Improved SCA-35/ST-35 Performance

for the DIY ST35, the Dynakit and every other PP EL84

Postby dcgillespie » Sun Jun 12, 2011 9:53 pm

WJW -- George at Audio Regenesis employed the industrial version of the regulator (LM235) on his board, as it is rated at an even higher maximum operating temperature than the LM335 device is that I specified in my article introducing the EFB modification. The LM235 is identical to the LM335 in every other aspect.

As for the regulator's operating temperature, when I was first proving the concept on my unit, I first ran the amplifier for one hour under quiescent conditions to obtain a normal operating temperature, and then drove the unit to full power output with both channels driven for a period of 20 minutes. Since my "heat sink" was the back panel, the temperature of the regulator rose very little from its normally cool operating temperature even with everything all buttoned up during that time. However, the full power output test employed a 1 kHz sine wave test signal, rather than a music signal that would simply reach full power on musical peaks. Using a sine wave is a much more severe test than a test using music would be, as music is so transient in nature, whereas a sine wave is very consistent in achieving full power conditions. When George was doing his own tests of the EFB application for his board, he was using a free air heat sink, a sine wave test signal, full power output tests with both channels driven, but for an even more rigorous 1 hour period of operation with everything all buttoned up. True to the very conservative approach he uses, he achieved an incredible 80+C safety factor for the internal operating temp of the industrial device. Also, before I published my article, and subsequently before George released his board, we both tried to put the circuit through our own form of torture test to induce failure. Since George was most concerned about temperature, he even ran a 337 version of the regulator in the circuit -- without a heat sink(!) -- at full power for one hour, all buttoned up, and even this commercial version of the regulator did not fail or shut down (and it was well over max temp spec too!). I was not as concerned about temperature with my installation, but more concerned about general and transient abuse, so I attempted several rapid AC power on-off sequences that did not induce failure either. Neither did removing the load under controlled full power conditions. So, between all the safe guards and testing we both did on our units, neither of us could induce failure, and temp safety margins of both my original installation, and the finished AR product are quite large indeed. The heat sink on AR's board will certainly become quite warm when all buttoned up under normal conditions (assuming 20C room temp, proper ventilation of the unit, and 50 C internal case temp in the general heat sink area), but due to the heat sink George employs, the internal operating temperature of the regulator itself is really quite low compared to its rating -- even under continuous full power test conditions.

To measure the power output and distortion of your unit, you will need a good THD test set (I use an HP 339A analyzer), two accurate 8 or 16 ohm load resistors capable of sinking at least 20 watts of power each, and a dual channel oscilloscope connected across the load resistors. The THD test set will provide the necessary low distortion test signal to drive the amplifier with, and an accurate AC voltmeter for measurement of the voltage developed across the load resistor, and indication of the THD produced as well. With such a configuration, maximum power output can be tested across the audio bandwidth (at the onset of clipping), as can of course Total Harmonic Distortion. Note that in the article, the THD figures provided are an average of that achieved between the two channels, and made at a power level 1 db down from rated power. This was done to keep the test procedure in accordance with that originally used by Dynaco for the specifications it originally published for the SCA35.

To measure Intermodulation Distortion, a separate IMD analyzer will be required to obtain that data. IMD is measured at the equivalent full rated power of the amplifier, or 17.5 watts developed in each channel, with both channels driven. The figure provided for IMD in the article is also an average obtained between both channels.

I hope this answers your questions!

Dave
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Postby rvsixer » Mon Sep 12, 2011 4:11 pm

When implementing the EFB 4-tube individual bias option, do the connections making up the grounded junctions of the two series resistors (GND/R13/R14, GND/R15/R16) need to be first broken, and then each bias slider individually connected to its respective grid return resistor?

I can understand the 2x2 tube/2 slider bias option having each slider attach directly to the GND/R13/R4, GND/R15/R16 junctions, but can't wrap my head around that working with the 4-tube/4-slider individual bias option (sick) .
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Postby dcgillespie » Mon Sep 12, 2011 6:23 pm

Hi Sixer -- When implementing either of the individual bias control options, you will first need to disconnect and isolate the end of each grid return resistor (R13,R14,R15,R16) that was originally connected to ground. Then, (in your case) connect the (newly) isolated end of each of these resistors to the wiper of the bias control you have selected for each particular tube. Therefore, there is no ground connection associated with any of these resistors anymore when you use either of the individual bias control options.

