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Old 1st March 2009, 07:31 PM   #31
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hey-Hey!!!,
I got the manual here:
http://www.tubes4hifi.com/downloads.htm

The cathode R is 7.8 Ohms, and with the traditional Dynaco-spec 1.56 volts across it we've got 50 mA/tube.
cheers,
Douglas
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Old 1st March 2009, 08:18 PM   #32
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Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
I got the manual here:
http://www.tubes4hifi.com/downloads.htm

The cathode R is 7.8 Ohms, and with the traditional Dynaco-spec 1.56 volts across it we've got 50 mA/tube.
cheers,
Douglas

Thx,

See page 20 voltage at cathode is 1.87vdc therefore 60mA.
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Old 3rd March 2009, 01:06 AM   #33
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"7. This is mod suggested in Sound Practices. Apparently will reduce IM distortion, altho at cost of slightly lower power output.

A.Disconnect screen lead from pin 4 of output tube (8417,6550,or KT88,etc). Connect to pin 6. (assuming pin 6 is empty.
If not install a small terminal strip and connect screen lead to ungrounded lug.

B.Install 80V, 5watt zener diode between pin 6 (or terminal lug) and pin 4. Banded end (cathode) must be connected to
lead from transformer!"

The quote above is from Uncle Ned on the conversion of the Quick Silver amps from 8417's to 6550/KT88's.

The quote is specific to placing a zener in series with the UL taps for "IM Distortion". Altho I have seen this for different reasons such as having the zener cause a garanteed voltage reduction on the grid in respect to the plate. I think its called the "triode trick" but don't quote me.

Can anyone comment to this?
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Old 19th February 2012, 03:55 PM   #34
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Hello,
Sorry to resurrect this old tread but I'm working on the same project. It seems, as suggested by SY, that a 12AX7 preamp/12AT7 phase splitter might be an option for this amp. (I have KT88's and a 12AU7/12AU7 LTP front end now.) Another option might be to use the Curcio 6922 cascode/6922 phase splitter designed for the Mark III as a front end.

Any thoughts?

Last edited by mr2racer; 19th February 2012 at 04:20 PM.
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Old 19th February 2012, 07:02 PM   #35
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Are you trying to add a gain stage to recover some of the loss gain that moving from 8417 to kt-88's results in or are you just trying to sub the 7199?
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Old 20th February 2012, 12:44 AM   #36
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I replaced the 7199 with the 12AU7/12AU7 front end to substitute. But that design was intended for 8417's. I was new to tubes then and was told to substitute KT88 all I had to do was change the cathode resistors and re bias. I have since learned that is not the case. When driven hard the front end distorts a bit. And I was hoping to clean that up. that is when I discovered the KT88's are less sensitive than 8417's.

I think the easiest thing to do is replace the 12AU7's with a 12AX7 and 12AT7. I have the 12AT7's already. It shouldn't take much to make the new tubes work. Also when I finished the front end the tubes didn't seem to draw enough current and I went down on the cathode resistors to get the current draw up. And haven't been back into the amps since.
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Old 8th September 2013, 05:24 PM   #37
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Hey Guys,
does anyone have a copy or a link for Kara Chaffee's article. I can't seem to find it.
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Old 8th September 2013, 10:11 PM   #38
roline is offline roline  United States
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Default Compactron MrkIII PCB Design

I did the following design concept for someone per their expressed request and then they did not want it. Compactron tripple triode for a gain stage and then LTP with CCS and MosFet followers for screen drivers. I've used this topology on other amps with great success. I used all the design features that I've tried including plate-grid feedback aka Schade for the output tubes.
This PCB design has a power supply bypass PCB with bias voltages that piggybacks the main PCB to simplify the chassis wiring. Designed around parts available from Newark. 6AC10 or 6AK10 would be the best driver tubes.
Attached Images
File Type: jpg MARKIII.jpg (98.3 KB, 106 views)
File Type: jpg Picture1_mark3 CHassis wiring.jpg (111.6 KB, 105 views)
File Type: png PCB-Pic1.png (103.5 KB, 105 views)
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Last edited by roline; 8th September 2013 at 10:16 PM.
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Old 13th September 2013, 05:30 AM   #39
olen is offline olen  United States
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Quote:
Originally Posted by mr2racer View Post
Hey Guys,
does anyone have a copy or a link for Kara Chaffee's article. I can't seem to find it.
Search for the March 2001 issue of AudioXpress. I was able to find a pdf online, but I don't have the link any more.
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Old 13th September 2013, 12:14 PM   #40
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Thanks Olen, what do you think of the new Bowie album?
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