All Octal Push-Pull Amp

[mcrracer]

I have a chassis laying around with five Octal sockets in it already. I was looking at an SE amp with 2 6SJ7 into a 6V6 and an 18watt with one 6SJ7 into a 12AX7 PI into a pair of 6V6 tubes. What I am thinking is an Octal preamp into an Octal PI into a pair of 6V6s. Does anyone know of an amp where this has been done? How about a schematic ? Any suggestions? Thanks

[Andy Le Blanc]

dig into the bargain bin at schematic heaven. pentodes are high impedance
You'll have to chose tone control accordingly.

[Phil_S]

Look at the Gibson GA-20 (not the GA20-RVT). This is a 1950's amp with 6SJ7 input. I built a one channel variant. 6SJ7>6SL7>pair 6V6. 5Y3 rectifier. I really like this amp. I think it is tonally very rich and has a nice bloozy tone. Push it with P90's and you'll feel like Jimi.

I had to reduce the last dropping resistor in the B+ ladder (don't remember to what) to get the voltage on the input pentode to an acceptable level. When I built it to Gibson's schematic, I think I only got about 90V at the plate and almost no clean headroom. I think there are voltages on the schematic and you should shoot for those if you build this amp.

Mine was a total bone pile build and worked out surprisingly well.
http://home.comcast.net/~psymonds/GA20.htm

OK, well, I found out the hard way that some of the bone pile wire I used was thermocouple wire. If you don't know what it is, you learn when you use it. There is iron in the wire and it functions like one big resistor. Not appropriate at all. I didn't know. Some of that red wire you see is no longer in the amp.

[mcrracer]

Thanks guys When I get some spare time I will play around with this and see how it sounds. I just wanted to get away from 12AX7s for awhile!

[David Root]

In a triode, I have built a few all-octal amps using the 6SL7GT (glass not metal envelope) in the preamp and PI. Great tube! The nominal plate resistance is 44K vs 62.5K for the 12AX7, so for example you would use 68K/75K plate loads vs. 82K/100K in the PI.

Caveat big-time--if you are going to these at high gain, which for this tube is anything over about 20, you will have to use a DC heater supply to ensure quiet operation. I've done them unregulated and regulated, if you have two 6V3 windings on your PT you can split them between power and pre tubes, otherwise regulate.

12SL7s used to be a lot cheaper than the 6 (audiophools like 6SL7s) and a 12.6V regulated DC supply is pretty easy to do.

[Phil_S]

I'm not questioning David's point about 6SL7's being noisy. I've seen this sort of comment more than once and it appears to be conventional wisdom.

My limited experience with the 6SL7-GT suggests that careful lead dress can give a good result. I've used it in 3 amps without any meaningful noise; twice as V1 and once as a LTP inverter in the V2 position -- not a high gain amp.

[selloutrr]

Try swapping out the 5y3 rec for a 5v4 it'll give you more output and tighter tone .

[M Fowler]

How about the Deluxe 5C3 octal pre into 6V6?

I have used 6SL7, 6SQ7 as well as GT versions of the same without noise.

The last batch of 6SQ7GT one out of three is microphonic no matter what position I use it in, so I'll some more of those.

[Cliff Schecht]

Second the 5C3. It's a surprisingly versatile little amp and isn't ultra-particular about the values you use in it (which makes it a great candidate for carbon comps). I built one with 6L6's and love how the amp sounds. I used 6SL7/6SN7 tubes instead of the 6SC7's but either way, it's a great little rock amp with a surprising amount of gain.

[tubeswell]

FWIW if you have an extra 300mA on the heater winding, why not put a 12AX7 or 6SC7 CF stage after the 6SL7 to avert tone stack lossiness (to give yourself some flexibility in tone stack design? Then you can use the other half of the 12AX7/6SC7 to drive the 6SL7 (as an extra gain stage). 5C3s are fairly clean amps in stock configuration (which might be what you are looking for, or it might not)

[Andy Le Blanc]

I seen some noise issues with some NOS 6sl7 and 6sn7.
The larger structure takes a little bit longer to heat so you get some thermal
noises, fine when they're all toasty.

High impedance controls have they're own response, don't be afraid to use them.
No need to force a -10 Db carving tone stack into the design with a CF.

drop the feed back and go with the paraphase in a 5b3 or a 5d3.
The thing has a tone and can just wail. Keep the zero bias pre.

And it is cathode bias, go to 100% plate dissipation, plate plus screens.

[azatplayer]

I built a B15N with 6SL7's and 6L6's, love those octal pres!

I bought three NOS 6SL7's before banging my head against a wall, very unhappy with what i got, all microphonic, and worse.

Ended up with a pair of new Tung-sol's, excellent tone, and very quiet.
B15N sounds killer with guitar too.

[labb]

Take a look at the old Fender 5B3, or 5C3 and then go take a look at the Rickenbacker M11. They are all 5 octal tube amps..They were made in the "50's I do believe...Been playing with an original Rickenbacker M11 and it sound great..It is the same circuit as the Fender 5B3 Tweed Delux amp..These amps run the plate voltages,both power and preamp tubes, pretty low. Think that the Rick runs about 325 VDC on the 6V6's.

[David Root]

The Phillips ECG 6SL7s are reasonably priced and a bit higher gain than some of the old ones. I like a 5691 in V1 position, lower gain but smoother tone. They are getting pricey tho', and can be microphonic.

I did have one ECG that one day just lost its vacseal and it just died immediately.