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    Alternative to Cathode Bias on ST-35

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    HifiGuy

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    Alternative to Cathode Bias on ST-35

    Post by HifiGuy on Sat Jun 15, 2013 7:07 pm

    I've been studying on converting my '35 to fixed bias. I like the ST35-BCU Bias Control Upgrade Kit at dynakitparts.com. Also found the EFB project on tronola.com with the associated mod board for sale. I really like the EFB idea but the mod still requires matched tubes. So... I reckon I'll go with the dynakit parts bias kit. Looks like I'll have to switch back to the "can cap" because my 35 is currently outfitted with a top of chassis cap board that has the PS caps and resistors plus the cap and resistor that makes the stock bias circuit.

    If anyone has had a go with this board, I have a couple of questions about it. I'd about bet money it does not use the fourth section of the can cap. (100uf @ 25Vdc)

    If that were the fact then maybe I could go with an alternate (with 3" height limit) on the multi-section cap to get additional capacitance.

    Should any audible benefits be expected after performing this mod? Probably worth the price of admission just for being able to use unmatched tubes.

    Thanks in advance for replies and comments.

    Chip


    tubes4hifi
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    Re: Alternative to Cathode Bias on ST-35

    Post by tubes4hifi on Sat Jun 15, 2013 7:53 pm

    if you didn't already have your amp, I'd highly recommend the DIYtube ST35 board.   I've built several.  
    I've also modded them with CCS for auto fixed bias, with a balance pot so you don't need matched tubes.
    You can't do a CCS on EACH tube because then they are fighting each other for current, each channel has to share a CCS for the push and the pull.
    For someone who already has the ST35 like you do, I'd recommend the EFB board by Gillispie (tronola.com)

    peterh

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    Re: Alternative to Cathode Bias on ST-35

    Post by peterh on Sun Jun 16, 2013 5:14 am

    What's wrong with matched tubes ??
    In my opinion it's the first step, then EFB or other improvements comes.

    Separate bias on dissimular tubes won't improve sound!

    Captain Coconut

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    Re: Alternative to Cathode Bias on ST-35

    Post by Captain Coconut on Sun Jun 16, 2013 10:03 am

    Go for the EFB for sure. According to Dave Gillespie, " the Z-565 transformer is optimally matched to 6BQ5 tubes when they are operated with fixed bias." I rigged up the EFB in my amp and am quite pleased with it. You can do individual adjustments for each channel or even each tube if you desire, but that adds to the complexity. 

    I got a matched quad of output tubes from Jim McShane over a year ago and they bias within 2%. For instance, I get 269mV on the left channel and 275 mV on the right. The target is 270mV with the EFB, so that's pretty darn tight.

    HifiGuy

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    Re: Alternative to Cathode Bias on ST-35

    Post by HifiGuy on Sun Jun 16, 2013 1:08 pm

    Hi Captain,

    I do like the regulated EFB mod for the reasons you mentioned and more but I wanted to be able to adjust the bias for each tube separately as the dynakitparts Bias Control upgrade does. I am looking for the quick and neat solution that a kit type mod would produce. Many folks say that matched tubes do not stay matched for long. Buying matched pairs is less costly and unmatched singles even less. If all the singles came from the same production lot they might be close anyway.

    Is the dynakitparts bias upgrade just maybe something like an adjustable cathode bias? (If there is any such bias scheme) There is no schematic or explanation of the circuit other than adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate “on board” precision bias pots.

    Thanks for responding to my post Captain. I may well get Mr. Gillespe's board. That way at least I would not have to get another multi-section cap. Roy recommended it as well so that's two votes for EFB.

    tubes4hifi
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    Re: Alternative to Cathode Bias on ST-35

    Post by tubes4hifi on Sun Jun 16, 2013 1:15 pm

    I'm not sure, maybe Kevin will chip in here, but I believe the DynaKitParts bias kit uses the same schematic as the DIYtube kit for biasing

    peterh

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    Re: Alternative to Cathode Bias on ST-35

    Post by peterh on Sun Jun 16, 2013 2:26 pm

    HifiGuy wrote:Hi Captain,

    I do like the regulated EFB mod for the reasons you mentioned and more but I wanted to be able to adjust the bias for each tube separately as the dynakitparts Bias Control upgrade does. I am looking for the quick and neat solution that a kit type mod would produce. Many folks say that matched tubes do not stay matched for long. Buying matched pairs is less costly and unmatched singles even less. If all the singles came from the same production lot they might be close anyway.

