Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on November 22, 2025, 04:09:15 pm
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I have a tweed bassman reissue PT that put out 491 B+ loaded in the reissue circuit.
The owner wants it stripped and a true 5F6A bassman circuit put in. Original schematic shows 325-0-325 PT w/about 435 B+.
He wants to stick with this PT for $$ reasons.
Given fixed bias what would the best way, if any, to put in a 40-50V zener diode? And are there any other considerations to think about? I have very little experience with zeners.
Then again, I'm sure original 5f6s run super hot at today's wall voltage so they are probably not far from these numbers.
Thanks!
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You could make a VVR circuit to knock the B+ down, with a zener and a mosfet and a couple of other parts - in a way that wouldn't affect the bias supply tap
Or, instead of a zener, you could try some chunky limiting resistance in series between each HT winding end and each rectifier plate. This wouldn't affect the bias voltage tap either. Experiment with the amount of resistance. e.g. 'whatever mA x whatever resistance = VAC dropped'. But, this will create some sag under heavy chord conditions (which may not be a bad thing depending on playing style), but it you don't overdo the limiting resistance it might not be that noticeable.
Otherwise add a bucking transformer for the HT winding (more expensive and eats up some chassis real estate). This can be done without affecting the bias supply tap.
Or change out the PT
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Thanks for the suggestions. Yes a new PT sounds easier but not to be I don't think.
Do you have a schematic for the mosfet circuit? I know less about them than zeners😂
Thanks again.
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Do you have a schematic for the mosfet circuit? I know less about them than zeners😂
Lots of stuff on the net about this - Google is your friend.
Attached example uses a power resistor instead of a mosfet -0 and will affect the bias supply tap as well, but you get the idea. There are plenty of examples using mosfets out there which won't affect the bias supply.
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Checkout this thread: https://el34world.com/Forum/index.php?topic=33012.msg366809#msg366809 (https://el34world.com/Forum/index.php?topic=33012.msg366809#msg366809)
I submit that you can use the tranny provided; and bias the power tubes for that voltage. It appears that actual amp components & voltages of that general era did not necessarily conform to the published schematics. The voltages in my Princeton 6G2 were way high. Hotblue convinced me to leave it that way. Also, e.g., the cathode bypass cap on the input gain stage was 100uF instead of 25uF per the schematic.
If the customer doesn't want to pay for a new tranny, then why incur the costs of parts and labor to adjust this tranny's voltages? The voltages you have will work.