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

[Jazz-O-Sonor]

Hi,
Please can anyone could describe how to make the conversion
of 2x6L6 to 2x6550 power tubes on a 45/50w Fender amp?
I need to switch to 6550 in order to set for a cleaner sound with
very, very late break up (or no break up at all) , for Jazz use.

I read that is raccomended to hook up the pre amp tubes to an aux 6,3V
3 amp auxliary filament transformer;
Since my Super Reverb PT is 220/230/240vac model ; which Filament
Tranformer would fit for the application? Any model/or brand to suggest pls ?

On circuit board I have 220K bias-feed resistors to which values
those need to be changed for 2x6550 ?

Should I also change the 470R screen resistors to 1K 5W ?

Below there is a description of the amp and of all the mods
that I already done . Everything has been done very neat and clean;

Fender SF Deluxe Reverb circuit ; I replaced all electrolytic caps
(cathode,bias,filter caps) with brand new ones , the phase inverter section has been converted exactly to SF TR specs

Filter Caps: 2x 220uF-350w and three 20uF-500

Tad Super Reverb PT this tranformer have the option of more windings taps for solid
state recto) I'm using the recto taps hooked to a TR style
Rectifier board w/6 5408 diodes

MM SuperSix Reverb OT 2/4ohm taps: hooked w/ 2ohm tap
2x10 8ohm-100w Delta Demon Emi's speakers

TR style Choke filter

Power Tubes: 2x6L6 SED winged C
Pre-Amp Tubes: EH12AY7 in V1 , Fender12AX7 in V2, EH12AT7 in PI

This amp actually sounds beautiful but starts to break up on 6 on the dial
and I wish to get a very clean sound from it (similar to a TR or a bass amp).


thank you , John

[überfuzz]

Hold the presses. There are some things you can do before getting your hands dirty.

1. Try different tubes? You can alter the break up with tighter tubes. Try to get a pair thats closely balanced.
2. Check the bias and see if you can nudge it down a bit. It might clean up the tone a bit.

Check the B+ voltage in you amp, what model is it by the way?

Off Topic: If you're looking for a cleaner sound you might wanna consider an other amp altogether. There are heaps of amplifiers with 'beautiful' jazzy voices. My opinion, go for a silver faced Fender, clean and powerful.

[Tom Phillips]

This amp actually sounds beautiful but starts to break up on 6 on the dial
and I wish to get a very clean sound from it (similar to a TR or a bass amp).

John,
I too would like to know what this amp is in the first place.
Note that the Fender design basically guarantees that the amp will be driven into distortion by the time you get to "6 on the dial." You could already be at the point that the amp is giving its all and possibly the switch to 6550s won't help.
Tom

[Steve Conner]

Yes. Just because it "breaks up at 6 on the dial" doesn't mean that it lacks clean headroom. If you want a clean sound, just don't turn it up as far as 6. See the Spinal Tap "up to 11" sketch for an explanation.

If you have this PT: Fender Power Transformer, 125P5D, 125P5DX, 022798, 022806, Super Reverb, Bandmaster Reverb,  Pro, Pro Reverb, Bassman Ten, 120/240V it has a 5 amp heater winding. What is the total draw of all your preamp tubes? It might be OK.

In my experience changing to 6550s gives another 10-20 watts. Worth it?

[MWJB]

The stock heater current draw is 1.8A for the 6L6s plus 1.8A for 6 preamp tubes, you mention that you only have 3 preamp tubes (? if not, don't risk 6550 with the full 6 preamp tube compliment), freeing up 0.9A. 1.6A each for the 6550 plus 0.9A = 4.1A. A stock SR PT can take this, but if you wanted to add the auxilliary 6.3VAC tranny (wise) use a 2A model and run the preamp tubes off this. Run the power tubes from the main PT, mount the auxilliary 6.3VAC tranny on the cap can cover (or similar) and don't forget a virtual CT.

If you have reduced the number of preamp tubes, did you remember to adjust the V1 pin 8 cathode resistor value? Should be 1.5K.

