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Old 18th June 2010, 03:59 PM   #21
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Originally Posted by jlsem View Post
The 7236 is of quite a bit different construction than the 6AS7 (and its family) as well as the 421A and the 5998. The elements are nowhere nearly as closely spaced and it is a lot less prone to arcing over.

John
We've had a number of customers try to use this tube as a replacement for the 6AS7G. Universally their experience and also IME it is more likely to arc than the 6AS7G. However the 6AS7G has fairly lightweight links from the tube base to the cathode which can blow out, severing the connection to the tube (during the arc, it can be considered to be shorted). With the 7236 the links rarely fail, so the internal short does not stop. So if you use this tube I recommend that you self-bias it and use a fuse to protect other elements in the circuit.
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Old 18th June 2010, 04:47 PM   #22
jlsem is offline jlsem  United States
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Were they the box-plate type or the Sylvania 7236 style of construction? The heater to cathode voltage rating is low, but I've run mine with over 300 volts on the plate for five or six years with nary a problem. I did try a 5998 once, but it arced.

John
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Old 18th June 2010, 04:51 PM   #23
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In my experience, pass tubes are best run in self-bias mode. I have primarily used the 5998/421A, 7236 and the 6336.

Jim
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Old 18th June 2010, 04:59 PM   #24
Bigun is offline Bigun  Canada
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How would you guys rate the performance of the 7236 in terms of linearity / sound in SE use (compared with 6AS7 if possible) ??
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Old 18th June 2010, 05:42 PM   #25
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Quote:
Originally Posted by jlsem View Post
Were they the box-plate type or the Sylvania 7236 style of construction? The heater to cathode voltage rating is low, but I've run mine with over 300 volts on the plate for five or six years with nary a problem. I did try a 5998 once, but it arced.

John
I have no way of knowing that. Are you self-biasing?
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Old 18th June 2010, 07:20 PM   #26
jlsem is offline jlsem  United States
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Yes, it's the amp I linked to on post #11. The thread is about a similar amp called the Cellini that the original poster hopes to build. I personally believe the Tungsol 7236 is the best tube for this type of amp. I don't know from Circlotron OTLs.

John

Last edited by jlsem; 18th June 2010 at 07:24 PM.
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Old 18th June 2010, 07:25 PM   #27
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Since 'atmasphere' is here, perhaps he can refresh my mind as to the value of adding a low value resistor (in series with the plate?) in helping with 6as7g longevity in an OTL application?

Thanks in advance.
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Old 18th June 2010, 07:31 PM   #28
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Originally Posted by Bigun View Post
How would you guys rate the performance of the 7236 in terms of linearity / sound in SE use (compared with 6AS7 if possible) ??
7236 and 5998/421A worked extremely well as SE driver tubes - the 7236 was a bit cleaner/brighter, while the 5998/421A produced a warmer sound.
All of my 7236s were Sylvanias with the metal base ring. The best 5998s rivaled the 421A, but 5998 exhibited quite a bit of sonic variation.
Overall, I preferred the 421A, but much will depend upon the rest of your system.
I did not try the 6AS7/6080, because I needed some gain

Note: Many pass tubes have notoriously mismatched sections...the 421A consistently exhibited the best section matching.

Jim
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Old 19th June 2010, 04:20 AM   #29
mach1 is offline mach1  Australia
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There is nothing remarkable about the design other than it was meant as a simple exercise for the beginner. As for carefully chosen component values, the resistors used are of common values and are repeated as often as possible. There are four 100kΩ, and two each of 1kΩ, 1.5kΩ, 4.7kΩ and 220kΩ, which could have easily been 100kΩ as well. All of the capacitors are 100F except the coupling cap which can be anything from .1F to .47F. I wired mine up in a couple of hours and was able to use components I already had on hand.
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5998

--------------------------------------------------------------------------------

Hi,

This the one you had in mind, John?

5998 power amp question 5998 power amp question

It's from the same hand as the Lilliput, BTW.

Cheers,
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Frank is Fdegrove. Lilliput is a Tim de P design incorporating H2 cancellation..

1. Agreed.
2. Because the design uses common values, you had the parts available and were able to knock one up in couple of hours has nothing to do with whether or not the designer attempted a degree of H2 cancellation.
3. I built an 'improved' version of the amp with ccs'd pc86 driver and led bias, and the amount of H2 it produced was horrendous, rendering it pretty well unlistenable. I was able to achieve much more satisfactory results playing around with different driver tube plate resistors and a small pot in each cathode.
4. if you are going to try distortion cancellation it is a good idea play around with input tube operating points USING THE SPEAKERS YOU INTEND TO USE, as the output tube loadline depends on the impedance of the speaker it is driving. To complicate matters, because speaker impedance also varies with frequency, H2 cancellation works best when driving speakers with a fairly flat impedance curve.
5. If I were to build the amp now, I would follow the schematic exactly, and then assess the effects of varying the input tube OP and loadline on distortion. I would definitely not use a ccs and led bias.
6. If the design was changed to incorporate cathode feedback around the output transformer to linearise output tube transfer characteristics to something approximating a good DHT, then a ccs and led on the driver may well produce superior results. This is now my preferred way of building with pass tubes.

This article may shed further light on the matter.

http://www.audioxpress.com/magsdirx/...rOddsEvRev.pdf

I completely agree with the author's view that it is preferable to design linear amplifying stages that do not require cancellation.
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Last edited by mach1; 19th June 2010 at 04:28 AM.
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Old 19th June 2010, 09:24 AM   #30
SY is offline SY  United States
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I would definitely not use a ccs and led bias.
...

I completely agree with the author's view that it is preferable to design linear amplifying stages that do not require cancellation.
Whiplash.
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