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JZatopa 1st May 2012 05:17 AM

Contemporary tradition breaking amp schematics.
Although there has always been a divide between "sand" and "glass", there has also been a divide between tube guys. One group feels that the traditional ways must be used to get a good sounding amp, the other has no problem using some SS parts. What I would like to know is if there are any truly contemporary designs that have cropped up over the last couple of years. The meteor and power-drive come to mind but I was wondering if there might be others. I guess I am just wondering if there are other "new school" designs out there.

jan.didden 1st May 2012 08:08 AM

Bob Cordell's C-70 amp would surely qualify but you would need to order Linear Audio Vol 3 for the article.
He uses tubes in the amp channel and SS in the power supply regulators for all stages, plus designed the amp based on solid engineering rather than religious dogma.

jan didden

Stormrider 1st May 2012 09:34 AM

Looks like the C-70 is actually in Vol. 2.

pauldune 1st May 2012 10:01 AM

Some people will disagree, but I think my contraption qualifies:

And no, it is not a "srpp".


JZatopa 1st May 2012 03:56 PM

Pauldune that is a very interesting design, did you ever make a final version? How would you compare it to other 2a3 amps you have heard?

The bob cordel design is interesting as well. I wonder if you could over build the outputs of other tube amps and use higher resistance output transformers to lower distortion and improve dampening further?

Tubelab_com 1st May 2012 04:55 PM

When I first posted the PowerDrive concept om my web site the reaction was mostly negative. Over time the reaction has changed and quite a few people actually use the concepts in their own designs.

I have continued to design my amplifiers from a purely engineering perspective, choosing the best component for the job even if it means sticking a silicon device in the middle of a vacuum tube amplifier. Many tube amp builders have grown to accept this, but every once in a while there will be an extremely negative voice with a loud PA system crying foul!

There is a thread about designing a guitar amp using less than $100 worth of parts. The "rules" weren't clearly stated, so I asked early on if there were any objections to a PowerDrive style mosfet follower. The replies agreed that it was OK.

About 1000 posts later up comes "is that a semiconductor I see in the middle of your design carrying AUDIO?" (post #1131) The $#*& storm started in post 1139 and it went downhill from there. Obviously there are still some strongly sandophobic amp builders out there that still share their opinion.

About 5 years ago I built the proverbial "bridge too far" I not only stuck some sand in a tube amp, I wrapped a DSP around it! A SE tube amp is terribly inefficient. 5 to 10% plate efficiency is common, with 33% being the theoretical max in class A and maybe 50% in class A2.

It is common in the cellular phone industry to use DSP techniques to improve RF power amplifier efficiency by adjusting the supply voltage on the final RF amplifier in real time according to the instaneous power demands of the device. In other words you change the supply voltage on the fly to keep the amplifier just above clipping, with zero headroom. In a phone this improves battery life during a call.

We can do the same thing in a tube amp. You need a power supply that can be varried from 100 to 400 volts at an audio rate. A dsPIC chip can do that. This allows the dissipation in the output tube to drop....a lot. Tube dissipation is the limiting factor in most of our tube amps, and all of the SE amps. If we can reduce the dissipation, we can increase the power output. Does it work? Well how else can you get 20 watts fron a single 6AS7 in SE?

The device was built for a magazine contest where it won a prize. I wrote an article for the magazine explaing it, but I don't think it was well understood by the typical embedded systems engineer that reads Circuit Cellar. 5 years have passed and the article and associated information have vanished from Circuit Cellar's web site. There is a photo and brief discription here.


The article did get me a phone call and a few emails from the owner of a well known guitar and amp manufacturing company. I explained that that particular design wasn't well suited for guitar amp use since it was very clean up to the point where the dynamic power supply runs out, where it snaps into gross distortion. A few discussions never went any where, but a few more cool and unique circuits are coming.

JZatopa 1st May 2012 05:57 PM

Tubelab I always love seeing my fellow FAU alumni doing awesome things and I think that you are on the right track and have been for quite some time. I understand the purist approach to things but as I have grown I have found that to be truly limiting. It does have it's allure though. The fact is that if you want to make the best "X" you can, you need to use all the tech and tools available to you.

That minitron sounds like it is doing what some car amps do to keep cool yet have big output. Aside from power output, how was the sound compared to a conventional power supply? What are the chances you would rebuild the minitron using an Arduino for DSP? If you were to do that, I think you would grab the interest of a large number Arduino users.

smoking-amp 1st May 2012 06:40 PM

2 Attachment(s)
"The bob cordel design is interesting as well. I wonder if you could over build the outputs of other tube amps and use higher resistance output transformers to lower distortion and improve dampening further?"

Do you mean higher primary impedance was used, or higher winding wire resistance? Over building a tube output stage is certainly do-able, but will run the price and power consumption up.


The "anti-triode" scheme (P-P emulation of SE) was used in an amp at one of the European triode festivals and won an award in a SE amp contest. Revintage entered it I think.


You don't need a DSP for the power supply control, just use a conventional SS amp connected in series with the output of a tube amp. The feedback for the tube amp comes off the combined sum then (by special design, and special OT) so as to produce overall tube sound using only a few tube Watts. It is effectively a class G or H amplifier then.

Image from an old thread: (and another oldee image for paralleling tube + SS to unload the tube amp, using a high Zout, current output SS amp)

TheGimp 1st May 2012 07:15 PM

Wayback machine ..

I don't know if the link will last, but one can go to the wayback machine and insert the url in George's post and it will take you there.

pauldune 1st May 2012 09:27 PM


Originally Posted by DJNUBZ (
Pauldune that is a very interesting design, did you ever make a final version? How would you compare it to other 2a3 amps you have heard?

Yes, I did build a final version of it. It is my main amp for almost 2 years now.
Since then I had the opportunity to compare it to a lot of other sytems, including a few 300B and expensive PP tube amps.
It is difficult to compare complete systems with eachother, because of all the variables that change.
But this I can say, it is still the best amp that I ever heard.
It has it shortcomings, though. Efficiancy is not very good,and it is only about 6 watts.
Distortion figures are a lot better than the prototype, because of the better linearity of the 2a3/300b. I'm using a TJ 300b/2.5 meshplate now.

I didnt post much of it, because it seemed almost nobody was interested.

I'm thinking about a rebuild for some time now, i have a few ideas to make it even better. Dc filament heating, higher B+, and maybe the switch to GM70 as powertube.


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