PSE Active Loaded 12B4A anyone?

[G]

Hi all, I had to get this down in schematic form before I got distracted and forgot about it. All of the values might not be optimal. If you have any constructive suggestions please feel free to give them. They are appreciated. I haven't worked out the PSU yet but it will be tube rectified with a CLCRC filter. I had monoblocks in mind for this design so the PSU will be for one channel, not two. I'll post it sometime in the next day or so. This should be a really nice sounding design.

[fdegrove]

Hi,

Nice work...

You may want to try to run some more current through the 6922 though.
I find them to sound a little too thin when run at 5mA.

Cheers,

[G]

Originally posted by fdegrove
Hi,

Nice work...

You may want to try to run some more current through the 6922 though.
I find them to sound a little too thin when run at 5mA.

Cheers,

Thanks Frank. I'll break out the data sheet and see what I can do. I might just use a 5687/7044/E182CC because I have plenty of them. What are your thoughts?

[Eli Duttman]

Gavin,

Are your 12B4s TIGHTLY matched? Dennis Boyle's combination bias is an effective tool for dealing with the reality of poorly matched "finals". You stand each 12B4 on an individual, unbypassed, 100 Ohm resistor. You use a C- bias supply with individual bias pots to dial the idle current of each "final" in exactly where you want it. The combination of exact idle current and local NFB from the unbypassed cathode resistors goes a long way towards overcoming the mismatch between O/P tubes.

You will need individual coupling caps. and grid leak resistors for the 12B4s, if you opt for combination bias.

[fdegrove]

Hi,

I might just use a 5687/7044/E182CC because I have plenty of them. What are your thoughts?

If you don't need the extra gain the 6922s are providing then the 5687 and relatives would be a better choice linearitywise.

Keep in mind that when optimising for max. mu the Zout goes up as well.
Not that it would be a problem in this case but still worth keeping an eye on.

Cheers,

[G]

Originally posted by Eli Duttman
Gavin,

Are your 12B4s TIGHTLY matched? Dennis Boyle's combination bias is an effective tool for dealing with the reality of poorly matched "finals". You stand each 12B4 on an individual, unbypassed, 100 Ohm resistor. You use a C- bias supply with individual bias pots to dial the idle current of each "final" in exactly where you want it. The combination of exact idle current and local NFB from the unbypassed cathode resistors goes a long way towards overcoming the mismatch between O/P tubes.

You will need individual coupling caps. and grid leak resistors for the 12B4s, if you opt for combination bias.

Hi Eli. You mean a fixed bias for each tube? Are the effects of loosely matched tubes in parallel bad enough to warrant the extra hassle? I've never done a PSE circuit before so I'm kind of feeling my way through this. I can certainly do it though. I was hoping that 12B4As out of the same sleeve would be matched closely enough. I could also buy them from the tube store matched if they have a big enough stock. Hmmmm.

[G]

Originally posted by fdegrove
Hi,



If you don't need the extra gain the 6922s are providing then the 5687 and relatives would be a better choice linearitywise.

Keep in mind that when optimising for max. mu the Zout goes up as well.
Not that it would be a problem in this case but still worth keeping an eye on.

Cheers,

I'm not sure I need the extra gain. The 12B4A only has a mu of 6 though. I have two versions designed. One with a 6922 and one with a 5687. I only need to swing 48 volts or so into the 12B4As though for absolute full output before clipping. I think you may be right. I think I'll go with the 5687.

[fdegrove]

Hi,

I'm not sure I need the extra gain.

You probably won't if you're using CDP as a source and/or use a active linestage to drive the amp.

I could also buy them from the tube store matched if they have a big enough stock. Hmmmm.

It can help for PP (often not worth the extra money IMHO) but PSE is rather harder as it's often best to have a means of "dynamically" matching the tubes in the circuit where they're used in, IME.
This is where Dennis Boyle's biasing scheme comes in handy but you'll need the extra PS to provide for the negative rail. (C-)

I'm not sure whether there's any info about this available on the net. Maybe Eli knows?
I haven't looked as I have the original articles as published in SP (?) some years back.
If everything else fails I can try to scan them for you.

Cheers,

[G]

Originally posted by fdegrove
Hi,



You probably won't if you're using CDP as a source and/or use a active linestage to drive the amp.



It can help for PP (often not worth the extra money IMHO) but PSE is rather harder as it's often best to have a means of "dynamically" matching the tubes in the circuit where they're used in, IME.
This is where Dennis Boyle's biasing scheme comes in handy but you'll need the extra PS to provide for the negative rail. (C-)

I'm not sure whether there's any info about this available on the net. Maybe Eli knows?
I haven't looked as I have the original articles as published in SP (?) some years back.
If everything else fails I can try to scan them for you.

Cheers,

I assume that the negative supply would be connected between the grid and ground? Would that eliminate the need for the cathode resistors? It seems that it would as that is all they do is bias the grid at a negative voltage when referenced to ground. I also assume that I would have to feed a ss bridge rectifier with a separate center tapped winding or transformer and use two separate adjustable voltage dividers between the negative rail and ground to get my biasing voltages? That shouldn't be too hard. Although it will take up some room.

[Gary P]

That little amp should sound good with provisions for balancing the output tubes.

I built a similar amp (the David) a couple of years ago. I'm only using a single 12B4 for output and a 6AN8 for the gain stage. With a mu of 19 the gain in just right. The David also has a CCS fed shunt regulated power supply for the driver stage. This project was an exercise in how much stuff can be crammed into a small box... The doner chassis was a pair of Motorola SE 6BQ5 stereo amps. In each amp the power supply was changed to a full wave bridge with choke input to get the supply voltage up for the active loading. One of the original SE ouput transformers was used as the power supply choke. Managed to cram 3 CCS boards into the chassis... The output transformer is a 70 volt line matching transformer, the cheap one that Radio Shack used to sell.

The pair of monoblocks are used as my lab amp and see almost daily use. It puts out 1 clean watt that is very transparent and has great bass. One of my friends uses a pair of Davids to power his Klipsh LaScala's. More power than he needs in a small room ;-}.

A couple of things you can do.

With CCS loading you can dispense with the cathode bypass caps. The constant current through the cathode resistor is basicall fixed bias.

On the output stage return the transformer to the cathode of the triodes, not ground. This will eleminate the cathode bypass cap on the output stage.

The Ixys part is pretty good, or at least it measures pretty good. I have not tried one out in a live circuit. To see performance measurments of various CCS's click on the CCS measurments link on my page.

Here's the schematic of the David.


Some pictures of one of the finished amps can be seen on my webpage. Go to the "link to old page" at the bottom then go to images.

Gary

GaryGary P's DIY page