Guidance needed for a beginner - Mark II restoration ?'s

So this is my first post here.  Browsing a little, I'm amazed by the depth of knowledge and resources!  Seems like this is the right place to go with questions...  I've messed with a little Dynaco stuff previously, but for all intents and purposes I'm a complete noob when it comes to tube gear.  This dirty pair came to my house the other night.  I'm pretty sure they're both MK-II's, but they've been fooled with.  Absolutely not mirrors of each other...  I'd love some suggestions about the best procedures to refurbish, rebuild, etcetera.  What's the "right" path if there is one?  I'm handy with a soldering iron and have cleaned up/repaired a lot of other vintage stereo equipment (mostly solid state) and speakers, but I'm ready and willing to learn more!

https://i.servimg.com/u/f39/20/16/68/95/img_8210.jpg - sorry I'm not familiar with the forum's UI - can't get this photo to rotate...

I studied the schematics and a lot of pictures of MK-II's and MK-III's and it looks like the old fellow that built these many moons ago added a 6AU6 to the circuit of one of the two amps.  I don't know what the purpose of this mod was...  It appears that line level signal comes into RCA input, hits the red terminal block screwed to the side of the enclosure (not a stock setup from what I can determine), through the 6AU6 via a coax cable then out to the PCB - points 8 & 9.  Any thoughts?
https://i.servimg.com/u/f39/20/16/68/95/img_2010.jpg
https://i.servimg.com/u/f39/20/16/68/95/img_2110.jpg
https://i.servimg.com/u/f39/20/16/68/95/img_8211.jpg
https://i.servimg.com/u/f39/20/16/68/95/110.jpg

I have not powered these up - I have a variac and a dim bulb tester, but I'll be patient...  I really would like to hear from those that are familiar what the best bang for the buck options are to get these up and running.  I was thinking maybe I should strip them down completely, clean, & paint the cases and transformers, and then rebuild following the original kit instructions, replacing caps and wiring along the way...  If that's the right path, are there affordable (I'm cheap) kit options out there?  Any suggestions would be appreciated.

This is the other amp without the extra tube.  It looks like the component complement on the PCB is different and potentially there are some missing components...?
https://i.servimg.com/u/f39/20/16/68/95/img_8213.jpg
https://i.servimg.com/u/f39/20/16/68/95/img_8214.jpg
https://i.servimg.com/u/f39/20/16/68/95/210.jpg

Thank you all in advance.

it looks like both of these MK2s are the later model which is near identical to the MK3, the only real difference is that MK3s changed to 6550 tubes instead of EL34.
EZ to fix up and improve. Take the entire amps apart, clean up everything, and then if you want better performance that stock, and don't want to use obsolete tubes like the 6AN8
then our MK3 board will work perfectly.
www.tubes4hifi.com/MK3.htm

Thanks for the feedback! And for any future comments! I will get my study on and plan a full rework of these. Cool stuff. I’m sure more questions will come up as I dive in.

Go for the MKIII. The only final difference will be the output and the fact that you will no have a 4 ohm tap on the output. I use the Triad Magnetics C-24x found here, https://www.alliedelec.com/product/triad-magnetics/c-24x/70218145/. It cost a woping $8 to $9. I ordered the original boards and populated it with new components. Use polystyrene's and silver mica for the Pf's. Most all can be found at JustRadios.com. Metal film resistors. You can still find some NOS 6AN8's out there. I have quite a few as I still have 4 MKIII's to get ready and sell. If I run out of them I will make an adapter to use the 6GH8a tube. I have a few hundred of these that are NOS Japan manufacture. I use these in all the ST-70 rebuilds and those who purchased them from me could not believe their ear's! Here are some photos of one that I need to finish. Regards, Jamey.

Thanks for your pictures.

What is the small circuit board on the bottom of the input board. The location is
upper-right in your picture, adjacent the input jack...

Thanks, regards to all,
Johnmil

Jbacik wrote:I ordered the original boards and populated it with new components.

Based on the photo showing the disconnected fuse holder you apparently are still working on that amp. If that's the case, I'd like to suggest changing the 1K grid stoppers back to their original configuration between output tube pins #5 and #6, with 1/2" leads, as stated in the original MK-III manual. When I rebuilt mine years ago I configured mine as yours are shown, with the 1K resistor between Pin #5 directly to the circuit board, figuring it was a shorter circuit path, no extra piece of wire necessary. Only later did I learn that the grid stopper resistor leads should be kept short to avoid oscillation, so the original Dynaco manual was the correct way to wire the amp.

And to the original poster, I think Bob's suggestion to convert your amps to MK-III status is a good one, but keep in mind that if you do use the VTA board, be sure to remove the 680 ohm resistor from the 16 ohm output tap to ground. Despite some well intentioned posts here some years ago, the 680 ohm resistor in the original MK-III circuit forms part of a voltage divider for the AC feedback network, and is NOT there "to protect the amp if it's powered up without a speaker connected". The resistor is not shown on the VTA schematic, and it should not have been suggested to leave it in place, for those of you using the VTA board.  

Additionally, whatever cap you use directly after the rectifier, be sure it has at least a 600 VDC rating, as the first cap sees a 585 V surge at start up, and the pF value caps (at least on the original Dynaco board) should be rated to 1000 V.

Good luck!

Regarding the 680 ohm resistor from the 16 ohm output tap to ground. In build pictures, Roy shows it being moved to the 8 ohm tap and going to ground. Are you saying that in that position, it serves no purpose and can or should be removed?

I always say for the best performance it’s better to strip them down and rebuild them with a newer circuit. It’s a great learning experience and just makes you more aware of what’s going on in the amp. Plus newer parts means less worry about aging parts. The VTA driver board is excellent.

If they are originally Mk2 then I would suggest keeping them that way. EL34 tubes are cheaper than the larger tubes. I didn’t notice much difference going from an ST70 to an ST120. The larger tubes are a little sexier but I don’t know if they’re really worth the higher cost.

EWALAA wrote:Regarding the 680 ohm resistor from the 16 ohm output tap to ground. In build pictures, Roy shows it being moved to the 8 ohm tap and going to ground. Are you saying that in that position, it serves no purpose and can or should be removed?

First, let me apologize for my delay in responding, as I was out of town.

I based my original comment on two things. The first being a comment by the late Bill Thomas (who knew his way around a Dynaco circuit). Back in 2008 he wrote, "The 680 Ohm resistor forms part of a Voltage Divider for the AC feedback Network". The second reason was when I finally got a chance to see the VTA MK-III schematic, the 680 ohm resistor was omitted.

Your question got me wondering, so I asked a tech who used to do original Dynaco warranty work years ago. He disagreed with Bill Thomas, feeling the resistor was indeed included for output transformer protection if run without a speaker attached.

The whole idea intrigues me, and I'm very curious why other manufactures, including the high-dollar Marantz and Fishers, for example, didn't include a similar "protection resistor". Either way, the former Dynaco tech felt the resistor does no harm, so it's OK to leave it there, even if it's not shown on the schematic.