Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on July 14, 2021, 09:52:15 am
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A friend brought me this and wants me to restore to original before he looks into blackfacing or modding it. Someone has replaced ONE of the original 20µ with an Illinois cap. Regardless, they're all getting replaced. The amp is making the frying bacon sound and this is likely coming from the 1/2w cc plate resistors? Although V1 & V2 have 2w 100k cc's.
Is it just good practice to replace the Mallory 25µ cathode caps?
Also, this amp has MAR 1972 stamped on the inside of the chassis and it has a 3 prong power cord AND the death cap. Should I still clip the cap out? Or is it not so deadly with the 3 prong cord?
And finally for now, the vibrato bug is damaged as in the lamp and the sensor are still soldered in but the heat shrink is gone-almost like someone deliberately removed it. Is it standard to replace the disk caps when replacing the bug?
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I think you can clip out the death cap or, as alternative, remove it and install an Y2 capacitor
the Y2 capacitors if broken didn't short, they will go open
https://www.mouser.com/ProductDetail/KEMET/C961U472MZWDBA7317?qs=xIpY4gsFVQFPciK2K3J94Q==&mgh=1&vip=1&gclid=CjwKCAjwlrqHBhByEiwAnLmYUCpmSRAS8U5lYAxSTOAdd1tVUT9PtK3VAUF4rM1LOiO1cY06CMDTvRoCHZ8QAvD_BwE (https://www.mouser.com/ProductDetail/KEMET/C961U472MZWDBA7317?qs=xIpY4gsFVQFPciK2K3J94Q==&mgh=1&vip=1&gclid=CjwKCAjwlrqHBhByEiwAnLmYUCpmSRAS8U5lYAxSTOAdd1tVUT9PtK3VAUF4rM1LOiO1cY06CMDTvRoCHZ8QAvD_BwE)
(https://www.mouser.com/images/kemet/images/C9-Z.jpg)
Franco
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You could replace the preamp plate resistors with 1W metal film. Replacing the cathode bypass lytic caps is up to you. I usually replace them as they are pretty old. You want to replace the bias lytic cap for sure, a failure there would not be good. 47uf 100v would be good. I make my own trem bugs using a Morley LDR and a bright white LED and then heat shrink them. They seem to work better than the stock type. I would't think the disc caps in the trem circuit osc.would need replaceing however to slow down the osc speed you could replace one with at .02uf disc cap. I've never had to replace one because it failed. I'd make sure the screen resistors were in good health too.
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Thank you.
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you didn't show is a pic of the Power supply / power tube side of he chassis..
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Its got the original paper caps except for one of the 20's which was replaced with an Illinois 22u.
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My view is that all components mounted to valve sockets will have been subjected to significant heat, vibration and wigglement stress over the decades of use. Hence the opportunity to replace them at a 50 year service should not be overlooked.
Leave a reasonable degree of ‘give’ in the leads of replacement components, to accommodate movement due to vibration, heat and valve removal / replacement.
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The amp is making the frying bacon sound and this is likely coming from the 1/2w cc plate resistors?
What about frying bacon smell? Have you checked the fuse holder to see what is in there?
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I would probably also replace the diodes in the rectifier and bias circuit with modern diodes if it was my amp.
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The amp is making the frying bacon sound and this is likely coming from the 1/2w cc plate resistors?
What about frying bacon smell? Have you checked the fuse holder to see what is in there?
It came to me with a 3A fuse. Now it has a 2A fuse.