I took the 6L6s out of my beloved Carvin XT112, a delightful amp with a single pair of power tubes, class AB, LTPI. I believe the transformer is the same as for their 100W quad 6L6 model. Schematics are here:
http://www.carvinmuseum.com/pdf/amps..._schematic.pdf
I added a bias pot long ago.
I installed JJ's 6V6S tubes, in spite of a 475V B+, and biased each of them at 25ma of cathode current. The goal was to get a bit closer to the Fender clean of my Deluxe Reverb re-issue, though the 6L6s do a pretty good job of warming things up. I powered up with a Variac at 100V, and the tubes didn't complain. I've since removed the Variac, and the amp's been playing happily for weeks. Apparently the JJ's 6V6S's reputation for taking high plate and screen voltage is deserved.
I note that under maybe 1W, I get the creamy tone with strong, tight bass that I crave. Initially, I set the output impedance selector to 4 Ohms for my 8 Ohm speaker. This worked well, but as I turn up the volume past "bedroom jazz" to "solo jazz" and "jazz trio" levels, the bass gets loose and boomy, and going louder, I get output tube overdrive sounds, including ghost notes in the bass, much, much below the point where the 6L6s would do it (in fact, they wouldn't overdrive on the clean channel). Surprisingly (at least to me), turning up the output impedance switch to 8 Ohms to match the speakers increased the "clean" range of volume.
So... I've got a hot B+, an output transformer with an impedance that's too low to match the circuit, and an NFB loop set up for EL34s. I can drop the B+, and I can change the output transformer, but I have a feeling that the feedback loop needs a boost and tuning to counteract the crossover distortion I'm getting at higher volume once the circuit passes from class A to class AB. I think the effect of the output impedance switch argues in favor of this. Does that sound reasonable, or should I just go and buy a transformer and some zeners?
At low levels, with the 6V6Ss, the amp really does a good job of producing "jazz to fall asleep to" in a classic jazz sort of way, with wonderful harmonic enrichment in the not-too-highs.
http://www.carvinmuseum.com/pdf/amps..._schematic.pdf
I added a bias pot long ago.
I installed JJ's 6V6S tubes, in spite of a 475V B+, and biased each of them at 25ma of cathode current. The goal was to get a bit closer to the Fender clean of my Deluxe Reverb re-issue, though the 6L6s do a pretty good job of warming things up. I powered up with a Variac at 100V, and the tubes didn't complain. I've since removed the Variac, and the amp's been playing happily for weeks. Apparently the JJ's 6V6S's reputation for taking high plate and screen voltage is deserved.
I note that under maybe 1W, I get the creamy tone with strong, tight bass that I crave. Initially, I set the output impedance selector to 4 Ohms for my 8 Ohm speaker. This worked well, but as I turn up the volume past "bedroom jazz" to "solo jazz" and "jazz trio" levels, the bass gets loose and boomy, and going louder, I get output tube overdrive sounds, including ghost notes in the bass, much, much below the point where the 6L6s would do it (in fact, they wouldn't overdrive on the clean channel). Surprisingly (at least to me), turning up the output impedance switch to 8 Ohms to match the speakers increased the "clean" range of volume.
So... I've got a hot B+, an output transformer with an impedance that's too low to match the circuit, and an NFB loop set up for EL34s. I can drop the B+, and I can change the output transformer, but I have a feeling that the feedback loop needs a boost and tuning to counteract the crossover distortion I'm getting at higher volume once the circuit passes from class A to class AB. I think the effect of the output impedance switch argues in favor of this. Does that sound reasonable, or should I just go and buy a transformer and some zeners?
At low levels, with the 6V6Ss, the amp really does a good job of producing "jazz to fall asleep to" in a classic jazz sort of way, with wonderful harmonic enrichment in the not-too-highs.
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