EFB tests on my diytube ST35

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

EFB tests on my diytube ST35

Postby Shannon Parks » Sun Feb 12, 2012 8:13 am

I finally did some decent bench testing on the EFB mod using my diytube ST35, a stock Rev C unit with mid 60's complement of transformers pulled from a SCA-35 years ago. I only had about half a day to dedicate to testing and modding, and focused on testing at 13.65W output, which is 1dB down from the 17.5W output (originally spec'd by Dynaco as never "exceeding 1% within 1 dB of 17.5 watts each channel"). This test was done on one and two channels over three amplifiers: a stock diytube ST35, then with all jumpered cathodes (like original), and then with the EFB mod (stock mod). The power tubes were two matched pairs of 6P14P. Voltages were adjusted to original specs with Variac. No tweaking took place. No IMD tests were done.

With all the testing, I realized I still liked the goofy individual biasing scheme of my original design. My 6BQ5/EL84 collection is a hodge podge, and the current design meets those needs. The difference - at least in these tests - just doesn't seem like enough to justify doing major layout changes. Note that the key data is with two channels being driven. So no Rev EFB at this time. I did start the layout changes, but with the individual biasing scheme I would have to invest too much time: probably several weeks of diytube time. My apologies. If I was doing this full-time, I would definitely do the mod.

I noticed a major performance hit from using some of my old US tubes with weaker transconductance (this seems obvious of course). So a new quad with stellar transconductance is the key to max output performance. The older tubes are handier for the guitarists. I think JJs would work particularly well with the EFB mod, as they have very high transconductance and bias hot which the EFB would handle. And the reduced bias requirements should keep those tubes running stronger longer. That could be a killer combo.

EFB mod tips: I used a Dremel tool with a cutting wheel for the screen & grid stoppers. I quickly was able to make eight precision cuts, and then carefully stripped the solder mask away from the traces with the same cutting wheel (scrape with Xacto if you don't trust your dexterity). I then used 1206 sized surface mount 1K and 100 ohm resistors. Don't worry about their voltage rating as the voltage drop isn't much. You can also use axials with trimmed leads. The entire cathode networks can be removed: R17-R28, R43-R46, C5-C8. I jumpered each side's cathodes at C5 & C6, and C7 & C8. Everything else can be put on perf board including the 5 ohm resistors. I used a clip-on heatsink for the LM337 which kept it cool. You can get LM337s on eBay for less than doing a full blown Digikey or Mouser order, too.

Results for One Channel (PDF)
Results for Two Channel (PDF)
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Postby dcgillespie » Sun Feb 12, 2012 4:25 pm

Shannon -- Thanks for taking the time to run the EFB tests and publish the results of your findings.

It would appear that in your distortion tests, EFB typically performed at least as well as the other biasing methods at some frequencies, and better at others. My own tests produced a much greater overall improvement with the application of EFB, which does leave me puzzled by some of your findings. However, without standardizing the test procedures, it would be hard to correlate the results.

Assuming the basic circuits are the same, and that the components are all within spec, the greatest variable could in fact be the tubes themselves. With all of the tube sets I tried (basically a dozen) during the development of EFB, I had no problem having any of them produce 13.65 watts at 1 kHz, with a THD reading of easily under .25%, with both channels driven. A couple of sets produced readings of under .10% THD in that scenario, but I used an average of all my readings for the published findings. That your results were typically 3X this level, and basically identical for all three biasing methods, makes me wonder if there were other issues at play. There also is classically a much larger drop in THD at 20 kHz, where the added current capabilities allowed by the EFB circuit allows the tubes to handle the OPT's increased shunt capacitances at this frequency. Again, of all the tube sets tested, none produced over .8% THD at 13.65 watts RMS at 20 kHz with both channels driven, while in stock form, it was impossible to have any set produce under 2.20%, with many sets not even able to produce the required power output for the test.