Steve is right, you will only see 15-20W more, but the tone will be markedly cleaner with harder dynamics.

No need to alter grid load resistors, 220K is fine.

Do change screen grids to 1K 5W.

Whenever fitting power tubes with a bigger heater draw, check for a significant drop in the 6.3VAC voltage, or more than 20v drop off the B+ at reasonable current draw.

If you really wanted a clean tone, I'd be keeping the 12AX7 in V1, replace the first stage cathode resistor with 2.7K/3.3K, bypassed with a 10-20uf cap. Adjust B+ dropping resistor to suit your taste. You might also find a 12AX7 will work fine in the PI too?

[JoeM]

2x10 8ohm-100w Delta Demon Emi's speakers

Why Delta Demons? You'll get more volume using one of Eminences more efficient speakers. Ragin Cajuns for example.

[Bruce / Mission Amps]

"MM SuperSix Reverb OT 2/4ohm taps: hooked w/ 2ohm tap
2x10 8ohm-100w Delta Demon Emi's speakers"

... never heard a 2x10 jazz amp I really liked.
Delta Demons... low sensitivity for a loud jazz amp and Eminence cheats on those dB/ watt figures anyhow.

DR amp with a 12AY7.... but with no vibrato? I can't tell from your description but the vibrato circuit kills a lot of unusable gain in the real amps.
Is yours modded to correct for that?
No, too much preamp gain hitting the phase inverter causing lots of distortion at higher levels?

There's a couple bottle necks right out of the gate.

[JoeM]

...
I wish to get a very clean sound from it (similar to a TR or a bass amp)...

If you want the amp to sound like a Twin, why not just buy one? One of the best jazz guitar sounds I've ever had was playing my archtop through a SF TR with JBLs. Heavy SOB though!

[Jazz-O-Sonor]

The stock heater current draw is 1.8A for the 6L6s plus 1.8A for 6 preamp tubes, you mention that you only have 3 preamp tubes (? if not, don't risk 6550 with the full 6 preamp tube compliment), freeing up 0.9A. 1.6A each for the 6550 plus 0.9A = 4.1A. A stock SR PT can take this, but if you wanted to add the auxilliary 6.3VAC tranny (wise) use a 2A model and run the preamp tubes off this. Run the power tubes from the main PT, mount the auxilliary 6.3VAC tranny on the cap can cover (or similar) and don't forget a virtual CT.

If you have reduced the number of preamp tubes, did you remember to adjust the V1 pin 8 cathode resistor value? Should be 1.5K.

Steve is right, you will only see 15-20W more, but the tone will be markedly cleaner with harder dynamics.

No need to alter grid load resistors, 220K is fine.

Do change screen grids to 1K 5W.

Whenever fitting power tubes with a bigger heater draw, check for a significant drop in the 6.3VAC voltage, or more than 20v drop off the B+ at reasonable current draw.

If you really wanted a clean tone, I'd be keeping the 12AX7 in V1, replace the first stage cathode resistor with 2.7K/3.3K, bypassed with a 10-20uf cap. Adjust B+ dropping resistor to suit your taste. You might also find a 12AX7 will work fine in the PI too?

Hi MWJB ,
thank you for the advices;
The amp in question is a SF Deluxe Reverb w/Super Reverb PT and SuperSix OT
(a beefed TR style OT)

yes I'm running all 6 preamp tubes and I really wish to do swtch to 6550.
I just need a later break up; actually the amp have plenty of headroom
but still need more clean.

About the 6.3v 3A auxliary Filament transformer ;can you pls link me to some
model/brand indicated for this application? thx
My PT is a TAD Super/Pro Reverb 220/230/240vac with optional/additional taps
for solid state rectification.
I've been looking around the web but I'm still confused about brands and specs

Why a 12ax7 in PI ? Should not be a 12at7 quiter and cleaner for phase inverter operation?


thank you, John

[MWJB]

Torres Valve Amplifiers in Pinner, UK do a 6.3/5VAC 3A transformer. £17-ish.

