Mark III project, help and advice needed

[Rex Everything]

I have my SDS and Poseidon boards finished and am ready to start on the amps. My basic knowledge and skill level is as a parts replacer. I'm not so great with schematics but can solder well and follow instructions(for the most part)

I've noticed that my lack of planning has left me without replacements for the two 50uF@75v caps and the diodes to replace the selenium rectifier. Any and all suggestions for the parts will be appreciated.

Other tips, advice and suggestions for completing these amps will be greatly appreciated. I hope everyone here is patient as I'll have many questions as this goes along. My goal is to have these amps up and running by the 28th of this month if possible. This means I'll need to get an oder in for anything else I need as soon as possible so please let me know your suggestions for the diodes, caps and anything else I may need.

Here is what I'll be working with. It looks like someone has already been inside the one to replace part of the can cap.

http://i10.photobucket.com/albums/a148/ampegmang/100_0207.jpg

http://i10.photobucket.com/albums/a148/ampegmang/100_0208.jpg

Any and all help will be greatly appreciated.

Thanks in advance.

[Flyquail56]

I notice that the SDS board for the Stereo 70 includes new parts for the bias supply, and the board for the Mk III does not. Interesting!

Anyway, replace the selenium rectifier with a 1N4007 diode. The electrolytics don't need to be anything fancy, just be sure to use something between 50 uF and 100 uF. Higher values than this will cause the bias voltage to come up too slowly when powering on, shortening the life of your output tubes. This is definitely a place where more is not better. You might be able to find what you need locally, as the caps aren't particularly high-voltage.

The silicon diode will cause the bias supply voltage to be slightly higher than with the selenium rectifier. If you find that you can't adjust the bias to the desired range, increasing the value of the 4.7K resistor will solve the problem. HTH

Mike

[Rex Everything]

Thanks for the input Mike.

I've made a bit more progress as I've received most everything I'll need for these amps. I'll probably change the inputs and maybe speaker terminals down the road sometime.

The work here is a bit sloppy but it's been getting better as I go. Planning should be much better for the second amp.

http://i10.photobucket.com/albums/a148/ampegmang/100_0231.jpg

http://i10.photobucket.com/albums/a148/ampegmang/100_0234.jpg

[Rex Everything]

The Mark III's are complete and singing.

This was a fun and educational project.

[DolbySProject]

Reviving an old thread; but, how do these sound with upgrade as compared to the original? Are you still happy with them?

I've consistently passed on the ST-70 (concerned about the power rating). I only recently found out about the Mark III's. I'm thinking about building a set of Mark III's to go along with my tube phono pre and preamp.

[DaveInVA]

I no longer own one but I have owned more than 24 of them in the past and even made replacement pc boards for them back in the 80's. Beefing up the power supply gave tighter bass and more midband power. Adding separate bias for each tube made a big difference cleaning up the sound also, Then if you don't mind changing the pc board you can use a different input/diver tube configuration for another major improvement. I used 2 pairs of MKIII's a side with the 4 ohm taps paralled to drive my 3 pair per side Strathern ribbons for many years and they were rock solid and sounded great.

Dave

[DolbySProject]

I no longer own one but I have owned more than 24 of them in the past and even made replacement pc boards for them back in the 80's. Beefing up the power supply gave tighter bass and more midband power. Adding separate bias for each tube made a big difference cleaning up the sound also, Then if you don't mind changing the pc board you can use a different input/diver tube configuration for another major improvement. I used 2 pairs of MKIII's a side with the 4 ohm taps paralled to drive my 3 pair per side Strathern ribbons for many years and they were rock solid and sounded great.

Dave

What are the small boxes with pots next to the MKIII's?

[DaveInVA]

What are the small boxes with pots next to the MKIII's?

Those were my homebrew active crossovers to limit 250hz and under from getting to the ribbons. The small boxes next to the Hafler 220's are also homebrew active crossovers for the subs and midbass drivers. This was my stereo for much of the 80's.

Dave

[DolbySProject]

Then if you don't mind changing the pc board you can use a different input/diver tube configuration for another major improvement. Dave

Would you mind delving into that a bit more? What did you feel were the best combination of input driver and power tubes? (I need to go cut wood; but, I will certainly be back to read your thoughts.) Thanks!

[DaveInVA]

Back then in the 80's before the internet you either had to design the mods yourself or get ideas from hobbyist magazines. The design I used came either from Audio Express or Audio Amateur (can't remember which right now) and it used a 12AX7 and 12BH7 instead of the 6AN8. I made pc boards up and modded 24 MKIII's and used 4 of them, kept a couple for spares and sold the rest. Back then you could buy them all day for $25-$40 and you could still get Gold Lion KT88's for $12 each. Modded I think I got a whopping $250 or so a pair. I also added separate bias control for each output tube, beefed up power supply and regulated the bias supply voltage and "Wonder Caps" for coupling caps (Those were the darlings of the day).
The circuit Dynaco used was pretty unique and one of the simplest designs out there, Was not just a simple variation of the common Williamson design like many other companies amps. It did some things better and some things not as well.
These days there are at least 3 different alternate input circuits. Off hand I can think of Triode Electronics, Allen Wright, and Curcio. Triode Electronics also sells power supply mod boards.
I no longer have any of the MKIII's but I plan on building some from scratch on a larger chassis so I can fit better non-electrolytic caps (like motor run caps for power supply). I plan on using GU50 output tubes and 6SN7/6BX7 for the input/driver tubes and point to point wire everything.

Dave