Comments for page: A New Look

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Posted by Bill December 30, 2016 - 03:17 pm
I use a thermistor (CL-90) on the A/C primary before the fuse. It's housed in small box with a couple of switches to run the ST-35 with or without the thermistor. With the thermistor engaged, I observe the following:

1) A/C gets ramped up slowly to about 2-3 volts shy of normal wall voltage.

2) output bias gets ramped up very slowly to about 0.270VDC. I use the EFB.

3) Plate voltage on the output tube starts up at 450VDC and drops to 220VDC (normal) after about 5-10 seconds.

It seems that the thermistor is doing its job with the bias but not the High Voltage on the plates. When I bypass the thermistor with the other switch (MBB), the voltage at the plate remains the same, but the voltage at the bias suddenly spikes to 0.300VDC and drops to 0.270VDC after a second.

When I don't engage the thermistor:

1) Plate voltage starts out at 450VDC and drops to 220VDC - just like with thermistor engaged.

2) Bias ramps up quickly but does not exceed 0.270VDC.

Dave, any idea what's going on here?

Posted by Bill October 12, 2016 - 07:01 pm
Great advise Dave. Thanks.

Posted by Dave October 12, 2016 - 01:27 pm
Hi Bill -- Of the caps you are considering, the 5 uF @ 35 vdc is the most ideal -- but all of them should work fine.

The capacitor's function is to help maintain stable operation of the regulator -- just as virtually all 3 terminal regulators require in most applications. It needs to be big enough to accomplish that task, without being so big as to limit response time to changes in B+ voltage. It would be very hard to distinguish any performance difference between the specified value, and that of any of your caps.

One other point to consider however is the rated working voltage of the cap you use. The cap normally sees about 15 vdc across it, making a cap rated for 25 volts ideal, and one rated for 35 volts perfectly acceptable. But as you go to caps rated for increasingly higher operating voltages, the capacitor -- over time -- won't hold its form as well as one that is more appropriately rated for the voltage it actually sees in service. Granted, it can take years for issues with form to occur -- but they will occur. Just something to consider.


Posted by Bill October 12, 2016 - 09:59 am
Hi Dave, one more thing: one of the caps in the EFBcircuit calls for a 4.7uF rating. I have 3, one measuring 5.0 uF @ 35 VDC; another that is 5.0 uF @ 50 VDC; the third that measures 5.9 uF @ 70 VDC. I like the build quality of the 5.9 uF cap but wonder if it's too far out of range?

Posted by Bill October 02, 2016 - 10:38 am
Thanks Dave

Posted by Dave September 30, 2016 - 06:42 pm
Hi Bill -- You can increase the value of the resistors if you like, but moving in the direction of increased resistance only serves to reduce power output, and work to reduce the effects of the Ultra Linear connection. Installation of Screen Stability resistors are by all accounts sonicly neutral, and once their value is large enough to damp out both large and small scale parasitic oscillations, then increasing their value beyond that point serves little purpose other than adding some insurance. Typically, tubes of the 6BQ5 class will remain stable with a Screen Stability resistor value of as little as 47 Ohms in most applications, so there is already some insurance built into the 100 Ohm value specified.

If you do increase the value of these resistors, I would not increase them beyond a value of 330 Ohms. Within the 6BQ5 family, EL84 tubes are true pentode tubes, and therefore draw more screen current than their beam power brethren. Raising the value beyond the limit suggested will only serve to limit amplifier performance when these tube types are used, while adding little in the way of insurance for the tubes.

Good luck with your project!


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