With the larger cathode resistor in V1 the amp will be cleaner, the 6550 will be cleaner still, the 6550 'may' need the bigger signal from the PI to fully exploit the better dynamic response. I once ran a Twin reverb with 2x SED 6550, the cathode resistor change noted (170 to 180-ish on V1)and an 8ohm speaker load (to correct impedances). It sounded huge, big, warm clean tone. A twin with 4x6L6 will still not have the dynamic response of 2x6550. The amp should be quiet with a 12AT7 or a 12AX7.

[Jazz-O-Sonor]

"MM SuperSix Reverb OT 2/4ohm taps: hooked w/ 2ohm tap
2x10 8ohm-100w Delta Demon Emi's speakers"

... never heard a 2x10 jazz amp I really liked.
Delta Demons... low sensitivity for a loud jazz amp and Eminence cheats on those dB/ watt figures anyhow.

DR amp with a 12AY7.... but with no vibrato? I can't tell from your description but the vibrato circuit kills a lot of unusable gain in the real amps.
Is yours modded to correct for that?
No, too much preamp gain hitting the phase inverter causing lots of distortion at higher levels?

There's a couple bottle necks right out of the gate.

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

[Jazz-O-Sonor]

Torres Valve Amplifiers in Pinner, UK do a 6.3/5VAC 3A transformer. £17-ish.

With the larger cathode resistor in V1 the amp will be cleaner, the 6550 will be cleaner still, the 6550 'may' need the bigger signal from the PI to fully exploit the better dynamic response. I once ran a Twin reverb with 2x SED 6550, the cathode resistor change noted (170 to 180-ish on V1)and an 8ohm speaker load (to correct impedances). It sounded huge, big, warm clean tone. A twin with 4x6L6 will still not have the dynamic response of 2x6550. The amp should be quiet with a 12AT7 or a 12AX7.

Hi MWJB ,
wow :: thank you for the link and the great advices
I already contacted Torres UK for the filament tranf.
Would you please explain in details how to install
the filament tranf (all connections es: to preamp tubes , filter caps ecc)

thank you , John

[Gibsonman63]

The JBLs will make is much cleaner... and heavier.

[Jazz-O-Sonor]

Yes. Just because it "breaks up at 6 on the dial" doesn't mean that it lacks clean headroom. If you want a clean sound, just don't turn it up as far as 6. See the Spinal Tap "up to 11" sketch for an explanation.

If you have this PT: Fender Power Transformer, 125P5D, 125P5DX, 022798, 022806, Super Reverb, Bandmaster Reverb,  Pro, Pro Reverb, Bassman Ten, 120/240V it has a 5 amp heater winding. What is the total draw of all your preamp tubes? It might be OK.

In my experience changing to 6550s gives another 10-20 watts. Worth it?

My PT is a TAD Super Reverb/Pro 125P5DX rated at 5,5A on the filament
Do you think could be ok for 2x 6550 ?
Im my amp I have 6 preamp tubes.

Thank you , John

[jrfrond]

I guess I am a bit more of a cowboy than some others, or perhaps, in my mind, playing with numbers is sometimes unproductive.

Change the screen resistors to 1K/5W. Install a pair of 6550's. Find the correct bias point area using the crossover notch method on a scope (you DO have a scope, don't you?), and run the amp for a few hours. If the power transformer gets too hot to keep your hand on for more than 10sec., you are drawing too much current. If you like the sound, you are golden. If not, then at least you know. And you can leave those screen resistors in. As a matter of fact, 470 ohms, in my professional opinion, is too low for current production Russian or Chinese tubes, which seem to fare better and realize a longer life span by limiting screen current a bit vs. the old 470 ohm resistors.

A 12AT7 might work better for you in tandem with the 6550's to ensure later breakup. However, I did say LATER breakup, not NO breakup. The fact that you need to play the amp on 6 in a jazz setting might mean that your rig is underpowered for your needs.