Dynaco Mark III Vacuum Tube Amplifier Static & Dynamic Balance Modifications

I should note to the viewer that the videos I make are made with components, i.e. the 6AN8 and 6550's, in this case, that are carefully selected to be reasonably matched and performing to OEM specifications. If the vacuum tubes, and other components in the amplifier, are in poor condition or unknown condition, these ideal results are not going to be realized.

[@djfrank59]

This is a very simple, yet effective modification! I have 2 Dynaco mono blocks coming in for a rebuild that have been laying unused since the early 60's. This is a great idea, and I'm going to do this at some point in the rebuild process! Keep 'em coming David...i always enjoy.

[@ElPasoTubeAmps]

Hi Frank - sounds exciting to fix up the two Dynaco's.I like them as they are easy to work on and pretty darn good stuff. 10% the cost of a Mac and almost as good. Have fun.

[@ElPasoTubeAmps]

I took the inspiration for the cathode balancing pot from the diytube Poseidon board. The variable load resistor on the triode of the 6AN8 was inspired by the McIntosh MA230 amplifier. In the MA230 they use separate pots to independently adjust the grid bias to balance the cathode currents of the 7591 output tubes but I think this works just as well and a lot simpler (and cheaper).

[@mikesamra9126]

Dave Wonderful and informative video as usual..You mentioned putting a 500k pot in place of the two grid to ground resistors which are 100k each..Wouldn't it be better to go with a 250k pot to be closer to the original values? I know some engineers like to change the rid leak to a higher as to make the driver see an easier load and get a little more driver.

[@ElPasoTubeAmps]

Mike Samra Hi MIke - I have to admit, I just lifted the "mod" from the diytube Poseidon board and it leaves the 100K resistors in like the original Dynaco board. With that 1 megohm resistor off the wiper of the pot, the grid resistors are essentially still 100K. I really do hope we can talk tonight. I have been into a bunch of projects. I did get in all the essentials for the 3-1000Z RF amp and the 810 audio amp. Too many projects and not enough time - what a burden, huh?

[@fullwaverecked]

Great video! Any advice regarding how I could make something like your "Awesome Contraption"? Thanks!

[@ElPasoTubeAmps]

It can be worth having with some amplifiers and it can induce instabilities in other amplifiers. You just have to figure out how you want to wire the connections from your tube sockets in the amplifier thru the tubes in the test jig and back to the amplifier. Keep the connecting cables as short as possible to decrease instability. The safest way it to put a 1 or 10 ohm resistor in the cathode circuit of the tube and measure the voltage across that resistor and calculate current - one tube at a time...

[@fullwaverecked]

@@ElPasoTubeAmps I really like the meter and switch. The switch is neat.

[@fullwaverecked]

@@ElPasoTubeAmps This video is very interesting. Your "Awesome Contraption" has got me to thinking: the way you have that switch wired and labeled, you are using one meter in place of many, when I finally caught it after... not my first viewing... Now I can build a compact meter panel with many vintage classic analog meters, many parameters (pun intended) one or two large nine position chicken's head switches and a large meter, that will display many aspects of the DUT: Amps, Watts, etc. Milli, Micro, Pico. This would be very handy, no? You are a genius. I want to run out in the street and shout "eureka"! But I don't want to get shanked or arrested... Or both. So you will just have to be content with a big Thank You. You build and produce these high quality tutorial videos with your own time and expense. Very generous to say the least. Though we may not realize it, you are helping many more than you can count. Thank you very much! .

[@fullwaverecked]

So, off topic, but I'm listening to black keys "go" through a Fender Mustang 1 (whilst jamming along on a Fender '81 Fender Bullet Deluxe) and I've noticed that when you turn the master volume down... It gets louder for a very short period. Why do you think that is?

[@frankgeeraerts6243]

An old trick for AC balance when you nothing to measure , is listening to the singing of the OPT at 1KHz.....

