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Old 17th June 2010, 11:08 PM   #11
jlsem is offline jlsem  United States
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If you are going to use the 7236, you might consider the operating points used for the output tube of the Triodino amp:

5998 power amp question

This amp would also be improved by using SY's CCS on the plates of the 12AT7.

John
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Old 18th June 2010, 01:50 AM   #12
Bigun is offline Bigun  Canada
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That looks encouraging, my Cellini amp is very similar, although I plan to use a choke load on the input tube instead of CCS (and a 6SN7). The 7236 was ordered today.
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Old 18th June 2010, 02:23 AM   #13
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Just be aware that the tube is prone to arc-over failure if operated in fixed-bias mode.
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Old 18th June 2010, 02:44 AM   #14
Bigun is offline Bigun  Canada
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Thanks for the heads up, I've no doubt we don't want arc-over, it sounds like a bad thing !

It's my first project and I want to keep thins relatively simple but for all the effort of building this I want it to sound good enough to keep around for a few years.

I'll be using cathode bias and tube rectifier so that the HT comes up after the output tube has had some chance to suck a good 2.5 amps. With low mu tubes I understand that fixed bias isn't always advised [my CELLINI triode amp]
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Old 18th June 2010, 05:14 AM   #15
mach1 is offline mach1  Australia
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Quote:
This amp would also be improved by using SY's CCS on the plates of the 12AT7.
Not necessarily: I believe it was designed by Tim de P who deliberately chose the driver tube, plate resistor value and operating point to provide a degree of 2H* cancellation. A ccs induced flat driver loadline would not produce the same result.


*which the 5998 produces in abundance
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Old 18th June 2010, 07:08 AM   #16
jlsem is offline jlsem  United States
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Quote:
Just be aware that the tube is prone to arc-over failure if operated in fixed-bias mode.
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.

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Old 18th June 2010, 07:17 AM   #17
jlsem is offline jlsem  United States
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Quote:
Not necessarily: I believe it was designed by Tim de P who deliberately chose the driver tube, plate resistor value and operating point to provide a degree of 2H* cancellation. A ccs induced flat driver loadline would not produce the same result.
Are you sure it was designed by Tim DeParavicini? I was under the impression it was designed by an Italian DIYer. And besides, distortion cancellation in a single ended amp is a bit of a canard, don't you think?

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Old 18th June 2010, 07:49 AM   #18
mach1 is offline mach1  Australia
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Are you sure it was designed by Tim DeParavicini? I was under the impression it was designed by an Italian DIYer. And besides, distortion cancellation in a single ended amp is a bit of a canard, don't you think?
1. No. However the information source is usually correct.
2. Depends. In instances where very high levels of H2 are being generated I think it can be justified. I have built two amps where it was definitely beneficial.
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Old 18th June 2010, 02:17 PM   #19
Bigun is offline Bigun  Canada
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I'd be more concerned about generating higher harmonics than cancelling out H2 - it must be difficult to generate a complimentary curve at the front end to cancel out some H2 from the output without generating some H3, H5 etc. ?
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Old 18th June 2010, 03:36 PM   #20
jlsem is offline jlsem  United States
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Quote:
Originally Posted by mach1 View Post
1. No. However the information source is usually correct.
2. Depends. In instances where very high levels of H2 are being generated I think it can be justified. I have built two amps where it was definitely beneficial.
I believe the amp was designed by Cristiano Jelasi and Ciro Marzio. 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.

John

Last edited by jlsem; 18th June 2010 at 03:39 PM.
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