As I mentioned above, the power supply seems to be slapped together - see the photos in the previous post and the cover vent holes below! I mean, use a drill press at least, or clamp it down a bit better! Oy!
Not complaining, the unit works!
Wire nuts in power supplies make me cringe.
-D
Hi!
could you check again the schematic of Melos power supply?
Regards Artem
IMHO, the GK is a PITA, but you gotta love them for their amazing sound! I rebuilt and slightly modded the outboard PS. It runs so hot that I added a fairly large computer fan to keep it from boiling itself to death. WHWRRRRRR!! Many of them must have cooked-off and died by now! Some cement R's literally char the stock board! And according to some professional advice, I added a much huger heat sink to one device and devised some for others that originally had none.
There are some parts such as some small diodes for which I simply cannot work up any data or specs. A pro would likely know what to use or substitute from just experience with familiar or similar circuits. I'm considering once again rebuilding the PS board — this time using point-to-point wiring.
I never did manage to find schematics for the GK-1 Melos.
Also, Melos was very careful to point out in the quaintly typed doc that special French capacitors were very carefully selected.
So, from what you say, I do not think you have an original power supply. And if you do not have some flash caps in it, you must have someone"s rebuild remedy! (I did not use flash caps in my rebuild.)
My suggestion is to rebuild the PS on a big chassis. Use HUGE heat sinks. Use a couple of those little high-output but quiet fans that are available these days. Use some mesh screen to increase airflow. If there are small kids about they should never be allowed to get a hand inside this sucker. Elevate the diodes and resistors that heat up fiercely. DO point to point wiring. Do NOT use a board. Use caps that are rated for 105 or more. Don't even consider 85 degree types.
This runs quite cool, although if it runs in a humid climate, some ventilation may still be a good thing. There are a number of electrolytic caps that should be replaced. And perhaps by now even a lot of the other types. ( I may be flamed for saying that — but that is my take on the subject.)
The switched gain controls that were used for gain and balance were a total PITA in this machine. Melos was very conscious of this, and replacements were supported by Melos Inc. My opinion ... the signal should be wired for optimum or flat. Perhaps balance could be attained by a passive attenuator box, but that introduces and whole other lists of possible "gremlins".
I am not gong to claim that this is the best preamp on the planet. But if you have one or encounter one needing a good home — then go for it. They are a very honest, open and true-to-life instrument. There is nothing of a bloated tubey sound. Some people might be convinced that they are listening to solid state. And they would be right because in its way it has a touch of the hybrid. — SS rectification for one thing. Depending on yer amp, you may never want another preamp. But you have to work hard to restore it and make it over so it will last another 30 or more years.
"Lorne - there seem to be some differences between the one you describe & what I have found. But I see that this is a GK-1 and yours is a GK1+1. The PS IS point-to point, and there are 8 tubes rather than the 6 you noted in your post. They are all 6GK5's,
You MIGHT be able to see in the pictures that the coupling caps are 4.0uf / 400v "Electrocube", the smaller silver caps are a mix of "Mial" and "Diel" brand, and the massive on-board filtering caps are standard-looking "Marcom" 750uf / 330v and strange to me PC-mount aluminum cans of 6.0 and 3.0 uf - possibly SCR brand -marked : / K200 MKP / 250v~DB / HPFNT 25-85deg. C / 4-85 (date?)
The green squat cap in the lower left hand of the first pix is a 1.0F (one Farad) 5.5v unit - which I thought was used mostly for power back-up for settings on memory chips - but there ain't no memory chips or apparent settings here to save....... So they must have been using it for... what? Armageddon filtering or one leg of the filament voltage? Surely something innovative....
... the pre amp itself seems well built, but the gloopy glue holding the caps in and the pilot light and the power supply cable bushing into the pre makes me think that at least this one was finished late at night to meet a shipping deadline."Hi, Chinacave. Some comments...
#1 - The GK1+1 is basically, a GK1 with the larger external power supply. The "+1" is the power supply. The GK1 and GK1+1 use eight tubes. Not six. The original used all 6GK5. The later GK 1+1 used four 6GK5 in the phono stage. The line stage used a 6FH5 for gain and a 6ER5 as a cathode follower. One interesting thing that I spot looking at you pictures. I see a resistor where a capacitor is on the later model. My guess is that your version uses direct coupling between the line stage tubes.
#2 - The Electrocubes are not coupling caps. They are bypass caps for the Marcons in the power supply. Most of the smaller silver looking caps do the same thing. They are bypass caps in parallel with the Marcon and Electrocube caps. These also shunt the z-diodes that are used to help regulate the plate voltages. Just curious... are yours valued at 680pf? Anyway, these are polystyrene. The PC mounted silver canned caps are actually your coupling caps. They are French made SCR caps. Basically fancy SOLEN caps in a can.
#3 - The green cap is indeed a supercap. It is used in the muting/relay circuit.
#4 - don't worry about the "gloopy" stuff holding down caps. CAT, Counterpoint and a bunch of other manufactures used this stuff and it is absolutely necessary for the SCR caps to stay in place.
Nice unit, i had a GK1+1 (so i was told but it was labeled just GK1) years ago. It had a wider power supply about as wide as the pre amp. Great sounding with the Perreaux 2150b I had at that time I wished I never sold it. I may have some left over papers from that amp on file but I think I gave them to the guy I sold it to. If I didn't you would be welcome to them. There was one for sale a couple weeks ago on one of the sites. I was tempted but resisted.
Jim
chinacave: Yeah ... it makes me think that this did not come from the Melos factory. Besides my unit being totally different with two large squarish transformers on each end, it had a cover with neatly pressed out slots and a wrinkle paint finish. When I do dig it out and take some photos, you will be surprised at how different the to PS's are.
MELOS GK 1+1 PREAMP
I have a resistor that I am try to figure out its ohm's value. It looks like it could be 300 ohms or maybe 300k ohms. Is the larger brownish red one to the far right side of the power supply near the two diodes and where the transformer hooks up at the bottom right side of the board. Its in series with a smaller brownish red one.
These have no marking and when it burned all that is left is what looks like 3(0.
I found the style they are High voltage Bar Glass Glaze Film Resistors 3watt. Some kind of ceramic resistor. Pre amp has worked fine for years. It was still putting out sound but at a much lower gain. This resistor feeds the lower power transistor it is ice cool because of the open resistor that feeds it power.
Hope you can help.
THANKS
Craig
Hi All
I recently bought Melos GK-1 with bad power supply - the main power transformer is fried but other parts are still here but unsoldered.
The preamp is in working condition
So, I need a power supply schematic in order to rebuild the power supply!
Could somebody from Melos GK1 owners (GK1+1) help me?
Details photos of PS inside and measurement of transformer's wires will be also very helpful
But, the alternate way - just to get from somebody info about voltages from PS OUTPUT and develop own PS
Regards Artem