Early MK II rebuild

Some photos of my recent Dyna MK II rebuild. As you can see these are early versions with the wider PC1 circuit boards. I want my amps to 1) look stock 2) match the original schematic part for part and 3) have tightly matched component tolerances. Obviously keeping carbon composition resistors  (mostly) meant measuring a number of parts to get good matches. Purchased polystyrene for the feedback caps and polypropylene for the coupling and by pass cap.

New 550V DC (600VDC surge) four section electrolytic cap was mandatory using the 5U4 and having 121 V AC line voltage. Nearly 590 VDC at turn on before the output sections starts to conduct.  

The original circuit boards were in rough shape with some lifted and missing traces. If you look close you can see where component (capacitors) leads were laid down the trace and soldered. Not the best solution but if you are careful they will work and should last. Another option is to make a plate that would cover the opening and cut out the middle to the size of a new replacement board. That may be a better solution but would not look vintage.

A new coat of paint and these look and sound great. A note on the use of the old feedback caps. Some try to reuse the domino shaped 390 pf cap. I tested mine and found one measured 283pf and the other 340pf. The new polystyrenes measure 388 and 390pf. Highly recommend all the caps to be replaced.

Re: Early Dyna MK II rebuild...

I have one of these... modded it significantly more.

A couple of additional things you might consider:

Buy some 20 ga silver coated teflon wire (I get mine from Apex Jr) and rewire from the input jack to board, the feedback loop from the output jacks to the board and from the board to the output tubes. Any additional rewiring may help but these would affect the signal the most.

Also looking at the pics, it would appear that the resistor on the right side output tube socket is burned, you may want to measure it and if it's bad, replace them on both tubes with metal film resistors as these will get hot from being attached to the tube socket.

Finally, add a CL-80 input current limiter to the AC power circuit. This will help with the initial power inrush and give both your power transformer and your output tubes an easier turn on cycle. This amp was designed for 115VAC and the volt or two you lose will not affect anything.

Regards,

-Ed

Ed – Thanks for the response. As stated in my opening comments I have some criteria that I want to keep these amps rebuilt to. Looking original and not deviating from the stock schematic are important to me. As such the 1) sliver coated Teflon wire, metal film resisters and 3) the current limiter would not comply with my criteria. All good suggestions if I am trying to modify the amp but no thank you. I have used the Teflon wire and metal film resisters on several other amps and preamp projects with good results. Then you get into discussions about what resister type technologies sound best. Pandora's box. It is bad enough that the replacement capacitors do not look like the originals. I did have to compromise somewhere.

I tested all resisters and even though the one resister looks burnt it is within 2% of the correct rating. I will publish the resister test results when one tough to find one is replaced. The results will be very good even by today’s metal film standards and are a huge improvement from what was there when I started. I had several at least 30% off with one over 50% high.

I get your point.

I'd still recommend the CL-80, even if you want this restored to original. Two reasons:

First, the original AC voltage rating was either 115 VAC or 117 VAC and you indicate you are currently getting 121 VAC. The CL-80 will drop that to something closer to the original design point. Plus it will bring the 6.3 VAC for the heaters closer to their original design point.

Second, the inrush current to light the heaters of the two 6CA7 or EL34 tubes plus the heater of the 5U4 is significant. Plus you have the surge of initially filling the DC power supply cap. The 5U4 comes on much faster than a 5AR4, increasing that surge. The CL-80 will slow that all of down, putting less stress on the power transformer AND the tube heaters.

Just a suggestion to increase the life of your tubes and power transformer.

(End of rant)

Another update to this earlier post for my Mark IIs. On the “for sale” sticky thread in the Forum Basket you will find the boards I helped develop and used. The following photos show the new boards.

A few more photos

Capacitor and driver update. Added an adapter and now using a 6U8A driver tube. Removed the coupling caps and the screen grid bypass cap. New Obbligato Gold Premium and a pair of Sprague Vitamin Q coupling caps. Need a little time to break in but should be a nice combo.