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Guitar Amp Build

108.247.77.70

Posted on August 19, 2014 at 16:35:41
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
Here is my latest amp.





SE KT88 Triode.
CLCLC with big chokes running 400v
OPT spec'ed at 50mA running at 60mA
MOSFET source follower driving the tone stack
Absolutely 100% silent with it dimed
300K load on the first 12AX7 plate

For the first time in a build like this I have no ideas on how to improve on the sound of the build.
Sounds really nice. :-)

 

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Cool peanuts!, posted on August 19, 2014 at 19:07:55
Is that a DIY OD box, too? Is it like a Tube Screamer?

Be nice to run that amp through a smaller cab. Maybe a 2 x 10-inch open back one. So you can run the KT-88 harder. SE designed 6550 and KT-88 can make nice overtones and harmonics.

 

RE: Cool peanuts!, posted on August 20, 2014 at 04:57:05
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
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.

 

I take it the KT-88 is cathode biased, posted on August 20, 2014 at 05:55:26
What value cathode resistor do you run on that single-ended KT-88? Is it bypassed with a cap?

Thanks, again. Nice DIY.

 

RE: I take it the KT-88 is cathode biased, posted on August 20, 2014 at 06:55:38
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
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.

 

"having one extra triode after the tone stack would be nice", posted on August 20, 2014 at 09:02:44
GSH
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).







 

And maybe a cascode input..., posted on August 20, 2014 at 10:48:32
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
I'd love to try Allen Wright's tube/FET cascode input design in a guitar amp front end.

 

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