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

[MWJB]

3.2A for 2x6550, plus 1.8A for the preamp tubes...there's 5A of your 5.5A right there. I'd want a whole amp, at least, headroom. I have run 6550/KT88/KT90 in amps with 022798 PT's OK, but only with 3 preamp tube circuits.

Either live without a channel, reverb & vibrato, or add the auxilliary transformer.

Auxilliary primaries connect to your PT primaries, disconnect the preamp tube heater wires & run them to the auxilliary transformer, making sure to add a virtual centre tap of 2x100ohm resistors to ground. Leave the power tube heater wires connected to the PT.

[Jazz-O-Sonor]

3.2A for 2x6550, plus 1.8A for the preamp tubes...there's 5A of your 5.5A right there. I'd want a whole amp, at least, headroom. I have run 6550/KT88/KT90 in amps with 022798 PT's OK, but only with 3 preamp tube circuits.

Either live without a channel, reverb & vibrato, or add the auxilliary transformer.

Auxilliary primaries connect to your PT primaries, disconnect the preamp tube heater wires & run them to the auxilliary transformer, making sure to add a virtual centre tap of 2x100ohm resistors to ground. Leave the power tube heater wires connected to the PT.

Hi,
I am seriously thinking about changing for a Twin Reverb PT with only two Tung Sol KT120 and 6 preamp tubes ; Can a stock Twin Reverb PT be fine for this application ?

thank you , John

[überfuzz]

What plate voltage are you aiming to get? I believe teh KT-600 is labeled as a 600V tube. I'm not sure the TW PT will cut it...

[Bruce / Mission Amps]

Hi Bruce ,
thx for aswering;

the MM SuperSix Reverb OT 2/4ohm taps (a beefed TR style OT) : is
hooked w/ 2ohm tap because I/m running only two 6L6 and this doubles the
amperage output to 4ohm.
The two 8ohm speakers are wired in parallel for a 4ohm load.

2x10 Emi's Delta Demon 100w each are loud enough for Jazz gigs in
small clubs and sometimes in medium places .

Bruce , my amp actually is very load and basically have a good clean
sonic character but it is still to much "sensitive" into break up and
getting a bit too "bluesy" and into unwanted discrete dose of distortion
when try to play clean rithmic funky chords .

I need some help to get more clean without sacrifing any of the
effective volume and power of the amp.
Pls, any advice about the 6L6 to 6550 conversion?

thank you , John

>>>>>>>>>>>>>
I don't think the conversion to 6550s is really going to help very much with the current power transformer installed.
I still can't tell if you have the classic black and silver face Fender opto vibrato circuit installed.
If not, then I think you should reduce the bass response a little bit and drop some preamp gain before the phase inverter.
My gut feeling is you need better speakers and the preamp gain before the phase inverter is a bit too high without the vibrato circuit installed.
You can dump some signal to ground right before the phase inverter coupling cap, (no more the 1000pF to a max of 1500pF here!)... and try a 100K to 220K in front of the coupling cap with a 68K to 100K resistor grounded before the 100K to 220K resistor.
It's just an inexpensive starting point before spending a bunch of money on unneeded transformers and tubes.... but really consider the speakers too.

That TAD PT looks like a Pacific transformer... if so, they make really good stuff and I bet those current ratings will be mildly conservative.

[dchang0]

What Bruce just said about dumping signal to ground works great. I reduced one grid load resistor on the last preamp gain stage from 1M to 220K, and without any other changes, the breakup now happens at 9 instead of 6. Overall volume is a tiny bit lower, but not by much.

[ThermionicScott]

Just checked the specs on the Delta Demons, and Bruce is right -- the best thing to do would be to move up to some efficient speakers. Dark-voiced speakers always seem quieter to me, as well.

John, I think you really need to let go of this fixation on modding your DR, and just start fresh with something that's big enough for your needs.

- Scott

[MWJB]

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.

[J M Fahey]

my amp actually is very load and basically have a good clean
sonic character but it is still to much "sensitive" into break up and
getting a bit too "bluesy" and into unwanted discrete dose of distortion
when try to play clean rithmic funky chords .

This says *exactly* "I do not have enough power" , only using more words.
You are going step by step towards a Twin by wishing Twin type transformers, only insisting in using 2x6550 instead of 4x6L6.
Just save yourself time and money (and frustration), get a Twin with approppriate speakers and be happy.
Or at least use more efficient 10's or even better, make a new frontboard and mount a loud fat 12" there, maybe a V30 or go all the way to an EV or JBL.

[Tom Phillips]

That correct. If you need a Mac Truck don't start with a little red wagon and try to modify.

[MWJB]

I agree to a point but it's worth pointing out that 6550 have a very different dynamic response to 6L6/5881. No matter how many 6L6 you put in parallel they will still compress like a 6L6 under load, 2x6550 will have a harder dynamic response than 4x6L6.

[J M Fahey]

Well, if that's your cake, you could build/mod into the loudest/cleanest Twin in the world, wih *four* 6550's in those four octal sockets, plus *two* EV's or JBL's.
Of course, God save your bandmates (and the Club's customers)

[ThermionicScott]

Sounds like these are some loud gigs already. SS-rectified 2x6L6 with heavy filtering and big output iron, and his amp is sounding "bluesy."

- Scott

[Jazz-O-Sonor]

Hi ,
Thank you to you all for the great inputs
Seems that the problem is solved now ;
the amp stays very clean up to 6 - 6 1/2 on dial without any loss of volume since I added a 0.002 cap to the reverb input
and added some new pre-amp tubes ;
I also use to have a hum noise coming from reverb that now is gone.
I think that actually this amp cant give more because of the 2x10 speakers , naturally with 2x12 speakers I would get a bigger sound
but I am very happy with the 2x10 arrangement , this is enough for my needs.
I like very much this little powerful amp;
I had to do some solder work to fix the reverb tank there was one of the little wires to resolder on a coil.
In these days I will test KT88 anyway , Im too curious to litsen how it will sound.

Thanks again for all your great teachings , John

[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.

Thank you for all the great advices John

[Jazz-O-Sonor]

>>>>>>>>>>>>>
I don't think the conversion to 6550s is really going to help very much with the current power transformer installed.
I still can't tell if you have the classic black and silver face Fender opto vibrato circuit installed.
If not, then I think you should reduce the bass response a little bit and drop some preamp gain before the phase inverter.
My gut feeling is you need better speakers and the preamp gain before the phase inverter is a bit too high without the vibrato circuit installed.
You can dump some signal to ground right before the phase inverter coupling cap, (no more the 1000pF to a max of 1500pF here!)... and try a 100K to 220K in front of the coupling cap with a 68K to 100K resistor grounded before the 100K to 220K resistor.
It's just an inexpensive starting point before spending a bunch of money on unneeded transformers and tubes.... but really consider the speakers too.

That TAD PT looks like a Pacific transformer... if so, they make really good stuff and I bet those current ratings will be mildly conservative.

Bruce, thank you for all your advices