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Dynaco MKii/iii triode kit project

Time for some measurements:
First off I decided to drop the line voltage down lower. Running 116 v now, vs more like 121v last night.
Then I recalled having this test socket to measure the tube voltage between pins 3 and 8. I also put the watt meter on the amp. Later I’ll check the cooler running one.
Then, after some math I decided to drop the current from 67 mA to 50mA. That lowered the power draw from the wall 19 watts.
I’ll see what difference all this makes.
I suppose I could run even less power through the tubes.
What are you guys running 6550’s at?IMG_9808.jpeg
 

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The MK ll/lll power transformer is wound to nominally operate from 115 vac. KT88 tubes were designed to conduct nearly 200 mA of current flow, and much more peak current. Against this, lowering the quiescent current by 20 mA or so will add little the effective life of these tubes, but will increase distortion significantly.

Dave
 
So unless I’m really worried about tube life, go back up towards 70 mA per tube.
Or put the amp on the distortion meter and see…
 
Does the pot RX1 here function just like a balance control on the bias/balance setup I’m more used to?
I would have expected it to be between R5 and R6.
IMG_9810.jpeg
 
Looks like AC balance for the phase inverter. Tweak for minimum THD or IMD, depending what you have for measurement tools.

That is certainly a way to draw that circuit. Took me a minute to figure out what the heck it even is. Its a long tail pair direct coupled to a twin triode operating in cascode.
 
Thanks I never knew what it really was.
I need to make a decent schematic of these things with component values.
 
Its really just a Mullard circuit, but using two triodes to make a pentode.


Now I'm suddenly wondering if one of the triode/pentode tubes has a triode section that matches some single triode. For completely annoying reasons I'm thinking it might be fun to make a Mullard front end from a tri-pent and a single triode.
 
Let me ask about the AC Balance pot RX1 again. Is it effectively the same as the dual bias setup shown here, but designed differently?
Bradbrews said:
Thanks for the suggestions so far.
The 25k pots are the recommendation for converting the board over to a dual bias configuration. Looking at it more critically now I wonder if it’s really necessary.
View attachment 3707382
The amp I’m looking at now has a 4.7k off the diode vs the 1k shown from Dynakit.
In the diagram above pad 2A & 2B head to the 100k resistors, point 2 on the original Dyna board.
I wouldn’t like it if I end up with a really coarse adjustment feeling.
Click to expand...
Would it be redundant to have both?
What’s the functional differences?

Does one change idle current and the other signal balance?
 
Last edited:
Different purposes. The AC balance changes how much audio signal gets to the output tube so both can be set for equal signal at the grid

The dual bias sets the DC voltage at each grid so the output tubes idle at the same current.
 
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