Dynakit Mark III w/ Octal Board - Out of the dry dock

Thought I would share my recent project. I put together a couple Mark III's from Dynakitparts and installed the Tubes4hifi octal boards utilizing 6SN7's. I picked up some 6550 output tubes here locally that were new and matched. After a bit of teething problems they are up and running sounding splendid!  Plenty punchy through my B&W CDM 7NT speakers. Assembly was easy, way more room and less work than assembling a preamp. I'll probably pick up a couple of cages for them in the near future but for now they remain naked. If I knew what I was doing I probably would have used lighter gauge hook up wire for the output filaments. Such thick wire makes for ugly solder joints.





A friend who has a VTA-120 told me that his chokes touched the bottom of his chassis, so I put a piece of thick white tape over the bottom edge of the chokes. Not sure if I really needed them. Didn't mean to censor the Dynakitparts logo.


Question about the output  tubes: I bought them from a local tube seller. They are JJ 6550 that have stickers from Tube Depot along the base. Hand written under the BIAS POINT is written 47 and 46 for the two pairs. What do you think they actually did to come up with these numbers and should I just assume these are the correct BIAS voltages for me to use. I assume they are talking mA in which case I would just bais  them at  0.47 and 0.46 V, but it doesn't say.  Finally, it seems to me that KT88's would be pretty close to each other in this arrangement but apparently they work fine.

"Question about the output tubes: I bought them from a local tube seller. They are JJ 6550 that have stickers from Tube Depot along the base. Hand written under the BIAS POINT is written 47 and 46 for the two pairs. What do you think they actually did to come up with these numbers and should I just assume these are the correct BIAS voltages for me to use. I assume they are talking mA in which case I would just bais them at 0.47 and 0.46 V, but it doesn't say. Finally, it seems to me that KT88's would be pretty close to each other in this arrangement but apparently they work fine."

You will need to contact Tube Depot to determine how the figures were determined because it depends on the equipment and setup they used.

Bias should not be set based on what is written on the tube but what readings you get when the tubes are used in the actual amp.

By the way nice job. What is taped under where the yellow and red wires join?

OK thanks.  Breaking it in right now, no complaints here and hope none from the neighbors.  I may contact the Tube Depot people and ask what they are actually trying to communicate on their tubes. If they are going to leave an ugly sticker on their tubes, it should be clear what exactly it means. I looked on their web site but couldn't find any info.

Is it normal to see a deep dark blue glow around the lower mica spaces on the output tubes during use?  Never saw that on my EL34's.

Nothing taped under the yellow and white wire join besides heat shrink tubing. I just tacked it down when I assembled it and never then removed it. Just didn't want it swinging loose down there.

It's probably a measurement of the tube's plate current at some predetermined plate and bias voltage. They probably won't say what plate and bias voltage they use, as doing so would not be in their own best interests (it would make it easier for you to purchase matching tubes elsewhere).

Yes, skriefal, I think you are right in that it's a measurement of plate current. Just wish they would have used some units. Almost every professor I had in physics or electronics always stressed the importance of using c
correct units.  Here is a new picture of the system. After about 5 hours the PS transformers are warming up. The preamp is Roy's Ultimate PAS parts. I keep the top up high because it  tends to get warm.

About he markings on the 6550 tubes;
It's most likley a marking made during test/matching. It _could_ be the current drawn when using a maximatcher ( mA at 400V with -48V on the grid).

You should, as others already said, bias as per the amp's instructions. The labels on the tubes are handy in the event that you shelf the tubes, then you can identify "a pair" by the labels.

Those look like .22 PIO caps, yes? If so any rationale for that size vs say .15 Roy also offers?


Thanks,

K.

When I get KT88s from Jim McShane they have "98" or "97" or "99" in greasepaint...assume they are measured in some way before he ships to get them as closely matched as possible as a pair. I put the ones within a digit or two in the same channel, some even match "perfectly" not sure what the measurement is, or the unit, but I would say use the tubes with the closest match in the same amp. I would not think this is an indicator or suggested measure to use for bias, and is only for choosing the most closely matched tubes performance-wise.

On closer inspection I also see a Neil Young CD laying near your rig....I would think Cowgirl in the Sand or Cinnamon Girl would sound most pleasing through your system. Turn it UP...Rust never sleeps!