MK III rebuild questions

for Dynaco Mark II/III/IV and DIY PP monoblocks

MK III rebuild questions

Postby charleyrose » Sun Aug 27, 2006 11:07 am

Hi All,
I recently picked up a pair of Dynaco MK III amps at an estate sale.
$125 for the set seemed like a good deal.
Anyway my plans are to restore these to original specs and here is where I have a few questions.
Both unit appear to be a more recent vintage with the Fiberglass PC boards which are in very good shape.
Lower chassis components have been removed which I suspect was the start of some modifications by the previous owner.
Two (2) manuals were included with these units but both are somewhat different ie.
The wiring pictorial one one (1) shows a silicone diode which is preceeded by a 4700 ohm 1/2 watt resistor connected at terminals (3) and (4).
Whereas, the other manual pictorial, I suspect an older version shows the selinum rectifier which is preceeded by a 1000 ohm 1/2 watt resistor.
Obviously, my rebuild plans will use a silicone IN4007 diode and my question is should I follow the newer version pictorial using the 4700 ohm resistor?
My last question regards a a loose page insert included with the newer Dynaco Mark III manual which states that "in order to accomondate the newest type of KT-88 tubes replace the 22K ohm resistor with an 18K resistor as located between the potentiometer lug #1 and ground lug on socket # V3".
Is this a good idea when using KT-88 tubes and/or if I decide to use 6550 output tubes should I stay with the 18K resistor.

Thanks in advance Charley
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Re: MK III rebuild questions

Postby Shannon Parks » Sun Aug 27, 2006 1:51 pm

charleyrose wrote:The wiring pictorial one one (1) shows a silicone diode which is preceeded by a 4700 ohm 1/2 watt resistor connected at terminals (3) and (4).
Whereas, the other manual pictorial, I suspect an older version shows the selinum rectifier which is preceeded by a 1000 ohm 1/2 watt resistor.
Obviously, my rebuild plans will use a silicone IN4007 diode and my question is should I follow the newer version pictorial using the 4700 ohm resistor?


Hi Charley,

First off, nice score!

I assume the selenium had more of a voltage drop or some other engineering change happened. So I would use the 4.7K resistor.

charleyrose wrote:My last question regards a a loose page insert included with the newer Dynaco Mark III manual which states that "in order to accomondate the newest type of KT-88 tubes replace the 22K ohm resistor with an 18K resistor as located between the potentiometer lug #1 and ground lug on socket # V3".
Is this a good idea when using KT-88 tubes and/or if I decide to use 6550 output tubes should I stay with the 18K resistor?


Actually, I used 18k on my friend's Mark IIIs, tested with Sovtek KT88s. I'm not sure which he is using currently, but I think 18K will be fine. I think that my stock biasing ended up right in the middle of the pot, so that seems good to me.
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Postby EdStiles » Mon Aug 28, 2006 6:37 am

Nice Haul

Concerning the bias pot, I would change it to 25K and use the 18K resistors to ground, this would give you flexibility to "roll" other tubes into the circuit : EL34s, KT77s, KT90s etc.

Do the transformers have plastic or cloth leads? The cloth are the older "original" Dynacos, the plastic are the later imported transformers. Both are fine, I've used both.

Also on the bias circuit, you might want to add a .1 or 1.0 200v cap to ground to get rid of spikes (in parallel with the 2nd bias cap).

Good luck,

Ed
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Postby Ty_Bower » Wed Oct 03, 2007 7:57 pm

EdStiles wrote:Concerning the bias pot, I would change it to 25K and use the 18K resistors to ground, this would give you flexibility to "roll" other tubes into the circuit : EL34s, KT77s, KT90s etc.


The 10k knurled shaft Alpha pot from Mouser (31VF401-F) is suggested in the Poseidon parts list. Alpha doesn't make a 25k knurled shaft pot. I guess the .33" smooth shaft is available in a 25k (31VA403-F). I can use the Dremel and notch the end of the shaft.

By my guess, the 10k pot gives you an adjustment range of about 23 volts or so (roughly -64 volts to -41 volts). The 25k pot would give a range of nearly 40 volts (about -68 to -28). Does anyone foresee a problem trying to dial in the bias when you've got that much range on a single turn pot?
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