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Hey guys!
I'm building this amp (schematic attached) mixing elements of various somewhat similar amps like a Fender 6G2, a Fender 5F10, a Gibson GA-8T, using a 12AX7 for preamp and 2 x RCA 6BM8 for a cathodyne PI, tremolo and a push-pull power amp. I'm using larger grid stoppers on the power tubes, and a 1M grid stopper on the PI input as well, like Merlin suggested in his website.
Well, first there was this annoying noise (kinda fizzy) when the notes were decaying, and the hum was too loud. It was ruining the clean tones so I started testing the components, experimenting with different grounding schemes, different filtering, and nothing seemed to help. Even removing V1 didn't change the noise. Then I just swapped the power tubes and the noise changed (for worst). In the end it was one of the 6bm8s that was deffective, despite being NOS. Well, I guess next time I'll remember to check this first... lol
The amp is running fine now, the tone is very nice for playing blues/rock jams, which was indeed my idea for this amp. But as I've never used 6BM8s before, I was wondering if the experienced guys could help me checking if my circuit looks good? I've seen larger screen resistors (2k2, 4k7) in other designs using 6BM8s, but I thought as I'm using a 1k resistor instead of a choke, I could make them smaller? Do my voltages look ok? Should I aim for almost 100% dissipation at idle for the pentodes, since this amp is cathode-biased?
I'm not sure about the value of the PI input cap, any opinions? 500pF is used by the GA-8T, and seems to give some more "clarity" compared to the 22nF used by Fender.
The cathode bypass cap at the 2nd triode deserves some experimentation too, it seems.
Well, any opinions will be appreciated! Thank you guys!
Best regards,
Christian
I'm building this amp (schematic attached) mixing elements of various somewhat similar amps like a Fender 6G2, a Fender 5F10, a Gibson GA-8T, using a 12AX7 for preamp and 2 x RCA 6BM8 for a cathodyne PI, tremolo and a push-pull power amp. I'm using larger grid stoppers on the power tubes, and a 1M grid stopper on the PI input as well, like Merlin suggested in his website.
Well, first there was this annoying noise (kinda fizzy) when the notes were decaying, and the hum was too loud. It was ruining the clean tones so I started testing the components, experimenting with different grounding schemes, different filtering, and nothing seemed to help. Even removing V1 didn't change the noise. Then I just swapped the power tubes and the noise changed (for worst). In the end it was one of the 6bm8s that was deffective, despite being NOS. Well, I guess next time I'll remember to check this first... lol
The amp is running fine now, the tone is very nice for playing blues/rock jams, which was indeed my idea for this amp. But as I've never used 6BM8s before, I was wondering if the experienced guys could help me checking if my circuit looks good? I've seen larger screen resistors (2k2, 4k7) in other designs using 6BM8s, but I thought as I'm using a 1k resistor instead of a choke, I could make them smaller? Do my voltages look ok? Should I aim for almost 100% dissipation at idle for the pentodes, since this amp is cathode-biased?
I'm not sure about the value of the PI input cap, any opinions? 500pF is used by the GA-8T, and seems to give some more "clarity" compared to the 22nF used by Fender.
The cathode bypass cap at the 2nd triode deserves some experimentation too, it seems.
Well, any opinions will be appreciated! Thank you guys!
Best regards,
Christian
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