Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Esquirefreak on November 17, 2021, 02:36:55 am
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Hello!
I just got a AB763 Showman and it's my first solid state rectified amp in a long time.
I thought that while I'll go over the caps and maintenance stuff, I also want to add 1 or 2 thermistors to slow down the current inrush during start up.
Where in the circuit will a thermistor perform its best? In series with the AC hot before the PT primary, or in series with one of each secondary HT?
Regards from Sweden,
Max
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The Ametherm web site is helpful and has the calculations you would make to determine the best unit.
https://www.ametherm.com/inrush-current/transformer-inrush-current.html
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The 65 TWIN REVERB REISSUE (https://el34world.com/charts/Schematics/files/Fender/Fender_65_twin_reverb_manual.pdf) (basically same as AB763 showman) uses a C60-11 THERMISTOR in the primary of the PT. Since you're in Sweden you may look at the Tube Amp Doctor replacement thermistor (https://www.tubeampdoctor.com/en/thermistor-like-used-in-fender.-replaces-c60-11).
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I like them in the live mains feed, eg between fuse and switch.
And in the HT supply, eg between the rectifier and reservoir cap.
Be aware they get hot in use, so allow plenty of free space around them.
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...I also want to add 1 or 2 thermistors to slow down the current inrush during start up....
Why? Are your house wires burning-up?
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Thanks, everyone. I'll probably put one between the mains switch and the fuse as suggested.
As to why - I thought that it couldn't hurt and my understanding is that it is easier on the caps, diodes, PT and pretty much the whole amp. Am I wrong?
/Max
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...I also want to add 1 or 2 thermistors to slow down the current inrush during start up....
Why? Are your house wires burning-up?
As I see it, a significant benefit is being able to use quick blow / F spec HT fusing for solid state rectified supplies, without needing to use a higher value fuse.
Hence my hypothesis is that the exposure to / collateral damage resulting from, fault current from valve shorts etc should be reduced.
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As to why - I thought that it couldn't hurt and my understanding is that it is easier on the caps, diodes, PT and pretty much the whole amp. Am I wrong?
The risk is that you are going to fit a new hot part in to the AC mains primary circuit of the amp - which is the most hazardous location for diyers to work in, especially if there is no convenient set of insulated terminals to easily and safely fit such a hot part. Imho that risk has more chance of hurt than any perceived feeling of making life easier on other parts.