| ampage Tube Amps / Music Electronics |
For current discussions, please visit Music Electronics Forum. |
ampage archive
Vintage threads from the first ten years
Never discount the factory factor
 9/28/1999 5:04 PM |
|
pumpstein | Never discount the factory factor I was working on a Silvertone 1484 this weekend and found that the reverb was really inducing feedback (side observation: this reverb circuit is truly one of the silliest wastes of 4 triodes ever known to man - if the chassis wasn't already sort of crammed, I might mod in a Fender circuit). I had already done the obligatory cap job, including the cathode bypass caps, and had swapped out tubes hoping that it was simple microphonics. No luck. I set about testing any resistors that might affect gain for drift - no real offenders. No faulty coupling caps. Then I noticed that the rev driver's plate was being fed by the wrong filter node, (the one upstream = more B+) and that the voltage divider network on the rev return stage had a 150k to gnd, rather than the stated 68k. The top resistor was 560k as expected. I left the driver alone and dropped in the 68k and viola! Crappy reverb just like I remembered from the last one I worked on. (but it worked - no more feedback). The amp looked like it had never been serviced, so I'm guessing that it was a production line change that didn't make the schematic. Now I wish I still had the other 2 1484s I sold last month, just to compare. I need to start taking polaroids of chassis' as soon as I crack the hood, so I can build an archive. Too many untouched wonders gone undocumented. |
|---|---|
| Replies: |
| Whit P, |