    Is the dynakitparts bias upgrade just maybe something like an adjustable cathode bias? (If there is any such bias scheme) There is no schematic or explanation of the circuit other than adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate “on board” precision bias pots.

    Thanks for responding to my post Captain. I may well get Mr. Gillespe's board. That way at least I would not have to get another multi-section cap. Roy recommended it as well so that's two votes for EFB.

    Matched tubes is the way to go. And should they divert later ( which some does) then change again ! This is however unlikely.
    What you _might_ be looking for is a way to measure the individual tubes for idle current, and that could be done by
    a small ( 10ohm?) resistor in series with the cathode. When using EFB you will have to measure across each resistor in order to
    measure current in each tube. But in my experience, most el84 will age nice so chances are that you tubes will age in sync.

    Using dissimular tubes will degrade sound whatever circuit tricks you do.

    Captain Coconut

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    Re: Alternative to Cathode Bias on ST-35

    Post by Captain Coconut on Sun Jun 16, 2013 4:45 pm

    Here's some info on the EFB. This will convince you that this is the way to go. And as I said about the output tubes, the two channels are biased within 2% of each other after more than a year of play. 

    http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf

    1973shovel

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    Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel on Mon Jun 17, 2013 1:30 pm

    I agree with the Captain 100%. The EFB is the way to go with amps using the Z565 output transformers (ST-35 and SCA-35). If you read Dave Gillespie's article, he explains why standard fixed bias actually sounds worse than the stock cathode bias Dynaco used.

    One of the things that drew me to the mod was the fact that it was easily accomplished with a few parts. Gillespie wasn't trying to sell anything. Yes, he's now selling the boards, but the mod is easy to do without them.

    This isn't meant to put down Kevin or anyone else selling the standard fixed bias kit. My post is simply to question why you'd want to do something to make your amp sound worse. Simply to use mismatched tubes? Not me, no thanks.

    My experience is that the EFB mod transformed my SCA-35 from what for years I called Dynaco's worst sounding tube amp, to something I actually enjoy listening to. It sounds better, it measures better, the output tubes last longer,  and it only costs a few dollars to do.

    It is also possible to build the EFB with individual tube bias, but on a stock ST-35 chassis you'd have to get very creative!

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Mon Jun 17, 2013 10:17 pm

    I have a Dynakitparts reissue Stereo 35 with the individual tube bias kit, and it does not make the amp sound "worse."  The main thing I noticed when going to the individual bias adjust was that the sound became cleaner and perhaps even a bit leaner and more detailed.  The original bias scheme sounded a bit warmer, rounder and richer.  You decide which you might like better.
     
    Personally, I like to adjust for any differences between the output tubes, and if I want the sound richer, I can swap the metal film resistors for carbons and/or the polypropylene coupling caps for K40Y9.
     
    It is my understanding that the Dynakitparts bias kit for the Stereo 35 is the same as in the diytube design.  If I recall, it should be possible to integrate the EFB mod while retaining the individual bias, but I do not know how this is done.  If I recall, before the PCB was produced by audioregenesis to facilitate the EFB, you could add the EFB inside the Stereo 35 with some discrete parts in the right locations.  Again, I have not investigated the details of how this is done, but it is a direction I hope to explore when/if I have the time.
     
    Regards,
    Peter

    1973shovel

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    Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel on Tue Jun 18, 2013 12:55 am

    Peter, you may be right. I have only hearsay to go on saying individual fixed bias sounds subjectively "worse" than the stock cathode bias when using Dynaco Z-565 output transformers. I stand corrected.

    What I should have said is that it mesures worse with individual fixed bias. Look on page 16 of Gillespie's "New look at an old friend", which Captain posted a link to, and read #10 "Popular Modification".  It explains it better than I could, instead of trying to repeat it here.

    Back to the subjective. I don't believe that measurements tell the whole story.  If they did, we'd be listening to solid state amps simply because they usually measure better. The individual fixed bias will cause the ST-35 to perform with "sub-par performance" (according to Gillespie). That's no reason for me to assume it will actually sound worse, simply that it will perform worse.

    I like the EFB becuse not only does the amp measure better (more powerful, with less distortion), but subjectivly, the amp sounds far better to me than with the stock cathode bias. Not bad for less than $10 in parts.

