Info or circuit diagrams on Melos modded Dynaco MK 4

Good Day!

I purchased in the 80's a pair of these from Melos's Dunellan, NJ shop. I have no documentation to maintain or upgrade these.
They are using a 7247 and 2 EL34's with quite a few capacitor upgrades. Is anyone familiar with these or have access to this info?
Any help would be greatly appreciated, I would like to keep these up and running.

Thanks

miniaussie wrote:Good Day!

I purchased in the 80's a pair of these from Melos's Dunellan, NJ shop. I have no documentation to maintain or upgrade these.
They are using a 7247 and 2 EL34's with quite a few capacitor upgrades. Is anyone familiar with these or have access to this info?
Any help would be greatly appreciated, I would like to keep these up and running.

Thanks

You may get the manual here at this forum, then new boards ( and other stuff that lacks from
the original). I assume the small board that has, in your case a 7247, and in the original a
6AN8A. All parts may be obtained at dynakitparts.com.
When restored to original config all will work and you will have accurate documentation.

Thank you for your reply. I realize there is the original Mk4 manual and circuit info available as well as a number of nice mods to that.

However, I have been listening to the Melos modded units for over 30 years now, and been very satisfied and I was looking for info on the mod and it's circuit.

I was curious as to whether it had been converted to a triode output and I was hoping if I had a circuit diagram it might be possible to add an autobiasing circuit and maybe an anti surge warmup circuit so I could get another 30 years out of it.

I am under the impression the proprietor of that shop passed away in the last decade and was hoping another customer or employee might have more data on his mod to the MK 4.

The units still work fine except I do not get a voltage off of one of the biasing points on one of the amps.

miniaussie wrote:Thank you for your reply. I realize there is the original Mk4 manual and circuit info available as well as a number of nice mods to that.

However, I have been listening to the Melos modded units for over 30 years now, and been very satisfied and I was looking for info on the mod and it's circuit.

I was curious as to whether it had been converted to a triode output and I was hoping if I had a circuit diagram it might be possible to add an autobiasing circuit and maybe an anti surge warmup circuit so I could get another 30 years out of it.

I am under the impression the proprietor of that shop passed away in the last decade and was hoping another customer or employee might have more data on his mod to the MK 4.

The units still work fine except I do not get a voltage off of one of the biasing points on one of the amps.

You have a choice : get a real honest Dynaco with documentation and spares
or
use an undocumented unsupported amp that _might_ be fixed when troube occurs.

That is the reason for the post. The mods were made 30-40 years ago. The mod inventor died 13 years ago.
I was hoping that there might be forum member familiar with his work back then or still has whatever documentation he handed out with his product.
Maybe he handed out or published the circuit for his mod, maybe one his former employees remember, that is the reason for this post.
The product was well regarded at the time and he went on and manufactured many well regarded audio products later..
Yes, ripping everything out, and restoring to the original with improved parts or a more modern mod kit is always an option,
but it has worked well since the 80's of the last century, how many mod kits and vendors can say that right now?

as usual, alot of answers but not to the question. The 7247 is a dual triode, one half 12AX7 and one half 12AU7. Likely the circuits yours has is very similar to the DIYtube ST35 circuit.
I'll assume for now you don't read schematics, but if you did I'd just trace the circuit you have and compare to that one, and with that info you could like find the problem.
Also, from your last post, seems the only real problem is you don't get bias on one of the amps. So compare that amp to the working one, find the difference, and you'll find the problem.
Very likely a solder connection has gone bad, or a wire broken off. Should be a simple fix.

I was more concerned about future ability to service the amp(s).
Rebuilding to original will be a relativly small effort, 2 pre boards from dynakiparts, a pair of tubes
and possibly some extra components. This might improve sound, but it will for sure make life
easier for the owner.
Keeping the undocumented mess will forever be - a mess.

S for lack of bias, if the original se rectifier is still in place, a Si bias kit from dynakitparts will freshen up the circuits. This is one area where a dynaco Mk4 is improved by modern conponents.

.

I am going to chime in here, and very strongly endorse Peter H and his approach.

