6SK7 as a driver for a 6L6 SE AMP

[Jerry garrcia]

Hi all. Planning on building a SE amp with a 6SJ7 up front and a 6L6 or a EL37 (probably 6L6GC). Simple Princeton tone stack. Aiming for maximum clean volume so plate voltage around 420 for the 6L6  and a 5K 125ese OT.
Read somewhere that the 6SJ7 isn’t sufficient to fully drive an 6L6. Have some 6SQ7’s but would like to keep them as spare tubes for my EH-185 clonish amp and for higher gain projects.
Have anybody used a triode wired 6SK7 as a driver for a 6L6 or is it just a stupid idea? Have a bunch of them from old radios and some of them checks out fine.

[DummyLoad]

6SK7 strapped in triode mode should be fairly close to 6SJ7 characteristics - E.g. AF of about 20 - or basically 1/2 of a 12AU7.


Refer to resistance coupled charts for 12AU7 - Should help zero in on some bias conditions. 


--pete

[Jerry garrcia]

6SK7 strapped in triode mode should be fairly close to 6SJ7 characteristics - E.g. AF of about 20 - or basically 1/2 of a 12AU7.


Refer to resistance coupled charts for 12AU7 - Should help zero in on some bias conditions. 


--pete

So what you are saying is that is that it would help since I’m not that interested in a high gain amp but to keep the clean tonal characteristics of the 6SJ7 that marries beautifully with an archtop. The best way should have been to go P-P but the cabinet that will accommodate the amp has not much room so a maximum of three tubes will fit. And since the cab is fairly big and has a nice 12” speaker I want it to be able to use at larger gigs.

So some more question:
1) is it true that an 6SJ7 wouldn’t be able to fully drive an 6L6GC?
2) what is the optimal characteristic of a good driver tube? What to look for depending on the power tube in use?  Only used the second tube to increase the gain in previous builds and therefore calculated the total voltage gain.
3) should a EF86 be better than the 6SK7? Aiming for all octal but since I’m not looking for distortion I’ve read that the EF86 is a good candidate.

[kagliostro]

You say the chassis has enough space only for 3 tubes

as well known a tube rectifier don't give advantages on an SE amp and you can go for a FW diode bridge

So why don't insert a 6SN7 between V1 and the Power Tube, you can also arrange a triode as CF and drive the Tone Control with It

Franco

[pdf64]

…
Read somewhere that the 6SJ7 isn’t sufficient to fully drive an 6L6. …

What was the context of the stuff you read?
What do you perceive the drive requirements of a 6L6 SE output stage to be?
The massive benefit of the introduction of output pentodes in the 40s was that they were really easy to drive, there being not much benefit in positive grid operation, unlike the previous output triodes.

[Jerry garrcia]

…
Read somewhere that the 6SJ7 isn’t sufficient to fully drive an 6L6. …

What was the context of the stuff you read?
What do you perceive the drive requirements of a 6L6 SE output stage to be?
The massive benefit of the introduction of output pentodes in the 40s was that they were really easy to drive, there being not much benefit in positive grid operation, unlike the previous output triodes.

Yep, that’s what I thought to but in some forums, don’t remember which, were writing that the single 6SJ7 didn’t have the capacity to drive a 5C1 with an 6L6 instead of the 6V6. Or others that the single 6SJ7 couldn’t fully drive the parallel 6V6’s in a GA-9. That’s the reason that I asked.

[Jerry garrcia]

You say the chassis has enough space only for 3 tubes

as well known a tube rectifier don't give advantages on an SE amp and you can go for a FW diode bridge

So why don't insert a 6SN7 between V1 and the Power Tube, you can also arrange a triode as CF and drive the Tone Control with It

Franco

I’m gonna go SS rectification, primarily due to cabinet space but also that I will be using one of my old radio PT’s. I’ll need the extra voltage to get the right plate voltage and many of them radio PT’s had selenium rectifiers.
Will use a diode bridge and will have to read up on the cathode follower. Never used it before. Do you have any tips for some schematics that utilises a CF to drive the tone stack?

[shooter]

maybe worth a read, He has a AC version also
The Valve Wizard

[pdf64]

… that the single 6SJ7 didn’t have the capacity to drive a 5C1 with an 6L6 instead of the 6V6. Or others that the single 6SJ7 couldn’t fully drive the parallel 6V6’s in a GA-9. That’s the reason that I asked.

Capacity in what sense?
What was the context of the comments? eg were the writers just plugging a 6SJ7 into a circuit intended for a different valve type?
ie rather than using a circuit specifically designed for 6SJ7.

