Kinghorn Fender Amp Mods

[Mars Amp Repair]

Hello folks,
Has anyone had any experience with this amp mod company?
I have a Fender Dual Showman that has a mod kit in it. Basically, it redesigned the entire amp. It runs on 6CA7 (EL34) tubes, and has a separate bias supply w/xformer to produce the higher bias voltage. Looks like it uses a regulator IC of some sort. I didn't look that closely.
The main issue with the amp as that the wires to the filter caps were apparently stressed & broke loose. The lack of filtering caused too high extended current thru the bridge diodes and shorted one of them.
The mod kit entails a new rivet board of high quality and a 3.5x3.5" board with a relay on it that appears to be for channel switching. About the only 'Kloogy' thing I was that this small board was wrapped in electrical tape and just kinda stuffed in the chassis. I subsequently zip-tied an insulator sheet to the bottom of the board in place of the electrical tape & affixed it to the chassis w/zip-ties.
The entire mod in general appears to be fairly well thought out & functions fairly well, except for some oscillation on extremely high levels. I tried to move lead dress to eliminate it with no luck. Since it is only at extremely high levels, I just left that way.
The master volume pot is the dual pot type that merely shunts the PI drive signal to the output tubes. It does attach between 4 coupling caps that serve to isolate the pot from PI plate voltage and Minus Bias voltage on the output tubes.
I did have an issue with the Mstr Vol clipping the positive sinewave to the power tubes that appeared to be resolved by putting the wiper of the pots on the PI side as opposed to the output tube side. Don't even ask how or why. Amps with mods have lots of weird artifacts that I either can't or won't try to explain even to myself!
Anyway,
Just wondering if anyone has seen these mods on this or other amps and what you think of them.
Thanx, Glen.

[g1]

I'm surprised about the bias mod. Normally EL34 require less bias voltage than the existing 6L6 types.
Are they maybe also creating an additional low voltage supply for the relays or something?

[Mars Amp Repair]

I'm surprised about the bias mod. Normally EL34 require less bias voltage than the existing 6L6 types.
Are they maybe also creating an additional low voltage supply for the relays or something?

HMM, I'm always hesitant to state anything as fact, but I seem to recall the EL34's require more negative bias for average idle wattage than the 6L6's, eg: Typical bias for a 6L6 for average idle wattage seems to fall in the -20 to -35Vdc zone. For the EL34, I seem to recall bias voltage in the -30V to -45V for average idle wattage.
Either way, you could be right about the added power supply. It's also possible that there were 2 supplies on that board. I just didn't look at it that closely.
Thanx, Glen

[Tom Phillips]

HMM, I'm always hesitant to state anything as fact, but I seem to recall the EL34's require more negative bias for average idle wattage than the 6L6's, eg: Typical bias for a 6L6 for average idle wattage seems to fall in the -20 to -35Vdc zone. For the EL34, I seem to recall bias voltage in the -30V to -45V for average idle wattage.
Either way, you could be right about the added power supply. It's also possible that there were 2 supplies on that board. I just didn't look at it that closely.
Thanx, Glen

Off the top of my head I'd say that the typical bias for a 6L6 for average idle wattage would be in the -40V to -50V range and the -25 to -35Vdc range for the EL34. Some modern production tubes differ significantly from the original production tubes. For a bias supply design, I usually allow for a wide adjustment range such as -40V to -55V for a 6L6 type.

[g1]

Glen, I wonder if you are thinking of bias voltage for 6V6 rather than 6L6 ?

[Mars Amp Repair]

Glen, I wonder if you are thinking of bias voltage for 6V6 rather than 6L6 ?

Hey guys,
I just realized I'm thinking of Bias current, not voltage DUH .
As we used to say as kids:
Oh Welp, anyways!