Trying to solve S/N issues with 12ax7 frontend. If I pull the 2 frontend tubes all the hiss goes away. I noticed that the volume control knob on one channel is very hard to turn and when I do it will start to motorboat and also affect the level of hiss. Looking for direction on troubleshooting. I have put new 12ax7 and lower gain 12xx7 in to replace but the same level of noise is still there. the hiss is louder than most tube amps I have dealt with.
Motorboating usually comes down to preamp stages not being de-coupled properly by the filter caps. See this de-bug page: New Page 1 upper left corner. Do you still have the original filter caps in the amp? Might be time for a cap job. RG really knows his stuff and this is one of the best fix it pages I've seen with lots of other great info too.
Thank you so much boobtube. I ve never come across that page before but already have idea of what to check and look for! Great help.
Just so you know, I have one of these amps and it has more than normal amount of hiss as well. Must be in the design. But Silvertones were made for the masses to get affordable amps so I don't worry about it as I can't hear any hiss when playing. It's got that sound that only cheap '60s amps can deliver, a great blues overdrive.
Just as a follow-up so that others might learn something also. The motorboating was traced back to one particular tube, input 12ax7. The hiss problem was improved by about 20-25 dB by replacing the plate resistors in all the frontend stages with metal film. thanks
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