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Old 9th May 2011, 02:02 AM   #1
opcom is offline opcom  United States
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Default The 7242

OK I got a pile of 7242's here, 6, or maybe 8 of them. But what to do with them? I see them mentioned by searching the board, but not found any really interesting use yet, not in an amp that anyone's built.
Maybe I missed something.

Push pull of some sort, I would prefer. How would they be used most efficiently? OTL is not going to perform the 1812 cannonade except in headphones.

It is a 100W tube, and I know from experience what a pair of US-made 845's or JAN 811's will do so I should expect some power.

Am I wrong to think that a pair in push pull with the right transformer and 300V on the plates would work pretty well? I have no idea what transformer, except the primary may be rather low impedance like 500-1000 Ohms CT.

It's an AB1 tube, no grid current. 0.9A peak @ 6V drop. I'd upload the data sheet but the forum does not allow that size. It can be found online at Frank's IIRC.

I'm not accustomed to low voltage triodes. Any suggestions?
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Old 9th May 2011, 04:57 AM   #2
jfitz57 is offline jfitz57  United States
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CIRCLOTRON!
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Old 9th May 2011, 07:59 AM   #3
opcom is offline opcom  United States
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CIRCLOTRON!
I have considered it and it might work with a transformer for a better match too, but there's no diagram with this tube in it. Not sure where to bias. Some feedback will be needed with these tubes also.
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Old 9th May 2011, 01:53 PM   #4
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http://frank.pocnet.net/sheets/127/7/7242.pdf
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Old 9th May 2011, 02:20 PM   #5
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As I have stated before, any time you venture away from the audio mainstream you must be prepared to experiment. I have seen this tube mentioned several times, and like you haven't seen much in the way of succesful designs. The 7242 is similar to the 6080 / 6AS7 and the 6528, but has higher gain. It may have the same non - linear behavior that the other regulator triodes exhibit too.

I found out about this tube a few years ago but haven't found any suitable samples to experiment with for a reasonable price. My usual tube suppliers have one or two in stock and they want $50 or more for them, which is far above what I will spend on experiments. They don't seem to be very common, so replacements may be hard to find down the road.


I was thinking that a circlotron or power cathode follower would be the right choice here. My cathode follower experiments worked at low power levels, but you can only suck so much power from a 6AS7 or 6336A. I was running these tubes with a constant 100 volts on the plate by using the augmented cathode follower circuit. There was a thread with that title going a few years ago. There were also a few other cathode follower designs going. All were SE. I did a simulation for a P-P cathode follower design, but never built it.

If you can't find a suitable design, I might be interested in trading you for some more common tubes, but I don't have the time to do any experiments now, in fact I am currently 1200 miles from home with no exact return date.
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Old 9th May 2011, 10:56 PM   #6
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I could use some new 838's for an old Norton amplifier here but they are as uncommon as the 7242. I'm pretty well full up on tubes and also short on time.

The 7242's are as nonlinear as the other pass tubes, but so are many of the tubes used in pass element power supplies and I've made a few of them sing. I guess that is admitting I am not the fussiest person about absolute performance. If it sounds good, listen to it! I'll search after the other threads.
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Old 10th May 2011, 02:18 AM   #7
jfitz57 is offline jfitz57  United States
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Originally Posted by opcom View Post
I have considered it and it might work with a transformer for a better match too, but there's no diagram with this tube in it. Not sure where to bias. Some feedback will be needed with these tubes also.
Why not build a custom higher impedance speaker system for an OTL circlotron? Say 24 or 32 ohms.
The only "audio" mag. I can get my hands on around here (S n V) is full of towers. Just make something like that and wire the drivers in series. Do you have or can you get sockets for those tubes?
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Old 10th May 2011, 02:55 AM   #8
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Quote:
Do you have or can you get sockets for those tubes?
If my memory is correct the 7242 uses the same socket as the 813. You can get new production Chinese sockets from several tube suppliers.
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Old 10th May 2011, 03:30 AM   #9
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Is it radioactive? It's graphite anode AFAIR is coated by Zirconium. But it may sound quite nice with 250V B+ and 750 Ohm SE load.
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Last edited by Wavebourn; 10th May 2011 at 03:47 AM.
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Old 11th May 2011, 04:37 AM   #10
opcom is offline opcom  United States
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The socket is like an 813.

I have not checked the tubes for radiation. I would imagine it is as safe and only slightly radioactive as any other tube, less so than some WWII spark gap tubes.
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