    Captain Coconut

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    Re: Alternative to Cathode Bias on ST-35

    Post by Captain Coconut on Tue Jun 18, 2013 8:48 am

    PeterCapo wrote:
     
    Personally, I like to adjust for any differences between the output tubes, and if I want the sound richer, I can swap the metal film resistors for carbons and/or the polypropylene coupling caps for K40Y9.
     

     
    Regards,
    Peter


    Peter, would the K40Y9 fit a stock ST-35? I would like to try them sometime but am afraid that they might be too big. I use Magnequest OPT's and their bell covers are larger than the original which left me no room to mount the coupling capacitors on top - at least the two close to the OPT's. I mounted those under the hood. The chassis is only about 3/4" deep and I guess if the K40Y9's are less than 3/4" DIA., they might fit inside. 

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Tue Jun 18, 2013 8:58 pm

    With regard to the K40Y-9 in the Stereo 35, here are two photos showing comparative size with Orange Drop 716P on the top side:





    The two K40s on the right are 0.1uf @ 400V.  The one on the left is 0.22uf @ 400V, where it really only needs to be 0.22uf @200V, but I don’t seem to be able to find them here – thought I had them, I guess not.  But, I have seen 0.22uf @200V available, and they would be smaller.
     
    I think all three might be made to fit on top of the PCB if using Dynaclone OPTs.  But if mounting underneath for any reason, I measured the diameter of the 0.1uf @400V at a shade over ½”.  The diameter of the 0.22 @400V measured between 9/16” and 5/8”, but, again, I’d get the 0.22uf @ 200V for an easier fit.
     
    I started reading through the Gillespie article at that link and I can see that he does not seem keen on the individual bias mod by itself.  It seems like the biggest advantage of the individual bias mod is that you don’t need closely matched tubes – which I think in and of itself is a valid consideration.  Might be interesting to see distortion measurements for tubes that are not closely matched before and after adjusting the individual bias pots.  But, I want to study the article further to try and zero-in on why he thinks the individual bias is worse...


    Last edited by PeterCapo on Tue Jun 18, 2013 10:03 pm; edited 1 time in total

    Captain Coconut

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    Re: Alternative to Cathode Bias on ST-35

    Post by Captain Coconut on Tue Jun 18, 2013 9:33 pm

    I use Multicaps in both positions since they are one of a very few that fit this amp. It looks like the Russian caps might work. Did you get them from Bob?

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Tue Jun 18, 2013 9:41 pm

    I got them a while back from somebody on ebay.

    1973shovel

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    Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel on Sat Jun 22, 2013 11:58 pm

    Hi Peter,

    It took me some time to remember where I read that the ST-35 sounded worse using standard fixed bias. Tonight I remembered, it was by a well regarded tube guy, Gary Kaufman, on the Audio asylum. Here's a link:   ST-35 fixed bias

    Kaufman's quote: "As you've guessed, the bias is not adjustable - the ST35 uses cathode/self bias. I've tried individual bias resistors in an ST35. Although it sounds like a good idea, it sounded really poor IMHO. I'd stick with the original design."

    In another post in the same thread, Mr. Kaufman continued: "I tried individual bias resistors (I seem to recall using 360 ohm) per tube, and also later per channel. Using one bias resistor per channel wasn't bad, but no improvement over the single resistor. Using a separate resistor per tube (with individual bypass caps) really sounded "unfocused". I have no explanation, frankly I would have expected the opposite."

    Mr. Kaufman made this observation in 2001. After reading Dave Gillespie's explanation of why Mr. Kaufman's ST-35 conversion to standard fixed bias "sounded really poor", I sent Gary Kaufman a link to Gillespie's article, but never got a reply.

    I hope you read Gillespie's findings on the Z-565 Dynaco transformers, and why the EFB works so well with them. Both from a measured as well as a subjective standpoint.

    To me, having to purchase a matched quad of output tubes is a small price to pay for the sonic bonus the EFB brings to the ST-35 and SCA-35. And, of course individual EFB tube biasing can be achieved by using a more complicated circuit. This is my plan when I build a larger chassis version of the ST-35, with an improved input stage.

    Enjoy your reading!

    peterh

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    Re: Alternative to Cathode Bias on ST-35

    Post by peterh on Sun Jun 23, 2013 5:33 am

    Yes, a matched set of tubes is a requisite for good sound. If tubes are dissimular nothing will make it sound good.
    The EFB mod is a very good continuation and a cost effective way of improving sound and power. EFB is available for SCA35 but i guess that the board might be mounted in a st-35 somehow.

