Pio1980
AK Member
It's taken some time, they are getting their stuff together with the tube production QC. Supposed to now produce the best KT88 obtainable, the early production from a couple decades past anything but.
Dynaco mk III re-animation guidance please...
- Thread starter dogwan
- Start date
dogwan
Dogwan
Made some more progress today. I ordered some nice repro-boards from ebay to replace the original red Phenolic boards. I stuffed the boards with all new components (except for re-using the new components I had already installed. It was a little disconcerting to have the wax bubble out of the phenolic boards when I desoldered the connections and removed by replacement caps. That doesn't instill confidence. The new boards now look nice and clean.
I also installed 100Ω .5W Screen Stability Resistors ala @dcgillespie mod.
I tested the amps out and they are sounding really good and behaving very well. The only things I have left to do are order a full set of new output tubes, and build some bucking transformers. I ordered a set of 6.3vct 4amp transformers and plan to build them so I can select between 3 and 6 volt drop. Right now my wall voltage is 120.5 and while testing I was getting 6.5v heaters and 505v B+.
PS, I decided not to change out the tube sockets for now. I might order some new hook-up wire to clean things up a bit and if I do I might then upgrade the sockets. But for now I don't want to tackle that mess.
I also installed 100Ω .5W Screen Stability Resistors ala @dcgillespie mod.
I tested the amps out and they are sounding really good and behaving very well. The only things I have left to do are order a full set of new output tubes, and build some bucking transformers. I ordered a set of 6.3vct 4amp transformers and plan to build them so I can select between 3 and 6 volt drop. Right now my wall voltage is 120.5 and while testing I was getting 6.5v heaters and 505v B+.
PS, I decided not to change out the tube sockets for now. I might order some new hook-up wire to clean things up a bit and if I do I might then upgrade the sockets. But for now I don't want to tackle that mess.
dogwan
Dogwan
Finally got around to making a pair of Bucking transformers for the MK III's.
I used Hammond 166N6 6.3vct 4 amp transformers. Wired them up to duplex sockets with the center tap to the top one and the full winding to the lower socket.
With the wall voltage at 119.4 volts at test time I got 4.6v and 8.6v drops under load (115.3v and 110.8v, or 3.8% and 7.2% drops respectively).
So, using the upper socket (the 4.6v drop) I ran each amps for about 30 minutes and then tested the voltages. Heater voltage was sitting at 6.22vac, and B+ was sitting right at 482vdc. This is very good I think. I noticed the power transformer ran much cooler and the output tubes were also a bit cooler and weren't as bright.
All in all I think this is a success and the amps will be much happier. Probably easier on the output tubes, right?
Heavy duty 2 gang wall boxes....
Cut off the mounting ears....
Mount the transformer in one side....
Wire up the duplex socket (used the commercial grade ones, more expensive but worth it). Not there wasn't much clearance between the transformer and the side of the sockets so I put adhesive rubber on the transformer housing for insulation near the screw terminals and wrapped the socket with electricians tape. I definitely might consider a 3 gang box next time.
All put together....
I used Hammond 166N6 6.3vct 4 amp transformers. Wired them up to duplex sockets with the center tap to the top one and the full winding to the lower socket.
With the wall voltage at 119.4 volts at test time I got 4.6v and 8.6v drops under load (115.3v and 110.8v, or 3.8% and 7.2% drops respectively).
So, using the upper socket (the 4.6v drop) I ran each amps for about 30 minutes and then tested the voltages. Heater voltage was sitting at 6.22vac, and B+ was sitting right at 482vdc. This is very good I think. I noticed the power transformer ran much cooler and the output tubes were also a bit cooler and weren't as bright.
All in all I think this is a success and the amps will be much happier. Probably easier on the output tubes, right?
Heavy duty 2 gang wall boxes....
Cut off the mounting ears....
Mount the transformer in one side....
Wire up the duplex socket (used the commercial grade ones, more expensive but worth it). Not there wasn't much clearance between the transformer and the side of the sockets so I put adhesive rubber on the transformer housing for insulation near the screw terminals and wrapped the socket with electricians tape. I definitely might consider a 3 gang box next time.
All put together....
dogwan
Dogwan
Updating...
On Friday I found a local guy who was selling two matched pair of Gold Lion KT88's NIB. He had bought them as backup tubes, but sold his amp before using them. They were originally from a known vendor. Got them for a decent deal, not great, but it was a fun drive through Yamhill wine country (nice twisty's) to get them.
Anyway, I installed them and then biased them up and ran them on the bench for an hour. Checked all voltages and bias remained incredibly stable. Then left them to cool down for a couple hours and came back and fired them up again. Re-checked bias and it read exactly the same after 20-30 minutes of playing music. Gave them another hour of on-time before shutting it down.
