Subbing 6SL7's in 12AX7 circuits

[ampgeek]

I have built 4 all octal 5F6A (Fender Tweed Bassman) amps using exactly the same cap and resistor values as the original 12AX* plan calls out and they sound fantastic!

I spliced in the standard two-tube reverb circuit using the typical 9 pin tubes in one of them and that sounded great as well.

All used red base 5692s (industrial 6SL7) and none ever had any signs of hum or microphonics.

Cheers,
Dave O.

[EtherealWidow]

I can't tell you how beautiful I think that is.

[Blackburn]

I was checking out the differences between 6SL7s and 6SN7s in my RCA manual and they have the same pinouts, as well as some different design centers, but I'm curious if they can be swapped without any change of values? Anyone done this before? I REALLY want to try out the N, as I hear it is quite harmonic with a fantastic rich tone.

[David Root]

You need to lower the plate and cathode resistors because the 6SN7's plate resistance is about 8,000 and the 6SL7's is 44,000 ohms.

Gain is 70 for the SL7 and 20 for the SN7. The 6SL7 seems to work fine at typical 12AX7 plate and cathode values, eg 100K/1K5, but the 6SN7 needs much lower values as it draws more current, similar to the difference between the 12AX7 and the 12AT7.

Tube audio freaks that like octals especially like the 6SN7 as a phase inverter. I've tried both as PIs in guitar amps and I preferred the 6SL7 in a long tailed pair. A 6SN7 as a cathodyne PI might be interesting with power tubes that don't need much signal, like the EL37, or even the EL34, but I haven't personally tried that.

[darryl_h]

I was checking out the differences between 6SL7s and 6SN7s in my RCA manual and they have the same pinouts, as well as some different design centers, but I'm curious if they can be swapped without any change of values? Anyone done this before? I REALLY want to try out the N, as I hear it is quite harmonic with a fantastic rich tone.

They can be swapped, the same way members of the 12A*7 family can be swapped with each other. The biasing may not be optimal, but they work fine.

I've experimented on the bench with 6SL7/6SN7 swaps using a recent build, but haven't yet had a listen to the swaps with guitar.

[img:800:600]http://i497.photobucket.com/albums/rr33 ... 81758c.jpg[/img]

[Blackburn]

Very interesting, guys. I'll probably just stick with using the SL7s because of how they run with 12AX7 values... And I'm more familiar with them. I've got my eye on a particularly nice one. I know SN7s are more audiophile tubes and rarely see use in geetar amps. Just thought I'd ask round here.

[C Moore]

I thought the N7 sound "fine".....but they have way less gain. Just about 30% of the L7 I believe.
best

[ampgeek]

Just noticed that I wrote 5692. Typo!! I mean't 5691. Sorry for any confusion.

I do have a handful of the 5692 (6SN7 equiv.) and have swapped them in. They do, indeed, exhibit significantly lower gain in the 5F6A plan.

Cheers,
Dave O.

[passfan]

ALRIGHT. As much fun as it is talking about the heaters alone, I still wonder, does anyone know for sure whether or not it's ok to use 6SL7's in pretty much any 12AX7 circuit?

I know that it was established that with typical 100k plate load resistor and 1.5k cathode resistor the circuit would be fine. What if that's not the case? In that link I posted with the grid leak cascode, would it be fine to substitute the 12AX7 for the 6SL7?

Passfan, in your 6SL7 Express, did you use the typical 1.5k Rk and 100k Ra?

Except for the PI , which was taken off Aikens Site. But I have done them both ways and all my amps are still tickin'

[passfan]

I was checking out the differences between 6SL7s and 6SN7s in my RCA manual and they have the same pinouts, as well as some different design centers, but I'm curious if they can be swapped without any change of values? Anyone done this before? I REALLY want to try out the N, as I hear it is quite harmonic with a fantastic rich tone.

I also do this in Octal Expresses and Rockets to cut the gain structure down to play softer styles or country. It works like swapping an ax7 for an au7.

[gingertube]

My own DIY guitar amp runs 6SL7.
Preamp is a London Power Standard Preamp (LPSP) with some very minor component value changes. These minor changes are to accommodate higher grid current levels in 6SL7 as compared to 12AX7, that is, I have tried to keep Rg1 values a little lower and in any case keep Rg1 to no more than 3 times the anode load resistor for that stage (RDH recommendation for high mu tubes in cathode bias). In one instance this meant raising the anode load rather than lowering Rg1.

Power Amp is 6SL7 Common Cathode Amp to Concertina phase splitter driving a pair of 6V6G.
I compare this to another amp I built for a friend, in that case a LPSP with 12AX7 and a power amp using the "inductry standard" differential splitter and a quad of 6V6G (50W Marshall Output tranny).

