Questions about the dual bias pots

Here I am with still more questions.

As I prepare the new VTA driver boards for the MK3 driver stage, I am posed with a question;
How much better or more significant is the VTA's dual bias pot configuration compared to Dynaco's original single bias adjustment for both output tubes?

The reason I ask is the more changes I make to the Dynaco, the farther I get away from stock design. Now I know the whole idea is to improve the unit, but is the change significant enough an improvement to warrant it's implementation?

Sorry, a bit on the theoretical here, but if the dual bias mod only provides a marginal improvement to the sound/operation of the amp, this is information that will help me decide the degree to which I modify/alter the original product. I am the kind of guy that just needs to know.

Appreciate the assistance and information. Love this hobby.

Follow up to my previous post...

Well, I took some time to study both the original Dynaco bias circuit as well as the VTA dual bias system. Looking at this in detail and reading up on the biasing steps from both, I came to realize the importance of this calibration. Dynaco went through the trouble of sourcing a special precision 11.2 ohm resistor in order that the bias currents through that resistor matched, in voltage, the level from a fresh D cell battery. This in order that a voltmeter reference could be established by the user to set the bias precisely. However this assumes that both tubes are matched and have the same level of bias current (how likely is that?).

If this level of precision was warranted back in the '60s, then I think an independent dual bias, one for each tube, could only allow a more precise adjustment on each tube to match the two sides.

So, I reasoned myself into a corner and will go with the VTAs dual biasing system, bypassing the original Dynaco bias pot.

I converted one of my Mark II to separate bias. Perhaps I have well-matched tubes, but I noted no difference in the sound with the ones that were in there.

Note that what you describe alters DC balance only. There could be another, more complex adjustment to adjust AC drive balance between the two tubes. Besides more circuit complexity, it also would demand a more rigorous way of measuring the adjustments for proper balance than just the simple "voltage drop across a resistor" that is used to measure the idle (bias) current through the tube.

It is specifically with tubes that are sufficiently unbalanced to require the dual biasing system that the requirement to adjust AC drive balance would be the greatest. If it's needed but can't be provided, then the use of unbalanced tubes is still suboptimal, even if we have dual bias adjustment.

Note also that when you separate the cathode resistor in a Dynaco amplifier into separate ones, as you must in order to measure the idle current in the two output tubes individually, you are also defeating a feedback mechanism that David Hafler used to reduce distortion in the output stage. So you have to weigh the advantages of using dual bias to compensate for unmatched tubes, and the inability to adjust AC drive balance that those same tubes would likely require, against the increase in distortion from separating the cathode resistors.

All in all, I am thinking that the next time I have to dig into that Mark II for other than routine adjustments, the original bias scheme and single cathode resistor will be going back in. Until then, in the words of Dynaco's tech writer Bob Tucker one time when I called him, it's "let sleeping amplifiers lie."

just my own opinion (and thus possibly biased, but I really try not to be) . . .
although the Dynaco amps have very good transformers (does it really cost much more to wind a great transformer than a poor one?)
don't forget that back in the day (50s and 60s) these amps were designed and sold at a very low price, not optimized for great fidelity or
using the best design (think of the alternative HK Citation amps and Macs).
Our VTA circuit mod uses modern technology and high quality parts to turn these amps into modern and very competitive hi-end amps
and although personally I don't believe the AC balance is a CRITICAL factor (because the newer circuit design is near optimum), it is nevertheless included in the design so that the circuit and the tubes CAN be PRECISELY dialed in for extremely optimium performance and fidelity, and having individual bias not only takes care of poorly matched tubes but also allows you to really KNOW the difference in the operating perimeters of the tubes as it's so easy to compare the settings with other tubes, it's the best test instrument for knowing the tube quality of the tubes in use under real-live conditions.

gp49, is right on the money


All in all, I am thinking that the next time I have to dig into that Mark II for other than routine adjustments, the original bias scheme and single cathode resistor will be going back in. Until then, in the words of Dynaco's tech writer Bob Tucker one time when I called him, it's "let sleeping amplifiers lie."


I have to say i have went with bias pots for each output tube,in amps from jadis defy7,ST70 an MK3s
an have pulled all out an went with one pot,an put cathiods of the outputs on one 10ohm res.the cathiods talk to each other an i get better bass,an more output,Mr Pass from pass lab,sets the gain on what would be the cathiods of his mosfet outputs of his preamps. I can,t let amplifiers
sleep

But this is just one mans finding

I would like to thank BOB for the site.

GP49 wrote:
Note also that when you separate the cathode resistor in a Dynaco amplifier into separate ones, as you must in order to measure the idle current in the two output tubes individually, you are also defeating a feedback mechanism that David Hafler used to reduce distortion in the output stage.

What if you restored the AC connection between the two cathodes with a large capacitor between them? Wouldn't this restore the feedback mechanism?

Or, how about jumpering the two cathodes back together after balancing, right at the front panel octal socket?

Jumpering the two cathodes would work to AC-couple the cathodes together within the audible range while maintaining DC isolation for separate idle current measurement. That's what I did on my Mark II. You need a GOOD capacitor, and in the range of capacitance required, it would either be a nonpolar electrolytic, or quite an expensive film capacitor. I used a film capacitor, originally a power supply filter capacitor in a conrad-johnson preamp, that I happened to find in the junk box; and bypassed it with a smaller, high quality film capacitor.

Thanks for your comment on using a capacitor to bridge the two cathodes.

When I also asked about jumpering, I was referring to using a jumper WIRE (on an octal plug) to bridge the cathodes at the front socket, except while balancing the bias. Any thoughts on that?

I think that would be a good idea, requiring only a properly-wired octal plug to make the re-connection, and much cheaper than a conrad-johnson film-type power supply filter capacitor.