6L6 to 6550 conversion for Fender 45/50w amp

[Jazz-O-Sonor]

A typical Twin reverb PT has to cope with 5.4A of heater current draw, subbing 4x6L6 for 2xKT120 gives a total draw of ~5.8A...close to the Twin PT's 6A rating. If you do try it, watch B+ voltage at reasonable idle current (60-70mA per tube...if B+ drops more than 20vdc you are probably stressing the PT) and watch for a drop in heater voltage (again, a sign of stress on the PT). If you are absolutely determined to run KT120 and you do find the PT overwhelmed, get a 2-3A auxilliary 6.3VAC transformer & use this to supply the preamp tubes. Run the power tubes only from the Twin PT heater winding.

MWJB , thank you for all your inputs and the great technical advices
thanks, John

[Steve Conner]

So, just for future reference, what was the problem? Your preamp tubes were old and worn out?

[Jazz-O-Sonor]

So, just for future reference, what was the problem? Your preamp tubes were old and worn out?

Hi Steve,
well I still cant belive that since I added the 0.02 cap on the reverb input
the amp start reaching the break up point later and the amp sounded
more clean and also elimininated a hum that I had on reverb.
I placed two brand new EH gold pin for V3 and V6 nad the sound improved a lot in dinamics and cleans.
Yesterday , I tryed 0.01 ceramic disk caps across pin5 to 8ground on each power tube sockets and I had an even cleaner sound very hi-fi (just like a pro reverb silverface I owned ) , not so rich of frequencies as it was before but a lot cleaner , so now I'm planning to add a little DPST switch and solder
the two 0.01 caps on it so I can switch between both sounds!

thanks for your inputs , John

[überfuzz]

Cool, that idea actually crossed my mind, but I rebuffed it thinking it would make the power section of my eye lamb such a hustle. Let me know how it vent.

[J M Fahey]

I added a 0.002 cap to the reverb input

What does it exactly mean?
Are they in series with the input (cutting bass) or in parallel (shunting highs to ground) ?
Please post the schematic and indicate what you did, so we all talk about the same.

[überfuzz]

J M Fahey - I thought he added a coupling cap somewhere to cut dc-leakage...

[m-fine]

I was going to guess the cap is in series and is cutting excess bass. That would fit the symptoms of breakup when hitting chords, and if he followed the silverface TR PI to spec, the input coupling cap gives a -3db point somewhere around 20-25 hz. The input to the blackface PI cuts at around 80 hz which is the fundamental of the low E if you dont down tune. I played around with mine and found a -3 db of about 50 hz allowed to bass to be a touch fuller than the BF but without the extra noise of the silverface. I think I used the 330k grids from the SF and a .0047 coupling cap to get there (as opposed to .01 for SF and .001 and 1m grids for the BF). At any rate, cuting bass can definitely help cut unwanted breakup.