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Old 1st March 2009, 05:49 PM   #21
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Sy,

One other question on this amp.

Plate is 500vdc
Screen at UL tap 500vdc
Grid -26

what is the ZERO SIGNAL current expected to be. I am having issues figuring it out.

Thanks
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Old 1st March 2009, 06:18 PM   #22
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Quote:
Originally posted by danFrank
Hi,
The Mark VI uses a 2200 ohm ct primary OPT.
The A451 is 2k2 a-a with U-L taps. The Mk.VI uses a 1k9 a-a OPT. It is near-trivial difference, perhaps one gage size up in the primary? I can find A451 coil wind design, but not taken a Mk.VI opt apart yet.
cheers,
Douglas
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Old 1st March 2009, 06:22 PM   #23
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Quote:
Originally posted by desperateaudio
Sy,

One other question on this amp.

Plate is 500vdc
Screen at UL tap 500vdc
Grid -26

what is the ZERO SIGNAL current expected to be. I am having issues figuring it out.

Thanks

Hey Bandersnatch

Any answer to my question above. THX!!!
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Old 1st March 2009, 06:32 PM   #24
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hey-Hey!!!,
To answer that, put a 1R resistor into the cathode circuit, and points to measure voltage developed across it. IIRC the 8417 has a 35 Watt plate rating, and at the top of the g2 voltage tolerance I'd want more like 25W into the tube...as in 50 mA/tube. 55 mA/tube is probably not dangerous, but I counsel conservatism at these operating conditions.
cheers,
Douglas
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Old 1st March 2009, 06:43 PM   #25
SY is offline SY  United States
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I think about 50-55mA per tube should be a good balance between distortion and lifetime, assuming you choose a good 6550 or KT88.
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Old 1st March 2009, 06:46 PM   #26
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i have 10 ohms in line with each cathode so i know what they are getting now.

i was asking if anyone had an idea about what the original current was expected to be on the dynaco mk vi as it has in the orig design 500vdc plate and ul and -26vdc on grid. 8417 sheets show more then what you discuss by a good margin.

take a look at:

http://tdsl.duncanamps.com/dcigna/tu...a/syl-465h.gif

and

http://tdsl.duncanamps.com/dcigna/tu...a/syl-466h.gif

the second link shows 145ma for 2 tubes at -25vdc on grid and 445vdc plate/ul with a 3.5k load.
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Old 1st March 2009, 06:50 PM   #27
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Quote:
Originally posted by SY
I think about 50-55mA per tube should be a good balance between distortion and lifetime, assuming you choose a good 6550 or KT88.

using 8417 as i have about 45 of them NOS or tested very good pulls. It is my experience that the current for the 6550/kt88 famliy is considerablly different from the 8417.

I am just wondering if anyone know what the ma would have been for the original amp? The manual does not say it just has expected voltage.

plate/ul = 500vdc
grid = - 26
load 1.9k
measurement at 30 ohm balance pot on output tubes in original circuit expected to be 1.8vdc
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Old 1st March 2009, 06:56 PM   #28
SY is offline SY  United States
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Ah, if you're using 8417, it's easy. The voltage across the cathode resistor in every Dynaco "fixed-bias" amp is 1.56V. Check that resistor's value, calculate current, and divide by four to get the per-tube idle.

My experience with 8417s in this and several other amps has not been good. Cherry plates at moderate dissipations and screen meltdowns are signatures of that tube.
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Old 1st March 2009, 07:02 PM   #29
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Quote:
Originally posted by SY
Ah, if you're using 8417, it's easy. The voltage across the cathode resistor in every Dynaco "fixed-bias" amp is 1.56V. Check that resistor's value, calculate current, and divide by four to get the per-tube idle.

My experience with 8417s in this and several other amps has not been good. Cherry plates at moderate dissipations and screen meltdowns are signatures of that tube.

Use RCA and Sylvanias and have never had a meltdown yet. I also run them most often at 350vdc plate in triode mode and some times pentode with 300vdc on screen.

Thx for the info although on the mk vi the cathode is 1.87vdc not 1.56. The formula should work though.

Thx again
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Old 1st March 2009, 07:31 PM   #30
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ok manual has 1.87vdc at cathodes with 7.8 ohm resistor to ground. That gives a total of 240mA of current for the 4 tubes in total or 60mA each.

Thanks SY & Bandersnatch as you guys hit it!!
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