Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sonny ReVerb on October 18, 2020, 12:03:51 pm
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So I built a tweed Super variant. It sounds really good, but it has some 60 hz buzz I haven't tracked down yet. The buzz is present mostly at low volume (say 0-3) and louder at high volume (9-12). It seems to be minimal at mid volume levels. I chopsticked it twice and checked the circuit matches the schematic. Still no luck.
My variant has a different power section, similar to later Leo models with a small choke and screen resistors. It also has a ground bus.
One thing I did out of left field - I used a multi-tap OT connecting the 8 ohm tap to a pair of 16 ohm speakers in parallel. The original used a 4 ohm tap and two 8 ohm speakers in parallel. To keep the NFB level consistent with the original, I connected my unused 4 ohm tap to the NFB. I've seen head cabs with multiple speaker outs use a potentially unconnected tap for NFB, so I didn't think this would be a problem.
The other question is do I need the separate ground from the input jack or does this create a ground loop? The original had a single ground wire from one input jack to ground. I put the brass grounding plate in, but it's only a contact ground - no solder connections.
Any suggestions?
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More pics...
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It also has elevated heaters using a pair of bleed resistors to form a voltage divider. Produces around 50VDC.
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This should be accurate except for the NFB connection (4 ohm tap).
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try disconnecting your elevated CT and just ground the CT. noise better or worse?
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shooter! Good call. It's definitely better with the heater CT grounded. Now it's quiet at low volume with increasing background noise (I'd say tube rush with some hum) toward max volume.
I still think it could be quieter with properly elevated heaters, though. I scoped the bleed resistor voltage divider and there's some saw tooth in there. Would a smoothing cap help across the bottom resistor? This is my first attempt at elevated heaters in a fixed bias amp. Any better methods to get a stable DC voltage for this purpose?
I found a thread covering the topic which does show a smoothing cap. They also connected the voltage divider further down the filter chain.
https://el34world.com/Forum/index.php?topic=15932.0
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http://valvewizard.co.uk/heater.html
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It also has elevated heaters using a pair of bleed resistors to form a voltage divider. Produces around 50VDC.
You also need a filter cap across the bottom vd resistor. And you used the dirtiest B+ in the whole amp. Move your voltage divider (and cap) to node C or D and all your buzz will be gone.
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Followup: That was the ticket. I put a 33uF/100V cap across the 33k resistor. The scope showed a nice flat voltage, so I reconnected the CT and it's beautifully quiet. Thanks for pointing me in the right direction, guys.