6AG7 Amplifier

[sorenj07]

I've been intrigued by these small metal tubes and have designed an amplifier to use them. What do you think?

http://www.shinjo.info/frank/sheets/093/6/6AG7.pdf

[Miles Prower]

Originally posted by sorenj07
I've been intrigued by these small metal tubes and have designed an amplifier to use them. What do you think?

Topology and loadlines look good. There is one potential fly in the ointment: the 6AG7 is really a small signal type. This VT is intended to be used as a wide-band or RF amp that has a large g(m) for good gain with small Rp's. The 6AG7's r(p)= 130K is an order of magnitude greater than that of the typical audio final. If you use it as a pentode, that's gonna either require alot of gNFB to get a good damping factor, or you're gonna have to live with sloppy bass from underdamped woofers.

Of course, the only way to be sure is to go ahead and do it. If you don't try, you won't know.

[ilimzn]

The 6AG7 has pretty high sensitivity, so it strikes me it would be ideal in pentode with plate-to-plate feedback to it's driver stage (of course it would have to be something other than a 6SN7 then) or a 'super triode' connection. As Miles said, it has a very high Rp but also high gm, so a high amount of NFB would seem expected. It is better to then have most of it without the output transformer in the loop.

[sorenj07]

Originally posted by Miles Prower
Topology and loadlines look good. There is one potential fly in the ointment: the 6AG7 is really a small signal type. This VT is intended to be used as a wide-band or RF amp that has a large g(m) for good gain with small Rp's. The 6AG7's r(p)= 130K is an order of magnitude greater than that of the typical audio final.

Thanks! Since the tubes are fairly plentiful, how does running them PPP sound? plate resistance should halve, and power output should double, and the 10K anode to anode OPT will make a nicer loadline. Does this solve a few of the problems inherent to the tube?

[Miles Prower]

Originally posted by sorenj07
Thanks! Since the tubes are fairly plentiful, how does running them PPP sound?

I have no idea. I haven't tried them myself, and don't recall seeing anything on the 'Net about them. Although the spec sheet gives some sample data concerning SE audio use. A THD= 7.0% for a SE pentode isn't bad since this usually runs ~10%, so someone must have been doing this at one time or another.

The original use was as a vid amp that could have an f(h)= 4.0MHz, and slam current into a CRT's Ci. They also frequently turn up as ECOs (buffered and/or with frequency multiplication) in ham rigs, and as horizontal deflection amps in o'scopes.

plate resistance should halve, and power output should double, and the 10K anode to anode OPT will make a nicer loadline. Does this solve a few of the problems inherent to the tube?

It should. It will cut the effective r(p) in half, and that will definitely help with woofer damping, and make it a good deal easier for NFB to drop it even further.

[sorenj07]

Cool. As an extra measure, I'm switching to a 6N7 driver tube. It seems to have similar headroom in the place of a 6SN7, and the extra gain will help feed some NFB. Any extra gain after that, I'll burn off with the volume pot. This is an integrated amp design.

As I mention over at AA, using all metal tubes is nice because I get to mount them inside a cool rackmount chassis without feeling bad. I'm assuming I'll use SS rectification, but are there any metal rectifiers that could put out around 150-175mA safely? That'd be icing on the cake

[Tom Bavis]

I have enough 6AG7s that I could use them as the input stage, PPP outputs, and a few more as rectifiers...

Seriously, the 5T4 is the biggest metal rectifier, but not all that common. It's 6L6 size, except for the earliest ones, which are bigger. I have a few metal 6X5s, 5W4, even 25Z6s.

[6f6]

This is slightly off-topic, but I will ask it anyway. I have a handful of these 6AG7 tubes and was wondering what to do with them. I contemplated making an audio amp, but I will probably use 6V6 or 6550. Do you guys think that a 6AG7 would make a good power final for small AM/SW transmitter (I plan to use it to transmit to vintage radios)?

[Miles Prower]

Originally posted by 6f6
Do you guys think that a 6AG7 would make a good power final for small AM/SW transmitter (I plan to use it to transmit to vintage radios)?

Fersure. 6AG7s are frequently used as oscillator/drivers in ham rigs. What is a driver but a smaller final?

[rcavictim]

I was going to try the 6AG7 as a driver for a bank of 6080`s parallelled up in SET. With the ability of the 6AG7 to swing big into a low plate load R I thought this might help deal with Mr. Miller.

The 6CL6 is almost identical to the 6AG7 and comes in a 9 pin miniature bottle.