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Sun Jun 23, 2013 8:08 pm

    I read the entire thread containing the postings by Gary Kaufman.  Sounds like he replaced the single cathode resistor with four separate resistors, and he got an unfocused sound.  But, notice there was no mention of a pot to set the bias current the same for each tube.  If all he did was to replace the single original cathode resistor with four individual resistors without adjustment pots, and if his tubes were not well matched, or if there was something else going on that was not apparent in his post, then I guess it isn’t surprising that there were problems.  I don’t know that we can directly compare Gary Kaufman’s experience to what Dave Gillespie discussed in his article.

    But, it would be good to keep Gary’s example in mind.  When we say “individual bias,” it could mean that you just broke-out the single cathode resistor into four, one per tube.  OR, when we talk about individual bias, we might mean the setup Shannon Parks and Kevin Devaney use, having a small resistor network including a potentiometer for each EL84.  You cannot match the bias current in each EL84 if you simply use four individual resistors off of each EL84’s cathode.

    Now, here’s something really interesting I found.  If you can, pick up a copy of the July/August edition of the absolute sound magazine, issue 234.  On page 122, there is a review of Bob Carver’s VTA20S “Black Magic” power amp.  The review includes quotations from Bob Carver talking about the circuit.  It uses four EL84s and two 12AX7s.  The 12AX7s are wired for voltage gain and then cathodyne phase inversion just like in the original Stereo 35, except Carver uses a 12AX7 instead of the 12DW7 used in the Stereo 35.

    Carver then goes on to describe the common cathode bias scheme he uses in the amp – with a zener diode to clamp the bias voltage at the single cathode resistor.  Sound familiar?  Dave Gillespie acknowledged the zener as a method to achieve the same goal as his EFB, but Gillespie also went on to discuss the zener’s potential drawbacks, as well.   The discussion in the absolute sound article reads like an abridged mirror of the discussion not only in the Gillespie article, but also of other discussion you can find in forums.  For instance, regarding the plate dissipation limits of different current production EL84s in the Stereo 35, where you would want to adjust the B+ so you don’t prematurely kill the EL84s.  You can find Jim McShane talking about this over at Audio Asylum.  Bob Carver’s comments in the absolute sound article are uncannily similar to Gillespie’s article and McShane’s posts.

    Talking about the zener bias regulation in his VTA20S “Black Magic” amp, Bob Carver stated the following: “The effect is quite significant, and flies in the face of what I have always been taught-that is to use separate cathode resistors.  Not a good idea, actually.”

    Maybe this is why he called his amp “Black Magic.”  And, on the face of it, it looks like Carver, in implementing something akin to Gillespie’s EFB, also thinks that individual cathode resistors are not a good idea.  But, notice here again, as in the case of the comments from Gary Kaufman in the AA post, Carver did not mention an individual bias adjustment pot for each EL84.  Maybe he would say a pot wouldn’t make any difference, but we really can’t tell from the article.  When he pointed-out that a number of different variations of cathode biasing schemes are in use in different amplifiers, Carver stated: “That tells me that there is no known consensus.  I like my way the best, especially with the clamp.”

    What I get from Dave Gillespie’s article is that the impetus for the EFB is to find a way to stabilize the EL84s’ operating points under load to compensate for B+ sag, especially with both channels driven.  I interpret him also to say that aside from the EFB, you could accomplish this by increasing the energy storage in the power supply with larger capacity electrolytics.  However, Gillespie wanted to find a simple, unobtrusive means of stabilizing the operating point – something simple, clever and cost-effective in the spirit of the original Dynaco philosophy.  Increasing the PS capacity would be more complex and expensive than the EFB.  (But, the cost-effectiveness doesn't seem to be there if you have to keep buying matched quads of EL84s with any regularity.)

    Item 10 on page 16 of Dave Gillespie’s article talks about the adjustable individual bias resulting in worse performance than the original bias circuit (no EFB).  What I have been wrestling with is exactly how he comes to this conclusion.  Evidence he offers refers to a test done by Triode Electronics in Chicago comparing their Dynaclone OPT to some other replacement brands.  The test was performed by dropping each OPT into a DIYTube Stereo 35 clone, which is known to use the adjustable individual bias.  Although Triode did not specifically say so, Gillespie assumes that the test results were obtained with only one channel driven, with the resulting distortion measurements equal to (in the midrange) or worse than (at the frequency extremes) what Gillespie found driving both channels of an SCA-35 having the original single cathode resistor scheme (no EFB).

