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Old 12th February 2012, 10:53 PM   #1
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Default Circlotron Amplifier Advice Needed

My son asked me to help him build a valve amplifier. Sounds simple enough but he really wants something to look impressive. My immediate thougths go to 845/211/811 etc. but to my mind the anode voltages are too high for a 'first amplifier', not to mention the difficulties and costs in finding suitable components.

I'm leaning towards a 6p33 circlotron design with 2x6P33 in each channel, 150-180V anode voltage, low impedance cathode follower grid drive, and maybe some small resistance in the 6p33 cathodes to give some bias stabilisation.

I'm also looking at using a low turns ratio output transformer (say 300Ohm/8Ohm) to isolate the speaker. I'm assuming that would allow more modest use of negative feedback (as output impedance is not so crytical), and resultant improvement in Transient Harmonic Distortion.

My question is, not having had any experience with Circlotron's, does this sound like I'm on the right track? As I'm planning to use an output transformer anyway, am I complicating things too much by using a circlotron over a conventional PP circuit, for no real gain?

Thanks
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Old 13th February 2012, 02:38 AM   #2
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The Circlotron approach is useful if you are needing a very high Z primary, since it only requires half the turns, and will thus allow better xfmr performance at HF. For the low impedance design you have selected, I don't see where it buys you anything useful, and it complicates power supply requirements greatly.

I also would definitely NOT recommend a dangerously high voltage design like the 845... mentioned, for any youngsters.
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Old 13th February 2012, 03:58 AM   #3
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[QUOTE=
I also would definitely NOT recommend a dangerously high voltage design like the 845... mentioned, for any youngsters.[/QUOTE]

Sorry, forgot to mention that he is a 23 yo quallifed mechanical engineer...but thanks for the advice. Maybe I should try something more conventional.
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Old 13th February 2012, 09:12 AM   #4
lechuck is offline lechuck  Netherlands
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What is the reason for choosing a circlotron output stage? You mentioned that this will be your son's first tube amplifier, perhaps it would be a good idea to keep things simple and start with a normal single ended or pushpull amplifier?

What are the requirements for the amp and what do you have in mind to spend on the parts?
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Old 13th February 2012, 09:40 AM   #5
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What is the reason for choosing a circlotron output stage?

I dont know really, other than it looks like an interesting circuit and it seems to have a good reputation. I have built a Murray SEPP amplifier using PL509's triode connected which gives about 30W rms with distortion cancellation, so I guess I just like something a bit unusual.

You mentioned that this will be your son's first tube amplifier, perhaps it would be a good idea to keep things simple and start with a normal single ended or pushpull amplifier?

I'm now thinking about using two 6c33 in class-A pp with some beefy CCS on their cathodes to overcome the drifting problem with those tubes. If I get the current balance close enougth I'm thinking I should be able to use a toroidal output transformer, and at those low impedances maybe even a toroidal power transformer would work to test the idea out.


What are the requirements for the amp and what do you have in mind to spend on the parts?

Cost isn't too much of an issue. Thanks for the comments.
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Old 13th February 2012, 11:56 AM   #6
lechuck is offline lechuck  Netherlands
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Quote:
Originally Posted by Philwait View Post
What is the reason for choosing a circlotron output stage?

I dont know really, other than it looks like an interesting circuit and it seems to have a good reputation. I have built a Murray SEPP amplifier using PL509's triode connected which gives about 30W rms with distortion cancellation, so I guess I just like something a bit unusual.

You mentioned that this will be your son's first tube amplifier, perhaps it would be a good idea to keep things simple and start with a normal single ended or pushpull amplifier?

I'm now thinking about using two 6c33 in class-A pp with some beefy CCS on their cathodes to overcome the drifting problem with those tubes. If I get the current balance close enougth I'm thinking I should be able to use a toroidal output transformer, and at those low impedances maybe even a toroidal power transformer would work to test the idea out.


What are the requirements for the amp and what do you have in mind to spend on the parts?

Cost isn't too much of an issue. Thanks for the comments.
If you like something un-ussual I might be able to help you out with schematics for a DC coupled differential amplifier using EL509 output tubes.

It's a good amplifier for any beginner (it's simple in design) and it works very well. If you want I can send it over via mail.
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Old 13th February 2012, 04:12 PM   #7
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Quote:
Originally Posted by lechuck View Post
If you like something un-ussual I might be able to help you out with schematics for a DC coupled differential amplifier using EL509 output tubes.

It's a good amplifier for any beginner (it's simple in design) and it works very well. If you want I can send it over via mail.
I'm interested too!

Thanks
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Old 13th February 2012, 05:11 PM   #8
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Be sure to check the Atma-Sphere M-60 OTL at atmasphere.com. Check the awards and the reviews...
The schematic is available here at diyaudio for free for personal use. The M60 is indeed very very good IME
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Old 13th February 2012, 07:04 PM   #9
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Quote:
Originally Posted by Philwait View Post

I'm leaning towards a 6p33 circlotron design with 2x6P33 in each channel, 150-180V anode voltage, low impedance cathode follower grid drive, and maybe some small resistance in the 6p33 cathodes to give some bias stabilisation.

I'm also looking at using a low turns ratio output transformer (say 300Ohm/8Ohm) to isolate the speaker. I'm assuming that would allow more modest use of negative feedback (as output impedance is not so crytical), and resultant improvement in Transient Harmonic Distortion.

Thanks
This should work quite well but given a 300 ohm primary, I think you will want to run a higher B+ to make power, 250V is where I would start. Distortion will be low without feedback.
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Old 13th February 2012, 08:03 PM   #10
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Quote:
Originally Posted by lechuck View Post
If you like something un-ussual I might be able to help you out with schematics for a DC coupled differential amplifier using EL509 output tubes.

It's a good amplifier for any beginner (it's simple in design) and it works very well. If you want I can send it over via mail.
Thanks, sounds interesting. When I'm back home tonight I'll dig up the Murray circuit also.
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