Reply
 
Thread Tools Display Modes
  #1   Report Post  
John L Stewart John L Stewart is offline
Senior Member
 
Location: Toronto
Posts: 301
Smile All Triode PP UL Amp Secret Revealed

That circuit I put on Audiobanter is something I looked at about 10 years ago. I had seen in the old tube manuals back more than 60 years ago some rather odd power tubes. Odd to me, anyway.

These would include 6AC5 & 25AC5, a pair of very high mu power triodes. The others are 6B5, 6N6G & 25B5, 25N6G.

All of these are now quite expensive, I guess the collectors need them to keep the old stuff running. I wondered about simulating them with easy to get tubes. If one thinks of it, most power pentodes when run as triodes with G1 & G2 strapped together have quite a high mu.

I checked some 6F6s in that mode & found mu to be about 50, so OK for some kind of trial. If we check the old tube manuals we find that the recommended driver for the 6AC5 was the 76, a medium mu triode. The 76 is the same as the 6P5G but on the older base.

The trial circuit is quite simple, a 6F6 DC driven by the cathodes of parallel sections of a 6SN7GT. The anodes of the 6SN7GT could be either connected to the B+ supply, the OPT CT or the plate connexion of the 6F6. That varies the amount of NFB applied to the anodes of the 6SN7GT. The resulting reach thru to the 6SN7GT cathode is (1/mu). I’ve put the test circuit & results into attachments.

I never did build the complete PP UL circuit. I realized that the driver plate(s) direct to B+ version was used commercially in some radios in the 30s. If you look at the base connexions on the octal 6N6G & 25N6G they are identical to many common power tubes such as the 6F6GT, 6K6GT, 6V6GT & so on. In the circuit they are self biasing, no cathode resistor required. But they did not catch on. Probably more expensive to build these tubes than the corresponding pentodes with there two cathodes, etc.

Anyway, an interesting exercise.

Here are links to a family of tubes suitable for an all triode PP UL amp.
http://frank.pocnet.net/sheets/049/6/6AC5GT.pdf
http://frank.pocnet.net/sheets/127/2/25AC5GT.pdf
http://frank.pocnet.net/sheets/127/6/6B5.pdf
http://frank.pocnet.net/sheets/084/6/6N6G.pdf
http://frank.pocnet.net/sheets/127/2/25B5.pdf
http://frank.pocnet.net/sheets/049/2/25N6G.pdf

Trying to add some schema's now, see what happens!

Snow & Sun today, so I guess it will be the ski's. Looks like the bike is parked till Spring.

Cheers to all, John L Stewart
Attached Images
  
Attached Files
File Type: doc 6SN7 6F6 Amp.doc (19.5 KB, 257 views)
  #2   Report Post  
Posted to rec.audio.tubes
patrick-turner patrick-turner is offline
external usenet poster
 
Posts: 119
Default All Triode PP UL Amp Secret Revealed

On Saturday, 30 November 2013 03:42:43 UTC+11, John L Stewart wrote:
That circuit I put on Audiobanter is something I looked at about 10

years ago. I had seen in the old tube manuals back more than 60 years

ago some rather odd power tubes. Odd to me, anyway.



These would include 6AC5 & 25AC5, a pair of very high mu power triodes.
The others are 6B5, 6N6G & 25B5, 25N6G.
All of these are now quite expensive, I guess the collectors need them
to keep the old stuff running. I wondered about simulating them with
easy to get tubes. If one thinks of it, most power pentodes when run as
triodes with G1 & G2 strapped together have quite a high mu.
I checked some 6F6s in that mode & found mu to be about 50, so OK for
some kind of trial. If we check the old tube manuals we find that the
recommended driver for the 6AC5 was the 76, a medium mu triode. The 76

is the same as the 6P5G but on the older base.

The trial circuit is quite simple, a 6F6 DC driven by the cathodes of

parallel sections of a 6SN7GT. The anodes of the 6SN7GT could be either

connected to the B+ supply, the OPT CT or the plate connexion of the

6F6. That varies the amount of NFB applied to the anodes of the 6SN7GT.

The resulting reach thru to the 6SN7GT cathode is (1/mu). I�ve put the

test circuit & results into attachments.



I never did build the complete PP UL circuit. I realized that the driver

plate(s) direct to B+ version was used commercially in some radios in

the 30s. If you look at the base connexions on the octal 6N6G & 25N6G

they are identical to many common power tubes such as the 6F6GT, 6K6GT,

6V6GT & so on. In the circuit they are self biasing, no cathode resistor

required. But they did not catch on. Probably more expensive to build

these tubes than the corresponding pentodes with there two cathodes,

etc.



Anyway, an interesting exercise.



