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
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
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.
I also would definitely NOT recommend a dangerously high voltage design like the 845... mentioned, for any youngsters.
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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?
What are the requirements for the amp and what do you have in mind to spend on the parts?
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.
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.
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.
Thanks, sounds interesting. When I'm back home tonight I'll dig up the Murray circuit also.
Murray Amp
For those who may be interested, here is the murray amp I built about 7 years ago. The distortion is very low at almost fully power (about 0.05%) and it does sound very good. An explanation of how it works appeared in Tube Cad at Murray Tube Amplifier
The first amp I built used the 12AX7 and then I changed to the Aikido cct with source followers to drive the EL509's. It's still going strong.
For those who may be interested, here is the murray amp I built about 7 years ago. The distortion is very low at almost fully power (about 0.05%) and it does sound very good. An explanation of how it works appeared in Tube Cad at Murray Tube Amplifier
The first amp I built used the 12AX7 and then I changed to the Aikido cct with source followers to drive the EL509's. It's still going strong.
I have a pair and it's true they are spectacularly good. I'd like to see the schematic but I've searched without success, can you point to it ?
Today I had a hard time too and found only the schematic 'm60 signal.gif" at diyaudio.com in several threads posted by atmasphere.
I found both schematics here
Ambience, room definition and dynamics is spetacular good on the M60 if you have the right speaker.
DIY Atmasphere M60 OTL Circlotron :http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp-2.html
Today I had a hard time too and found only the schematic 'm60 signal.gif" at diyaudio.com in several threads posted by atmasphere.
I found both schematics
Thanks for digging up the cct.
Excuse my ignorance (I'm an old RF guy and usually play with SMD components I can't see) but I'm finding the input arrangement a bit perplexing, can someone explain the operation of V3? Why not simply use a simple FET ccs here?
Also, I'm wondering how this cct would go with only 2 output tubes, say 6c33's, and a low turns ratio output tranny or even an auto-transformer.
I guess that would be defeating the purpose but, as the output impedance wouldn't be such an issue, would it allow less NFB?
You can use a FET as CCS but then you have a solid state device their close to your signal... You can replace V3 with a resistor if you want a simpler circuit and also a gain tube if you don't need all the power.
If you consider to use a OPT why not build a well proven PP-design?
You can consider an auto former like this.
The M60 can be used without feedback.
I'm down in Melbourne on business checking my diyaudio post at a cafe....sick eh!
This is the issue to me, circlotron or standard
pp. I think the distortion profile is the same,if so the only advantage I can see would be in the lower output impedance allowing a simpler transformer with more leakage inductance. Then again the cost saving in the pp simpler power supply could pay for a high-end output transformer.
With 2 x 6C33 per channel as parallel choke loaded cathode followers , you could build a class A OTL amp . A touch of feedback between op stage and driver will bring the feedback down to a few ohms . Should yield a handful of watts and sound pretty good if my 3 x 6C41 SE OTL is anything to go by...
316A
The feedback as shown in the schematic is only 2 db. The circuit runs fine without it.
The CCS concept is a high impedance circuit used where there is already a B- of equal but opposite polarity. This is very nice for a differential amplifier! It allows the voltage amplifier to operate without change in performance while the power supply might vary due to line voltage variation.
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