Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on May 03, 2024, 12:26:46 pm
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I've resurrected a 1948 RSA/Selmer amplifer (electrolytic caps, leaking coupling caps & drifted resistors replaced, plus one input adjusted for guitar), and added an isolation transformer for safety. Adjusted schematic below. The amp runs well, but has excessive 50hZ (I'm in the UK) hum which increases as you turn either volume control up, and is also affected tone-wise as you rotate the tone control. I'm currently examining lead dress and grounds for the AC heaters, which are series connected from rectifiers to first amplification stages and then chassis, as I suspect this is the main cause of noise entering the first gain stages.
Physically I have four chassis ground points:
- The safety earth cord is terminated to a bolt of the isolation transformer inside the chassis, and above the chassis (same bolt, different tag washer) I have the ground reference for the isolation transformer.
- The first two filter caps are grounded to tags on another bolt of the isolation transformer inside the chassis.
- The output tube cathode resistor, speaker jack and phase inverter transformer (L1) ground at a tag located on one of the rectifier tube socket bolts.
- All preamp/PI grounds connect to a bus, terminating to the chassis at a tag on one of the V1 tube socket bolts...this is also where the series heaters are grounded.
It's worth noting that the control pots and associated wiring physically have shielding which is grounded to the chassis plate they are mounted on.
I'm assuming that it's probably not a great idea to have the AC heaters running direct to the chassis at the same point as the DC preamp circuit grounds? How would folks advise approaching the grounding scheme?
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Ideally, I would only have two ground points plus the earth safety ground. Three in total.
Probably not possible with PTP wiring but try to keep left side grounds away from right side grounds.
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Thanks, great advice…it is a bit of a challenge consolidating the grounds, but I can definitely divide and group as suggested.
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Well, I’ve done some work on separating and tidying the rats nest of point-to point wiring round the rectification and first two filters, plus separating the grounding points. Also upped the preamp’s 8uF filter to 33uF. Doesn’t seem to have made any improvement in the hum though…I was hoping I’d start noticing some incremental reduction.
I’ll have to dig deeper…the front panel does have the volume and tone controls, plus cap. It’s all shielded cable, but the mains fuse and switch are also located on the panel, so the proximity might not help. However I’d assume that if that was a huge issue the amp wouldn’t have been made that way to start with 🤔
My iso Tx is highlighted with a red ring.
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I had overlooked this earlier... The rectifier is only half wave. That means the B+ ripple frequency will be 50Hz rather than the normal 100Hz we would see with a full wave rectifier. And that means your 50Hz hum could be due to the B+.
Try to determine which stages affect the hum. Does the hum go away when you remove V1 and V2? Does the hum go away when you remove V3? These old tubes could also be the source of the hum.
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Ah yes! I hadn’t thought about that! They’re U31 half waves…I know the hum is altered tone wise by the tone control, and of the two inputs the unaltered mic volume increases the noise most noticeably…I’ll try grounding the stages in turn to test. If I pull tubes it cuts the heater path as they’re series heated.
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If I pull tubes it cuts the heater path as they’re series heated.
Duh huh! Not thinking very clearly today. :BangHead:
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So I’ve continued the lead dress, grounding and layout tidy…with no joy yet.
The rectification and first two filter caps, along with the isolation transformer “neutral/ground reference lead) are to one bolt of the power transformer. Safety earth to another. All amp circuit grounds and thirds filter cap are connected now to a buss bar, referenced to the chassis close to the input. I’ve also lifted the volume and tone ground references from their pot casing to a wire and onto the buss. No impact on the 50hZ hum, but I’m pretty sure behind that it’s helped clean the sound up a smidge.
Turning up (or down) the volume of either channel doesn’t affect the amount of hum, but turning the tone control right down (max treble roll-off) does affect the hum level by rolling off a lot of the hum.
Disconnecting the coupling cap from either V1 or V2 to remove that portion of the signal path doesn’t impact the hum. But grounding the anode of V3 eliminates the hum. So next step is to focus in on V3.
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Try replacing the filter caps
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If the treble control affects the hum, then it must be before the grid of v3.
I would check all connections to the pots. Perhaps clean the contacts where they mount to the faceplate, and maybe check connections between the faceplate and chassis.
Is that shielded cable connected in more than 1 spot? Looks like it could be creating a ground loop between the 3 potentiometers.
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Try replacing the filter caps
Great call…I should have mentioned that I’ve replaced the filter caps with new F&T. The couple caps someone before me replaced with some fairly generic big RS polyester box caps.
If the treble control affects the hum, then it must be before the grid of v3.
I would check all connections to the pots. Perhaps clean the contacts where they mount to the faceplate, and maybe check connections between the faceplate and chassis.
Is that shielded cable connected in more than 1 spot? Looks like it could be creating a ground loop between the 3 potentiometers.
Great call as well…the place to chassis, and pot jacks to read good continuity, but I’m probably going to renew the shielded cable and tone pot itself. I’ve already renewed the two volume pots as they weren’t reading consistently even after cleaning, but I’ll also check that Tone pot too. V3 anode doesn’t have a shielded cover, but that’s original so it never did to start with.