I will be using one of my old radio PT’s

Will use a diode bridge

A full wave bridge rectifier will produce over twice the required DC HT voltage, when used with a winding intended for a 2 phase push pull rectifier.
Unless half the winding is left unused.
I think you probably need to use a 2 phase push pull rectifier, of course suitable silicon diodes can be used to increase the DC output voltage a bit.

See http://www.valvewizard.co.uk/bridge.html to help get your head around it.

some schematics that utilises a CF to drive the tone stack?

The Fender 5F6A Bassman and Vox Top Boost circuits are classic examples of that.

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.gif

https://el34world.com/charts/Schematics/files/Vox/Vox_topboost_preamp.pdf

[kagliostro]

In my previous post I forgot to Say that a 6L6 tube has a trsconductance of 4700 vs an EL34 that has a trasconductance of 11000 this will result in a much less drive required for the EL34 tube

(you told  .... a 6L6CG tube or an EL34 tube)

Franco

[PRR]

Are we talking 6SJ7 "capacity" as Input level or Output level?

With careful bias and ample B+, I'm sure a 6SJ7 can smack a 6L6, only IF there is enough input to drive the 6SJ7 fully. An e-guitar may not make it unless you have Jack Armstrong wrists and a titanium pick; not for long sessions.

Much tone-control may make it even tougher.

This is 1999 (at least?). You can stick a JFET before the first tube.

[bmccowan]

I do not have the electronics knowledge of others here. But that never stops me from throwing out ideas. Merlin's preamp book covers driving pentodes in V2 position. I have done that a few times with good success. One is a Bogen CH-8 that I rewired for guitar. I have a 6AV6 driving a 6SJ7 driving a 6L6. Its a nice tone machine. Another is a variation on a Matchless Clubman with a 6SL7 driving a 6SH7 into 2x6V6. It seems to me that driving the pentode is a good way to go. I have also built and/or rebuilt many Gibson and other circuits with pentodes in V1, but if more than 1- 6V6 SE, they all have had an additional gain stage. 

[Jerry garrcia]

In my previous post I forgot to Say that a 6L6 tube has a trsconductance of 4700 vs an EL34 that has a trasconductance of 11000 this will result in a much less drive required for the EL34 tube

(you told  .... a 6L6CG tube or an EL34 tube)

Franco

That’s one of my knowledge gaps. Not that well read up on trasconductance and drivers. Is there a book or page that covers tHis stuff. I think this is probably the reason I’m asking and why the question came up.
Previous I’ve just focused on mixing different preamp tubes that I like, calculating the voltage gain so a two channel amp will gave aprox the same output volume, different paraphrase PI’s and power sections.
Is there a book or page that covers t

[Jerry garrcia]

Are we talking 6SJ7 "capacity" as Input level or Output level?

With careful bias and ample B+, I'm sure a 6SJ7 can smack a 6L6, only IF there is enough input to drive the 6SJ7 fully. An e-guitar may not make it unless you have Jack Armstrong wrists and a titanium pick; not for long sessions.

Much tone-control may make it even tougher.

This is 1999 (at least?). You can stick a JFET before the first tube.

I think this is also a black hole for me and why the topic came up. The guitar utilised will be mainly archtops with CC or P90 pickups and aiming for cleans. So no “hot” pickups here resulting in naut to much “in” signal.
Ok. As I looked around I’ve quite a few 6SN7’s so will probably use one as @kagliostro suggested as a CF and a more complex tone stack than the Princeton one.
A triod wired 6SK7 would have been more fun though…

[Jerry garrcia]

Went with the unbeaten path. 6Sk7 driver and got me a new PT since none of the ones I have will give me thiehigh B+. Uncertain about the triode strapping of the pentode but found some lecture that this should be the optimal way. The cathode resistor might be tweaked a bit. Looks ok?

[DummyLoad]

One of the caps in the edit will keep you from destroying the 6L6.   

--Pete

[Jerry garrcia]

One of the caps in the edit will keep you from destroying the 6L6.   

--Pete

Thanks and sorry. Forgot to post the last edit.
Forgot to put in the coupling cap after the 6SK7 in the first one and have changed the 1M master volume pot to a 500K but could also go with the 1M and add a 470K grid leak due to the specs of the 6L6.
I was planning to leave room for the bypass cap but might include it from the beginning and make it switchable?
This is one thing that I love with this forum (and some others) is that people really help.

One other thing is the hookup of the 6SK7. A bit uncertain of the large cathode resistor. Hard to find any info regarding triode wiring of a 6SK7.
🙏