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07-09-2020, 03:16 PM
Hi Guys
The 470 Rk value for the reverb driver is actually okay, but the effect on the tube gain may be problematic for given layouts In an old Fender chassis I used for trying circuits out in, the stock R was 470 even though the schematic suggested a different value.
It is important in the SE circuit to have a high-value good-quality filter cap for the related supply node. Remember, PSRR is nonexistent in such a circuit, so the supply has to be quiet on its own otherwise you get IM products into the reverb tank. In the worst case that can sound as if the reverb is wildly distorted and in the best case simply that the reverb is muddy. Obviously, a bad filter cap can cause a lot of supply-modulated bad sound.
You can scope the drive into the OT using a 100x probe (if using a DSO). Visual distortion of a sine wave represents quite a high figure of THD, but it's what most people have to go by. You can also send a signal into the return path to make sure that section is clean unto itself.
EL-34s dissipating 15W is no problem; if you meant EL-84, then it is a problem and Rk needs to be split (ideally) and/or made higher in value. Those 100R screen-stops will cause the amp to eat tubes.
have fun
The 470 Rk value for the reverb driver is actually okay, but the effect on the tube gain may be problematic for given layouts In an old Fender chassis I used for trying circuits out in, the stock R was 470 even though the schematic suggested a different value.
It is important in the SE circuit to have a high-value good-quality filter cap for the related supply node. Remember, PSRR is nonexistent in such a circuit, so the supply has to be quiet on its own otherwise you get IM products into the reverb tank. In the worst case that can sound as if the reverb is wildly distorted and in the best case simply that the reverb is muddy. Obviously, a bad filter cap can cause a lot of supply-modulated bad sound.
You can scope the drive into the OT using a 100x probe (if using a DSO). Visual distortion of a sine wave represents quite a high figure of THD, but it's what most people have to go by. You can also send a signal into the return path to make sure that section is clean unto itself.
EL-34s dissipating 15W is no problem; if you meant EL-84, then it is a problem and Rk needs to be split (ideally) and/or made higher in value. Those 100R screen-stops will cause the amp to eat tubes.
have fun
