Need help with biasing MKIII

Hey,

I picked up a pair of Dynaco MKIII the other week and started checking them out. I started with the .pdf of the manual. So far so good with voltage checks. Everything looked good, tossed in the rectifier, everything looked good, and today I put in a set of tubes. No smoke, everything came up but I cannot get the bias voltage above 1.3V. These things were modified by the previous owner so I may need some assistance decoding what he/she did.

Here are some pictures of it:


















BTW I am running KT88 in them. Thanks!

With a modification, anything goes. That measurement (what some call "bias voltage") will vary depending on the value of the cathode resistor(s) in the output stage; so depending on what the modifier put in there, it could be correct. What is actually most important is IDLE CURRENT which can be derived from the measured "bias voltage" and the value of the cathode resistor.

Ohm's Law is your friend.

You might consider returning the bias circuit to factory stock--especially if you intend to run 6550/KT-88 with tube rectification. The stock bias circuit is very simple... Bob provides the schematics and layout diagrams for you. Using those you could restore the bias circuit. Most everything you need will be available from your local and convenient neighborhood Radio Shack--other than the precision wire wound 11.2 Ohm resistor if that has been changed. I think Dynalitparts.com would be the only source for that resistor.

yeah, I'm going to need to bump up the value a tad. Once I find the stock value, yes ohms law will tell me what to choose. I am at the extreme of the bias adjustment up top right now. I will have to choose something that will park me more in the middle of the bias pot.

Found out that white pot on the driver board allows you to balance the current between the two output tubes so they are able to be biased exactly. Pretty cool. I just can't get it above 1.3 volts or so. Each one has it's own bias resistor down by the preamp connector. Those are the ones I am going to have to change their values on.

You can see them in the third picture but are only 10.0 ohm 2% resistors

Now correct me if I am wrong, the point is the really get the correct current flow to ground off of pin 8 of the power tubes. The resistor value needs to be chosen around that in my case because I have a modified amp and cannot just go to the stock value of 11.2 ohms.

Right now I have lets say 1.2 V across a resistor that measured 10.2 ohms = 117.6 mA
The stock design calls for 1.56 V across 11.2 ohms = 139.3 mA

I need to choose a lower value to increase the current flow. I need approx 16% more current so I would need to choose a value approx 16% lower than my measured value of 10.2 ohms. So something around 8 (8.6 wold be exact) ohms would get me there with a tiny bit of adjustment head room.

With the 8 ohm resistor I would expect to set the value around 1.11 V.

Am I thinking about this the right way?

Those currents in the output tubes are way too high.

The recommendation is 40-50 mA per tube, that is 80-100mA per side. If you are messuring on a catode resistor on 10 Ohms, the correct messure is max. 1V.

The stock catode resistor is 15,6 Ohms which equals a current on 100mA at messured 1,56v.

thank you for the information. I think I realized where I went wrong now. I was looking for 1.56 V, what I should be looking for is 1/2 that since this amp has two bias points, 1 per tube to monitor! On top of that I know it is using a 10 ohm resistor measured to 10.2 ohms. Therefor I would want around 510mV for a 50mA setting. I knew I was missing something.

You got it right

Anyway, I would suggest you to go a little lower, 50mA is in the high end for the tubes, give it a try at 40mA.

Got it. I will park it around 40-45mA

Only one adjustment left to figure out. What is the small blue pot for up on the driver board. It is on one of the resistors that hangs off of pin 6 I believe of the 6AN8.

Thanks for the information Bob. I will check that out and confirm. If it turns out to be the case would I monitor the balance between Dynaco's #1 and #3 eyelet to ground (referring to the MKIII circuit design schematic)?

those 10 ohm resistors you are referring to sure do look like 220 ohm resistors to me (red-red-brown) or am I looking at the wrong ones?
That would make me think the former owner put those in for "self-bias" and the values are about correct for that method.
If they really are 10 ohms then you are correct, you want to measure about 0.500v to get 50ma

OK, yes, now I see them (2 - 10 ohms on the test socket) but what are those 220 ohm resistors connected to pins 6 to ?

haha, you got me man! If you a referring to the two large 220 they are connected to pin 4.

All I know is these things are DEAD QUIET. No hum, sound awesome. Whatever the guy did, he did it well. They hold bias well now and pump out some serious umph.

Bob Is All pentide, like this pr of MK3s,Can you tell us how the pin 4 on the output tubes are set up,Look likePentode Set up is before the Choke i see the 2w 22o ohm on pin 4 but what is the B+ at pin 4,All so i have never seen NO 1k grid stopers used on the output tubes pin 5? You can see in the pix that the ultra-linear taps are not used.

thanks For the pix



This is BOB trip vary cool

HOW IT ALL STARTED



It was a hot mid-summer's night and I was in Chicago at the consumer electronics show.

I was in my booth pitching my wares, a new amp and preamp, and it was hot inside. Not the amplifier, but the convention floor, and I was getting thirsty. I wanted something cold to drink, so I decided to sneak away from my booth for a moment to get a coke. As I was sneaking, a colleague happened to walk by and said, " Hey Bob, have you seen Stu Hegeman's new preamp?"

I found myself getting excited, as Stewart Hegeman, the master designer of so many classic vacuum tube amplifiers and my very own hero, was here! Stu Hegeman was a true genius, having designed the Citation I, II, probably the III and countless other amplifiers for Sidney Harmon and for Lafayette.



