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07-09-2020, 04:23 PM
Yes, of course I meant EL84's, my mistake. I split Rk/Ck and adjusted the resistor values to cool down the tubes. 168R per side took the dissipation down below 12 watts. I increased screen resistors' values to 1K and I don't think that changed the sound much.
The amp was an interesting case and I have learnt quite a lot working on it. I was only to switch the PT wiring to 230V but I noticed the amp hums a bit too much so I had to investigate that. There were many random ground connections and I had to convert everything to galactic ground. I then experienced all sorts of unpredictable behaviour when the amp would stop humming and buzzing one day and start over again the next day. Turned out the reverb driver tube was oscillating and upsetting the V1, which was quite misleading at first. I fixed that by installing a bypassed 2K2 Rk usually found in typical Fender-style reverb circuits. The tube stopped oscillating right away but the sound coming out of the reverb was noticeably harsh. I scoped the tube to see what's going on and found the distorted output. I then only had to find the optimal Rk value so there was enough signal swing to drive the reverb without distortion and the tube was not oscillating. Typical values didn't work as expected and the circuit had to be carefully tweaked to get decent performance. Three other 12AT7's behaved exactly the same way in this position so the problem was not caused by a faulty tube.
The offending tube was on the screen supply which was well filtered, but didn't have an individual filter cap as such.
The amp was an interesting case and I have learnt quite a lot working on it. I was only to switch the PT wiring to 230V but I noticed the amp hums a bit too much so I had to investigate that. There were many random ground connections and I had to convert everything to galactic ground. I then experienced all sorts of unpredictable behaviour when the amp would stop humming and buzzing one day and start over again the next day. Turned out the reverb driver tube was oscillating and upsetting the V1, which was quite misleading at first. I fixed that by installing a bypassed 2K2 Rk usually found in typical Fender-style reverb circuits. The tube stopped oscillating right away but the sound coming out of the reverb was noticeably harsh. I scoped the tube to see what's going on and found the distorted output. I then only had to find the optimal Rk value so there was enough signal swing to drive the reverb without distortion and the tube was not oscillating. Typical values didn't work as expected and the circuit had to be carefully tweaked to get decent performance. Three other 12AT7's behaved exactly the same way in this position so the problem was not caused by a faulty tube.
The offending tube was on the screen supply which was well filtered, but didn't have an individual filter cap as such.