    Per his discussion in the article, one channel of the SCA-35/Stereo 35 performs considerably better than both channels being driven together (due to B+ sag).  So, if one channel driven in the Triode Electronics test results in equivalent or worse distortion than Gillespie found driving both channels of an SCA-35 with a single cathode resistor (without EFB), then this seems to be the reason why he thinks adjustable individual bias is “worse” than the original scheme.

    Problem I have with this is that the Triode Electronics test Gillespie references may or may not have been driving one channel only – we really do not know for certain if this was the case.  There may have also been other variations of test parameters that may not be obvious.  Even if Gillespie referenced distortion measurements from the original Z-565, how do we know that there weren’t substantial differences even among different samples of original Z-565s?  The point being that Gillespie did not conduct his tests with the same amplifier having the same set of Z-565 transformers that Triode Electronics had, and if Triode’s test showed anything it is that there can be variation among different transformers, never mind any other differences between the amplifiers.  I know Triode highlighted differences between different brands of OPT, but it seems to me that there could be significant variations in performance among original Z-565s, especially if they were manufactured by different vendors over a span of many years.

    Gillespie may be exactly right, but I’d like to see tests done under carefully controlled conditions rather than drawing inferences from another party’s test that was done for a different purpose, in Triode Electronics’ case, testing for differences between brands of OPTs.  I’d like to see tests done with one amp having one particular pair of OPTs only, measuring distortion both with and without closely matched tubes for: 1) single cathode resistor without EFB, 2) individual bias adjust with potentiometer (no EFB), and 3) basic EFB without individual bias adjust.

    Again, Gillespie could certainly be exactly right.  But, even by his own words, there are other ways to deal with B+ sag.  And, here I will come full circle.  Hifiguy began this thread considering either the adjustable individual bias or EFB.  With either choice he would have to replace his cap board and go back to the quad-section can.  I don’t know what the total capacitance of HifiGuy’s cap board is, but keeping the cap board could possibly obviate the need for the EFB.  Also, going from low ESR electrolytics rated for 85 C or 105 C on the cap board to a quad-section can rated for 55 C could introduce its own change to the sound that may or may not be desirable.

    Gillespie’s basic EFB seems an efficient and effective solution with the caveat that you need to keep matched tubes in the amp.  If you like to roll different kinds of tubes that may not be matched very well, or if your tubes do significantly drift apart over time and you want full use from them, then I think the individually adjustable bias may be the best way to go.  Pending evidence from a more targeted and closely controlled test, I think the jury is out on whether the individual bias adjust is really worse than the original single resistor (without EFB, that is).  Perhaps it is worse.  But, again, when I installed my adjustable bias, I did not get an unfocused sound like Gary Kaufman reported.  If anything, the focus improved.  But, then again, I listen at moderate levels, and any “worseness” might not show up readily unless the amp were pushed hard...

    Regards,
    Peter


    Last edited by PeterCapo on Tue Jun 25, 2013 10:35 am; edited 18 times in total

    Captain Coconut

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    Re: Alternative to Cathode Bias on ST-35

    Post by Captain Coconut on Sun Jun 23, 2013 8:59 pm

    There's a wealth of information on this subject at diytube.com. It's unanimous that EFB is the way to go. Anyone that has incorporated this into his/her amp praises it. It's a no-brainer as the kids like to say.

    You appear overly concerned about matched tubes. As mentioned before, my inexpensive tubes from Jim McShane have not drifted at all in well over a year. They may shit the bed tomorrow or bias flawlessly for years to come. I checked bias today and got 267mV in one channel and 275mV in the other. Unless you're obsessive about this stuff, this is pretty darn good.

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Sun Jun 23, 2013 9:24 pm

    Just to be clear, I'm sure EFB is a fine choice with the caveat that it needs matched tubes, I understand that it is popular, and what I wrote suggests nothing to the contrary.  With regard to matched tubes, I was quoting Dave Gillespie himself who said EFB needs matched tubes.  Glad your tubes stayed matched closely - do you think that would always be the case with all sets of matched EL84s everywhere?  And, not everyone buys their tubes from Jim McShane, so there might be a number of ostensibly matched sets running out there that weren't matched all that well in the first place.  Just offering real world considerations and not taking things for granted.


