Amp is an Ampeg J12J Reverbojet, 1968 or so. When it was brought to me by my nephew the volume was very low. This is an 18W amp, with much less volume than a 5W.
A look inside showed a fried electrolytic, and testing other electrolytic caps showed they were all way off original values. Figured I would do a much needed cap job to start. Once I got the caps in (all electrolytics including the cap can), the amp was noisy, but still no volume. Measured DC voltages, determined the OT was no good. Replaced OT, now we have some volume, but still have some noise. Pulling tubes indicated something in the 6U10 circuit was indicated. Replaced eventually nearly everything in that circuit. No one component made a big change, but gradually the amp quieted down somewhat. Now pulling tubes one at a time ... it's pretty quiet all around. Still a bit of noise with V4 out (12A7X - 1st pre-amp/trem tube) but not bad.
With all tubes in however, the hiss is above what I would say is acceptable (worse than other tube amps I have used, and it has just a bit of "raspiness to it)".
I happen to have another of these amps around (long story) that is basically original (needs new caps, etc) but a side by side comparison - noise is about the same between the two.
These are early PCB amps, and I am wondering:
1) Is this level of noise normal for these amps?
2) Can I do something about lead dress to reduce the noise?
For example, while nearly all the wiring is on the board. the OT wiring all comes through one small hole in the chassis. Would I be better off to make a second hole, seperate signal from power, etc?
Also, the original OT had leads for the speaker and the neg feedback coming right out of the OT. The replacement only has one set. I branched these near the OT and sent a set to speaker and a set to the NFB. I read (somewhere) that the NFB should come only from speaker contacts. Not sure why that would make a difference, but that's not how it was originally, but is that better for some reason? I did disconnect the NFB leads, and the noise is unchanged.
I have also tried different tubes, disconnected the reverb cables, etc. - no difference. Any suggestions to quiet this beast?
The best schematic is on Hoffmans site. It's the J12R .pdf (not the J12R.pdf) if you are interested.
http://www.hoffmanamps.com/charts/Diagrams.htm
Thanks for any suggestions!
Chris