Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernest on November 25, 2025, 12:04:53 pm
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Hi, I haven't been into playing with amps for a few years but I picked up a 5f1 clone someone else was building on marketplace and have a couple questions. I've been busy building a cab and covering it so I'm just now getting around to looking at the circuit. I haven't looked at the bias of the power tube yet since I don't know if I need to fix a few things on the circuit beforehand. The amp sounds fine but I'm not sure if it's running properly. First off, it has a 47uf 500v in C3 the first filter cap. Secondly, it doesn't appear to have a cathode resistor on V1 preamp tube. The 68k grid stopper resistors are wired off the input jacks, not on the board, with shielded wires to tube which is fine but I don't see the cathode resistor. How is the preamp tube biased and should I fix the C3 filter cap to a 16uf 450v?
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did it come with a schematic?? a pic is worth 1,000 words
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did it come with a schematic?? a pic is worth 1,000 words
No schematic, the guy I bought it from was not the builder. a Pic was uploaded to the first post. did you not see?
thanks
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If the amp sounds fine there must be a cathode resistor for V1. Measure resistance from v1 pin 3 to ground. It should be 1.5K. I'm betting you will find the missing resistor while doing this.
Don't worry about the power tube bias. I would change that 47µF to 20µF or 22µF just to protect the 5Y3.
Your pic needs to be much bigger for us to tell much about the circuit. I suggest at least 10X as big as the little one you posted.
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Your pic needs to be much bigger for us to tell much about the circuit. I suggest at least 10X as big as the little one you posted.
Okay sorry about that.
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++ If it sounds good, it's ok.
Looks like an F&T (Mersen) and maybe some Sprague Atoms, and the work looks very neat -- so someone went to some trouble. I do agree that a 47uF filter cap for a 5Y3 is an error and will lead to lower tube life. Changing that to 22 or 20 should not change the tone much if at all.
Aside from the picture being a bit small (I don't think it's so bad), we can't see the way the tubes are wired and connected to the board, and thus infer a schematic. So more angles showing the connections would be great.
Overall it looks good, though.
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Those octal tubes are not seated very well. If they are inserted as far as they will go, I suggest replacing with better quality sockets.
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^^^ what he said;
I can solder pretty good, those ceramics though will wick-up n change the diameter at the bottom.
after di-secting the ceramics with a 3LB hammer, I switched to these n never looked back.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=TubeSockets&ORDER_ID=!ORDERID!
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That link does not work because the exclamation point at the end of the link must be included as part of the link.
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If there's no cathode resistor on V1, it's probably grid leak biased. (You'd be able to tell quickly by measuring the grid leak resistor, which would be something like 5M or more.) The early tweed champs were grid leak bias in V1 (6SJ7 pentode)
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If a 22uF as First Capacitor after the rectifier results a bit noisy and make the amp respond not as you want you can arrive till 32uf as value
https://www.r-type.org//pdfs/cv1268.pdf (https://www.r-type.org//pdfs/cv1268.pdf)
Franco
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here's a pic
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Missing total insertion of the tube PIN on the sockets may be due to excess of tin when soldering the sockets
Or may be it is only a problem due to bad insertion caused by the base tube retainers
Which of the two ?
Franco
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the tubes are just not seated all the way. They were removed when the chassis was taken out of the cabinet, and lightly put back in to keep them from rolling off somewhere for a brief moment. Not even the tubes I am using to run it with. Thanks for the tip on the sockets.