One thing you might check is the effect of the bias setting. While 10 sets produced a null in distortion with a quiescent current setting of between 26 and 28 ma per tube. Two sets were radically different, with one requiring 22 ma (!) to null THD, while the remaining set required about 32 ma to null. The effect of the bias setting is significant. While I am confident that a setting of 27 ma will be the optimum setting for the vast majority of tubes out there, the effect of the null is significant, such that operating (say) 3 ma either side of optimum would causes a significant rise in THD -- likely doubling it. However, since even that distortion level was so much lower than the stock performance, it still represented an improvement in my tests. In any event, you might check the effect of your bias setting on your results when you get a chance.

The other element not highlighted in your tests is the increase in maximum power output available, where I would imagine that the cathode bias formats were likely at their power limits with both channels driven in your tests. With EFB, my amplifier was able to achieve 17.5 watts RMS from each channel with both channels driven, from 25 Hz to 20 kHz, before the onset of clipping. This is performance that the traditional cathode bias formats simply cannot match.

In the end, I certainly respect your decision not to go forward with an EFB revision. You have to do what makes the most sense for your business, and particularly so with the current economy we are in.

Thanks for conducting the tests, and contributing so much to our hobby!

Dave
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Postby Shannon Parks » Mon Feb 13, 2012 10:28 am

It was my pleasure to try this mod out, Dave. Many thanks to you for your testing, documentation and support for the EFB circuit. I think anyone can probably mod their Rev C or Rev D in about an hour and I'm happy to provide any guidance there, now that I've actually done it.

The ST35 probably has excessive phase shift >10kHz due to the 47K grid stopper at V1 combined with its set-to-the-max gain (300K plate resistor). The 7199 is probably very linear in comparison. So reducing the 47K resistor could help that issue. I may test this.

Dave, I think this would sell well, but the more I worked on updating the ST35 layout, the more I wanted to do. I would proably end up totally redesigning it! So being swamped in my other project, I just can't commit with my limited tube time. I would love to get away from the shared 12AX7 and 12AU7 tubes, and switch the phase splitter to an LTP maybe, and why not just go all the way with a fixed bias amp and Edcor trannies? Alas. :)

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Postby rmyauck » Mon Feb 13, 2012 3:00 pm

What about other tubes than 12AX7 if your going to change everything. It has it's disadvantages as you know of distortion and high end roll off. I know 12AU7 can be subbed for CCG7, 12BH7A etc. I know you would have to use in production tubes, so what about the JJ ECC99? EF86? etc. The 12AT7 is another to look at.

On diyaudio the Floating Paraphase is being discussed with a few saying it has merit & others willing to try it.

Maybe ST-70 style driver boards could be built which would simplify the boards.
I think what looks great on paper doesn't always sound great so that's why I like the idea of trying the floating paraphase with adjustment pots like Scott did on their first 299's. The later ones went to a split phase like the ST-35 probably to get away from the adjustment.

The separate PS like the ST-70 , Scott's etc. used should work the same as the EFB mod right? This also might satisfy those who don't want SS transistors, IC's etc.
An Antec or similar cost Bias supply PS transformer could be had for low $ if desired.

The LTP is the Mullard I believe and the Split or Cathodyne is related to the Williamson.
Some on diyaudio thought ST-35 type of circuit had more detail and preferred it over the Mullard.

I think coming out with different driver boards in the Style of the ST-70 is the answer as different circuits could be made available and many different tubes could be rolled.
Since everyone seems to have different hearing and preferences maybe this would be an easy way in that the bias system of preference could also be used.
Also the boards could drive other easy to drive tubes such as 6973 or 7591 family.

Also CCS could be optional on the LTP boards if desired.

I guess you would have to keep the ST-35 board with the same layout as Triode Electronics uses it in their kit too. That would also sell to those who what to keep the spirit of the original ST-35 with more tube rolling options etc.

These new driver boards could even be sized to fit fit the ST-70 chassis for lots of upgrade or modification options too.

Just some thoughts!

Regards!

