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Quicksilver 8417 Bias
Quicksilver 8417 Bias
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Old 20th June 2019, 03:42 PM   #11
meq123 is offline meq123
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Thank you for your replies. They forced me to get off my a** and plug one of the amps in (no tubes) and measure the voltages at the bias resistor points - see pic.

Interestingly, the bias voltages are much lower than your assumptions @PRR, with the actual bias voltage set at ~23volts - from last time they were operational.
Given the voltage-drop measurements, I'd have to assume the resistances are actually 10K, 10K(pot) and 30K as @Alllensoncanon suggested.

This *is* definitely surprising though, since it also somewhat conflicts with @Burnedfingers dual bias circuit, which implies a much larger 'hot' range.


UPDATE: I was just reading that, indeed, 8417s require a much lower bias voltage (in the order of -20V) than others octals such as 6550 (~-40V). I assume that is why the difference with Burnedfingers circuit (and now Ketje's) which would also work with replacement tubes such as the 6550/KT88s
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Last edited by meq123; 20th June 2019 at 04:11 PM.
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Old 20th June 2019, 04:02 PM   #12
meq123 is offline meq123
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Quote:
Originally Posted by Ketje View Post
My 2 cents
Mona

Hi, Mona, I didn't see your reply before I posted. So you have interesting differences with Burnedfingers' dual bias circuit. I have a couple of questions if you don't mind:

1) Why the much lower value wiper capacitors than the original design?
2) Why add the 1M wiper resistors?
3) Most important: Why did you join pins 1 & 8 on the output tubes?

Waiting in Antici....pation


Thanks.
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Old 20th June 2019, 04:30 PM   #13
Ketje is offline Ketje  Belgium
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1) First i don't like useless fat cap's and smaller cap's means less leaking current.
2) When the wiper loses contact the grid is floating, no good, the 1M prevents this.
3) In a KT88 the cathode is internaly conneted to the g3 (dp) and pin 1 is N.C.
But tubes like the EL34 have the g3 on pin 1, with 1 & 8(cathode) together is allways ok.
Mona
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Old 20th June 2019, 05:45 PM   #14
meq123 is offline meq123
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OK, cool. Thank you.
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Old 20th June 2019, 06:18 PM   #15
Andrewbee is offline Andrewbee  Jamaica
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Take a look here at yet another bias Modification.http://www.triodeel.com/8417.htm
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Old 22nd June 2019, 03:30 AM   #16
PRR is offline PRR  United States
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Quote:
Originally Posted by meq123 View Post
...the bias voltages are much lower than your assumptions @PRR, .... reading that, indeed, 8417s require a much lower bias voltage (in the order of -20V) than others octals such as 6550 (~-40V).....
Vast apologies!!!

I know the 8417 well (had an array of twenty at one time) and *forgot* that its Mu(g2) is higher than most.

(I'm having medical issues and am not thinking straight.)

Someone who can do voltage-divider math clearly should be able to work this out.

(In my defense: if you had blindly followed my plan, nothing would burn-up, just run cold, and "hoarse" on small signals.)
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Old 22nd June 2019, 04:39 PM   #17
meq123 is offline meq123
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Heheh. PRR, after I posted my "indeed, 8417s require a much lower bias voltage" update, I went back and noticed that where I saw that info was actually in one of your posts!

As for the voltage divider math, that's what I used to determine the correct stock OEM resistor values once I'd measured the voltages.

Thank you for your replies and posts!

Last edited by meq123; 22nd June 2019 at 04:43 PM.
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Old 25th June 2019, 02:45 AM   #18
meq123 is offline meq123
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To complete this for posterity, I'm attaching both the updated/corrected original circuit for my amps, as well as the dual-bias modified circuit. For the latter I've made a bit of an amalgam of both Burnedfingers' and Ketje's circuits, though I may reduce the bias capacitor values depending on space. Also using a stereo 1/4 jack for the test points to save extra holes, and I'll use 10-turn pots to avoid needing to be too careful with trimming - given the new much wider range of voltage.

One final question, can anyone tell me the wattage requirements for the 23K2 resistors on top and bottom of the 12AU7 splitter - I'm not sure how much current flows through that circuit? One of my resistors is showing strange resistance readings and I think it may be on it's way out (or already out) but I can't find any replacements at Mouser/Digikey/Arrow/Jameco of that resistance or near, that are more than 5W. I'm worried that may not be enough?

Thanks
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File Type: gif QuickSilverCct_DualBias.gif (68.7 KB, 42 views)
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Old 25th June 2019, 12:36 PM   #19
Ketje is offline Ketje  Belgium
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2W is enough, 3W is safe side.
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