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In Reply to: RE: Cool peanuts! posted by FenderLover on August 19, 2014 at 19:07:55
Thanks. It is just a Hammond walnut box. The front plate is a piece of 1/4 inch thick aluminum plate off ebay. The circuit is loosely based on the AX84 site. I started out with the traditional 100K on the first 12AX7 plate and it was fine, maybe a little sharp. Putting in the higher value resistor, lets the amp run from fairly clean to distorted. Now that I think about it, having one extra triode after the tone stack would be nice Combined with the twin 15" 1960's JBLs it has a little more chime than the vented 12" Eminence. I have built amps with 6SL7s, but I will have to admit it is really hard to get the level of nice distortion even with a handful of them as you can get with a single overloaded 12AX7.
What is really great is combining audio type construction with a guitar amp. The B+ power supply and DC heater supply are Hi-Fi. Sprague Atoms and Solens.
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Or even before the stack, but another stage for sure. You have twice the bias on the 6550 as a 6V6, so to get saturation, this has to be overcome.
You could add a 6AV6 to the front and drive the output off the low impedance sourced stack which will retain more EQ range. Or add it after, where it could be an octal. I find octals are never as good as miniatures for the first input stage.
Another option if you're going to drill another hole, is another twin triode, but decrease the gain on all 4, by using a pair of 12AY7's.
See the front end on a mid 50's tweed twin 5E8-A, (loose the phase inverter).
I'd love to try Allen Wright's tube/FET cascode input design in a guitar amp front end.
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Buy Chinese. Bury freedom.
What value cathode resistor do you run on that single-ended KT-88? Is it bypassed with a cap?
Thanks, again. Nice DIY.
Yes, cathode biased with a 450 ohm resistor bypassed with 100uF. There is 27 volts positive on the cathode for 60mA across the bias resistor. The KT88 is running just over 20 watts, and it is maxing out the output transformer to get a bit of OPT saturation. I have a 55v bias tap that I was going to play with, but maybe later.
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