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    JJ E34L Bias in the ST-70

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    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    JJ E34L Bias in the ST-70

    Post by calinet6 on Wed Jun 01, 2011 1:55 pm

    Hello all, first post (and first ST70) here...

    When I picked up my ST-70, it had 3 original Mullard tubes and one RCA replacement, and one of the tubes had glowing plates... I figured a set of new tubes was in order. Based on my budget and reading a few quick reviews, I settled on the JJ E34L's and went for it.

    I plugged them in and biased them around the same as before (1.4ish due to what I've been reading about it not sounding any different and increasing tube life). Since then I've been a little disappointed in their performance; they weren't sounding as smooth or detailed as I remember even the old mixed set of tubes did. A little washed out, even grainy at times.

    So I took the amp out of commission for a day, replaced the bias electrolytic caps, replaced the selenium rectifier diode with a modern one, replaced the speaker terminals with some nice gold banana plugs, replaced the aging power cord... the simple stuff.

    When I plugged it back in and powered up, everything worked beautifully and the bias seemed to be more stable than before. I set it up at 1.4v again, but the sound was still thin and disappointing.

    I did some reading and some suggest that the JJ E34L's should run with more current than standard EL34's... so I figured I should at least run them at the standard 1.56v. I moved it up to the high side of 1.56v and let them loose.

    Man do they sing now! Full, detailed, and solid midrange. At least as good as I remember the amp sounding initially, if not better.

    So, my questions: first, anyone else have similar experiences or recommendations on the E34L's? How far can I push the bias voltage/tube current without worrying about damaging the tubes or other components (power transformer I assume would be the concern). What is the risk associated with more tube current? Should I just go with what sounds best, within reason?

    Thanks in advance, I'm just learning the circuit so any advice or tips are much appreciated.

    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by calinet6 on Fri Jun 03, 2011 11:44 am

    Sorry - let me simplify the question...

    How far can I push the bias voltage? How exactly does it relate to the current through the tubes? And how does that affect the power transformer and put load on other parts?

    Thanks Smile

    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by calinet6 on Fri Jun 03, 2011 12:12 pm

    I think I found some good information in this thread... still trying to find more information about exactly how the E34L's are different from EL34's if at all. I could just be making this up, I don't know.

    http://dynacotubeaudio.forumotion.com/t173-nailing-down-bias?highlight=bias

    Bob Latino
    Admin

    Posts: 1970
    Join date: 2008-11-26
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by Bob Latino on Fri Jun 03, 2011 12:14 pm

    Hi,

    Dynaco biased the output tubes a little on the high side. The 1.56 volts DC recommended bias voltage gave a bias current of 50 milliamps per each EL34 output tube. IMHO that 50 milliamps per each output tube should be considered the upper limit for biasing EL34, KT66 or KT77 output tubes.

    If you bias an output tube too high in a Dynaco tube amp ..

    1. The amp will stay in class A longer before switching over to class AB1 (no real problem there)
    2. The output tubes will wear out sooner.
    3. You will ask your power transformer for a little more current and it wll run warmer.

    What you try to do is select a bias current that will be a "compromise". If the bias is not high enough, the output tube will not be at its most efficient area of operation. If the bias is way low the amp probably won't sound right. Correct bias is somewhat comparable to automobile efficiency. If you drive your car at 10 MPH down the highway - that's too slow for your auto to be efficient. If you drive at 60 miles MPH down the highway your auto is now in an area that gets good mileage and doesn't wear the engine too much. If you drive at 100 MPH, your auto is now well out of it's proper efficiency area and your auto's engine will probably wear out much quicker at 100 MPH than it would at 60 MPH.

    Conventional wisdom is to bias EL34's at about 40 milliamps. On a stock ST-70 with the stock board that is about a 1.25 volt DC bias setting for each of the two pairs of output tubes. On a VTA boarded ST-70, that equates to a bias voltage of .40 volts DC. Some amp companies don't even recommend that high a setting. For the Jolida JD202 EL34 amp, Jolida recommends just 37 milliamps of bias current per tube.

    Bob

    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by calinet6 on Fri Jun 03, 2011 12:24 pm

    Thanks so much for the reply, Bob, really appreciate it.

    Good analogy with the car efficiency, that makes sense to me though I know it's not exactly the same.

    So what I'm getting if I bias the (stock) amp at 1.54 is 50mA per tube. I assume that's probably too high. I will play with lowering it to 40ma (which would be 1.23 V). Seems very low but I will see how it sounds!

    Thanks again for your help.

    Sal

    Posts: 222
    Join date: 2009-02-05
    Location: Central New Jersey Dynaco-ST70.com

    Re: JJ E34L Bias in the ST-70

    Post by Sal on Fri Jun 03, 2011 3:10 pm

    Maybe just maybe the tubes need to break in? I have read about that phenomenon, I never noticed it myself but I don't do to much tube rolling..

    Sal

    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by calinet6 on Fri Jun 03, 2011 3:19 pm

    That's possible. I listened to them last night at stock levels (1.54 V) and they were sounding very good. I've probably put only 20 hours on them so far though, so maybe after a while I will be able to drop the bias voltage and listen again. I'll keep researching and see what I find.

    calinet6

    Posts: 17
    Join date: 2011-06-01
    Location: Massachusetts

    Re: JJ E34L Bias in the ST-70

    Post by calinet6 on Tue Jun 07, 2011 2:51 pm

    Another update on this - I installed new filter capacitors in the input stage (the Sprague 716P orange drops) and now when setting the bias much lower at 1.3 V or so, everything sounds much better than before.

    The sound has the same general oomph and presence now even at way lower bias. Does this sound plausible? What am I hearing by replacing the filter caps? To me everything sounds much clearer, more defined, a little more airy in the high end... I'm liking the sound very much.

    Anyway, I'm glad I replaced the filter caps in any case, the old ones looked very worn out and one of them had some goop on the bottom that I can only assume was not good. I think I'll leave the amp this way for a while before installing the VTA board to get a good feel for the "stock" sound... thanks again all.

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