[@du6fztech]

I've been looking for a circuit that I could balance both 6550 tubes on my mark iii. Please correct me if I'm wrong., Looking at the modified schematic, it looks like you have cut off the dc fixed bias that goes to both signal grids of the two 6550 and it is a cathode bias now. Also when you installed that 39k ohms in series with 20k ohms pot that goes directly to the ground, what happen to the 12mmf and the 20uf 525volts capacitors, are they still connected? Looking forward to hear from you and please explain in full detail. Thank you.

[@ElPasoTubeAmps]

The output tubes still have fixed (adjustable) bias. This modification does not make the tubes cathode biased. The reason Dynanco used an 11.2 ohm resistor, in the original design, was because a fellow could use an old style D cell battery to calibrate their voltmeter. Nowadays the old style batteries are called "heavy duty" and not alkaline. The voltage across a new old style D cell was 1.56 volts. The idea was to use the old style D cell battery to "calibrate" the voltmeter used to make adjustments on this amplifier. Once the 1.56 volts from the battery was noted on the voltmeter, the idea was to set the bias on the amplifier to this same point on the meter, thus ensuring a correct bias voltage. In my case, I separated the cathodes and put matched resistors in each cathode circuit to ground so that we could monitor the cathode current on each tube separately. The actual value of 11.2 ohms is irrelevant to the performance of the amplifier and is that value only because of what I noted above. As described in the video, the static balance pot simply allows a small range of adjustment of the bias voltage to each tube separately so that the cathode currents can be equalized as read across each individual cathode resistor that has been newly installed. Once the tubes are balanced, as best as can be done, the original Dynaco bias adjustment can be made to set the bias to exactly 0.7 volts across the 10 ohm cathode resistors ensuring 70 mA of static (bias) current through each tube. The 12 mmF and 20 uF 525 volt capacitors have not been changed. Just simply remove the existing 47K resistor from the PC board and install in its place the resistor and pot shown in the video. This allows a dynamic adjustment of the driving signal to the grids of the 6550's and must be done with a sine wave signal (1 KHz) driving the amplifier, usually at full output. Naturally, I would "circle through" these adjustments a couple of times as one adjustment may have some slight effect on the other. I hope this helps.

[@kevinobrien8803]

Nice video! Enjoyed that. How much would one of those testers cost when they were new! Looks very expensive. $5 must have felt nice, a steal.

[@mamoynas]

why don't you just connect the dvm to the 2 TPs and aim for Zero volts?

[@ElPasoTubeAmps]

mamoynas That does work for the cathode current (bias) balance but it does not work for the dynamic (AC) balance.

[@mamoynas]

ElPaso TubeAmps Ok! keep up the good work!

[@dhpbear2]

There's a MUCH easier way to do these balance adjustments. Measure from grid-to-grid and tweak for zero volts.

[@amplifierexperts1983]

Driving 6550's with a cathodyne inverter is not a very good design in the first place. Ah well, it's cheap and keeps the tube count down.

[@ElPasoTubeAmps]

Amplifier Experts Hi Mark - economy seems to always play a big part, doesn't it. When I look at some of the 1950's era guitar amplifiers, I am amazed at the cleverness of the design probably just to save a part. It is also interesting that those designs and builds are held in such high esteem for their (sound) with the best of them having a triangular waveform (for a sine wave) with the full spectrum of descending amplitude harmonics. You know this because you and I have measured this and this (sound) is what makes people "cry" with joy and want to duplicate. I also try to remember that 60+ years ago, a tube socket, a 0.1 uF 400 V capacitor and a 100 ohm 5 watt resistor was not as easy (and as cheap...) to come by as it is today. On the other hand, I often times stand in respect for just how long some of these devices have lasted and still work. I think you have seen my comparison of the new driver boards for the Dynaco and compared to the original 6AN8 design. For reason I can't quite qualify, I think I like the original 6AN8 design better. Maybe it is just nostalgia. From a philosophical standpoint, in many cases, feelings, belief and superstition will trump data and logic - in all of us, at one time or another and in one way or another. Good to hear from you.