6sn7 - 2 watt amp

[Prattacaster]

Looking for a jumping off point here. I plan to build is a 6sn7 based push pull 2 watt amp. I need some direction as to which 6sn7 to use as the power tube. This amp will also have a 6sn7 cascode input gain stage. Which tube would likely be best for the power tube and which is best suited for cascode. Tubes available are:
(2) green label sylvanias triangle plate
(3) white label black plate tungsols spiky spacer type
(1) RCA silver label flat black plate bottom getter
All NOS of course. I dont expect to get an overwhelming response with these uncommon tubes, its good to be hopeful.

[riz]

I'm interested in doing something like this myself. Keep us posted! I myself am catching up on some tube-related reading while taking care of some other things in life, but when I have the time to get back into it I will hopefully be better equipped to hang with this forum, and start designing some amps! Post up a schem if you come up with one! I wouldn't worry yet about using what BRAND of tube where, the tube specs should be the same regardless of position, no? The individual TUBES' circuits, biasing, etc., will be different depending on position in the AMP circuit, but... Roll 'em around till you get the best sound after you build it!

[MWJB]

+1 on what Riz says, if going NOS I'd stick to GTA/GTB types (450vdc 5W per side) for the greater voltage handling. I'm running my 6SN7 power tubes at 300-370vdc on the plate for ~2W in PI driven mode, about 1W in self-split mode.

What kind of OT are you looking at?

[Prattacaster]

You will be dissapointed because of blatancy but the hammond 125C just seems like the best way to go. The 125B probably has enough current handling but its more expensive suprisingly enough. I actually have a scheme and a layout I drew. Its a very minimalistic design, just single volume, single tone. SS recto so I have a standby. Tone pot will be a push pull style to allow for a mid-boost. Tone is just a muff style with tweaked values to fit the 6sn7 cascode output impedence about 47k. The mid boost switches to a different cap and resistor to ground simltaneously. What this does is gives a near flat response at Tone "6" and also boosts the overall signal level. The PI - LTP 12at7 gives plenty of output to drive the output. I could keep with the octal idea and go 6sl7 as I have plenty of high quality octal sockets and the chassis is punched for octals....any thoughts on that? MWJB what is self split mode?

[MWJB]

Self-split doles away with the phase inverter tube & the power tubes themselves become "self inverting", somewhat like a
125C looks like a fine choice, of my 2 6SN7 powered amps, one likes 17K primary best, the other prefers 32K.

LTPI. The in-phase tube is modulated by its grid, the 2 tubes share a common cathode resistor (no bypass cap) & the out of phase tube is modulated by the shared cathode, the grid is grounded.

For a cathode biased amp to convert to self split, ground the out of phase power tube/triode's grid (don't try this with a fixed bias amp as you will take away bias voltage & kill the tube) & lift any power tube cathode bypass cap, if you replace the power tube cathode bypass cap, you now have the same amp operating in SE, unground the out of phase tube's gride and you have PP, PI driven...with a choice of with/without bypass cap.

[Prattacaster]

Very good, thanks for the explaination. So the 125C is cool with SE mode? I'm going to start with a 22.5k primary with the OT, and from your experiences it sounds like a good place to start. What do you use as the phase inverter?

[MWJB]

One amp has no PI, permanently wired as Self split, the other uses a 5D3 style PI, I run a 12BH7 in there at the minute (looking for headroom), but it will take any 12A#7 tube, a pal of mine also has a couple, one running a LTPI with a 12AU7, I'd say base it around 100K plate resistors, tweak cathode & tail resistors to suit...see what floats your boat & revise if necessary.

[Prattacaster]

Well you know how similar a 6sn7 and a 12au7 are. But 100k seems awful high and doesn't get you much more gain than say a 47k. Do you know how much voltage is required to drive a 6sn7 push pull? By my calculations and bias point it seems about 20v p-p. Does that seem right? Mine is cathode bias with 12.5v dropped over a 750 ohm resistor, into 22.5k at 330v. It would be really cool to use all 6sn7's, but would that be too much of the same??? another thing is a LTP 6sn7 even with 100k plate only produces a voltage gain of 9v, with 47k its 8v. Like i said before, if advisable i would love to use all 6sn7's. Opinion's??

[MWJB]

If you use 100K then you might also try 6SL7 in the preamp & PI, if you use 6SN7 accross the board you might find gain a bit on the low side. I'm using 470ohms shared cathode resistor for 9v @ 335vdc on the plate (uncorrected), a shade over 3W per side idle...still 470ohms up to 350/360 on the plates. I tried 660ohms but tone went overly fuzzy, no cleans.

[Prattacaster]

Do you suppose that was crossover distortion? I guess thats why you bias cathode bias closer to 90-100%. My design is at 70%. I was playing safe but now i see that its going to sound bad. Thanks, that saved me from at least one troubleshooting step!