Transformerless push-pull tube circuit??

Hi guys,
I have been trying to figure out how to build a transformerless tube push-pull circuit but haven't had much luck in finding a sample circuit to learn from, can someone point me on the right direction to build such a circuit???

Can this be done???

Thanks in advance!!

I don't quite get the question?

E.g the SRPP stage in the pultec clone is push-pull (somewhat) - and does not NEED an output transformer as such..

Jakob E.

[quote author="gyraf"]

E.g the SRPP stage in the pultec clone is push-pull (somewhat) - and does not NEED an output transformer as such..

Jakob E.[/quote]

arh......I see, right. I was under the assumption that a tube push pull circuit was restricted to using 2 tubes connected together by their cathode driving by a phase splitter with their plate coupled by a CT transformer, since that's all I can find in the RDH under class B amplifier and looking at the transistor push pull it has PNP BJTs where there is no PNP tubes.......

BTW, whats the main difference between the SRPP and a WCF??? Also, looking at the pultec clone's SRPP's EQ buffer how do you adjust the feedback to control crossover distortion??? By connecting a variable resistor from the plate of the bottom tube to the grid of the top tube?? I would assume that would alter the output gain also? increase the resistance would increase gain but also distortion as well?

[quote author="Learner"]whats the main difference between the SRPP and a WCF?[/quote]

Different beasts. Do some searching here: http://www.tubecad.com/

Peace,
Al.

Over at tubecad.com (in their back issues) there's some great articles about these things..

Jakob E.

There was a compendium of amplifier circuits done in Japan some years ago. I don't have it, but I recall a 6AS7-based direct coupled amp for headphone drive. It used feedback to reduce the already low output Z of the AS7's. It was a balanced design if memory serves rather than a SRPP or White follower, and must have had something to keep the d.c. output nulled close to zero.

Learner, do you want an OTL (Output-Transformer-Less) tube power amp?

[quote author="Learner"]Hi guys,
I have been trying to figure out how to build a transformerless tube push-pull circuit but haven't had much luck in finding a sample circuit to learn from, can someone point me on the right direction to build such a circuit???

Can this be done???

Thanks in advance!![/quote]

Well if you are talking about an OTL PP power amp you could try finding Bruce Rozenblit's Beginners Guide to Tube Design (or something like that) and he has an OTL circuit and he explains the design process. It uses parallel 6as7 tubes to lower the output impedance without transformers.

There's alot of stuff around for OTL amps if you google too.

Read up on Julius Futterman. He may not have "perfected" the OTL power amp, but he's considered the father of the practical OTL.

As for line-level stuff, the WCF and SRPP have already been mentioned. In layman's terms, the main differences between the two are as follows: you'd use a WCF if you wanted a beefy cathode follower with very low output Z that can drive a fairly tough load. You might use an SRPP if you need just "low-ish" output impedance and some voltage gain to boot. I think the SRPP is kind of over-used and trendy these days, but that's just my opinion.

Futterman---that's who I was trying to remember. Thanks NYD.

As I recall though, he did use an output coupling cap. No transformer though.

The earliest commercial OTL amp was made by the Stephens Manufacturing Company, using 2A3's. Needed a 500 ohm speaker.

New York Audio Labs and Fourier Components did variations on the Futternman.

Currently the most well known OTL's are the Atma-Sphere models.
Based on the Wiggins "Circlotron" output circuit. Fully balanced, no caps, no nothing. Straight into the 8 ohm speak, so use a fuse!

http://www.atma-sphere.com/reviews/tas/

cj

hey thanks a lot guys, for all the info!!!

Actually what I was looking for was some sample circuit/schematics to see how things are done, found these on the net based on the very helpful info suggested above.

http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm

http://www.unibase.com/~ahucula/futterman/page2.html

After seeing the schematic it has made realized that by trying to avoid using a coupling transformer, in exchange I would require a dual polarity high voltage power supply instead :?
Ha! trying to find a short cut just to face with a bigger dilema...... I think I'll be better off sticking with the coupling transformer... unless there are other ways around it??

