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Old 15th June 2008, 12:58 AM   #11
SY is offline SY  United States
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You got it. Pretend the load is in the plate, calculate the distortion, then divide it by the feedback factor (about the tube's mu).

ECC88 is a standard and excellent choice. For your service, a 12AT7 will also work fine. Pentodes make great CFs, but at a higher parts count (need to swing the screen with the cathode).
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Old 15th June 2008, 01:19 AM   #12
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Hmmm... just noticed that there is a 6FH5 in there. Might be worth a look.

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Old 15th June 2008, 01:19 AM   #13
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Hi,

I did a 6GH8A mu-stage linedriver once, pentode on top. Worked pretty good and they are a dime-a-dozen.

Cheers!
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Old 15th June 2008, 11:06 AM   #14
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Thanks Geek, will make a note of that use for those tubes, that could come in handy.

As to the 6FH5 my main concern is that using a single triode to mix the two channels might compromise the channel separation in the other filters. The plan is to feed all three frequency range filters from the volume pots which should be fine but doing single channel sub ties the L&R together through the input resistors.

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Old 15th June 2008, 12:02 PM   #15
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A dual triode won't cause any separation problems. Remember, the plates are grounded at AC.

6FH5 will work fine assuming it isn't microphonic (which many tubes of its class tend to be) and you take care to prevent it from oscillating.
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Old 15th June 2008, 05:35 PM   #16
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Thanks SY,

Quote:
A dual triode won't cause any separation problems. Remember, the plates are grounded at AC.
Actually my concern was using a single triode with both left and right channels feeding its single grid as shown.

Any possibility of causing crosstalk problems with the stereo filters and amplification stages that will be fed off of the same input?

Quote:
6FH5 will work fine assuming it isn't microphonic (which many tubes of its class tend to be) and you take care to prevent it from oscillating.
Grid stopper adequate or do you think other more heroic measures will be necessary?

As far as microphonics goes operating at line level with gain <1 should make that less of a problem than it would be otherwise I suppose.

I guess I can always "bread board" it and do a quick and dirty measurement of crosstalk. The 6FH5 intrigues me because of its combination of relatively high mu (not important in this application of course) and high gm and current capabilities. Seems like it might be a nice little tubie.

mike
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Old 15th June 2008, 07:36 PM   #17
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SY.....to cut a long story short.....I've been using this config (below) in studio stuff for donkeys (25)years. Fooling everyone because it's so darned simple and sounds sharp.....double section ECC88 paralled parafeeding into a Sowter stepdown. The catch is an excellent quality coupling cap and worst of all, the transformer spec. The small caps on the B+ is testament to the RF nature of the ECC88 tube. As expected, layout is somewhat critical and must be kept to pins. I've omitted the CCS as to avoid any noise contribution, already -90dBu down.
Current through tube is pretty well what you find.
I could have used a split cathode resistor i.e bootstrapped to g1's to up the input Z, but in every case I found thd (already dead low) somewhat unpredictably increases vs. load Z. Perhaps I should revisit this problem. Notice the actual o/p Z !

richj
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Old 15th June 2008, 09:07 PM   #18
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richwalters, thanks for the very interesting pointer. However I can't find that transformer (9818, 3:1) on the Sowter site, so is it a special and could any other (Sowter or otherwise) be effectively substituted in?
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Old 15th June 2008, 09:47 PM   #19
SY is offline SY  United States
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Rich, what do you use for that 68u cap? Of course, as you know, any CCS noise is divided down by the parallel combination of the transformer Z and the cathode source Z. Not that you'd want a CCS there anyway, the load impedance is dominated by the transformer.

mike, if you model each of your sources as a Thevenin equivalent, you'll see how much separation suffers (it won't be much). Regarding the care and feeding of RF triodes, use a grid stopper, a cathode stopper, and bypass the heater pins individually to the closest possible chassis points with some 10nF ceramics. Make sure the plate is bypassed to signal ground with short leads and a high-quality cap so the tube won't think it's got a load at VHF.
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Old 15th June 2008, 10:40 PM   #20
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Quote:
Originally posted by SY
Rich, what do you use for that 68u cap? Of course, as you know, any CCS noise is divided down by the parallel combination of the transformer Z and the cathode source Z. Not that you'd want a CCS there anyway, the load impedance is dominated by the transformer.
-> Yup quite right. Somewhere I should have a graph of output level vs. loading plus thd thrown in. Will dig it out.

cap is a. ICW wopper size 160V low leakage polypropy'e. Your end I should think Cornell Dublier should have something sim.


http://www.icwltd.co.uk/claritycap/pwa/size.html



rj:-
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