I found him in his booth; we began talking about preamps and amplifiers, ultimately leading to a quiet corner in a close-by restaurant. Very close, as it was part of the convention itself. I finally got my coke. I could not believe I was in the presence of THE Stewart Hegeman, and he was talking to ME! We talked and talked about big solid state amps, tube preamps, solid state preamps, loudspeakers, ionic tweeters, recording lathes and finally tube power amps. And what a scientific talk it was! As the hours went by, he admitted to having been caught up in the ultra-linear fad of his day, that it was the biggest blunder of his career, and he did it only once and would NEVER design an amp with an ultra-linear output stage ever again.

And he didn't. The Citation V was pure pentode, as was the subsequent Lafayette 550 and everything else he designed from then on. I asked why. He explained that when the plate pulls the top of the output transformer winding towards ground, the ultra-linear tap pushes the screen grid so low that it renders the tube unable to drive difficult loads. In addition, he pointed out that the normal idle potential on the screen grid regularly exceeded a safe voltage, often causing output tubes to blow up. The tube manufacturers hated it, but had to go along or lose market share, and so changed the specification for screen voltage in order to allow ultra-linear output stages. I can’t help but wonder if they REALLY changed the tube design, or simply changed the screen voltage specification. This conversation led to me confessing that I had always wanted to build a big tube amp that was painted the same color as my first car, a '49 metallic burgundy Mercury automobile. So I started on it. And here it is!



A BREND NEW POWER AMPLIFIER, THE SILVER SEVEN NINE HUNDRED



Several months ago the new Tung-Sol KT120 and new KT 100 mk II vacuum tubes became available, and I just had to try them! I found them to be utterly remarkable, with bandwidth and fidelity as good as anything I have ever experienced. In order to obtain the best from these remarkable new tubes it was necessary to design a new output stage , and what better one to start with than my own Silver Seven vacuum tube amplifier?



Success !

I was rewarded with a new and powerful amplifier that sounded as sweet and as expansive as my original Silver Seven, perhaps even better, yet easily outperformed it by delivering substantially more power and with less distortion (not that it needed it) than the original.





The Mighty 6550 vs the KT120 and the KT100 mk II

Back to Stu. Anyway, when I asked him about his favorite output tube, he said the mighty 6550 was the one to use, and when I queried about the "kinkless" tetrodes, (the KT tubes), he held up his little pinkie finger as if to hold a tea-cup and said in a mock British accent. " Brits you know, if you want watered down tea."

He loved to use wide-band video pentodes in his amp designs, and did so when he could. (Read that as cost-no-object.) They were expensive then. "Why would anyone want to use a triode front-end when they could use a pentode?" he mused. Stu was single handily responsible for one of the worlds great amplifier topologies, the wide-band video pentode design. Never been done before, and it added a new category of stunning topologies to our universe.

This design uses Stu's front end approach using a 12BY7. For this new Silver Seven I've used the more powerful, lower distortion version, the 12GN7 and I’ve updated and modified this amplifier to represent an expression of my latest thinking regarding power amplifier design. These modifications include a return to a pure pentode output stage, allowing substantially more output drive current than possible with, in (Stu Hegeman's) own words, the devil-begotten ultra-linear output stage. Strong words, but as we shall see, Stu was right. This amp has a DC filament supply, unheard of in power amps, but a necessary feature when using high-bandwidth pentodes in the front-end. This is the lowest noise power amp ever, with an A-weighted signal to noise ratio of minus 120 dB! Power output is hard to believe, and the sound is even more difficult to believe.



THE DESIGN OF THE NEW SILVER SEVEN

I started the design of this new amplifier by changing the color in order to protect the innocent. I changed the color from black to a beautiful strawberry burgundy red with chrome and silver highlights, just like my car. This new Silver Seven has every known de-lux circuit embodiment known to man, woman, or minor gods. As mentioned earlier, I used a 12GN7 vacuum tube for the front-end. I followed that high speed tube with several class-A drivers for the output tube grids. A cascode voltage amp is used for the gain stage that runs the dual regulator tubes, a pair of power output tubes. In the old days, huge amounts of energy storage were not practical. But today, in the vast intervening gulf of years, the technology associated with capacitor energy storage has allowed an increase of approximately ten fold in power supply capacity. This amplifier has had its energy storage increased substantially by the addition of HUGE capacitors that Stu could only dream about. This, combined with the additional DC restorer circuit eliminates every last vestige of “DC bounce” on musical transients. DC bounce and low-frequency stability have always been the result of compromises made by all amplifier designers in order to design an amplifier for the real world, that could be BUILT in the real world. Not any more, thanks to the DC restorer, HUGE capacitors, and my new output transformer which I wound my very own self. Not only that, the DC restorer works by keeping the DC component on each output tube grid the exact correct value through the entire audio signal swing, allowing perfect performance up to and even beyond clipping.



The DC Restorer and Super Long-Lasting Output Tubes



The DC restorer allows simultaneous low distortion and low idle power, allowing extreme longevity for the output tubes. The final distortion level in this amplifier is so low that I am embarrassed to write it down here, and the output tubes should last 50 years unless they have a catastrophic failure or won't bias up. This amp has a lifetime guarantee. It's my life and as long as I'm alive I will fix it free. I'll replace the output tubes free if they should become defective, even if you drop the amp on them. It's the least I can do. I know they will not become weak in our lifetimes.



Features

Four output terminals; common ground, one, two, four, eight and sixteen ohms. A bias meter and a bias control. It has a volume control and a built-in tube tester for the output tubes. It comes in four chassis; two power supplies and two amplifiers. Built by me and Tubular Joe. Shipping weight is about 325 lbs for all four chassis plus the tubes.

One more thing: I accept ANY form of rational payment, and the pictures are of me, Bob Carver.
Bob Carver