The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


+2
Roy Mottram
HifiGuy
6 posters

    Alternative to Cathode Bias on ST-35

    avatar
    HifiGuy


    Posts : 8
    Join date : 2010-09-09
    Location : South Carolina

    Alternative to Cathode Bias on ST-35 Empty Alternative to Cathode Bias on ST-35

    Post by HifiGuy 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

    Alternative to Cathode Bias on ST-35 St-35_11
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Roy Mottram 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
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by peterh 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!
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest 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.
    avatar
    HifiGuy


    Posts : 8
    Join date : 2010-09-09
    Location : South Carolina

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by HifiGuy 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.
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Roy Mottram 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
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by peterh 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.
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest 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
    avatar
    1973shovel


    Posts : 45
    Join date : 2009-12-13

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel 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!
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Mon Jun 17, 2013 10:17 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:28 pm; edited 1 time in total
    avatar
    1973shovel


    Posts : 45
    Join date : 2009-12-13

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel 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.
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest 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. 
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Tue Jun 18, 2013 8:58 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:29 pm; edited 2 times in total
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest 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?
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Tue Jun 18, 2013 9:41 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:29 pm; edited 1 time in total
    avatar
    1973shovel


    Posts : 45
    Join date : 2009-12-13

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by 1973shovel 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
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by peterh 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.
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Sun Jun 23, 2013 8:08 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:29 pm; edited 19 times in total
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest 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.
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Sun Jun 23, 2013 9:24 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:31 pm; edited 2 times in total
    corndog71
    corndog71


    Posts : 840
    Join date : 2013-03-19
    Location : It can get windy here

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by corndog71 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.  Alternative to Cathode Bias on ST-35 Sm_nono 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. Alternative to Cathode Bias on ST-35 Duh  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.
    avatar
    Guest
    Guest


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Guest Sun Jun 23, 2013 11:03 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 3:31 pm; edited 1 time in total
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Roy Mottram 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.
    avatar
    sailor


    Posts : 269
    Join date : 2011-04-04

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by sailor 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.
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Roy Mottram Tue Jun 25, 2013 12:45 pm

    Sailor, you are right on!

    Sponsored content


    Alternative to Cathode Bias on ST-35 Empty Re: Alternative to Cathode Bias on ST-35

    Post by Sponsored content


      Current date/time is Tue Mar 19, 2024 7:50 am