Shuguang KT66

If they glow, this is the place to be
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Paul Barker
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Post by Paul Barker » Mon Mar 16, 2015 8:07 am

To be more exact

1/ is the B+ 450v and the bias fixed so that the Anode voltage is 450v?

Or

2/ is the B + 450v and the bias via cathode resistor or current sink, and so anode voltage is B+ less cathode voltage?

Or

3/ a hybrid of both, in which case, what then is the cathode voltage?
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RobW
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Post by RobW » Mon Mar 16, 2015 8:32 am

Paul Barker wrote:To be more exact

1/ is the B+ 450v and the bias fixed so that the Anode voltage is 450v?

Or

2/ is the B + 450v and the bias via cathode resistor or current sink, and so anode voltage is B+ less cathode voltage?

Or

3/ a hybrid of both, in which case, what then is the cathode voltage?
Hi it’s how you mention in example 1, it’s a fixed bias amp, the cathode is grounded. It’s not a cathode biased amp or hybrid. :D

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Paul Barker
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Post by Paul Barker » Mon Mar 16, 2015 10:16 am

Thank you.
I can do it right or I can do it now, but I can't do it right now.

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Paul Barker
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Post by Paul Barker » Mon Mar 16, 2015 10:29 am

Have you followed the GE recommended anti parasitic oscillation measures of 10k to 50k control grid stopper and 100 to 300 ohm screen grid stopper?

Have you heeded the GE advice to use a stiffly regulated B+ supply for push pull circuit to avoid "excessive quiescent anode dissipation"?
I can do it right or I can do it now, but I can't do it right now.

RobW
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Post by RobW » Mon Mar 16, 2015 12:07 pm

Paul Barker wrote:Have you followed the GE recommended anti parasitic oscillation measures of 10k to 50k control grid stopper and 100 to 300 ohm screen grid stopper?

Have you heeded the GE advice to use a stiffly regulated B+ supply for push pull circuit to avoid "excessive quiescent anode dissipation"?
Funny you should mention anti parasitic oscillation measures. The original amp spec had no grid stoppers and this one is like the original, but I have come across a schematic today by Criatone in which they use 5k6 resistors on the control grids in their JTM45 clone. I’ve also read somewhere that shuguangs sometimes suffer from high frequency oscillation whereas the GEC didn’t or so I believe. The screen grids have the usual 470 ohm resistors and the power supply is also standard JTM 45 spec. The sound of the amp would be compromised by stiffening up the power supply. Regarding quiescent anode dissipation, it’s not pulling that many amps at idle I worked it out at 60% dissipation, so it could be the parasitic oscillation you mentioned. It's only one of the valves that red plating, swapping the tubes around doesn’t cure the problem so I will try that. I just wish I had a graph of anode dissipation, but the data sheet for the Shaguangs is quite vague. Thanks 8)

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Post by RobW » Tue Mar 17, 2015 9:53 am

In conclusion I have now tried grid stoppers, no difference. Turn up the heat last night to 40ma, 70% dissipation, still only one of the valves red plating slightly at the edge. Tested output transformer for shorts and reflected impedance all ok. So tried some old GE 6l6GC from the 70s in it and they were great, even sounded better, ok at 45ma; so it must be just a faulty valve. Thank you eBay 8)
Last edited by RobW on Tue Mar 17, 2015 2:16 pm, edited 1 time in total.

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Paul Barker
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Post by Paul Barker » Tue Mar 17, 2015 1:25 pm

That's the way it has to be then!
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Post by jack » Mon Apr 27, 2015 3:39 pm

Recently, I bought a bunch of supposedly NIB but unmarked valves of a particular type from eBay - there were in OEM packaging, i.e. waiting to be marked up for the final vendor - this is not uncommon - tubes bought from wholesalers, e.g. RELL, are often still like this.

The physical construction was a bit different to other, known original, tubes of that type, so I characterised them on the uTracer and compared that with the known good one.

Completely different - yes, they were double-triodes and physically very similar to the ones I was after, but electrically they were miles away.

Vendor refunded, but only because I had him bang to rights...

Might be worth having that tube looked at...

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Post by RobW » Fri May 01, 2015 5:55 pm

Hi I've replaced them now and all is as it should be. Like you I purchased off a certain ebay seller, can't say who but most sellers are genuine, this one was selling off his not up to scratch stock on there, still is. The ones I have now are from a different source and are brilliant but still off ebay. I should build a U tracer. I think its worth the effort.

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