer or bench meter? You should do the same with other amplifier monoblock and compare the two. Then the goal will be to get the offending build as low in hum/noise as the amp monoblock that is “very quiet.”
Best,
Anand.
You can also let us know what you mean by ”low level”…i.e. how many millivolts in AC rms (with inputs shorted)? Do you have scope pictures or have you measured the noise/hum at idle (with inputs shorted) on an audio analyzer or bench meter? You should do the same with other amplifier monoblock and compare the two. Then the goal will be to get the offending build as low in hum/noise as the amp monoblock that is “very quiet.”
Best,
Anand.
This pic shows the leads of the first choke. I won't name the very highly acclaimed (and retired) maker of the four custom chokes in these because we all goof up from time to time. I, for one, spent considerable $$ recapping and many hours revising the grounding layout, bias, and filament supply. But not having the first choke in the PS be a short makes all the difference! 1/10 of a cent of heat shrink and the problem that it appears has been plaguing these amps for around two decades is gone! I should have started with measuring the simple things ....
Paul
Paul
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Thanks. Very long story short (just typed a huge post about the saga and lost it), I feel badly I didn't notice instantly. I knew it had problems, but assumed it could be parts I planned to replace anyway, so I didn't measure much before the teardown.
Once I actually went node by node, the problem was obvious. The offender had the same AC ripple both before and after the choke, so I was not surprised when it measured as a short. Because it is a custom choke at the center of the chassis, finding a suitable replacement would have been hard. (I have electrically suitable options on the shelf, but they wouldn't physically fit.) I guessed it was likely a problem with the leads. With no easy alternatives, I thought I should open it and have a look.
Now, the question is whether to enjoy my success or pull the two-chassis stereo amp out that I shelved after also having a terrible hum post-modification!
Paul
Once I actually went node by node, the problem was obvious. The offender had the same AC ripple both before and after the choke, so I was not surprised when it measured as a short. Because it is a custom choke at the center of the chassis, finding a suitable replacement would have been hard. (I have electrically suitable options on the shelf, but they wouldn't physically fit.) I guessed it was likely a problem with the leads. With no easy alternatives, I thought I should open it and have a look.
Now, the question is whether to enjoy my success or pull the two-chassis stereo amp out that I shelved after also having a terrible hum post-modification!
Paul