I will assume that you have switched tubes one-amp-to-the-next and the issues did not follow the tubes.

a) Obtain the OEM manual, and go through the OEM Circuit design until you understand it, at least as a picture.

b) Understand that "upgrading" capacitors is, after basic QC is met and Application Suitability is understood, 99-44/100% snake oil. And that which is not snake-oil (36/100%) is salamander oil.

c) Tracing between the "good" amp and the "bad" amp, as previously noted, is the first place to start as you have no information *right now*. First, start this with the amps cold and unplugged. Essentially, you are measuring component values to the extent that you can when they are in-circuit. A good VOM and some care is all you need here. Look for any differences - and if found, make a note of where/when/how-much and report here before making any changes.

d) If you are exceedingly careful, and understand that there are lethal voltages at play, and that if you cause a short you may be spreading hot molten metal about, you might start testing key voltages when the amps are plugged in and on - as points of comparison. This would be ONLY if nothing turned up on the cold-tests.

e) Do you have the means to actually test tubes? Quality, shorts, gas? If you do, start there and at least eliminate the most obvious potential problem(s). And, if you are anywhere near southeastern PA (Philadelphia), I have a decent tube tester that will also allow matching if needed.

Point of all this is that it will most likely be something small like a failed part - and I am leaning to the bias rectifier if it is retained in that mod. OEM was selenium, and that is the most-likely-to-fail part in my direct experience. Yes, selenium rectifiers will 'fail by inches' until they go, but when they go, they will clear the area. They really do STINK. It should be your first removal. And then do the resistor mod to get your bias range back.

Ideally, you will go all the way back to OEM (but change the bias rectifier!). The MK-IV is known as 'Half-a-70' and is subject to all the issues related thereto.

I would like everyone for their prompt,considerate, and thoughtful insights on my quest.

A couple of updates on info. The amps have no rectifier tubes I'm guessing they replaced with silicon.

The amps are running fine with no hiss or hum. I have not yet swapped tubes to see if bias setting issue follows.

I do have some spare EL34s. I dug up old Heathkit TC-3 and dusted off. Have not fired up yet. The paper chart with all the tube test swich settings detached from drum due glue age.

Will search on web for updated switch settings if the old geezer fires up.

Going to order new 7247's off web.

I have attached 3 detailed images, hopefuly not to large for server.

Thanks for all the help so far, and maybe someone knows someone or something about th eold Melos shop in NJ

Down the road I would like to see if there is someway to incorporate an automatic biasing circuit and and/or a slow warmup circuit, if I can sort everything out.

https://melosaudiorestoration.com/

Will take you to their chief tech for many years - and who remains active and in business.

Hope that helps. Not exactly next door, but less than 90 minutes away.

That would be cool. Is he in NJ? I see you are in PA, I live in Long Island.

miniaussie wrote:That would be cool. Is he in NJ? I see you are in PA, I live in Long Island.

Middlesex, NJ per their website. About 30 - 45 minutes southwest of NYC.

.

When would be a good time? Would it be easier to meet at his place?

miniaussie wrote:When would be a good time? Would it be easier to meet at his place?

I have a feeling that all that you need could be handled via a phone call if that gentleman does have any of the old paperwork.

But, writing for myself, and to help clear things up.

I am (still) working full-time, so I do not have a lot of down-time. We are in the midst of selling a vacation property in Columbia County, PA (on water), so once Spring is sprung, we will be spending much of our time up there. We have a wedding to go to in May, and we will be in Kenya for most of the latter part of May - eco-safari and all that.

I have offered to run a hands-on clinic on vintage Dynaco here - it never really happened for any number of reasons, but with sufficient notice, I have let people come to the house for some diagnostic teaching. No charge, of course as this is still 100% a hobby for me. Fair warning - we have two large, long-haired cats and two very active dogs who love _EVERYONE_, one large Golden, one Scottie.

Peter W.
Thank you,

If you do put something together, I would be very interested.

I have a brother down the road from you whom would always be nice to visit.

If I can sort out this amp, I would like to add an auto biasing board if possible.

Regarding that link to the Melos Restoration tech I have left a form on his website. I do not believe there is a email address, Phone# or address listed on the site.

Good Day!

I swapped the EL34's and 7247 between the 2 MK 4's.
The MK 4 that prior was able to show voltage at the two bias set test points in an acceptable range of 1.56 volts at both points still did so with the other amp's tubes.
The MK 4 which before the tube swap was indicating .2 volt and .6 volt at the bias set test point before the swap still did after the tube swap.
Both sets of tubes tested good on the Heathkit TC3.
Audibly and using an uncalibrated android sound pressure level checker output on speakers sounded the same loudness level.

I strongly suspect your bias rectifier. If the selenium device is in place, get rid of it. ASAP. The marriage you save may be your own....

Several people have recently asked about the toxicity of selenium and
its compounds since selenium stacks were common as rectifiers in many
Boatanchors. Like others here, I too have learned the hard way just
how bad a selenium rectifier can smell when it is overheated or
shorted. One of my first electronic projects has a small selenium
stack that I wired in backwards. Naturally I didn't plug it into the
wall socket at my workbench - I carried it into my bedroom to test it!
I couldn't sleep in there for three days!


Barry Ornitz