Preamp problems on a guitar amp

I recently acquired a Guitar combo. The power amp had been removed. Consequently the power to the preamp board was also missing. I have bought a hy2003 to replace the power amp ( with dedicated psu and transformer. I have built a 15v power supply + regulator for the preamp. Unfortunately the only signal that comes through the preamp is intermitant and seemingly when the input is at its strongest. Is my 15v transformer too low a current ? (rated at 6VA) or is it something more serious ? :?:

Reply to
Peter F
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"Peter F" wrote in message news:58ednSRI4qf6HwbeRVn

Are you sure that the preamp only needs +15v ? In my experience with guitar amps, and I do repair a lot of them, it is usual for the preamp to require a negative supply as well, as the circuitry is usually based on 4558 or similar opamps.

Also, the supplies for the preamps are very often derived on-board, from the main +/- 50v or whatever rails that feed the power amp. So +/- 50v goes across to the preamp, where it is zener'd down to +/- 15v, with a couple of wirewound resistors and 5 watt zeners.

The clues you should be looking for are does the preamp employ opamps ? Where the power comes onto the board, are there two electrolytic caps for decoupling ? Where the power comes into the board, are there any signs of wirewound resistors ?

The sound problems you are describing could well point to only one rail on an opamp stage.

If it's a ' famous ' make, then the schematics for it are almost certainly available, probably for free, from one of the many excellent guitar amp schematic sites on the 'net.


Reply to
Arfa Daily

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