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Old 12th March 2014, 08:49 AM   #1
coffin is offline coffin  Taiwan
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Default 6F12P-6Ф12П passive RIAA

During the search of proper tube for building RIAA, I came across a site introducing a triode-pentode tube called 6F12P (6Ф12П) which has very high mu and high Gm for each section. So I purchased a few from ebay. Last week I read the article of Kevin discussing his future MICRO project for 6F12P, thinking it is a good time to try this unfamiliar Russian tube.
After some rough calculation and reference from internet, I drew a schematics as attachment.
Triode section is used for input stage, then direct-coupled to pentode section using pentode connection. Then 5:1 opt out.

It is really simple, so I use a versatile PCB hard-wiring a circuit. As photo.

I don't have much experience about small signal pentode, only hoping all is alright. After power up, the working points seem OK, but each tube had different gain, so I just kept swapping 6F12P to get two channels balanced.

And the sound is pretty good! Although a little bit hum, but this is a good stuff.

I'll manage to build another with different layout.

cheers

Coffin
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File Type: jpg 6F12P-RIAA.jpg (119.6 KB, 211 views)
File Type: jpg BOARDASSEMBLE.jpg (162.7 KB, 197 views)
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Old 12th March 2014, 02:18 PM   #2
coffin is offline coffin  Taiwan
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I found that one of the 6F12P flickered during powering up. Then I check other tubes to find that 3 out of ten will produce this flicker. I have biased the heater at about 50V, which is between the cathode voltages of two section.

weird.


www.youtube.com/embed/yy8pR42VPkM
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Last edited by coffin; 12th March 2014 at 02:22 PM. Reason: add video.
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Old 12th March 2014, 02:48 PM   #3
Bigun is offline Bigun  Canada
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sometimes the tubes need a chance to 'clean up', I had some 6C4C that needed a bit of time before they were settled.
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Old 12th March 2014, 05:17 PM   #4
kevinkr is offline kevinkr  United States
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Not sure what you mean by flickering, do you mean that you saw arcing during warm up, or did the filament "flash" when the power was applied? Can you describe it in a bit more detail.

FWIW I have a design on paper which I will start to work on shortly. In my case the pentode is used as the input in pentode connection and the triode is used as the output. More details will follow in the thread once I start to build a prototype.
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Old 12th March 2014, 07:25 PM   #5
adason is offline adason  United States
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I believe his video shows it well...
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Old 12th March 2014, 11:26 PM   #6
kevinkr is offline kevinkr  United States
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Yes, it is internal arcing, and it is an indication that the voltages are too high between one electrode and another when the filaments are cold. I would suspect the pentode grid to cathode as one possibility, another very likely scenario is suppressor grid to plate. This will destroy the tube in short order. I suspect this phono stage is powered by an unregulated solid state supply?
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Old 12th March 2014, 11:37 PM   #7
AJT is offline AJT  Philippines
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Quote:
Originally Posted by coffin View Post
I found that one of the 6F12P flickered during powering up. Then I check other tubes to find that 3 out of ten will produce this flicker. I have biased the heater at about 50V, which is between the cathode voltages of two section.

weird.


6F12P flicker - YouTube
hi coffin, it's been a long time....
i am not even sure that you need to lift the heaters at all, since you are not using direct coupling between stages...perhaps just grounding the heater centertaps is good....i could be wrong though, but i think it is worth a try...

OT, do you still sell generic polypropylene caps?
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Old 12th March 2014, 11:54 PM   #8
kevinkr is offline kevinkr  United States
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Umm, it is direct coupled, have another look..

Not a bad idea IMO, but need to watch what is going on with the power supply while the tubes are warming up.. FULL plate voltage is applied to the pentode grid and the spacing between grid and cathode is tiny.. This is one of the few cases where I would recommend a clamp diode from grid to cathode that conducts any time the grid is more positive than the cathode.

I am also very concerned about the large electrolytic on the cathode of the pentode section, it could be a lot smaller and would help.
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Old 13th March 2014, 12:28 AM   #9
AJT is offline AJT  Philippines
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sorry my bad....

in that case a neon NE2 from grid to cathode of the perntode might help,
or a diode may be tried....

agree on the cap, more like 3.3uf is more than enough, so that film caps can be used instead...
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Old 13th March 2014, 02:43 AM   #10
coffin is offline coffin  Taiwan
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Quote:
Originally Posted by kevinkr
Yes, it is internal arcing, and it is an indication that the voltages are too high between one electrode and another when the filaments are cold. I would suspect the pentode grid to cathode as one possibility, another very likely scenario is suppressor grid to plate. This will destroy the tube in short order. I suspect this phono stage is powered by an unregulated solid state supply?
Kevin, That was something I didn't notice. Maybe I can use 6X4 or EZ81 as slow turn-up.
I'm using MOSFET regulator based on Audio Research design.


Quote:
Originally Posted by AJT
hi coffin, it's been a long time....
i am not even sure that you need to lift the heaters at all, since you are not using direct coupling between stages...perhaps just grounding the heater centertaps is good....i could be wrong though, but i think it is worth a try...

OT, do you still sell generic polypropylene caps?
AJT,
It is direct-coupled. So elevating the bias voltage of heater is necessary.
And yes I still have some caps on hand…..


Quote:
Originally Posted by kevinkr
Not a bad idea IMO, but need to watch what is going on with the power supply while the tubes are warming up.. FULL plate voltage is applied to the pentode grid and the spacing between grid and cathode is tiny.. This is one of the few cases where I would recommend a clamp diode from grid to cathode that conducts any time the grid is more positive than the cathode.

I am also very concerned about the large electrolytic on the cathode of the pentode section, it could be a lot smaller and would help.
Yes! Clampping is a great idea! Thanks.
About the 330u/100V cathode by-pass cap, what I know is when using triode, the real Rk is [(RL+ra)/(u+1)]//RK], but I don't really know how pentode works, so I use the same concept…..please correct me if I'm wrong.


Quote:
Originally Posted by AJT
in that case a neon NE2 from grid to cathode of the pentode might help,
or a diode may be tried....

agree on the cap, more like 3.3uf is more than enough, so that film caps can be used instead...
I've never used NE2, is it similar to zener diode?
Or can I use a zener? or a P-JFET as diode? or just simply a 1N4148?


Thank!!

Coffin
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