Good thing I don't build these for a living...

[Ty_Bower]

Dunno if this helps any, but here's some photos of my unfinished work:



You can see the 0.02 uF disc cap on the 3-lug strip. It ties the 6.3V heater center tap (green/yellow) to ground. I wouldn't consider that cap part of the bias circuit. Did you get other ceramic disc caps in your kit?

Put the 1K ohm grid stopper resistors between pins 5 and 6. The resistor should be as close to pin 5 (grid) as reasonably possible. Pin 6 is not connected to anything inside the tube. It is just being used as a tie-in point, to hold the end of the resistor. Wire the Poseidon board to pin 6.

You might notice I also put 80 ohm resistors on pin 4 (screen). These act as "ballast" resistors to help prevent screen arc-overs, or so I've been told. I figure it's good insurance and can't hurt anything. You probably won't have these resistors in your kit.

You want to connect pins 8 and 1 together, and tie them to ground through a 10 ohm 1% precision metal film resistor. Do this regardless of whether you intend to use EL34 or KT88. The EL34 require it. The KT88 don't, but they aren't bothered by it being there.

[FATMAN]

thanks for the PICs, Nice I can enlarge the picture and see how it goes. I am just waiting for the chokes from triode electronics they are back ordered it there a hammond subsitute I can use untill they get stock?
can't wait to warm up the horns with 60watts!!

[Ty_Bower]

I think the P-T156R is real close, although the DC voltage rating seems a little on the low side.

http://www.tubesandmore.com/scripts/fox ... em=P-T156R

[FATMAN]

Thanks Ty , your amp is looking good! I will look up that choke How much voltage will there be? also I noticed that on the original MK3 they join both out put tubes together at PINs 1 and 8 are then joined to 1 and 8 on the second tube,then use a single 11.2 resitor to ground, I wired mine like in the poseidon schematic with pins 1 and 8 joined then through a 10 ohm 1/4 watt resistor to ground,and the same for the second KT88. is this a big enough resistor? 1/4 watt seems pretty light, I will add the other resistor that you added to pin 4 too, if you think it is nessesary.
Andrew

[Ty_Bower]

The original Dynaco Mk3 has a shared bias circuit for the tube pair. Shannon's design improves upon this with an additional multi-turn trim pot located on the Poseidon board. This allows you to individually set the bias for each output tube. In order that you can measure each tube's idle current, separate cathode sense resistors must be installed.

1/4 watt is plenty. Each tube should draw 70 mA at idle. Across a 10 ohm resistor, you will see a 0.7 volt drop (V=I*R). Power dissipated in the resistor will be only 0.05 watt (P=I*V). If my math is right, the maximum current through the tube should be no more than 186 mA during impossibly loud peaks. Even at that current you'll only burn 0.35 watt in the cathode resistor.

The idea behind the cathode resistors' size is that it will act as a fusible link. If you are continuously running more than 1/4 watt through that resistor, something has gone wrong. Hopefully the resistor will burn out before your expensive output transformer does.

I couldn't say if the screen ballast resistor is necessary. Obviously, Dynaco felt it wasn't required. Then again, I disagree with several of their other design decisions, such as the usage of 525 volt rated power supply capacitors. Many people suggest the screen resistor is a prudent addition, which could potentially extend the life of the output tubes. I have only read one negative remark concerning the screen resistor; it suggested the amp sounded better without it. If I can find the article that promotes the concept, I'll post it here.

Edit - Here it is: http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf
Look at the part starting on page 3 under the heading "Simple Solution".

Forgot to answer your first question. My P782 makes just under 900VCT with no load. If you've got a solid state plug in rectifier which you intend to use, the DC rail coming off the rectifier could be as much as sqrt(2)*900/2, or 635 volts.

[Ty_Bower]

Well, now I've got two of them "mostly" wired up.

[FATMAN]

Ty thanks for the information I guess the hamond choke will be underated at only 400 volts, I phoned about my order at triodelectronics and my chokes are finally on the way so I can go further on finishing mine when they come. I think they should be here buy next week end. thanks for your help hopfuly i wired it up right and it will work. Your amps look nice with original tranny's. Next project I have a harmon Kardon citation 1 to recap next for my preamp. Cheers Andrew

[Ty_Bower]

Yes, the Hammond's working voltage rating seems low. Too low, when you consider the construction appears identical to other units with voltage ratings in the kV.

I don't know why it didn't occur to me sooner - I bet the Triad C-24X would be a fine drop-in replacement for the C354.

http://www.alliedelec.com/Images/Produc ... /C-24X.PDF

[Ty_Bower]

It's about time. How long ago did I start this project?



It's not really happy with the cheapo 6L6 tubes. I had to crank the bias pots all the way counterclockwise to keep away the red plates. Idle current is about 40 mA per tube. I shudder to think what the B+ must be like with such a light load on the power supply. It's a good thing the SDS board is rated up to 630VDC. J & K are both about 50 volts higher than they should be. Seeing as how nothing has blown up, I'll probably put the good tubes in later tonight.

Special thanks to Dave, who finally inspired me to get off my duff and finish these things.

[TomMcNally]

Nice job Ty ! There is a simple resistor mod in the Dynaco
manual to get more bias voltage for tubes that need it.
The only thing I would have done differently would be to
mount the board components on the bottom, to keep
them away from the heat of the tubes.

Those amps will rock the house - they are sweet !

[FATMAN]

[img][img]http://i493.photobucket.com/albums/rr295/Bentwood_photos/DSC01026.jpg[/img]

I see if my amp pics can come though[/img]

[FATMAN]

wow Big picture here'a one of the bottom feel free to advise![img][img]http://i493.photobucket.com/albums/rr295/Bentwood_photos/DSC01027.jpg[/img][/img]

[FATMAN]

HONK your Horns![img][img]http://i493.photobucket.com/albums/rr295/Bentwood_photos/DSC00094.jpg[/img][/img]

[Ty_Bower]

They're coming along very nicely. Those horns are going to be loud. I can't remember if you mentioned it earlier, but that's definitely an Uncle Ned kit if I ever saw one.

I bet you can't wait to get those chokes. Probably should have put in the bolts for them. It's going to be tricky to get the bolts in underneath the power transformer with all the leads attached.

I can't decide if you have your SDS board turned around 180 degrees, or maybe I do. It looks like the end of your bias supply filter cap (the Sprague Atom) is flying. I'd tie that down to something. The last thing you want is for the bias supply to short out against the chassis bottom while the amp is running. By the way, where's the red/black bias tap going? It looks like it's landed on the SDS board. Did you leave the diodes on the board? I guess there's no reason not to mount the bias rectifier there, but be sure to cut the trace on the circuit board to separate the bias rectifier from the rest of the high voltage filter circuit.

Curious method of mounting the 1K grid stopper resistors. Not exactly textbook, but I suppose it should work.

Generally, the AC wiring is twisted together to minimize leaked hum. That would mostly be the 6.3V and 5.0V heater wiring and the high voltage secondary leading to the rectifier tube socket. Most people also twist the leads to the C354 filter choke as well.

[Ty_Bower]

I nearly missed it the first couple times I looked at your photo... the kit included a 600V mylar film cap to go between the heater center tap (green/yellow) and ground? Interesting. I guess that's the way it's done these days. In the 60's Dynaco used a ceramic disc capacitor there. That's what I put in mine too. But, I do have another amplifier with a more modern design. The designer recommended a film cap be used at a similar location in the circuit. I wonder if there is any technical reason why one is better than the other.