Mark III project, help and advice needed

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Mark III project, help and advice needed

Postby Rex Everything » Tue Mar 10, 2009 8:29 pm

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.

Image

Image

Any and all help will be greatly appreciated.

Thanks in advance.
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Postby JW34 » Wed Mar 11, 2009 4:47 pm

Well, I will try to help as much as I can, but don't quote me until this is confirmed by someone else.

1. According to the schematic, you need (1) 100uf @100V per Mark III. I haven't sourced these yet as I need them too.

2. From what Hilldrweller told me and from what I read, You want a fast recovery diode as long as the power supply can support it. I'm not sure if the SDS can or not. Mouser part# 625-UF5408-E3. My Power supply is pretty hefty so I shouldn't have a problem using the fast recovery diode.

You can also do a search on "diode" or "fast recovery" on this web site and a bunch of stuff pops up.

Jay
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Postby Ty_Bower » Wed Mar 11, 2009 5:25 pm

Rex Everything wrote: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.

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.


Use 100uF, 100v rated caps for the bias supply. Sprague Atoms are nice, but expensive. I used Mouser 140-TG101M2A-1322-RC in mine and they work fine. You'll need two for each amp (four total).

The diodes should be something like a UF4007. You could use a UF5408, but you really don't need the amp rating. There's almost no current drawn from the bias supply. You need one diode for each amp (two total).

You will want a bunch of multi-lug terminal strips. If you bought four of the 3-lug strips (Mouser 158-1003), you'd probably do just fine. You'll use one 3-lug strip to replace the original one in the Mk3 (where the ceramic disc cap is soldered - did you buy them already?) and another 3-lug strip where the selenium rectifier is located. This one is used as a place to solder the UF4007 diode. Look at the photos of my Mk3 (here and here) to see how they fit in. Just don't fly the end of the Dale like I did - tie it to something solid, use a 4-lug strip if you have to. It was supposed to get tied to the grounding lug on the nearby tube socket, but the replacement sockets I used didn't have grounding lugs.

You'll also need two 4.7K and two 18K ohm resistors (one of each for each amp). Some people report the 4.7K resistor doesn't work for them, and they end up needing to swap it out for something smaller (2K or so). You might save yourself some trouble and just start out with 2K. Use whatever is economical - I used 1/2 watt Dale parts 71-RN65D-F-18.2K and 71-RN65D-F-4.75K.

Finally, you should replace the 10K one-turn pot. If yours still has the original pot, I wouldn't trust the carbon track any more. I bought a cheap Alpha pot (31VF401-F) but I wish I had spent the extra money and bought a nice PEC hot-molded carbon pot. These things are expensive, but they are really nice pots. I think this is the right one, but check dimensions carefully as I have not actually used this part: RV4L103C-ND.

Oh, almost forgot. You should add a 100K "safety" resistor between the wiper and the top side of the bias pot (where it meets the 4.7K part). Geek detailed the changes here. Don't look for it in my photos - I haven't put it in mine yet, but I will real soon. I promise. :)
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Ty_Bower » Wed Mar 11, 2009 7:43 pm

Oh, by the way - I'd recommend you take the tubes out before you flip the amp over. Those things are heavy, and you're just asking for a smashed tube.
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Postby JW34 » Wed Mar 11, 2009 7:59 pm

That is why I don't like to give advice. Sorry I misread the schematic.

By the way, this helped me out too.

Thanks Ty,

Jay
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Postby Rex Everything » Thu Mar 12, 2009 6:04 am

Thanks for the info guys.

I'm not sure if I did the wrong thing or not but I went ahead and ordered the bias supply kits from Dynakit parts before I saw this. I also ordered a couple of new pots for the bias from them. I hope these are good alternatives to your suggestions.

I'm gonna see if I can get the other parts mentioned locally.
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Postby Ty_Bower » Thu Mar 12, 2009 6:52 am

Rex Everything wrote:I went ahead and ordered the bias supply kits from Dynakit parts before I saw this. I also ordered a couple of new pots for the bias from them.


Excellent choice. They'll include everything you need to rebuild the bias supply in one bag. It saves the hassle of pricing parts out separately from Mouser. They've got nice pots too, especially the "military style" which appears to be a PEC unit. Their pricing is fair all around. Good luck on the rebuild.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Rex Everything » Fri Mar 20, 2009 4:12 pm

Ok,

I was hoping that the instructions for the SDS and the Poseidon would be a bit more "step-by-step" but they are not.

Can anyone here tell me what needs to run to the SDS board, what to the Poseidon and what is not needed at all?

Very clear detailed photos are always appreciated.

