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Ping SY

Hey SY,

Did you ever draw up a schematic of your 150w pp sweep tube amp, the one that was screen driven operating in mostly class B? If you did, I'd love to take it a look at it if you are willing to share. I'd love to study it for learning purposes.

P.S. I found myself in the possession of a small quantity of some 6FW5's.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Agggh, I haven't forgotton, it's just become more complicated. Thank you Microsoft, my scanner will not work with my laptop or my "normal" computer because of Vista incompatibility. So I have to scan on my XP machine, which is not hooked up to a network, then move it physically with a memory stick. If I can escape child-care tonight, I'll do it.

Basically, it's a SYclotron input stage using a 6SL7 and 2N5114 pFETs direct coupled to a long-tail pair of 6SN7. Cap coupled to source followers (IRF820) which are direct coupled to the screens of a pair of 6LF6. Control grids are shorted to the cathodes. IIRC, B+ was about 700V. Plate to plate load is 1k2, idle current is 3mA (not a typo). I would NOT try 150W sustained, but it has no problem hitting that in tone bursts without clipping.

I'd do things differently these days for the input and drivers, but the basic output stage configuration is a keeper.
 
Hi Stuart

I'd do things differently these days for the input and drivers, but the basic output stage configuration is a keeper.

What would you do different? I remember the thread where you presented the SYclotron driver, including some measurements on the excellent performance of this circuit. Is there something 'better' around to produce large, balanced and undistorted sine waves?

Greetings, Erik
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
I'd probably do a long tail CCS for the first stage with something like a 6AQ4 or triode-connected D3a, then 6S4 for the second stage. Or a pair of dissimilar triodes like 6GF7 with the same topology.

What the SYclotron really brings to the table is a reduced input capacitance, the Achilles' Heel of high mu input tubes.
 
This seems like a very interesting learning/experimenting project. Thanks for the details, SY.

I have a pair of 1.25K p/p OPTs from Onetics. Bud said they were 150w cores. They were meant for another project but I can use them to test this idea.

I will search for details on your SYclotron input stage. It sounds like what your suggesting now is something akin to the glass palace input/driver, but with different tubes and mosfet source followers instead of cathode followers. Does that sound about right?

It seems on paper the 6FW5 should be able to operate similarly to the 6LF6's. 6LF6s don't seem to be cheap anymore. I got 38 6FW5's for $4/ea. I've read that electrically the 6FW5 is the same as the EL36 and I've seen schematics for the EL36 with screen drive.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
I haven't tried them, but paralleled 6JN6 seem to work; that was the approach Berning took when updating the EA2100.

The tail in the LTP could indeed be cascoded MOSFET (especially DN2540 or similar depletion mode). It could just as easily be a cascoded bipolar. The swing only needs to be one-half the input signal, making the design a bit easier.
 
I got 38 6FW5's for $4/ea. I've read that electrically the 6FW5 is the same as the EL36 and I've seen schematics for the EL36 with screen drive.

Did you buy them from a vendor in Florida? Were they all he had left? I got 10 from him.

I built a screen drive P-P amp using 6AV5's. It made 80 watts easilly. I tried the 6FW5 in that amp before I dismantled it. The 6FW5 is an uprated 6AV5 and one of only 3 tubes with the same pinout. The 6AU5 is the third. I was told that the 6FW5 was built from 6DQ6 parts and may indeed be a 6DQ6.

There are pictures of my 6AV5 P-P on this page. I have a paper schematic and I can post it as soon as I have time to photograph it and process the picture.

http://www.tubelab.com/6AV5.htm

It seems on paper the 6FW5 should be able to operate similarly to the 6LF6's. 6LF6s don't seem to be cheap anymore.

The 6LF6 is related to the 6LW6 (same tube different base). These can eat 6FW5's for lunch. My testing with the 6LW6 shows that any of them can handle 40+ watts of dissipation. The best versions can operate at 80 watts with no signs of glow in a dark room. I have run them at the 100 watt level with only a hint of redness on the plate. The 6FW5 is rated for 17 watts and will take about 25.

Class B screen drive has very low static dissipation, but requires large peak cathode currents while remaining linear. The 6LW6 - 6LF6 can pass over one AMP of peak cathode current with good linearity. I have tested it for low voltage cathode follower operation. It was the best of dozens of tubes tested with the 6336A coming in a close second.
 
I have an HP204C + HP333A that gives me about 0.1% measuring capabilities. It showed about 0.5% THD at 150vpp single ended output. I may have been using my wavetek 166 at the time, which has 0.5%thd sine, I can't remember, I should have taken notes:(. B+ was 340V, I've attached the circuit. Single-ended gain is about 35 V/V. I just got an HP 8903A and could redo the experiment if you wanted, I'm curious now myself. I don't have a DSA for spectra, I've been meaning to rig up an attenuator for my soundcard, someday...
 

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I was editing my previous reply while you were posting. I will post my schematic as soon as I can.

Now that you have them all, have you seen what they fetch on Ebay. They are GE's by the way. I plan on doing some more screen drive experiments in the future, but I have been infatuated by cathode followers lately. The 6FW5's are for the screen drive experiments.
 
SY said:
I'd probably do a long tail CCS for the first stage with something like a 6AQ4 or triode-connected D3a, then 6S4 for the second stage. Or a pair of dissimilar triodes like 6GF7 with the same topology.

Thanks for the answer! DC coupling two differential stages has been extensively discussed in some other threads which I will have to re-read now!

BTW, the shop where I bought a small stock of 6AQ4/EC91 hasn't got any more in stock. Seems that diyaudio rumours really spread fast!

Erik
 
I think you already posted the schematic, or have you changed it in the meanwhile?

Yes, that is it. I knew that I photographed it, but I couldn't find it on my hard drive. I must be getting senile! I built that amp after SY told me the wonders of screen drive. I had to dissassemble it to use the breadboard for another project. Later I built a P-P screen drive amp using 6LW6's to test out some 1250 ohm 400 watt OPT's that I bought. It made about 150 watts, but I never took the time to optimize it. The cathode follower spell took over my thinking and I haven't been able to shake it yet. There is some good magic in CF's and I am still developing some circuits that keep my breadboard occupied.

BTW, the shop where I bought a small stock of 6AQ4/EC91 hasn't got any more in stock. Seems that diyaudio rumours really spread fast!

Those were on my shopping list at the Orlando Hamfest last weekend. The tube vendors that had any were asking $15 each. I guess that I am too late on that tube.
 
Those were on my shopping list at the Orlando Hamfest last weekend. The tube vendors that had any were asking $15 each. I guess that I am too late on that tube.

That's a lot of money for the small tube. It is all to blame on Gregg who has shown them in a thread, and on Stuart who did the distortion spectra.

But seriously, for that kind of money one can buy the D3a, which at least on paper should outperform the 6AQ4.

Erik