6SN7 PP PA Stage Question

[surfsup]

I would like to run two 6SN7s in push pull but with each tube paralleled as in the attached schematic. I'm trying to figure out the proper impedance, etc. If I am going to feed the plates 350V, according to valvewiz, I can get a rough estimate by using the plate dissipation from the datasheet. But it says 5W as a triode or 7.5W with both triodes operating:

Max. Plate Dissipation (each plate) 5W
Max. Plate Dissipation (both plates) 7.5W

Does that mean parallel, or just if they are both on? I assume parallel, acting as a single tube.

[img:337:242]http://chicagocadcam.com/ChrisHahn/schem/6SN7_1.jpg[/img]

[Andy Le Blanc]

the total dissipation for one tube for both sides

Ive seen hammond 125 series universal replacement OPT used alot
get from 1200 to 25k for loading

I hope your using current production 6sn7

[surfsup]

"the total dissipation for one tube for both sides"

Ok thanks, so I would then use 3.75W for each triode or 7.5 for each tube as parallel.

"I hope your using current production 6sn7"

This statement implies you know something I don't. I bought two NOS 6SN7 Y plate tubes (seem new never used still in box) for this. Why do you ask this question?

[Andy Le Blanc]

NOS 6sn7 are getting more valuable for HI FI all the time.
a really great sounding tube, not to waste.

Its a hard thing, current production just isn't on par, nos 6sn7 are a commodity.

[M Fowler]

Well that is a NOS tube I have in stock and plan to use up someday.

I think the tubes from China Shungay? are very good tubes and used by Hi-Fi guys as well.

Mark

Why do people say OMG-WTF

[surfsup]

well if current production is not on par, sounds like I would want to use these in the amp? Even though I'm not using them in a hifi.

[Phil_S]

A 6SN7 has the same operating characteristics as a 12AU7, so you can look for guidance there. The big difference is that the big bottle 6SN7 will run up to 450v on the plates. I am thinking one tube (2 triodes) want to see about 22K on the OT. Two tubes want half that. Triodes are very tolerant with respect to the OT primary load. Don't get hung up in this. I think you'd be fine with anything from about 8K to 12K. Don't expect a loud 15W from this.

This was my attempt at a 6SN7 amp. http://home.comcast.net/~psymonds/6SN7amp.htm I got what I wanted, clean tone, bit of OD, not a lot of volume, and a good sounding amp.

You'll notice that I used a concertina. I started with a floating paraphase like Ampeg uses, but it was just horrible. I concluded you can't hit the triodes with too much voltage swing. They distort very easily.

Curious, why are you planning for fixed bias?

[Andy Le Blanc]

you might want them later on in your building career is all

when they get harder to source...

I tend to use bench tube (NOS) on the bench but put the amp out with new...

[surfsup]

phil, as I understand it the fixed is more efficient and the PA won't sag as much. Can use higher voltages and the tubes will last longer. Maybe I have that wrong. EDIT: oh and I think fixed being more efficient won't break up as early so with a high gain design like this I'm thinking that might be helpful? Originally I started with the PS of a TW express which used fixed and kept it- though I think I should redo it for separate bias sources for each tube, but I wonder if running them in parallel the way I have it is best. I see you just ran them as four separate triodes.

I see you use cathode bias and an option with this is removing a tube. I am thinking I might like that option. Thinking aloud here but is it possible to put in a switch to have both in the same amp? I'm thinking its doable and I'm sure many have so will google that.

Andy thanks for the tip. One has to wonder how long tubes will be manufactured. One day the cost/profits will likely kill the industry. Who knows maybe they will be a key ingredient in cold fusion and available for pennies 50 yrs from now.

[tubeswell]

Does that mean parallel, or just if they are both on? I assume parallel, acting as a single tube.

You assume correct. The 7.5W maximum is for the whole dual triode in parallel configuration (or 3.75W per triode)

[Phil_S]

surfsup, don't confuse the schematic with the layout. I simply wired it so that one tube could be pulled and it would still be an ordinary PP config. If you wire one bottle for each side of the inverter, then you can't pull a tube for half power. Instead of pulling one tube, I suppose you could lift the two cathodes from ground with a switch and shut off the tube. Because you still need to adjust the speaker impedance, you won't be doing this on the fly. So, why bother with a switch? That's my thinking.