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845 with kathode bias
845 with kathode bias
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Old 28th June 2011, 07:22 AM   #1
Dogie_Hogan is offline Dogie_Hogan  Netherlands
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Default 845 with kathode bias

During more than 19 years I have 2 home-made SE-VT4C amps. Have a pair of RCA and G.E. VT4C's.
As neither of these tubes is after those years 100%, I like to replace them by a matched pair of 845B's, with kathode bias too.
(And I don't dislike somewhat more output...)

Because 211's and 845's are DHT's, a lot of fixed-bias parts (like 4 diodes, CRC, potmeter) are in the audio path, so I prefer kathode bias. And I dislike the weakest link: the potmeter (a bad contact causes a broken tube, as Rk is 10R).
845 kathode bias :
I estimate Rk 1K5, Ia ca. 84 mA; and the by-pass cap wil be Solen/SCR 200uF/250V. MKP (as audio-proof 200V. caps/elko's are hard to find).

Today the VT4C's have kathode bias (Rk 1K0, Ia 53 mA, B+ 1025V., caps: 2x 47 uF/100V. Black Gate).
The HV powersupply: Sec. 800V./300mA, 4x BY228 (in 1992 I wasn't familiar with the 866A-family yet), than 1 parallel 5R4WGB ('soft-start'), than CLC (C's: 51uF/1500V. Solen/SCR MKP's, L: 15H/200mA). Hook-up wire: Kimber TCSS.
The pre-drivers (2 G.E. 6SN7GTB with DC-coupling, B+ 320V.) have an independ PS.
Circuits with 845 kathode bias are hard to find; found only this one.

Anyone familiar with 845 tubes (valves) in kathode bias? (I am aware B+ will drop)

Last edited by Dogie_Hogan; 28th June 2011 at 07:44 AM. Reason: spelling
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Old 28th June 2011, 07:37 AM   #2
Yvesm is offline Yvesm  France
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Why do you want to vaste precious voltage just to heat a huge resistor

Yves.
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Old 28th June 2011, 07:50 AM   #3
Dogie_Hogan is offline Dogie_Hogan  Netherlands
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Quote:
Originally Posted by Yvesm View Post
Why do you want to vaste precious voltage just to heat a huge resistor Yves.
2 reasons: fixed-bias: many parts influence the final -audio- result (kathode bias: just risistor + cap) and safety (bad contact in potm., tube drift: checking neg. supply regularly).

************

edit
Attached: 845 data (source: "Transmitting Tubes", RCA 1938)

Last edited by Dogie_Hogan; 28th June 2011 at 08:01 AM.
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Old 28th June 2011, 09:20 AM   #4
tricomp is offline tricomp  Denmark
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[QUOTE=Dogie_Hogan;2621808]2 reasons: fixed-bias: many parts influence the final -audio- result (kathode bias: just risistor + cap) and safety (bad contact in potm., tube drift: checking neg. supply regularly).

Actually the Fixed Bias parts are not in the Audio-path, Cathode-resistor and capacitor are!
You may design the fixed-bias pot so that when you have a bad contact in the pot-meter the negative voltage increases, cutting-off the tube. Just use the pot. as the divider-resistor to ground. If it fails, the negative voltage just increases and keeps your tube safe.
It is probably true that a cathode-biased system will correct for wear on tubes to some extent but I find it more usefull to just check the fixed bias once in a while.
With fixed-bias you are not throwing away watts into heating a resistor. The watts should go to your speakers

rgds,

/tri-comp
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