Recaping a MusicMan HD130
Im about to order new caps for my old HD130, but some af the caps are hard to find.
It takes 5 x 150uF, 50V, which I cant find nearby (at a reasonable price).
I know I can parallel a 47uF and a 100uF, but I dont like the look of that solution.
I read somewhere that it is ok to move up to the next standard value, 220uF, as the tolerance is -50/+100%.
Can you confirm that?
Yes, that should work fine. As an added bonus, it may tighten up the bass just a hair - probably not substantially, but a little.
Thanks for your fast reply.
Two of them are fitted on the rectifier board, two in the bias circuit and one in the driver circuit.
Will you please explain why it will affect the sound?
Not to be disrespectful - trying to become wiser :)
Capacitors act as batteries, more or less. The ones in the power supply (the rectifier board) are storing energy so that when the amp requires more current (a loud transient, for instance) than usual, the extra energy is there in the caps to cover it. It's like having a resevoir of extra energy in the power supply. When the requirements of the amp exceed the ability of the caps to provide energy, the voltages in the amp drop and the amp starts to compress. This is often called "sag" by guitarists who use tube amps. Larger caps have a larger capacity to store this energy so they provide you with more clean headroom and tighter bass (since bass requires the most energy to reproduce).
That was a VERY simplified explanation, but it should give you the basic idea.
The caps in the bias supply serve another purpose altogether. They provide stored energy as well, but the function of them is to keep the bias voltage stable so the tubes behave consistently. Larger caps in that part of the amp will keep the bias stable and also help to provide clean headroom, but they take a little longer to charge so the amp will take a little longer to stabilize. It's a good idea to let the amp warm up for a full minute or two before playing anyway, but even more so if you increase the size of the bias caps.
Enjoy your amp!
Thanks for the explanation, Lo-E, it makes sense.
I will go for 220uF caps then.
I look forward to hear the amp play again. It has been sitting in a closet for years.
The tubes have not played many hours, but I had to put it aside because of a large bulge in the rubber on one of the 100uF caps. I didnt like to take the risk of destroying the output stage.
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