Thanks
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Pictures added

Postby Rex Everything » Fri Mar 20, 2009 8:00 pm

If you all could let me know what needs to be removed and anything else that needs done I'd appreciate it.

The red and white wire is the center tap for the output transformer, correct?

Does it matter which side of the choke leads goes to which hole on the SDS board?

I need to put a 100K resistor from the right most leg of the pot to the back of the pot, correct?

10K resistors need to go on the power tube sockets, correct and where?

Anything else? I appreciate all the help so far. I'm sure it's easy to tell I'm a newbie to this :)

Image

Image

Image

Image
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Re: Pictures added

Postby Ty_Bower » Fri Mar 20, 2009 8:27 pm

Rex Everything wrote:If you all could let me know what needs to be removed and anything else that needs done I'd appreciate it.

The red and white wire is the center tap for the output transformer, correct?

Does it matter which side of the choke leads goes to which hole on the SDS board?

I need to put a 100K resistor from the right most leg of the pot to the back of the pot, correct?

10K resistors need to go on the power tube sockets, correct and where?


Yes, red/white is the center of the OPT primary. It'll get landed on the SDS board.

No, it doesn't matter which way the choke goes.

The 100k "safety" resistor needs to go from the center leg of the pot to one of the side legs... can't remember which one. It's the "top" side - the side closest to the diode.

The 1/4 watt, 10K precision metal film resistors go between the cathode and ground. Cathode should be pin 8 on the output tubes.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Ty_Bower » Fri Mar 20, 2009 8:40 pm

Do watch the orientation of the diode, and the 100uF caps.

The red/black lead off the PT needs to go to the banded end of the diode.

The first cap must have its positive end soldered to ground, and its negative end soldered to the non-banded end of the diode.

The second cap must have its postive end soldered to ground, and its negative end soldered to the center leg of the pot.

From your photos, it appears you've got it wired correctly. Even still, I would consider powering up the amp without any tube installed (neither rectifier nor power tubes) and verifying you've got proper bias voltage (-70 to -40 volts) present on pin 5 of the output tube sockets.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Rex Everything » Fri Mar 20, 2009 8:53 pm

Thanks for the input on that. I was watching closely to make sure everything was going just as it was shown on the directions with the bias supply kits.

I'd like to finish cleaning up items that are left but not needed. If you have time and don't mind please let me know what else needs removed from the amp before I move on.
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Postby Ty_Bower » Fri Mar 20, 2009 9:50 pm

Rex Everything wrote:I'd like to finish cleaning up items that are left but not needed. If you have time and don't mind please let me know what else needs removed from the amp before I move on.

There should not be many "leftover" parts that need to be removed. The stock can cap is gone and replaced by the SDS board. Your photos show the can cap is gone, and I'm confident you can wire up the SDS board correctly. The only other piece of the stock circuitry which gets eliminated is the 11.2 ohm resistor. It is replaced with separate 10 ohm resistors on pin 8 of each output tube. Cover those bases, and you should be golden. Check your bias voltage before you start up, and adjust it to maximum negative. Should be minus sixty volts at least, preferably more. The Mk3 is an excellent amplifier, and well worth the effort of rebuilding. Good luck - your efforts will be rewarded.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby Rex Everything » Fri Mar 20, 2009 10:43 pm

I only ask because when I look at the photos of yours and others on the site I see many things in the picture I just posted that are not in yours and others pictures(ie twisted wires and resistor on the pre amp power socket) and see several thing in yours and others that are not in my pictures(ie extra resistor on the left side of both output tube sockets, extra wire running from the right tube socket all the way around the right side and back of the amp to somewhere...).
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Postby Ty_Bower » Sat Mar 21, 2009 5:53 pm

Good points. The extra twisted wiring leading to the preamp socket is to provide 6.3V heater power to the preamp. The resistor going to the preamp socket serves a similar purpose - it provides high voltage DC for the preamp. I have no compatible preamp to plug into the socket, so I didn't connect these items.

I've put resistors on the power tube sockets between pin 4 and the screen taps of the output transformers. There is a post around here which describes what it does and why. You'll find some info in this post - look for the part talking about the "screen ballast resistor". http://www.diytube.com/phpBB2/viewtopic ... 3&start=14

I'm not sure exactly which wire you see running from the right socket around the side of the amp. There are wires attached to both KT88 sockets which lead around to the preamp socket. One wire goes from pins 1 & 8 of the power tube V1, and ends at the preamp socket pin 5. The other wire goes from pins 1 & 8 of the power tube V2, and ends at the preamp socket pin 8. These are used to measure the voltage at the cathode while the amp is running. You use the cathode voltage to calculate the idle current.

Be sure to take a look at the Poseidon manual, if you haven't already done so.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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