6AS7 Parallel Feed Amp

[lcsegeng]

I built a 6AS7 based parallel feed SE amp and I get only about 3 watts out of it with both sections of the tube in parallel. The 6SN7 driver tube cannot provide enough swing.

The 6528 tube looks much better. It has a higher Mu at 9 versus 2 for the 6AS7 and the 6SN7 could drive it nicely. However, I would only need one section per channel. My question is there a tube out there that would have the same plate characteristics as a single section of the 6528.

[Jeb-D.]

I know most people here are Pentode phobic, but Pentodes can do a really good job for driving low mu/rp output tubes such as 6as7.

But being as your looking for low gain, it might rule out the pentode. I've managed some 2 stage 6as7 and 6c33c designs using a Pentode front end.

[Shoog]

I know most people here are Pentode phobic, but Pentodes can do a really good job for driving low mu/rp output tubes such as 6as7.

Using a pentode as a driver would allow the application of a bit of plate to plate feedback which would help to linearize the not two linear 6AS7. It would also tighten up the bottom end which is still a bit sloppy even with the low rp of the 6AS7. Simply lash a 250K pot between the pentode and the 6AS7 plate and dial it until things sound better.

The 6528 tube looks much better.

It looks a whole lot better, but its difficult to get hold of and costs a hell of a lot more. It might be better to try to overcome the limitations of the 6AS7.
I found that the real world gain of the 6AS7 was less than unity as you are not going to get the full Mu as gain with a transformer as a load.

Shoog

[Jeb-D.]

Using a pentode as a driver would allow the application of a bit of plate to plate feedback which would help to linearize the not two linear 6AS7. It would also tighten up the bottom end which is still a bit sloppy even with the low rp of the 6AS7. Simply lash a 250K pot between the pentode and the 6AS7 plate and dial it until things sound better.

Yes, I definatley plan to experiment with that form of feedback, especially for an SE amp.

Right now I'm using a cross-coupled, screen-feedback scheme that works rather well (It's a Pentode driven 6as7 PP amp)

[isaacc7]

I own a parallel 6as7 amp and I believe it puts out somewhere around 15 watts per channel. It uses both a 6sn7 and a 6sl7 up front, but I'm not sure of the configuration. Sounds plenty good to me. You can see the amps here:

http://www.audiomirror.com/amps.html

Isaac

[alexg]

I built a parafeed 6as7/6080 and it sounds excellent, though I have not measured the power. Am using a 93db single driver speakers and it is more than loud enough for my 6 meter X 6 meter listening room.

I tried several drivers on it (6n1p, 6j5) and I finally settled on plate loaded 12at7.

Originally posted by isaacc7
I own a parallel 6as7 amp and I believe it puts out somewhere around 15 watts per channel. It uses both a 6sn7 and a 6sl7 up front, but I'm not sure of the configuration. Sounds plenty good to me. You can see the amps here:

http://www.audiomirror.com/amps.html

Isaac

Isaac, does your amp uses two 6as7 per channel (I can't determine if the two ST tubes at the back are both 6as7 or one 6as7 and one rectifier).

Thanks.

[lcsegeng]

Thanks for the responses.

I should add a bit of info. I obtained the amp schematic from a local tube buff who had built a couple of 6AS7 parallel feed amps that were fixed biased and used two power supplies, one for the input and driver stage and one at a lower voltage for the 6AS7. He got 10 watts out of the amps and advised they sounded great.

The schematic he gave me (the amp I built) is cathode biased operting at one voltage which ended up to be much to high for the 6AS7. He advised later that he had made a mistake, however, I have a lot invested in the amp including a nickel plated chassis which cost me $160.

The input is 1/2 of a 12AT7 followed by 1/2 of the 6SN7. The 6SN7 swings 170 v P-P.

The 6AS7 is cathode biased to about 100 v. The supply voltage is about 310 v which gives a plate voltage of 210 v. If you take a look at the plate curves for the 6AS7, the tube is operating at the far right close to pinch off. I could use a lower plate voltage, but this means a new transformer which I don't want to do. I also tried a fixed bias at -80 v but the current draw was way too much at over 400 ma. The chokes are rated for only 250 ma

I have found a tube that should work, the 7236 which has a higher Mu of 4.8 versus 2 for the 6AS7 and the tube should operate closer to the middle of the plate curve. The 6SN7 will provide more than ample voltage swing.

I've ordered a couple and will report the results.

[tubelab.com]

I have found a tube that should work, the 7236

I have experimented a lot with the 7236 and other similar tubes. Most of my testing was for cathode follower output stages though.

The 7236 is found in two variants. The most common variety has a box plate that looks like a typical tube. It is less tolerant of over dissipation than the other variety. The rarer variety looks like a 6080 only the grid wires are wound with a tighter spacing giving a higher Mu.

The 5998 is essentially the same tube only the cathode is not rated for "long term operation in cutoff". This does not matter for audio use. The 5998 is of similar shape to a 6AS7G. The 5998A looks like a 6AS7GA or a 6080.

The 6336A is like a 6AS7 on steroids. It has a solid graphite plate and slightly higher Mu. The 6528 has the same construction with more Mu. Both are expensive. I have found used 6336A in military surplus junk. Many of them work fine including one with cracks in both plates from serious abuse.