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HK Citation V

24.27.41.140

Posted on April 29, 2021 at 10:31:50
Triode_Kingdom
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This amp has two resistors in series with the filament leads of the 12BY7 input stage. It appears this drops the filament voltage by about one volt, to 5.3V or so. Any idea why HK would have done this? None of the other tubes in the amp are treated this way.












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RE: HK Citation V, posted on April 29, 2021 at 11:24:03
Story
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is this pentode sensitive to changes? Maybe an update to a higher AC line voltage remedy. What was the original spec...115vac? Then 117, 120, 122...it's been climbing ever since but mostly years ago when it was in production.

I can think of no other reason because the drop is more than 6%. Question is - why only that tube? I'd like to read the answer to this.

BTW my AC line varies from 127 to 114 throughout the day/night according to demand and the sucky local PSE&G in NJ. My Panamax tames it.




 

RE: HK Citation V, posted on April 29, 2021 at 11:30:29
sony6060
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Sometimes manufactured lowered the tube filament voltage for lower noise operation.

 

That's the original schematic from the '60s, posted on April 29, 2021 at 12:09:41
Triode_Kingdom
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There must have been a reason other than line voltage, but darned if I know what it could be.










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Interesting, posted on April 29, 2021 at 12:19:17
Triode_Kingdom
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The only parallel to this that I've seen has been the old Akai/Roberts reel-to-reel tube amp. The EF86 head preamp has its own filament winding and hum pot. The winding is labelled 5.7V, not 6.3V. I've never known why this was done. Noise reduction would make sense, although I'm not sure it would be so beneficial in a line-level stage like the 12BY7.





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RE: That's the original schematic from the '60s, posted on April 29, 2021 at 12:28:42
Tre'
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Maybe the designer just like the parameters of the "new" tube that was created by running the heater at a lower voltage?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: HK Citation V, posted on April 29, 2021 at 13:07:35
Alpha Al
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That is my guess also. Heater to Cathode leakage results in hum when the filament is heated with AC. Lower Cathode temperature would reduce it.

 

RE: HK Citation V, posted on April 29, 2021 at 14:56:35
Jim McShane
Dealer

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That's exactly the reason on both the Citation II and Citation V.

 

RE: HK Citation V, posted on April 29, 2021 at 18:07:58
Triode_Kingdom
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So by "noise," we mean hum, is that correct? I wouldn't have thought that would be an issue in a line-level stage. OTOH, it's a high gain pentode, and the amp has a spec of -85 dB. I can see how that might be a difficult number to meet.

This all relates directly to my current project. Thanks for everyone's comments!






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RE: HK Citation V, posted on April 29, 2021 at 19:51:21
elblanco
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on the scott 4310 one of the tubes had a lower filament voltage for noise reason.

someone I used to know called Daniel Von Recklinghausen (HH Scott chief engineer)about twenty years ago, (I think he passed away in 2010), and when subject was brought up DVR told him the noise reduction really comes into play when using lowered filament voltage in conjunction with a certain anode current for given tube.

or something like that.

 

RE: HK Citation V, posted on April 29, 2021 at 21:35:37
Triode_Kingdom
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Thanks, good info, I didn't realize this was a common technique. My current project is slated to use an EF86 in the front end of each channel. I was already thinking I might want to use DC on those due to some things I've heard regarding heater-cathode leakage on the newer varieties. The power transformer has an unused filament winding, so I'll just rectify that with a few Schottkys to be sure the tubes are quiet.





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RE: HK Citation V, posted on April 30, 2021 at 10:13:38
Tre'
Industry Professional

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Location: So. Cal.
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I believe it's the thermal noise (tube rush) that is reduced at lower heater voltages.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

So... Noise, not Hum?, posted on April 30, 2021 at 14:12:16
Triode_Kingdom
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Location: Texas
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Well, that's completely different. Maybe I should plan to reduce filament voltage on the EF86 like Akai did. Don't want to waste time creating a DC source if that's not the problem. Do you know of any references online that might help to nail this down?





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RE: So... Noise, not Hum?, posted on April 30, 2021 at 14:54:24
Tre'
Industry Professional

Posts: 15153
Location: So. Cal.
Joined: February 9, 2002
I find some anecdotal stuff but that's all.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Bench did work that showed filament starved tubes {not output} had lower distortion, posted on May 2, 2021 at 13:38:21
Cleantimestream
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Perhaps HK was ahead of the curve.
The Mind has No Firewall~ U.S. Army War College.

 

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