Here are links to a family of tubes suitable for an all triode PP UL

amp.

http://frank.pocnet.net/sheets/049/6/6AC5GT.pdf

http://frank.pocnet.net/sheets/127/2/25AC5GT.pdf

http://frank.pocnet.net/sheets/127/6/6B5.pdf

http://frank.pocnet.net/sheets/084/6/6N6G.pdf

http://frank.pocnet.net/sheets/127/2/25B5.pdf

http://frank.pocnet.net/sheets/049/2/25N6G.pdf
Trying to add some schema's now, see what happens!
Snow & Sun today, so I guess it will be the ski's. Looks like the bike
is parked till Spring.
Cheers to all, John L Stewart
+-------------------------------------------------------------------+
|Filename: 6SN7 Reach Through W.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=363|
|Filename: 6SN7 6F6 Amp W.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=364|
|Filename: 6SN7 6F6 Amp.doc |

|Download: http://www.audiobanter.com/attachment.php?attachmentid=365|

+-------------------------------------------------------------------+


I will look at schems. Some older ideas can always be revisited with a fresh approach to see what happens. But age old discipline are still required to see if anything can be done to justify the visitation.
Patrick Turner.

  #3   Report Post  
Posted to rec.audio.tubes
patrick-turner patrick-turner is offline
external usenet poster
 
Posts: 119
Default All Triode PP UL Amp Secret Revealed

I will look at schems. Some older ideas can always be revisited with a fresh approach to see what happens. But age old discipline is still required to see if anything can be done to justify the visitation.
Patrick Turner.

I just had a look at schem with paralleled 6SN7 driving 6F6 with Ia at 13.65mA and Ea at +250V. Maybe not much class A PO?
Sure, trioded pentode with DC drive to g1 connected to g2 does give tube more gain than usual, and high THD, and much FB must be used and I could not see any.
Patrick Turner.
  #4   Report Post  
John L Stewart John L Stewart is offline
Senior Member
 
Location: Toronto
Posts: 301
Cool

Quote:
Originally Posted by patrick-turner View Post
I will look at schems. Some older ideas can always be revisited with a fresh approach to see what happens. But age old discipline is still required to see if anything can be done to justify the visitation.
Patrick Turner.

I just had a look at schem with paralleled 6SN7 driving 6F6 with Ia at 13.65mA and Ea at +250V. Maybe not much class A PO?
Sure, trioded pentode with DC drive to g1 connected to g2 does give tube more gain than usual, and high THD, and much FB must be used and I could not see any.
Patrick Turner.
This exercise is meant to explore a concept. I had no intention to build a complete amplifier with all its warts & otherwise. Read on & you will realize anyone with an amp using SE or PP output such as 6K6, 6V6, 6F6 & so on needs simply to plug 6N6 into the same socket, whether it be UL or otherwise. The only wiring change would be to remove the bias network. Under ordinary conditions the hookup described is self-biased.

I did get some useful information, see below. One needs to go well beyond RDH4 on a regular basis. Otherwise, missing quite a bit!!

Look again at the table showing the test results. As the plate of triode one is taken from the B+ lead & moved to the OPT CT the Rp goes down. That is NFB. And further still, as one would expect when the plates of triodes one & two are strapped together. The change would be even more pronounced had I used a lower mu triode as the driver.

A while back on RAT there was some discussion of pentodes wherein G1 & G2 are strapped together. That forms a zero bias triode useable in a PP power amp. These were for the most part modulators in HAM transmitters. Right after WW2 there were quite a few built using 807s which were plentiful on the surplus market. Later TV sweep tubes such as the EAR example were used.

As a rule Class B PP has a problem with crossover distortion, in particular at low levels. A simple fix applies a little +ve bias to the driven grids. But most folks don’t bother. Oddly one shows up in a Ham modulator schematic in the 60’s. A pair of Class B 807’s are direct driven by the cathodes of a 6SN7. I would think that would work quite well. But the standing current goes up somewhat, something the mobile guys were allergic to!

My early copy of Electronic Workbench CAD has modeling for triodes & includes some examples. There are no pentodes, so the usefulness of the software is somewhat limited as far as vacuum tube applications are concerned. And I have yet to master SPICE.

But there are both voltage & current controlled voltage & current sources. So quite a bit of AC modeling can be accomplished. Still no way of connecting feedback such as UL or otherwise to a screen connexion.

That is when I thought ‘why not try the composite triode hookup’ such as the 6B5/6N6G, Etc. It turns out a very good AC model of things like 6V6, 6K6, 6L6, Etc can be constructed with two triodes in that connexion. That way another terminal becomes available to simulate the screen in a pentode. All works in the circuit very well.

The simulation uses a somewhat low mu triode for the input side, say 6 to 12. For the output side a mu of 50 to 120. If you look at the characteristics of any common pentode or beam tube, the triode mu is most likely in the range 6 to 12. Sweep tubes come in even lower.

If the G1/G2 strapped characteristics are used, then the resulting triode mu tends to be in the range of 50 or more. So the model simulates all very well, indeed.

Cheers to all, including the doubters!!

John L Stewart
Attached Images
 
Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The MiddiotBot Secret Identity Revealed GeoSynch Audio Opinions 35 September 20th 06 01:46 PM
MIDDIUS REVEALED !!! Annika1980 Audio Opinions 0 June 1st 05 03:18 PM
mmdir2002 language REVEALED! Tony F Car Audio 17 March 3rd 05 05:12 AM
Using power triode or power pentode wired as a triode as a split-load phase splitter tube? at Vacuum Tubes 9 August 27th 04 11:24 AM
Using power triode/pentode wired as triode as a split load phase splitter tube? at Vacuum Tubes 4 August 27th 04 10:47 AM


All times are GMT +1. The time now is 05:03 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"