converting 6sc7 amp to use 6sl7

[dmartn149]

I have an old PA amp that I've converted to a guitar amp. It has 6sc7 preamp and PI tubes and I want to rewire the sockets to use 6sl7 tubes. It looks to me like all I have to do is move the lead from pin 3 to pin 1, then add a jumper from pin 6 to pin 3. Is that correct and is that all I need to do? Should any other components be changed? Thanks

[Jazz P Bass]

Your assumption on the rewiring seems correct.
Don't know about 'what other components to change' but I would question why you want the tube change.

Those 6SL7's are notorious for microphonics.
I think it is because of the shared cathode.
Either that or the 'shield to reduce hum' is the cause.

I do not care for that tube.

My 2 cents.

EDIT: I am confused on the tube numbers>

I'll get better.

[dmartn149]

Thanks for the input Jazz, but I think you may have the SC and SL mixed up. The 6sc7 has a common cathode and is very micronophic. That's why I want to try the 6sl7

[Jazz P Bass]

Thanks for the input Jazz, but I think you may have the SC and SL mixed up. The 6sc7 has a common cathode and is very micronophic. That's why I want to try the 6sl7

Yup.

Confused am I.

So rewire it & let us know.

[dmartn149]

Yup.

Confused am I.

So rewire it & let us know.

Will do.

[Enzo]

In the shared cathode, the cathode resistor carries the current for two triodes, so for a given resistance, the voltage across it will be twice that of a single triode. So to set the desired bias voltage, you'd have to double the value for each single triode.

For example, the common Fender circuit with 1500 ohm single cathode resistor becomes an 820 ohm resistor when cathodes are shared.

[dmartn149]

Thanks Enzo. I think the cathode resistor for v1 is a 2.2k. with a 22uf cap. I don't remember how I arrived at those values. I think it was on advice from this forum (Loudthud maybe) It was originally grid leak biased. v2 has it's original 1.8k And the PI has its original 1.5k So anyway I thought I would jumper pins 3 and 6 tying the cathodes together and leave the resistors alone. What do you think?

[Enzo]

Try it, it isn't like it would hurt anything. If you like it, then it is good.

[catalin gramada]

Hi,
shared cathode will not work well into cascaded configuration (series gain stages). I found 6sl7 to be microphonics in a sense it have afinity to develop a lot of EM couplings (more if you keep the HT supply pretty high) but have not any wories about severe oscilations into audio frequency range. You can even use a metal shield over bulb to improve SN ratio if you feel, not a classic solution still.
Cheers
Catalin

[dmartn149]

shared cathode will not work well into cascaded configuration (series gain stages).

Do you mean where one triode overdrives the next for distortion? This amp is not like that. It's two channels, two triads each, then PI. Not much preamp distortion.

[mac dillard]

I have built a couple of Fender 5B3/ Rickenbacker M-11 that I ended up using the 6SL7's in because of Microphonic problems with the 6SC7's . I think they sound pretty good. Sovtek and Tung-Sol have new production 6SL7. I first used the Tung-Sol but it has got to expensive for me. About $35.00. The Sovtek is about $12.00. I don't hear good enough to tell much difference. I didn't change the cathode resistor in V2. It is 2.5K. V1 is grid leak biased with 5m resistors. Left them the same too.

[catalin gramada]

I have built a couple of Fender 5B3/ Rickenbacker M-11 that I ended up using the 6SL7's in because of Microphonic problems with the 6SC7's . I think they sound pretty good. Sovtek and Tung-Sol have new production 6SL7. I first used the Tung-Sol but it has got to expensive for me. About $35.00. The Sovtek is about $12.00. I don't hear good enough to tell much difference. I didn't change the cathode resistor in V2. It is 2.5K. V1 is grid leak biased with 5m resistors. Left them the same too.

Have to agree, new 6sl7 Tung Sol are pretty quiet relative to old productions, but if you take a look inside will see it is diferent build tube than old one. Anyhow , expensive enough

[dmartn149]

I finished the conversion, and it's a big improvement. The 6sc7's were so microphonic that even a light slap to the side of the cabinet would cause noise from the speaker. The 6sl7's are very quiet, and actually they aren't even real 6sl7's These are NOS Soviet 6N9S tubes. I know that they don't have the best reputation, but they sound just fine to me, and they were only $4.00 ea. Of the three I ordered one was DOA, so I'm subbing a 6SN7 that I already had as the PI and the tube seller is sending me a replacement for the dead tube.
Next project is to use the old 6SC7 bases to build adapters so I can sub 12AX7's

[Gingertube]

I really like 6SL7.
Here is my take on a London Power (Kevin O'Connor) Standar Preamp with 6SL7 in lieu of 12AX7.
I'm running it with a Fender 5E3 style 6V6 push pull power amp with another 6SL7 for the poweramp common cathode gain stage and concertina splitter.
A number of local professional players have used this amp and I have had only positive comments.
Cheers,
Ian

[baddog]

Dmartn149,

That sounds right to me. What cathode resistor is currently being used on the 6sc7?