If you look at Fig 10 on page 18 of the original article, your version of the installation would use four 5K pots in place of the two 2K pots shown. The wipers of these four controls would then attach the the isolated ends of the four resistors discussed above.

Of course, in this instance, you would also simply use a 10 ohm resistor for each individual output tube cathode connection, and eliminate the common cathode connection between the cathodes in each channel.

Using this approach will let you adjust each tube's current draw with the new individual 5K bias controls, while the original bias control now effectively becomes a bias "range" control, that allows adjustment of all four tubes at once so that the individual bias controls can be brought into range for the particular group of (mismatched) tubes you are using. This approach allows for great flexibility, allowing tubes of very wide range to all be adjusted correctly for proper current flow, while still maintaining all the performance improvements that the EFB circuit provides.

Thanks for your interest, and I hope this helps!

Dave
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Postby rvsixer » Tue Sep 13, 2011 11:09 am

Dave,

Thanks for the clarification regarding the grid resistor connections. That kept me from letting out some smoke =:o . I had two years of electronics a couple of decades ago, so a bit rusty.

Just to confirm, if implementing the four individual bias control option, each 10ohm cathode resistor goes to EFB Output (LMM 387 out)?

I just read with interest the direct cathode connection advantages you speak of in "A New Look at an Old Friend". I assume the proposed setup would negate those advantages (unless you jumper the direct cathode connection after each bias adjustment)? If so, I am now beginning to see the complexity of a full and proper implementation. And KISS driving me towards the 2-channel bias solution as being more practical (while still realizing some tube/channel matching capability).
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Postby dcgillespie » Tue Sep 13, 2011 4:41 pm

Hi Sixer -- Your understanding for implementing the individual bias option for four tubes is correct. Each output tube cathode would have its own 10 cathode resistor connected to the output of the LM337 regulator. And yes, "reconnecting" the output tubes of a given channel together after the adjustment has been made is the best of both worlds.

As for complexity, I would offer two thoughts. First, not only because of the performance advantages it offers, this is partially the reason that I encourage the simpler four matched tube installation. Yes, you pay a little more for matched tubes, and it represents a needed purchase if you don't have a matched quad available. But a matched quad is ultimately the best approach when funds permit, as it not only allows for balanced conditions between the two tubes in each channel, but between both channels as well. It eliminates just one more possibility of mismatched performance between the channels. Also, since the output tubes last so much longer with the EFB modification, the return on investment for this approach is significantly increased to offset the added expense.

Finally, if you should go the matched quad route, let me also mention that Audio Regenesis sells a complete power supply board designed for installation in an SCA35 amplifier, that includes the exact EFB circuit as published. If you have the room, and are able to modify your existing board to allow interface with the AR board, that would provide your power supply filter caps and EFB modification all in one easy step. You would have to judge for yourself if it would work well in your particular application. I should mention that I have a relation with AR regarding their use of the EFB circuit on their power supply board, but I am hardly mentioning it here for any personal reasons, rather, it is merely offered to give you an additional approach to consider with your project.

Keep us posted on your progress!

Dave
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Postby rvsixer » Tue Sep 13, 2011 8:21 pm

Dave,

Thanks again for your help and patience, and most of all for sharing your EFB design with us. This will actually be for a new build (still undecided PCB-13 or diytube PCB), but wanting to make sure I understand EFB circuit implementation. The diytube board is of special interest, as there seems to be real estate for EFB on it already (with a little circuit reconfiguration).

I understand and appreciate the idea behind going matched tubes. This will be my first tube amp since growing up listening to my father's McIntosh, and I seem to recall him chasing around for elusive "matched sets" back then. So the thought of independent bias, and not having to worry about unmatched sets certainly appeals. Maybe my memories have it incorrect, maybe matched sets really are that nowadays, and they don't become "unmatched" over time?

I did see the SCA-35 board, certainly something to consider will check it out.

Rob
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Postby dcgillespie » Tue Sep 13, 2011 8:52 pm

Hi Rob -- Theoretically, any "matched set" of anything will lose its match over time for some reason, and tubes are no different. However, if they start out the same (or nearly so), the idea is (which does largely hold in reality) that they will slowly degenerate together at the same rate, therefore till holding some degree of matching through out their lifetimes.

It is interesting that your dad chased after such tubes for his Mac equipment, since that equipment is likely some of the most immune equipment, relative to mismatched output tubes and performance. Still, it's nice to have, but the most unnecessary with that type of equipment due to the design of the output stage used.