[quote author="CJ"]Learner, do you want an OTL (Output-Transformer-Less) tube power amp?[/quote]

hehe....it will be a loooong while before I am ready to venture into the land of power amplifiers, still yet to grasp & overcome building prototypes of class A, B and D pre amplifiers.... not to mention variation of dynamic controllers & filters....

"After seeing the schematic it has made realized that by trying to avoid using a coupling transformer, in exchange I would require a dual polarity high voltage power supply instead"

What's wrong with R-C coupling? Is your objective to pass d.c. and eliminate coupling caps?

You will find virtually all the topologies OTL use here:

http://www.audiodesignguide.com/otl/index.html

[quote author="NewYorkDave"]You might use an SRPP if you need just "low-ish" output impedance and some voltage gain to boot.[/quote]

I don't know nearly enough to make an educated statement about this, but to my ears it seems like a lot of SRPP circuits I've heard are kinda... errr... 'distortiony', if you will. It's a good-sounding richness of some sort, but they definitely aren't the cleanest amps out there.

Peace,
Al.

Great site Marik!

They are missing the Alan Kimmel design.

I got to build one of those beasts!

The thought of 300 volts dc sitting millimeters away from an 8 ohm speaker, talk about listening on the edge of your seat!

Learner, did you ever build that LA2?
:?

Once, when I was into audiophilery/audiofoolery, I built a lot of different power OTL configurations, including Futterman and Graf.
None of them sounded right to my ears as OTL with low ohm speakers. I had a couple of 200 Ohm old speaker drivers, and with them or OPT they were fine. They needed lotsa NFB and were quite unstable.

The best sounding OTL I made was with 845 big transmitting tubes, directly driving Martin Logan CLS electrostatic panels. No global NFB, huge and inefficient, but together with plasma tweeters and TL subwoofer, they sounded actually pretty amazing. With them in winter time I did not need to turn on a heater. It was in Phoenix, Arizona, so unfortunately 10 months a year I had to pay extra AC bills, to cool the room not only from the wheather, but also from these beasts.
Then I met my girlfriend (now wife) and she won--the system had to go.

> by trying to avoid using a coupling transformer, in exchange I would require a dual polarity high voltage power supply instead

The Old Men were not stupid.

The only sane way to get big work out of a vacuum tube is to transformer-couple. Remember that most loads are conductors (voice-coils) and the vacuum inside a vacuum tube is a very BAD conductor. Tubes are 20KΩ-1KΩ, loads are 600Ω-4Ω. That's terrible for power transfer. A transformer is a 95% perfect solution to this basic problem.

For small work, you can R-C couple. You can even redefine "small" as "big" if you use a "very-very-big" tube. This is not rational design. (But most present-day tube work is irrational.)

> whats the main difference between the SRPP and a WCF???

SRPP works best at ONE specific load. And has voltage gain.

WCF never gets as great as a precisely-loaded SRPP, and has unity voltage gain. But when you put a heavy load on it, it sucks less than any comparably simple design. It isn't true Push-Pull (neither is a SRPP) but can be designed to stay mighty close to push-pull operation over a useful range of very low load impedances. In this range, it is very inefficient: no worse than any other transformerless design with comparable tubes, but very much worse than a transformer coupled design.

If audio transformer pricing were rational, we wouldn't even consider such kludges.

[quote author="bcarso"]
What's wrong with R-C coupling? Is your objective to pass d.c. and eliminate coupling caps?[/quote]

No, there is nothing wrong with RC coupling. In fact, that exactly what I am looking to do but I can not find any push pull circuit being done that way to see how its done. Is it because back in the day they want/needed low impedance output to drive long cable so transformer was a popular choice for all the circuits??

Anyway, I have tried it on SPICE and it seems to work OK..... Here is the schematic...



Here is the result .....



Now the important part, is this a reasonable prediction by SPICE?? If not, what do you think SPICE has left out or is there any additional factors that would need to be taken into consideration for building and testing??

[quote author="Marik"]You will find virtually all the topologies OTL use here:

http://www.audiodesignguide.com/otl/index.html[/quote]

hey thanks for the link Marik!!! That was mighty helpful! :thumb:

:guinness: