Radford Low Distortion Oscillator Series 2 - Page 2 - diyAudio
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Old 28th December 2011, 02:20 PM   #11
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I'd switch your meter to DC and take a look at offset. Both with the AC output turned up just below clipping and down to almost no output. Might point you into correct section. If it was Solid state I'd look for a soft -V cap. But with tubes I couldn't say. An output transformer should negate any offset.
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Old 28th December 2011, 02:31 PM   #12
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Thanks, the circuit is a 'Bridged T' in the feedback loop with additional bootstrapped feedback to the first Valve (EF184) in a pair with the second one (EL822) doubling as the Output in Cathode follower mode without a transformer.
Thinking something may have gone 'high' in the cathode follower output stage to reduce negative half cycle current availability.....

No full schematic yet so doing this the 'hard way' at present from the generalised cct below
Mik
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Last edited by audiomik; 28th December 2011 at 02:50 PM. Reason: add generalised schematic
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Old 28th December 2011, 03:53 PM   #13
DRC is offline DRC  United Kingdom
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A cathode follower would explain the -ve rail clipping - output valve is not passing any current !!

dc
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Old 28th December 2011, 04:18 PM   #14
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Yup, need to spend some more time circuit tracing and component checking!

With date stamps on the Caps of April '67; things will be expected to have drifted in value which is the most likely cause.

Anyway interesting to do as haven't seriously worked with Valves since the '60's

Mik
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Old 29th December 2011, 10:49 AM   #15
DRC is offline DRC  United Kingdom
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I was trying to point out that a cathode follower (or even emitter follower) would not be biased for symmetric clipping if you want to get low distortion. I would assume a capacitor coupled output so I would check that cap for leakage.

Is the O/P voltage, at clipping, 3.2V RMS ?
What's the max indicated O/P voltage ?

dc
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Old 29th December 2011, 12:56 PM   #16
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At the output of the cathode follower, there should be a bit over 10volts of signal available with ability to drive the attenuator.

In the test I am running with 10dB of attenuation; it isn't quite achieving the 10volts without the asymetric clipping happening. The attenuator loading causes the clipping which appears just below full output. Have so far checked the output cap for ESR and it appears to be OK.

Haven't had time to investigate further yet, but biasing of the output cathode follower could well be one issue, in which case the anode or cathode resistor(s) of V1 might be high.....

Mik
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Old 1st January 2012, 01:24 AM   #17
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Since my last post, I've ordered a pair of new Valves for the Oscillator circuitry and when they arrive in a couple of weeks will have had the opportunity to properly check the associated passive components and acquire replacements as necessary.

So hopefully some further results to post in a couple of weeks or so

Mik
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Old 9th November 2012, 09:03 PM   #18
jimb0s is offline jimb0s  United Kingdom
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Unhappy posting on the yahoo Radford group

Quote:
Originally Posted by dchisholm View Post
Is it worthwhile to join the Yahoo groups "radford" at < radford : Radford Electronics >, or "radfordfiles" at < radfordfiles : Radford Electronics Files and Database >?

Dale

Thanks Dale,
but has anybody sucessfully posted a thread or guestion on these sites. unfortunately I have not been able, but it may be it's me and not finding the correct procedure.
i am still looking for information on the control element in the RADFORD LDO3 oscillator, and wanted to expand my search field but had my querry fail with the following statement:-
"Your message was not delivered because it was sent to an
announcement-only group, where only the moderator may post."

jimb0s
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Old 10th November 2012, 05:09 AM   #19
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jimb0s,

It DOES appear that the "radfordfiles" group allows only the moderator to post. However, scanning the "Messages" list from the "radford" group, I see posts from about a dozen different ID's over the last couple of weeks.

Dale
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Old 21st January 2013, 06:37 AM   #20
richiem is offline richiem  United States
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Just found this thread -- that circuit you show is a Twin-T, not a Bridged-T. Has nothing to do with low output or high distortion, but just wanted to mention....
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