Improved SCA-35/ST-35 Performance

[dcgillespie]

mm -- Good questions. The 4.7 mfd is a 35 volt unit, the 15K resistor is a 1/2 watt unit, while the 5K bias control can be any low wattage unit of your choosing. It should have a linear taper to it however. The current flow through these resistors is very small (about 1 ma), so the dissipation is also very small.

The only resistor within the EFB regulator itself that is discussed at any length in the article is the 360K resistor. It can be made up of any series combination of resistors, as long as the total dissipation capability of the string is 2 watts. This is done to minimize any potential for drift in the bias setting as the amplifier components heat to a stabilized operating temperature. In my unit, I used two 180K 1W resistors in series to achieve 360K at 2 watts.

At the output tubes, all of the new grid resistors are 1/2 watt units, while the two new common cathode resistors are each made up of two 10 ohm 1/2 watt units in parallel to achieve 5 ohm 1W resistors in each location.

Thanks for the interest!

Dave

[Marshman]

Thanks Dave!!

My sons stereo 35 has eaten up its first set of power tubes (aprox 3 years of hard use).

Although it is an outstanding amp, I have heard the "compression" at higher levels and the tube life did seem short compared to my ikes.

I think I will suss out your EFB solution and give it a try!!

Thanks again,

mm

[Marshman]

I have implemented the EFB circuit in my ST-35.

It seems to be working in that I am getting a bias to the tubes, but it is not adjustable. It is running at what appears to be the extreme cold end and I cannot seem to be able to adjust it upwards. The unit passes sound, but it is weak and slightly distorted.

I have re-traced the schematic many times, swapped out regulators, double checked the pot, resistor and cap values/orientation. I have checked continuity to the tube pins from the board and from the board to ground.

So I must conclude that it is something small and stupid that I am overlooking.

Any suggestions as to what I might look at next??

Thanks,

mm

[dcgillespie]

Hi mm --

The main question I have is: Is the bias is adjustable at all? If not, then there is a basic wiring/component problem. If it is (as I suspect) but you are just not able to bring the bias into range, then it is likely just the bias needs your particular tubes have, versus the general range of bias the circuit offers.

The reason I believe that is the issue is this: If the regulator was open, the amplifier would not work at all. If it was shorted, there would be no bias at all. By the way, I subjected my amplifier to all manner of endless (but safe) abuse to try and destroy the regulator, but could not. So this experience, coupled with the fact that these are not the conditions you are describing, is the reason I think a simple tweak is all that's needed.

Looking at the circuit, as the bias control is rotated towards the 360K B+ dropping resistor, the voltage drop across the regulator is increased, which increases the bias (cathode voltage) for the tubes, which in turn reduces current draw through them. As the slider is rotated in the opposite direction towards the 15K resistor, the voltage drop across the regulator is reduced, reducing the bias for the tubes, increasing current draw through them.

In my amplifier, with the bias properly set for the required current draw, the voltage at the Output terminal of the regulator was 14 vdc, while the voltage at the Adjust terminal was 15.25 vdc. Of course the In terminal is grounded. These voltages were obtained with the bias control at about 1/3 rotation from the low current draw end of rotation (the end with the 360K resistor). If your particular tubes need less bias in order to obtain the proper current draw, first confirm your unit will produce this general bias voltage at the regulator output at some point in the rotation of the bias control, and that the bias is in fact adjustable. Then, if all is OK there but more current draw is needed, then reduce the 15K resistor to a 12K or even 10K value. This will then allow less bias to be developed across the regulator, and increase current flow through the tubes.

If you are still having problems after this, it will be helpful if you can supply some basic voltage reading to help in the trouble shooting process. I have no doubt that this will be easy to correct. Let me know if I can help any further!

Dave

[Marshman]

Dave,

I will get some readings tonight and post them. IIRC, we were getting ~388v at the 360k, then something like ~20v after it going into the 5k pot.

I am measuring mVDC across one of the 5 ohm resistors? Correct? I used a 25 turn 5k trim pot, and I can go end to end with no change on the meter.

I have a feeling I am going to be kicking myself over something silly stupid here.

Thank you for helping with this.

Marshall

[Marshman]

Got it!!

It appears on these boards, the 100mf cap to ground is required.

I installed one, and wa-lah, biases right up and sounds fantastic!!

Thanks so much Dave!!

mm

[dcgillespie]

Hi Marshall -- I am certainly glad to help, but happier you found the problem!

As I mentioned in the article, the 100 mfd cap is not required to help create a low virtual ground for the cathodes of the output tubes to operate from -- the regulator does that, providing an extremely low impedance at the cathode rail, just as would be required in any traditional fixed bias amplifier. It does however work to satisfy a requirement of the regulator to maintain it's stability. Therefore, like virtually all 3TRs, some cap is required in this position. The existing 100 mfd Dynaco cap was left on the schematic to serve this purpose, and serves the purpose just fine, although it surely does not need to be this big. In keeping with Dynaco tradition however, I was just trying to keep the EFB modification as simple as possible, so I simply used the original cap for that purpose.
Without any cap in that position, the regulator would obviously lock up, and create a no adjust condition.

