SCA35 photob 850

Dynaco SCA-35

A New Look at an Old Friend

Think the popular mods to your Dynaco SCA-35/ST-35 achieve it’s best performance? Think again. Here’s a new mod that really does improve performance, and has the documentation to prove it. See the article here:

A New Look At An Old Friend thumb

Dynaco SCA-35/ST-35 mod article (1.4MB)

Table of SCA-35 performance, before and after the mod

Power increases about 30% at low frequencies where it’s needed. Distortion at 1kHz is reduced 5X !

EFB Modification for Dynaco SCA-35
Dynaco SCA-35 mod connections at the "can" capacitor

EFB™ circuit at the heart of the Dynaco SCA-35 mod.

Complete instructions for an easy mod!

 

Reader Comments


Posted by Jonathan March 24, 2018 - 12:59 pm
Hello,
I'm looking for guidance gauging the health of my amp. I have an SCA35, my first tube amp, restored with all Tronola boards, including the EFB. The voltages to the output tubes all run a little high, but I believe are within an acceptable range (I get 17.35VDC to pin 3 and 392VDC to pins 7 and 9).

However, I initially installed a quad of new Sovtek EL84's, which I thought I had biased around .27VDC, but I now assume were never stable and, within maybe 10 hours of playing time, redplated a tube. I have now installed a quad of matched Sovtek EL84M's from Jim McShane. These appear quite stable and I have set the bias around .27VDC as per the EFB instructions, but I am now checking the bias at regular intervals and within 8 hours or so, they have gone from 10mV difference left to right to 20mV apart.

Is this a normal amount of drift or is it a symptom of a problem. If so, what tests can I perform? Any advice is greatly appreciated, and apologies if these are too vague of questions.

Best,
Jonathan

Posted by Bill January 10, 2018 - 03:59 pm
Thanks, once again, for your insight Dave.

Posted by Dave January 09, 2018 - 09:20 am
Hi Bill -- Looks like you're getting to the bottom of things regarding your question. I appreciate Steve stepping in during my absence.

A couple of points that your exercise points out:

1. Unfortunately, in the vacuum tube world today, it is not entirely uncommon to find as much as a 20% variation in Eg1 voltage required to achieve a set current draw This is particularly true if you are comparing American manufactured tubes to the Russian tubes produced today.

2. The good new is that, assuming a good tube, this variance from one tube to the next is not the prime consideration; it is the quiescent current flow through the tube that is most important. Granted, the amplifier will produce the best performance when the characteristics of all four tubes are (nearly) the same, and particularly so at the LF end of the spectrum. But whether they are all matched at a characteristic that requires an EG1 of -13.5 vdc or -16.5 vdc, that is of far less importance than the fact that they are in fact matched at the same characteristic. Now technically, tubes requiring a lower negative Eg1 versus those requiring a greater value will theoretically have a performance edge due to the increased open loop gain they will provide. But we're talking performance differences that would need a proper lab to demonstrate.

3. Your exercise also points out what really good tube testing can tell you. Steve's new VTA is of priceless value in this regard. It has the ability to check the characteristics of a tube at precise, real world (or unit identical) operating voltages to ensure they are all the same, and within the range that a unit was designed to operate at. I have done something similar for years in testing the actual power output that a tube can produce, which is another indicator of a tube's capability as well. At issue then, consider this:

If tubes that require -16.5 vdc Eg1 voltage are well within the characteristic norm, then are tubes that require only -13.5 vdc simply out of norm by normal manufacturing production variation? Or are they out of norm because their cathodes are well worn requiring reduced grid voltage to produce a set, target cathode voltage?

Conversely, if -13.5 vdc Eg1 is the norm, then tubes requiring -16.5 vdc would typically be considered as having a wide tolerance from the norm, or in today's lingo, be considered a "hot" tube. But again, without thorough testing at honest real world operating voltages, you just won't know if the issue between these two sets of tubes is production variation related, or due to worn emission capabilities. Emission testers that check this quality at very low operating voltages are near meaningless as the target current flow is also quite low: Therefore, a well worn tube can still perform quite well in an emission test.

Thanks for posting your observations. It gives a chance to address basic issues that no doubt others grapple with as well!

Dave

Posted by Bill January 05, 2018 - 11:54 am
My suspicion has been realized. I had the bias set at .270 with the new tubes, took them out, and put the old ones back in. Bias was around .070. I adjusted the trimmer to achieve the desired .270. Lo and behold, the cathode voltage was 13.6. Put the new tubes in, readjusted the trimmer to .270 and cathode was 16.6. I had no idea there was that much difference with EL 84ís, especially the same brand. I guess my original tubes matched those used by Dynaco when they published their spec of 13.5. I

I think I read somewhere that Dave designed the EFB to accommodate most EL84ís.

Posted by Bill January 04, 2018 - 04:55 pm
Yeah, Iím probably worrying about nothing except for a couple of years I was always getting 13.4 - 13.64. In fact, just before Christmas I was getting 13.64 and then a few days later, 16.4 or so. Same wall voltage, same everthing. I always have bias set at .270 VDC. WEIRD!

Posted by Steve L. January 04, 2018 - 03:59 pm
Okay Bill, the B+ seems roughly okay. From your readings, the LM337 reference voltage is 17.52-16.30 = 1.22V, which is fine. Bias voltage on the output tube cathodes is 16.6V. You mentioned on 1/2/18 that the original spec is 13.5V. That's true but recall that the EFB mod reduced output tube bias current from 35mA to 27mA each, so a higher than original cathode voltage is to be expected. Your readings indicate that the drop across the 5R resistor is 16.60-16.30 = 0.3V. Since the nominal value is 0.27V, the bias current is, if anything, a little higher than nominal.

I just put an unused Electro-Harmonix EL84 on the Vacuum Tube Analyzer (VTA) with Va=Vg2=368V, which represents about the same conditions as in your ST-35. I adjusted grid voltage (Vg1) for the nominal 27mA plate current (Ia). The value of Vg1 was -17.14V. That suggests the cathode bias voltage you are seeing (16.6V) is about normal. From what I can see, there doesn't appear to be anything wrong. You might want to adjust the bias to make the voltage across the 5R resistors closer to 0.27V. That's about all the light I can shed on this.


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