In later years, better output transformer design allowed for near Mac performance without all the Mac complexity and special transformers that design requires. If you are using the Dynaco Z565 transformers or well executed clones of them with your project, your amplifier will provide very superb performance in deed!

Dave
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Postby jgf » Wed Sep 14, 2011 9:37 am

rvsixer: Are you building in a dynaco chassis or your own build? To me that would be the deciding factor on board choice. The diytube board has some big advantages: less wiring hassle and more tube choices (improving the 12au7 section with 12bh7)
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Postby rvsixer » Wed Sep 14, 2011 10:29 am

jgf: Hopefully not straying too far off topic, but wanted to acknowledge your post. To my way of thinking, with amps function comes before form (but I must admit I find the original ST-35 chassis/layout MUCH more appealing than the diytube PCB layout/chassis form factors I have seen).

Still, I am leaning towards diytube PCB, since it looks like with some modification EFB can be incorporated on it directly (with less wiring as you mentioned, and the additional tube choices are a big plus).
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Postby rhing » Mon Oct 17, 2011 5:07 pm

I recently installed the EFB modification in my refurbished Dynakit Stereo 35, and I have to say this is an incredibly impressive modification. I really liked the Stereo 35 in relatively stock form after I refurbed a rusty Craigslist find, but the EFB has really taken this amp to another level of sonic performance. It actually runs cooler now too. Can't wait to roll some more tubes in this amp.
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Postby cedricb » Wed Oct 26, 2011 8:55 am

Here is my interpretation of the EFB mod with individual bias for the diytube ST-35 version.

http://cbompart.files.wordpress.com/2011/10/st35-efb2.pdf

@dcgillespie, I'm not really sure how to implement the jumpers to disconnect/reconnect the output tubes... :/

Please let me know if I've missed something!
Last edited by cedricb on Thu Oct 27, 2011 9:59 am, edited 1 time in total.
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Postby dcgillespie » Wed Oct 26, 2011 1:11 pm

Cedricb -- Your schematic is perfect! It accurately depicts the proper way to have the bias on all four output tubes become individually adjustable.

As for the necessary modifications to the diytube 35 board, I wish I could be of more help in guiding you in your installation. However, I do not have one of those boards, so any input I could give would only be general in nature.

Since this board already has individual cathode resistor networks, I assume that using those spaces (with some modification) will work for the new individual 10 ohm cathode resistors as well. It is likely that the addition of the grid stopper resistors and screen stability resistors for each output tube may require the most work. If you search back through this thread, I believe there was someone early on who added the standard EFB circuit to a diytube 35 board, so that may be of some help.

Please keep us posted on your progress, and good luck!

Dave
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Postby cedricb » Wed Oct 26, 2011 2:32 pm

Your understanding for implementing the individual bias option for four tubes is correct. Each output tube cathode would have its own 10 cathode resistor connected to the output of the LM337 regulator. And yes, "reconnecting" the output tubes of a given channel together after the adjustment has been made is the best of both worlds.

@dcgillespie, I was referencing the your above comments, so how do I add this "jumper" functionality to my schematic?
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Postby dcgillespie » Wed Oct 26, 2011 8:53 pm

Ah! I'm sorry I didn't understand your question better.

The comment refers to connecting the cathodes (pin #3) of each channel's output tubes together after the bias adjustment is made. This both minimizes distortion and maximizes power output.

This can be accomplished by either using a DPST switch to short the cathodes together in each channel after the adjustments are made, or, if a test point is provided at each cathode, an external jumper can be installed between the test jacks of the tubes in each channel. Either way, the point is that the ideal approach is to allow for the cathodes in each channel to be separate when the bias adjustments are made, and then connected together afterwards to effect maximum performance.

I hope this clarifies my comment for you.

Dave
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Postby cedricb » Thu Oct 27, 2011 2:19 am

Thanks Dave for the clarification. So if I understood correctly, I need to add a jumper in parallel with each cathode resistor (10 ohms)?
So what's the procedure to adjust the global bias and the individual bias? :'(
- short the right and left inputs
- use 8 ohms 20W resistors to load the speaker outputs
- open the 4 jumpers
- power the beast!
- use the 4 jumper pads as test points for the individual bias and set to 0.27VDC ??
- close the 4 jumpers (can I do that when the power is on?)
- and don't know about the TP for the global bias...
Sorry if I'm a bit slow but I don't think I've grasp the ST-35 design philosophy... :|
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