Then I spent most of yesterday making a system change in my small listening room. Here they are pushing a pair of Chapman T-7's.
Just for reference here is the same system with my Acoustat Model X's (view from listening position)....
Here are the amps on my bench (ahem...kitchen desk)...
On Friday I found a local guy who was selling two matched pair of Gold Lion KT88's NIB. He had bought them as backup tubes, but sold his amp before using them. They were originally from a known vendor. Got them for a decent deal, not great, but it was a fun drive through Yamhill wine country (nice twisty's) to get them.
Anyway, I installed them and then biased them up and ran them on the bench for an hour. Checked all voltages and bias remained incredibly stable. Then left them to cool down for a couple hours and came back and fired them up again. Re-checked bias and it read exactly the same after 20-30 minutes of playing music. Gave them another hour of on-time before shutting it down.
Then I spent most of yesterday making a system change in my small listening room. Here they are pushing a pair of Chapman T-7's.
Just for reference here is the same system with my Acoustat Model X's (view from listening position)....
Here are the amps on my bench (ahem...kitchen desk)...
dogwan
Dogwan
Like Pio and gadget said, mainly a big concern is the heater voltage. But also, if you read back through this thread you will see that when my AC was @ 120.6v I was getting B+ at 505vdc (note: original can caps were rated at 525v) and heater voltage at 6.45ac. The design center according to the manual is 475v.
What I've noticed was that by dropping the AC through the Bucking Transformer is that both the tubes and the power transformer seem to run cooler. Don't get me wrong, it's still a hot running amp with the output tubes and the PT so close together, but it made a noticeable difference. Also, the output tubes look less "angry". By that I mean the color is a lot further away from red-plating. Additionally, I also noticed the PT have a lot less mechanical hum.
So, while I am sure the voltages running off the wall AC were still within safety margins I believe running the amps closer to the design center is much better in the long run for both tube longevity and the continued operation of these 50+ year old amps (mine have 1969 date codes on the Bias pots). Anecdotal evidence that I've seen littered all over the internet is that the Mk III's are hard on output tubes and rectifiers. I suspect this can all be traced back to the issue of higher wall voltage and a bucking transformer is a great solution.
Thanks for response. Are you biasing at the prescribed 1.56? Im biasing at1.46 as prescribed by Doctor Dawgs video in order to keep the Kt88's happy. Although the previous owner now deceased had boxed these up in original boxes with original Genelex KT-88's (blue rectangle label) and Mullard GZ34 rectifiers many many years ago. the Kit boxes have postal label 1960 something as it is hard to read. One of the Mullard's was cooked all other tubes read over a 100 on my tester and run dead quiet and not even a hint of red plating. I did the typical cap job with Orange Drops and F&Ts Can 550 volts and the two electrolytics in the guts. Great Sounding amps. Question> can one use a veriac to run these two amps at 117? My veriac only has one input but can I run two strips to acomodate both amps>?
dogwan
Dogwan
Nice!
I too am setting my bias a little low, and that's mainly because I want a little wiggle room if my line voltage creeps up. The bias will go up if the line voltage rises too. I think I am at about 1.48v.
However, I am now having a problem with my AC line voltage being a lot lower than normal. Of course it's Murphy's law that the day after I got everything set up for them my line voltage has dropped down and has been floating anywhere from 115.2vac up to 118.3vac. Whereas up until last Friday my line voltage has always (I mean without fail) been around 120 to 122. So now tonight I am going to try running them without the Bucking Transformers and see where the bias is and the B+ is at.
Re: running a variac...
my understanding is that it can be done, but it's not the best idea for the following reasons:
- Most Variac's were built around the 117v times, not modern 120v. So, the dials are horribly inaccurate. Which means you will be constantly checking the output voltage manually unless it actually has a voltmeter built in.
- Almost all Variac's can be run "over" line voltage so it's very easy for a friend, or a curious child, or a domestic servant to inadvertently bump the line voltage up into damaging territory very quickly.
- The way Variac's are constructed is such that the wiper is always bridging windings. They are not intended for permanent solutions. If left in one spot on a constant basis the windings can develop burnt or dead spots under the wiper, which can cause headaches later.
MRL_Audio
RIP 2022
I use a Tice unit that let's me set the actual voltage to all my tube gear. Usually have it set at 117 to minimize wear and tear. Not exactly a variac but similar.
I think I read that some where about the Variac also. So bucking transformers or as below mentioned MRL a "Tide Unit" which I suppose I will google for info. Yep Murphys Law or Mercury Retrograde take your pick. Everything is in a Circle. Im also working on a HH Scott 222c Amp that has seen a bit of butchery yet sounds very nice for 20watts. What I really like about the Dynaco 111's are how simple the design is.