Twin triode as one - advantages/disadvantages

[funkgang49]

What would be the advantages/disadvantages in running a twin triode as a single gain stage?

[xk49w]

What would be the advantages/disadvantages in running a twin triode as a single gain stage?

Assuming you mean paralleling them, lots of discussion on this.
Example.
Example.

[funkgang49]

Thanks xk49w for those examples - good reading. However let me tell you what I'm trying to do. I've built a SE amp similar to the Weber Maggie:
https://taweber.powweb.com/store/maggie_layout.jpg

Difference being that I'm using a single input. I'm also using 2 volume controls for 2 separate preamp tubes (V1-6SL7) & (V2-12AX7) with a shared tone control. Output is then blended and sent to a driver stage (V2b) then onto the power stage. I'de like to get a nice clean, high headroom output from V1 and a hotter, quicker breakup from V2. The 6SL7 has slightly less gain then the 12AX7 so I thought that I might be able to compensate for the slight disparity between the tubes by tying the 2 triodes together (shared A/K/G) in the 6SL7. Would tying the 2 triodes of the 6SL7 allow me to acheive my goal?

[tubeswell]

Would tying the 2 triodes of the 6SL7 allow me to acheive my goal?

Only one way to find out really.

[gingertube]

With 100K as the load resistor and the stage loaded with 1M Ohm to give effective load of 90.9K
ANS assuming an rp of 40 KOhms
Then theory says gain will change from:
48.6 with a single 6SL7 triode
TO
57.4 with parallel triode sections
With the same conditions the theory says a single 12AX7 triode will give a gain of 59.5

In practice I doubt you would notice the difference.

6SL7 is my favourite preamp tube - I use circuits originally intended to take 12AX7 and have never noticed a lack of gain, what I have noticed is that the higher gain circuits go into and come out of overdrive much more smoothly and it is much easier to set gain levels to achieve the overdrive sound you want. The only thing to watch is that the 6SL7 has much higher grid current than a 12AX7. That sometimes means changing the circuit slightly to accommodate a lower resitance back to 0V when looking away from the grid.

I have used parallel triodes on the input stage but for a different reason. I use dissimilar operating points to give some harmonic emphasis. That is I use separate cathode bias resistors and bypass caps on teh 2 triodes but wire the anodes together and also wire the grids together.

Example:
https://tubeamparchive.com/viewtopic.ph ... x&start=15

Cheers,
Ian

[Leo_Gnardo]

6SL7 is my favourite preamp tube - I use circuits originally intended to take 12AX7 and have never noticed a lack of gain, what I have noticed is that the higher gain circuits go into and come out of overdrive much more smoothly and it is much easier to set gain levels to achieve the overdrive sound you want.
Ian

Hi Ian, what 6SL7 do you reckon is your favorite? I have a pedal steeler who wants me to build him a preamp with octal base tubes and Baxandall EQ. NOS SL's I find way too often are noisy, tend to clank & howl. Have you found the Russian ones to be OK? I may try a grid leak input to get some squeeze and also separate the guitar/volume pedal from the input grid DC-wise. Thanx, LG

[funkgang49]

Only one way to find out really.

I found out! Wired everything up yesterday:
6SL7
Plate-51K
Grid-10K
Cath-910ohms
(bypass-47uf@50v)
12AX7
Plate-100k
Grid-15K
Cath-1.3K
(bypass-25uf@25v)
Results: WOW! The 6SL7 sounds fantastic with both hums. and sings. Clean, open, big sounding and quiet. I thought the 6SL7 might be a little dark sounding or slightly muted - no way! As a matter of fact the 12AX7 is a tad dark in comparison. I might tweak it to be a little brighter but before I start changing out components I'll try swapping out different tubes first.

With 100K as the load resistor and the stage loaded with 1M Ohm to give effective load of 90.9K
ANS assuming an rp of 40 KOhms
Then theory says gain will change from:
48.6 with a single 6SL7 triode
TO
57.4 with parallel triode sections
With the same conditions the theory says a single 12AX7 triode will give a gain of 59.5

In practice I doubt you would notice the difference.

Output wise the 12 has slightly more output than the 6 but in comparison it seems just about right. I might drop the Cathode resistor from 910 ohms to 820 ohms on the 6SL7 (from 1.8K to 1.6K effectively) which should get the outputs to balance more closely, but for now I'm just gonna play and listen. Some might say that would be splitting hairs but hair splitting is what we tone-tweakers do, eh?

Lots of headroom - the 6 doesn't start to breakup til 2 o'clock, the 12 starts at noon on the respective volume controls (with humbuckers!).
I might adjust output in the driver stage but, as in the preamp (12AX7), I want to try out a range of different tubes in the power section i.e., EL84(w/adapter), 6V6(as tested w/5AR4 recto), 5881, 6L6, EL34, KT77, etc., before I make any changes.
Overall I am very pleased - way more useful and versatile than a standard Champ and very pedal friendly but still with that slightly raunchy Tweed Champ crunch. Me likey!

[funkgang49]

Hi Ian, what 6SL7 do you reckon is your favorite? I have a pedal steeler who wants me to build him a preamp with octal base tubes and Baxandall EQ. NOS SL's I find way too often are noisy, tend to clank & howl. Have you found the Russian ones to be OK? I may try a grid leak input to get some squeeze and also separate the guitar/volume pedal from the input grid DC-wise. Thanx, LG

I am using a vibration damping ring on the 6SL7 as well as silicone rubber feet from a reverb tank to isolate the chassis from the head cabinet. I also have large rubber feet mounted on the cabinet bottom to isolate it from the speaker cab. As tested, with a NOS 6SL7, I have not encountered any microphonics or noise whatsoever. I have a bunch of russian made 6SL7's on hand and will be plugging them in and out today. I'll let you know what I find.

[Leo_Gnardo]

Hi Ian, what 6SL7 do you reckon is your favorite?

I am using a vibration damping ring on the 6SL7 as well as silicone rubber feet from a reverb tank to isolate the chassis from the head cabinet. I also have large rubber feet mounted on the cabinet bottom to isolate it from the speaker cab. As tested, with a NOS 6SL7, I have not encountered any microphonics or noise whatsoever. I have a bunch of russian made 6SL7's on hand and will be plugging them in and out today. I'll let you know what I find.

I'll have to score some of those tube condoms & russki 'SL's. Thanks much! LG

[gingertube]

6SL7 is my favourite preamp tube - I use circuits originally intended to take 12AX7 and have never noticed a lack of gain, what I have noticed is that the higher gain circuits go into and come out of overdrive much more smoothly and it is much easier to set gain levels to achieve the overdrive sound you want.
Ian

Hi Ian, what 6SL7 do you reckon is your favorite? I have a pedal steeler who wants me to build him a preamp with octal base tubes and Baxandall EQ. NOS SL's I find way too often are noisy, tend to clank & howl. Have you found the Russian ones to be OK? I may try a grid leak input to get some squeeze and also separate the guitar/volume pedal from the input grid DC-wise. Thanx, LG

I'm working my way thru' a whole batch of NOS 6SL7 tubes - many different brands. Philips ECG are what I use in my HiFi Amp, I find the NOS tubes better than the modern production Russian 6N8 although they are pretty good in guitar amp circuits too.

Cheers,
Ian

[jjman]

I had to replace 3 fairly new Sovtek 6SL7's in a friend's Ampeg since they hummed. NOS did not hum.