Hopefully, you should notice more undistorted reserve power now, particularly in the bass region, and a cleaner overall presentation. And at 27 ma per tube, it runs cooler, and the tubes will definitely last longer!

Thanks for the interest, and congrats on a great job!

Dave

[nyazzip]

one question about this modification: my tendency is to listen to my Diytube ST-35 at very low volume levels(bedroom, night time, others in the house sleeping)....at these listening levels, would the modification be worthwhile?
i have cranked the amp up on a few occasions, and was a bit surprised at the early onset of lo-fq breakup, but again, it is not my habit to use the amp in that manner.
i will have to tear into my amp soon to replace some overheated resistors, so i have been looking at this thread a little....

[cedricb]

@Marshman: have you done the mod on a normal ST-35 or with the diytube board?

[dcgillespie]

nyazzip -- I understand that with low volume use, the obvious effects of high volume breakup are not your primary concerns. However, I would still encourage the EFB modification, if only for the improved tube life and cooler operation it promotes, as well as being able to stand ready for those occasions when hi level performance is required. Here, it makes itself known rather quickly.

Be assured that the conversion to EFB operation does not change (i.e. hurt) the low level distortion performance of these amplifiers in any way. These are Class AB1 designs. Therefore, at low power levels they operate in class A with, or without the modification. The modification reduces the standing current of each tube from 35 ma each to 27 ma, so it does change the point at which Class B operation commences. In the original unit, this happened at about 4 watts RMS output. With the modification, it happens at about 2.5 watts. Within their respective Class A power level ratings, the distortion performance of the original and modified amplifier is very low, and virtually the same. Once class B operation commences however, distortion starts rising rapidly in the original design (with both channels driven), where as it remains low with the modification throughout the available power range, whether both channels are driven, or not.

Thanks for your interest!

Dave

[Marshman]

@Dave I misunderstood about the cap, and the ST-35 used 4 separate caps, so I removed them as part of the old bias and never thought to replace them with a single cap. It finally came down to that being about the only thing I had not tried! :-)

The amp is a COMPLETELY different beast. More open, MUCH more detail, way more dynamics. It was a good amp before, now it is a great one!! I cannot believe what a HUGE difference this mod made. All that and longer tube life! NICE!

Thank you so much!!

@Cedricb I modded the ST-35 PCB. Not the prettiest thing, there is probably a better way to do it, but it works. I had remove all the cathode bias parts and had to cut 4 traces to add the 1k swamper resistors, other then that, no mods to the board itself. I did the rest of the EFB on a small breadboard and ran flying wires where needed. Was not very difficult other than my own built in headwind!!

[dcgillespie]

Ah -- So it was a modification of a modification! Very easy to see how the cap could get left out in that situation. Glad you finally traced the problem down!

The modification really does add significant performance to these amplifiers, and the improvements achieved are clearly audible, and measurable. The Z565s are such wonderful transformers, but using them with 6BQ5s operating with any kind of traditional resistive cathode bias is really a poor procedure. The EFB modification takes care of that, and power supply regulation issues as well, all in one easy step.

There is at least one transformer that does work very well with these tubes using traditional cathode bias. Since it is traditional cathode bias we're talking about, it cannot produce the absolute performance that the Z565 transformers can with the EFB circuit, but otherwise it performs very well indeed, and certainly better than the Z565 transformers do with traditional cathode bias. I am finishing up a short article on it now as a followup piece to the original article, and should have it posted on the tronola.com web site in a day or so.

Glad you're happy with the mod! Thanks for giving it a try and posting your results here.

Dave

[cedricb]

I modded the ST-35 PCB. Not the prettiest thing, there is probably a better way to do it, but it works. I had remove all the cathode bias parts and had to cut 4 traces to add the 1k swamper resistors, other then that, no mods to the board itself. I did the rest of the EFB on a small breadboard and ran flying wires where needed. Was not very difficult other than my own built in headwind!!

Can you post some pictures please?

[Ty_Bower]

There is at least one transformer that does work very well with these tubes using traditional cathode bias.

You won't even give us a hint?

[dcgillespie]

Hi Ty -- I wasn't trying to keep the transformer a secret -- just trying to not take the post too far off topic. I should have the followup piece posted in the next day or so in my section of the Tronola website.

The transformer is the Heath 51-29 used in so many of that company's 6BQ5 offerings. It doesn't get too much press regarding it's performance capabilities (to my knowledge), and only got just a (good) honorable mention from EB in the original transformer shootout thread. Therefore, I thought it would be helpful to post some real world working results for these transformers, and then analyze the results from the perspective of which type of bias works best with them. When that information is compared to that of other transformers and which type of bias they work best with, then it can help folks make the best decisions to get the best out of what they have.

Dave