1962 Fender Twin Caps

[Tom Phillips]

Keith,
Your explanation covers some of what is going on in an amplifier power supply. There is an excellent write up at The Valve Wizard which I suggest you read since it covers the subject very well and will be a quicker source of information than us writing here to cover the same subject.

Edit: See the additional information which I added to post #29.

Cheers,
Tom

[pdf64]

Sorry I'm a bit too late now, but when recapping one of these old Fenders, if there's time it's worth checking the underside of the cap board.
Sometimes the hidden link wires can run close by eyelets and if there's a big blob of solder associated with the eyelet, a short between them might occur.

[g1]

Those bleeders are 220K... the first dropper is 27K...

Justin

It's the other two 27K droppers I'm concerned with. In the following pic, their multiplier band looks yellow, the same as can be seen on the bleeder alongside the cap. The droppers should be 27K, with orange multiplier band.

[Justin Thomas]

Okay, I was looking at the "before" pic in post #24... but yeah, that "after" shot you posted - definitely look 470k. Can . We get a verification?

Justin

[keithb7]

Justin the dropping resistors left to right, as seen in upper image in post 20 are:
Red, purple, orange, silver. Supposed to be 27K.

However they measure in at 21K and 22K. Then 3.7K and and finally far right 22K.
Either they have drifted a fair bit, or are they lower as they are soldered in place with capacitors connected?

[Justin Thomas]

I was just f following up on what g1 said. Must be a trick of light; I'll trust your eyes over a picture. As far as the values, I'd replace them. They're well out of the 10% tolerance.

Justin

[Enzo]

Very rare for resistors like that to read low, especially all four of them. But it is quite common for parallel resistances in circuits to make resistor reading appear low. If those resistors are a little low like that, the only result is the downstream voltages will be a little higher. it won't blow fuses or cause amps not to work.

The caps often have small residual voltages. Try this, measure one of those 27k resistors, then swap meter leads and measure it the "other direction". If the reading changes at all, then you have residual voltages.

Your meter uses a small voltage to make resistance readings, and even that small voltage, will charge into caps. You can also verify with the volt meter whether there is a half a volt or a volt or something hiding in those caps. or you can lift the cap leads or one end of each resistor to verify the real resistance. I'd be sure before just wholesale swapping out a bunch of resistors.

[keithb7]

This is the best forum anywhere. Bar none. Great advice. Thanks, yet again.

Before I settle in and digest what everyone has said, and their instructions, I sat down and drew the following crude layout diagram. This is the stock circuit. The dotted green lines indicate under the board connections.
Have a look. I'll get back to reading and digesting soon. Then heat up the iron again and do it right. I have learned a lot from taking the time to draw this out. I understand much better how each cap provides reserve DC voltage to each part of the amp circuit.

[keithb7]

I hooked up ground wire to adjacent caps. Drained any residual millivolts. Resistors now read very close to stock specs. Thanks Enzo.

[keithb7]

I have rewired it to look like this. Have I got it right now?

[keithb7]

Success. Thanks for your help folks. Cathode caps next.

https://youtu.be/G1EDSBWmpyw

[g1]

Sounds good Keith. Maybe make yourself a before and after recording for the cathode caps, to see if there is an audible change. And capacitance measurement would be a bonus.

[keithb7]

Sounds good Keith. Maybe make yourself a before and after recording for the cathode caps, to see if there is an audible change. And capacitance measurement would be a bonus.

I have often wondered if changing cathode caps, can actually have a sonic effect. My understanding is line signal does not go through these caps.

[Enzo]

Cathode bypass cap affects gain and response of the state. It does not have to be in the signal path to affect the signal path.


I mean "stage".

[Chuck H]

I have often wondered if changing cathode caps, can actually have a sonic effect. My understanding is line signal does not go through these caps.

What Enzo said. And...

What constitutes "through"? Would that be ONLY the shortest wire distance from the input to the output as determined by series connected lead wire, board traces and components??? Not at all. Signal DOES pass through bypass caps. Isn't that which is bypassed just as relevant to tone as that which is "passed"? And when you consider a bypass caps function it becomes even more clear that it has a significant affect on what goes through the signal chain. And is, therefor, part of the signal chain. Bypass caps bleed AC from the cathode circuit where it would otherwise create negative feedback. How much, how well or at what specific frequency the bypass cap affects the cathode circuit would be proportionate to the amount of "through" signal that NFB gain reduction represents. So, if you had a 20dB gain increase for a given amplification stage and it's circuits, and the cathode bypass cap for that circuit can affect the gain at frequency, say, +/- 6db, doesn't that make the quality and characteristics of the bypass caps importance roughly 30% that of the coupling cap? 30% control over what is going "through" the amp isn't insignificant at all.