Randy
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Postby Shannon Parks » Wed Feb 15, 2012 6:56 am

Not that I even have anything like this on the back burner, Randy, but here is Eric Hayes' TAL1773 amp (not sure if it has been posted before):

Image
Image

He used a 6GU7 as the phase splitter. Sweet little amp.

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Postby EWBrown » Thu Feb 16, 2012 10:38 am

If a CCS for the LTPI is used, then the Poseidon or ST70 driver boards could be used, just change the resistor values, and make any circuit changes as needed.

Way back when... I had made a trade for two of Eric's PPP EL84 1773 OPTs, which are the same dimensions as a PA774, and approx 4K A-A. He had sent me the schematics, but in the intervening years, and the big move south, the schemo's got misplaced or lost.

And of course, The original e-mail with them is long gone, that computer is no longer around, and I didn't have a chance to copy its e-mails, as its HDD had crashed and burned :'( It was an old W98 POS on its last legs, anyway, and it lasted far longer than it realistically should have...

My plans at the time, were to make this a dual mono PPP EL84 system, with one PA-774 per channel, and using modified Poseidon boards one for each channel. This would leave me the option of assembling it as dual mono on a single chassis, or as two separate monoblocks. Essentially, I was considering making a design, similar to that of the VTL (Vacuum Tube Logic) Tiny Triode 25 Monoblocks. I still have everything needed, and ready to go, except for the chassis(es) and the ambition. :$ :/ :|


/ed B
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Postby corndog71 » Thu Feb 16, 2012 11:42 am

You guys are crazy! I LOVE my EFB'd ST35! I just recently installed a Genelex Gold Lion 12AX7 and 12AU7 with balanced triodes and resolution took a couple of major steps forward. The soundstage has blown way out side of the speakers.

I think I want to rebuild mine and include a Bottlehead Quickie to create a killer integrated amp. This will help cut out a pair of intereconnects and associated RCA jacks. I'll also add dual volume pots for easier balancing of the preamp tubes. It's hard to find matching pairs.

It would be nice to have it all on one PCB but then again I'm really digging p2p wiring. Plus I think it allows one to be a bit more creative.
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TAL 1773 looks interesting!

Postby danf » Mon Feb 20, 2012 3:20 pm

Hi Shannon (or Eric),

The TAL 1773 looks interesting. What tube is used for V1 with these circuit values? I was thinking of using Dave's EFB circuit and pentode connection with this front end.

Best,
Dan
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Re: TAL 1773 looks interesting!

Postby Shannon Parks » Tue Feb 21, 2012 6:20 am

danf wrote:Hi Shannon (or Eric),

The TAL 1773 looks interesting. What tube is used for V1 with these circuit values? I was thinking of using Dave's EFB circuit and pentode connection with this front end.

Best,
Dan


Looking at the values used, it must've been a 12AX7, though I don't remember. Eric Hayes RIP.

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Postby Shannon Parks » Tue Feb 21, 2012 6:21 am

corndog71 wrote:You guys are crazy! I LOVE my EFB'd ST35!


Aren't we all crazy? (lol)

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TAL 1773

Postby danf » Tue Feb 21, 2012 7:05 am

Thanks Shannon. I'm really sorry to hear about Eric.

Best,
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Postby TerrySmith » Tue Feb 21, 2012 10:29 am

Since the Diytube ST70 driver is very similar to Eric's 1773, I wonder if it could be tweaked for EL-84 use?
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Re: EFB tests on my diytube ST35

Postby wicked1 » Thu Aug 14, 2014 8:44 pm

Hi,
Sorry to bring back an old thread.. I was last here about to do the EFB mod in 2011, when Shannon had posted his ideas in the "Improved SCA-35/ST-35 Performance" thread, page 9. Basically a mini efb circuit for each cathode.
I like the plan there, as it keeps most everything on the PCB and gives adjustment for each tube. Did anyone ever build one this way?
Is it better join the cathodes as per the original EFB design, and as he did here?
I'm finally going to finish this. I'm giving it to my dad, who has a bad habit of forgetting to turn off his amps.. I figure anything to keep it cooler is good.

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