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There was another thread on these forums I based the idea for the amp on.
Here's a link to the schematic I'm put together, with some voltages I'm reading now that I've built it.
http://vwtweaked.ca/images/6k6%20amp.jpg
After some tweaking, the build is pretty stable, fairly low noise, and sounds really really good to my ear for most of the stuff I want to use it for.
I wanted this amp to push the power tubes into overdriving naturally, so I set it up fairly low gain in the preamp. I have 3 stages, but no cathode bypass caps. I did end up putting a switchable one in on v1b just in case I want some more gain.
The amp does indeed push into a really nice power tube overdrive though, and not at earsplitting volumes. So basically I've accomplished what I wanted to here.
Except...
One issue I'm having with the amp is that with any sort of palm muting, or heavy-ish notes the bass sort of flubs out, even with the cathode bypass cap engaged in the preamp. I also reduced the bass in the amp somewhat by reducing the size of some of my coupling caps, it's still got just a bit too much flub for me though.
I know it's not my cab or speakers doing it. So what I'm wondering is if it's likely my OT is not sufficient. It's a Hammond 125D (10W, 100hz-30khz). The frequency range seems odd to me for a guitar amp. I'm wondering if it has something to do with it.
According to datasheets, a pair of 6k6gt power tubes running at 285V on the plates and screens, with a 400ohm shared cathode resistor (which biases them at max plate dissipation) should put out 9.8W roughly, so the OT should be rated fine power-wise.
My PT (Hammond 269JX, 250-0-250) is also probably close to it's max ratings, of 60mA on the secondary HT winding.. Part of me wonders if this could be creating part of the issue.. The only reason I say that is because at one point I had a 5th node in the power supply, before the node that the OT primary is powered from. It was decoupled with a 500ohm power resistor. I figured some extra filtering couldn't hurt.. But it did, big time. The amp had seemed to have this extreme compression happening, like I'd hit a chord on the guitar, and the volume level it was putting out would drop way down.. So it made me think with relation to this flubby problem, that the power system in general could have a part in this problem.
I'm not against trying some different transformers in the amp.. The only slight concern I have is finding an different OT with a primary impedance of 12,000ohms and a secondary at 16ohms.
I guess I could use something like the Hammond 1760H rated at 20w and 75Hz-15Khz which has 6,600ohm primary and then use the 8ohm secondary tap as 16ohms..
I'm also open to any other suggestions regarding the problem.
Here's a link to the schematic I'm put together, with some voltages I'm reading now that I've built it.
http://vwtweaked.ca/images/6k6%20amp.jpg
After some tweaking, the build is pretty stable, fairly low noise, and sounds really really good to my ear for most of the stuff I want to use it for.
I wanted this amp to push the power tubes into overdriving naturally, so I set it up fairly low gain in the preamp. I have 3 stages, but no cathode bypass caps. I did end up putting a switchable one in on v1b just in case I want some more gain.
The amp does indeed push into a really nice power tube overdrive though, and not at earsplitting volumes. So basically I've accomplished what I wanted to here.
Except...
One issue I'm having with the amp is that with any sort of palm muting, or heavy-ish notes the bass sort of flubs out, even with the cathode bypass cap engaged in the preamp. I also reduced the bass in the amp somewhat by reducing the size of some of my coupling caps, it's still got just a bit too much flub for me though.
I know it's not my cab or speakers doing it. So what I'm wondering is if it's likely my OT is not sufficient. It's a Hammond 125D (10W, 100hz-30khz). The frequency range seems odd to me for a guitar amp. I'm wondering if it has something to do with it.
According to datasheets, a pair of 6k6gt power tubes running at 285V on the plates and screens, with a 400ohm shared cathode resistor (which biases them at max plate dissipation) should put out 9.8W roughly, so the OT should be rated fine power-wise.
My PT (Hammond 269JX, 250-0-250) is also probably close to it's max ratings, of 60mA on the secondary HT winding.. Part of me wonders if this could be creating part of the issue.. The only reason I say that is because at one point I had a 5th node in the power supply, before the node that the OT primary is powered from. It was decoupled with a 500ohm power resistor. I figured some extra filtering couldn't hurt.. But it did, big time. The amp had seemed to have this extreme compression happening, like I'd hit a chord on the guitar, and the volume level it was putting out would drop way down.. So it made me think with relation to this flubby problem, that the power system in general could have a part in this problem.
I'm not against trying some different transformers in the amp.. The only slight concern I have is finding an different OT with a primary impedance of 12,000ohms and a secondary at 16ohms.
I guess I could use something like the Hammond 1760H rated at 20w and 75Hz-15Khz which has 6,600ohm primary and then use the 8ohm secondary tap as 16ohms..
I'm also open to any other suggestions regarding the problem.
, as well as the paul ruby mod, since I had some extra zeners and it did seem like I had some blocking or crossover distortion. (I measured my idle cathode voltage to be 18V, which rose to 23V under heaving load. So 20V zener seemed a good value to parallel with the cathode resistor. And all I had left was a couple more 25V zeners, which seemed like they would be a decent enough value for the paul ruby addition)
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