    Last edited by PeterCapo on Tue Jun 25, 2013 11:13 am; edited 1 time in total

    corndog71

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    Re: Alternative to Cathode Bias on ST-35

    Post by corndog71 on Sun Jun 23, 2013 10:50 pm

    Just to add to this discussion I would like to throw in what Roger Modjeski says about matched tubes.  

    "Here's an analogy for the mechanically-minded. Imagine a VW-type engine where the cylinders are opposed and made separately, and the manufacturer puts together engines where the pistons, rods, and cylinders are not very uniform. In this engine there is only one arm on the crank (like the driver in our amplifier), and all the rods are connected to it so that they all have the same length of throw. One "pair" of pistons should top out together when the other pair hits bottom. If any of them has a short rod, it will achieve less compression and therefore less power. If he matched the rods and selected cylinders of the same bore, he would then get equal power per constraint of that one crank, as we have with our single (symmetrical) drive voltage. With unequal push rods, we're going to end up with very different compressions at the end of the stroke, and thus very different output power from each cylinder."

    "...with unmatched tubes, you will never attain maximum performance."

    He also has some interesting things to say about Carver's amp:

    "According to Dick Olsher in the most recent TAS Bob Carver has a new EL84 amplifier with one cathode resistor.   Although Dick Olsher states in the review that he previously thought individual cathode resistors are better he appears to have been won over by Bob's ability as a hawker to believe that now that one is better.   To make matters worse Bob has put a 14 volt zener diode across the cathode resistor which will likely short out the first time a tube shorts or flashes. Then there will be no bias resistor. After the tubes meltdown the amp can visit the shop to have its zener diode replaced, which is there only so the amp can do 20 watts sine wave (remember Bob likes specs). I have not seen a schematic, would love to, however its pretty clear to me what the circuit looks like from Olsher's description. Oh one other thing. It appears Bob did something with the feedback to raise the output impedance to 1.5 ohms making the damping about 5. I guess he wants the amp to sound real tubey."


    Ok, I didn't add much but it's an interesting discussion.

    PeterCapo

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    Re: Alternative to Cathode Bias on ST-35

    Post by PeterCapo on Sun Jun 23, 2013 11:03 pm

    It’s a good post, corndog71.  To have a broad view is good.  People just think differently about things and the broader our perspective is, the better understanding there is.

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    Re: Alternative to Cathode Bias on ST-35

    Post by tubes4hifi on Mon Jun 24, 2013 11:48 am

    one problem no one has mentioned.   The bias has to be IN PROPORTION to the B+.  Although Carver has made some great amps, I believe he is making a mistake (unless there are some other fine points to his design he hasn't mentioned).    Using a zener will not keep the bias from fluctuating.   It will keep the bias VOLTAGE from fluctuating, but NOT the CURRENT.  At least with a fixed resistor, the current is proportional to the B+.   Curcio has made this mistake several times, and that's why many of his amps blow up.  
    I've seen WAY too many Curcio boards that were burnt like toast.
    You either have to regulate every voltage attached to a tube, or none of them.   I prefer none of them, it sounds WAY better.
    The very best solution in my own experience is exactly what Gillispie has done - use a CCS.   That guarantees bias current that won't fluctuate, and stays at the correct setting
    no matter what the B+ is doing.   It also reduces distortion.

    sailor

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    Re: Alternative to Cathode Bias on ST-35

    Post by sailor on Tue Jun 25, 2013 12:28 pm

    Interesting that Carver chose the 12AX7 as a phase splitter. The impedance miss match between the split outputs is many times worse with a 12AX7 than with a 12AU7. Also there is no amplification advantage because the phase splitter will always be about .9 gain no matter what triode you use. The third problem is the 12AX7 has much less available amperage and more prone to microphonics. I think it was a bad choice all the way around.
    Which brings up another point I have been harping on for quite some time. The ST and SCA both have positive feedback. This feedback comes from one side of the phase splitter. I don't like the use of positive feedback in this type of amp to begin with and Dynaco only put it in to increase the output of the driver stage. But forget that, the main problem it creates is no matter how matched your EL84s are the split driver signal is miss matched because part of the output from the cathode is being pulled off to run the positive feedback loop. The result is distortion which gives the amps a very pleasant dispersed tuby sound. Most people actually like this distortion and call it to detail.

    tubes4hifi
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    Re: Alternative to Cathode Bias on ST-35

    Post by tubes4hifi on Tue Jun 25, 2013 12:45 pm

    Sailor, you are right on!

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    Re: Alternative to Cathode Bias on ST-35

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