Post PI cathode follower for 50 watt amp

[crbowman]

Greetings Guys,
I have a question for you guys who are a bit more knowledgeable about circuit design than I am.
Seeing as we now have a pretty good idea of what the SSS post PI cathode follower looks like, I was wondering what changes would need to be made to implement this in a 50 watt output stage. Is it as easy as simply dropping one leg each off of pins 3 and 8?
Any input here would be greatly appreciated.
Thanks in advance,
CB
https://tubeamparchive.com/files/sss_00 ... 74_781.pdf

[tictac]

Is it as easy as simply dropping one leg each off of pins 3 and 8?

Yea, pretty much... that's how my stereo direct-coupled amp is wired...

TT

[crbowman]

Thanks TT,
OK, so the 470K resistors to ground. I'm assuming that's a ground reference.
Can they be omitted as well?

[tictac]

so the 470K resistors to ground. I'm assuming that's a ground reference. Can they be omitted as well?

I don't use them in my amp... the circuit seems to work just fine without them...

TT

[212Mavguy]

To the OP,

What are the reasons for wanting to do that in the first place?

[crbowman]

I have a donor 2 holer chassis from a gutted Laney 30 watt ProTube that I'm thinking about building into a clean 50 watt amp. Chassis is drilled for two octal and 4 nine pin noval sockets.
There's been a lot of discussion here about whether this post PI cathode follower has any real sonic advantage at anything less than full bore. Some say no real advantage, some say (and Merlin seems to agree) there is some additional clarity and compression. Seeing as clarity and compression is kinda what I'm shooting for I figured I'd use the extra stage to try this out and see if I like it or not. Easy enough to remove and do something else with it if it's a no go.

[wjdunham]

I have a donor 2 holer chassis from a gutted Laney 30 watt ProTube that I'm thinking about building into a clean 50 watt amp. Chassis is drilled for two octal and 4 nine pin noval sockets.
There's been a lot of discussion here about whether this post PI cathode follower has any real sonic advantage at anything less than full bore. Some say no real advantage, some say (and Merlin seems to agree) there is some additional clarity and compression. Seeing as clarity and compression is kinda what I'm shooting for I figured I'd use the extra stage to try this out and see if I like it or not. Easy enough to remove and do something else with it if it's a no go.

The post PI follower makes a difference, no question. Works as well with the 50W amps as the 100W, I have built quite a few of both. The difference is more pronounced at higher volumes, but it's there across the range to some degree, mostly on the attack of the lower notes. I've been using 12AT7 as the follower tube, I've found that they tend to match up better side to side than the 12BH7's. Side to side matching of the followers is critical to get the power tubes to balance up correctly.

Bill
www.sebagosound.com

[crbowman]

Hey Bill,
Awesome! Thanks for that man.
Still using the faux #40 clone you helped me on awhile back. Love it!

[LeftyStrat]

Let us know how it turns out. I've been wanting to build a 2x50w and was considering this as well.

[JazzGuitarGimp]

I have a donor 2 holer chassis from a gutted Laney 30 watt ProTube that I'm thinking about building into a clean 50 watt amp. Chassis is drilled for two octal and 4 nine pin noval sockets.
There's been a lot of discussion here about whether this post PI cathode follower has any real sonic advantage at anything less than full bore. Some say no real advantage, some say (and Merlin seems to agree) there is some additional clarity and compression. Seeing as clarity and compression is kinda what I'm shooting for I figured I'd use the extra stage to try this out and see if I like it or not. Easy enough to remove and do something else with it if it's a no go.

The post PI follower makes a difference, no question. Works as well with the 50W amps as the 100W, I have built quite a few of both. The difference is more pronounced at higher volumes, but it's there across the range to some degree, mostly on the attack of the lower notes. I've been using 12AT7 as the follower tube, I've found that they tend to match up better side to side than the 12BH7's. Side to side matching of the followers is critical to get the power tubes to balance up correctly.

Bill
www.sebagosound.com

I imagine you could compensate for this by changing the two 220K resistors to 200K, placing a 50K-B pot between the two resistors, and applying the bias to the wiper of the pot. That way, the output stage could be perfectly balanced a al the Dumble balance pot.

[JazzGuitarGimp]

Hi All,

I hope this isn't too much of a thread-hijack: I've been thinking about CF drivers for the output tubes lately as well, primarily because I want to hear what class AB2 sounds like. I am wondering about the 1.5K grid leak resistors at the output tubes. If you wanted to push the grids to 5V more positive than the cathodes, the resistor is going to limit the current to the grids to about 3mA. I suspect this won't be enough current to actually get the grids up to +5V. Is there any danger in lowering the grid leak resistors to, say 470R, so that you can get more current (and therefore, more voltage) into the grids?

Thanks,
Lou

[wjdunham]

I imagine you could compensate for this by changing the two 220K resistors to 200K, placing a 50K-B pot between the two resistors, and applying the bias to the wiper of the pot. That way, the output stage could be perfectly balanced a al the Dumble balance pot.

Yes, that would work - very nice idea. It has the added advantage of being able to compensate for slightly mismatched power tubes, at least getting the DC bias balanced.

Bill
www.sebagosound.com

[ToneMerc]

Anyone done anything else with this style of circuit in 50 watt? I had this on hold, but think I' m going to fully commit to a "lite" version.

TM

[crbowman]

I'm working on one right now, but it's going very slowly seeing as my down time is pretty much nonexistent these days. Two EF86's in the preamp, two dual tridodes for the PI and CF stage, and two 6L6s. Very bare bones to start with. If I like it, it may get a little more attention down the road.

[ToneMerc]

I'm doing basically a dual KT88 small special with a CF driver; 12ax7, 12au7, 12bh7 and upright Twin power. Taylor made me a head box awhile back, I just need to have face and rearplates made.

TM