I find the 6SL7 pre easier to use. It goes into and out of overload more smoothly and it is easier to dial up the gain level you actually need to achieve a particular overdrive / sustain level. It is also easier to drive the "overdrive" level fronm the guitar, that is by how hard you pick/strum.

How much do I like it? Well suffice to say that I have collected 30 or more 6SL7 and a few 12SL7 for future guitar amp builds. They are my "default" guitar amp , preamp tube. I have also listened to it with modern production Russain / Chinese 6N8 and they work well too.

I did approach Kevin about the possibility of posting the schematic but he doesn't want that LPSP design in teh "public domain". Buy one of his TUT series books and it is all there. If you ONLY want to pay out for one of hiis books I'd recommend TUT3 or TUT5.

Cheers,
Ian

[Cliff Schecht]

I pick up 6S*7 tubes whenever I can find them. To be honest 30 of them is not a lot. I find myself always sorting through a lot of these to find good ones. Some are low noise yet still microphonic in V1, others are low microphonics but noisy. They're all over the place really, but it's fun to roll a bunch through an amp and see what sounds best.

I've been pleasantly surprised many times by putting tubes that I thought were going to sound cruddy (badly mismatched power tubes, weak looking getters, etc..) and they sounded fantastic (and vice versa for tubes that *should* sound fine). Every circuit and really every amp is different and I do enjoy experimenting to see what sounds good where. This is one of the big reasons I love cathode biased amps, they let you plug and play without the fuss of having to rebias.

[renshen1957]

Very interesting, guys. I'll probably just stick with using the SL7s because of how they run with 12AX7 values... And I'm more familiar with them. I've got my eye on a particularly nice one. I know SN7s are more audiophile tubes and rarely see use in geetar amps. Just thought I'd ask round here.

Hi,

Earlier Garnet Amps commonly used 6Sn7 in the PI before converting to 12Au7 (and finally) to 12AX7. The Canadian Garnet and Traynor were of the loud and clean school for their basic amp philosophy for the most part.

Best Regards

Steve

[renshen1957]

My own DIY guitar amp runs 6SL7.
Preamp is a London Power Standard Preamp (LPSP) with some very minor component value changes. These minor changes are to accommodate higher grid current levels in 6SL7 as compared to 12AX7, that is, I have tried to keep Rg1 values a little lower and in any case keep Rg1 to no more than 3 times the anode load resistor for that stage (RDH recommendation for high mu tubes in cathode bias). In one instance this meant raising the anode load rather than lowering Rg1.

Power Amp is 6SL7 Common Cathode Amp to Concertina phase splitter driving a pair of 6V6G.
I compare this to another amp I built for a friend, in that case a LPSP with 12AX7 and a power amp using the "inductry standard" differential splitter and a quad of 6V6G (50W Marshall Output tranny).

I find the 6SL7 pre easier to use. It goes into and out of overload more smoothly and it is easier to dial up the gain level you actually need to achieve a particular overdrive / sustain level. It is also easier to drive the "overdrive" level fronm the guitar, that is by how hard you pick/strum.

How much do I like it? Well suffice to say that I have collected 30 or more 6SL7 and a few 12SL7 for future guitar amp builds. They are my "default" guitar amp , preamp tube. I have also listened to it with modern production Russain / Chinese 6N8 and they work well too.

I did approach Kevin about the possibility of posting the schematic but he doesn't want that LPSP design in teh "public domain". Buy one of his TUT series books and it is all there. If you ONLY want to pay out for one of hiis books I'd recommend TUT3 or TUT5.

Cheers,
Ian

Hi Ian,

Speaking of TUT3 there is the Ampeg Portaflex amp project that uses 6SL7 preamp tubes (and the option to go to 12AT7).

I have more experience with the 6SC7 (the single cathode cousin of the 6SL7) in the Glass Tube variety. I am more inclined for either tube to change the Plate resistor to the 220k.

I hope all is well in Oz

Best regards,

STeve

[Nichesound]

This uses a 6SN7 preamp with 82K ohm load resistors in a cathode follower config into a Phase inverter 6SL7 cathodyne with 6V6 power tubes and a 5Y3 rectifier.

I use a 6J7 (pentode) that goes into the 6SN7 then a preamp volume pot into a single tone pot into a volume knob that is before the PI.

https://www.youtube.com/watch?v=2Kfcd-yeODw

for those out there that think they can't use a 6SN7 in a preamp config, well there you go...

ps..I am a drummer per trade but dink with guitars and build amps for fun and sometimes for delighted buyers -These are for sale as of 12-2014.