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Question about 6922 design

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Posted on June 13, 2020 at 11:36:49
jedrider
Audiophile

Posts: 15166
Location: No. California
Joined: December 26, 2003
 Is there any design parameter that causes some preamplifiers to be hard on tubes? If I measure filament voltage to be on the high side, will that give me some clue? Maybe, one of those switching filament supplies could be a good upgrade for a preamplifier with four 6922's.

 

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It is possible, posted on June 13, 2020 at 12:39:18
richardl
Audiophile

Posts: 3555
Joined: September 5, 2002
 However, when the heater is high, oftentimes IME I have experienced noise. Is it noisy?

The obvious way to burn tubes (small signal tubes especially) is simply to run them at high current and plate voltage. The measure the plate voltage and measure the voltage across the cathode resistor and compare the current and the voltage to the tube specs.

 

RE: Question about 6922 design, posted on June 13, 2020 at 12:43:44
Story
Audiophile

Posts: 10423
Location: NJ
Joined: December 11, 2000
 I remember values being given as a +- from design center but even my Langford Smith doesn't list the percentage - but I seem to recall 5%.

Even the RCA doesn't list but only says "may cause unsatisfactory operation/reduced tube life (low v) & may cause rapid evaporation/shorten tube life (high v)

the only other parameter that would be hard is plate dissipation



 

RE: Question about 6922 design, posted on June 13, 2020 at 13:28:55
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 How do you know your tubes are bad?


Dan Santoni

 

RE: Question about 6922 design, posted on June 13, 2020 at 16:54:52
Steve O
Audiophile

Posts: 12359
Location: SE MI
Joined: September 6, 2001
 Many preamp designs would run 6922/6DJ8 very hard with near max spec cathode current and anode dissipation. When operated this way, most versions of the series would become noisy at an early age and generally unusable. If I were to design something using this tube, I'd keep Va < 120V and Pd ~ 0.5W or less. As always, YMMV.

 

RE: Question about 6922 design, posted on June 13, 2020 at 21:34:21
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 I agree.

For what it's worth, I recall people saying (some 20years ago?) that 6DJ8s fail much more rapidly if their quiescent plate voltage exceeds 90vDC; however 6922s do not suffer from this.

I've aimed for 70v and 4mA, with the heater between 6.0 and 6.3v. For most tubes, spec on heater voltage is +/-10% (a few high-performance or long-life tubes specify 5%). You can run on the low side if the cathode current is low enough.

 

RE: Question about 6922 design, posted on June 14, 2020 at 07:27:59
jedrider
Audiophile

Posts: 15166
Location: No. California
Joined: December 26, 2003
 They just become noisy and I need new tubes then.

I've switched to solid state preamplification for now.

But, I'm going to investigate what's up and, maybe, modify. I hate to take apart a nicely laid out unit, but fetching price of an Anthem Pre 1L is not all that high anyway. I have an idea that I want to insert a different preamplifier board in the chassis.

 

One of the main ways to junk an input tube is to violate the Heater to Cathode voltage ratings, posted on June 14, 2020 at 19:27:16
Chip647
Audiophile

Posts: 2647
Location: The South
Joined: December 24, 2012
 The 6922 has a 60 volt max heater negative with respect to cathode. This preamp uses stacked sections meaning that the top triode cathode sits at the plate voltage of the lower section. My guess is that they did not raise the heater voltage bias up above ground so that the tube is exceeding the heater to cathode rating.

There is an easy fix for this, but you will have to cut some circuit traces and create a voltage tap with 2 resistors and a cap. Read this:

http://www.valvewizard.co.uk/heater.html

 

RE: Question about 6922 design, posted on June 15, 2020 at 04:12:03
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 I've found a couple of reasons for this.
Tubes that are run too close to their limits in plate voltage and/or current.
I've also found the a lot of old stock tubes get microphonic really fast compared to new manufacture.

Dan Santoni

 

RE: One of the main ways to junk an input tube is to violate the Heater to Cathode voltage ratings, posted on June 15, 2020 at 04:20:39
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 You are right about that as well. Definitely a probably cause.
Interesting thing though, if you look at a 6DJ8 datasheet, it shows the H-F voltage to be 200V plus or minus.
So while the two tubes are very similar, they are not exact.
Also the E88CC is a 6922.


Dan Santoni

 

there are 2 ratings: heater positive and heater negative, posted on June 16, 2020 at 19:47:29
Chip647
Audiophile

Posts: 2647
Location: The South
Joined: December 24, 2012
 Also, in a 6DJ8 "Unit 1" has only a 50 volt average rating for heater to cathode and "Unit 2" goes up to 130.

The best "tube" for the top of a stacked pair is a MOSFET

 

RE: there are 2 ratings: heater positive and heater negative, posted on June 17, 2020 at 12:57:31
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 As I shouldn't look at these things late at night. My Amperex data sheet is incorrect. Page two is actually from a 6CG7. Never noticed before.
So you are correct, and that is probably the OP's problem.
I'd like tom know what preamp it is.


Dan Santoni

 

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