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    ST 70 Power out versus EL34 Bias point

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    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    ST 70 Power out versus EL34 Bias point

    Post by Tubes4ever on Tue Aug 25, 2015 8:37 pm

    Since the original st70 recommended a bias point of 50 mA and Bob recommends a bias of 40mA on the EL34 tubes, I was wondering what the effect of the two different bias points has on output power and harmonic distortion?

    I do understand the benefits to tube life with the lower bias setting.

    Has anyone made these measurement?

    Bob, I'm sure you have some useful data concerning this.

    Thanks!

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: ST 70 Power out versus EL34 Bias point

    Post by PeterCapo on Tue Aug 25, 2015 9:29 pm

    Along with other things, your question is discussed to an extent in this document: http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf   With regard to the original Stereo 70, measured stock performance is on page 2.  Discussion of lowering bias current is in the section "The Power Output Stage - Pt. 1" on pages 5 and 6.

    I believe tube longevity is more an issue of plate power dissipation than actual bias current by itself.


    Last edited by PeterCapo on Wed Aug 26, 2015 11:31 am; edited 1 time in total (Reason for editing : Correct misspell.)

    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: ST 70 Power out versus EL34 Bias point

    Post by Tubes4ever on Wed Aug 26, 2015 12:32 am

    Thanks Peter!
    That was an excellent read. My takeaway was that running the EL34s at 50mA is optimal as far as distortion in concerned. Also, running them at the currently recommended 40mA increases distortion and doesn't radically increase tube life. The 50mA bias point gives the lowest distortion values.

    So, if you want the best possible sound run the tubes at 50mA.

    Bob Latino
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    Re: ST 70 Power out versus EL34 Bias point

    Post by Bob Latino on Wed Aug 26, 2015 8:40 am

    Comparing the proper bias point on an original stock Dynaco amp with the original driver circuit to the proper bias point on a new Dynaco type amp with a VTA driver circuit is like comparing apples to oranges. The original Dynaco amps ran a rather high 50 milliamp per each output tube and it worked well because Dynaco used the extremely durable Mullard EL34's which would take this rather high bias point in stride. Modern EL34's are not as durable as the old Mullard's and will wear out quicker if run at 50 milliamps on a steady basis.

    Another thing > The original Dynaco ST-70 was designed to run at the standard AC voltage of the 1960's which was 115 - 117 volts. This gave a B+ of about 435 VDC. Running an original Dynaco ST-70 at 120 VAC will run the B+ that those EL34's have to look at to maybe 460 VDC. This will put even a little more stress on those modern EL34's.

    Check out some of the bias recommendations of Rogue and Jolida amps. Many of the amps from both companies that have a fixed adjustable bias recommend a bias in the 35 to 40 milliamps per each output tube. Below is a link to the Rogue Atlas EL34 based amp. On page 6 of the manual they recommend 35 milliamps per each of their four EL34 output tubes.

    Rogue Atlas manual

    Bob

    PeterCapo

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    Re: ST 70 Power out versus EL34 Bias point

    Post by PeterCapo on Wed Aug 26, 2015 12:24 pm

    A tube seller who I shop agrees that most current production EL34s do not last as long as those of yesteryear – with one exception: the EH 6CA7, which he feels can definitely take the stress.  Last time I checked, on his website I believe he points out its sheer increase in plate area and extra cooling fins vs. EL34s.

    As far as current production EL34s, I am kind of inclined to say "just let 'em burn."  If you don’t use your amp every day, it might not end up being much of a problem.

    But, as it has already been said in threads like this one, we might not be able to hear much if any difference by lowing the bias a bit (though I still feel kind of reluctant to do so).

    If you have an original Stereo 70 having a PT wound for 117VAC mains, then one or two (maybe even three) appropriately rated thermistors in series with the primary should help to drop the internal voltages to whatever level you feel is appropriate.  If it’s too much of a reach for thermistors because your line voltage is too high, then some folks run off of a variac or bucking transformer or whatever.  So, that part should be solvable one way or another.

    The Dynaktparts.com PT, last I knew, is wound for 122VAC mains, and I have one in my dynakitparts Stereo 70.  With this newer transformer, and a thermistor, my 122VAC-125VAC mains results in something less than 400VDC (original spec=410VDC on plates with 117VAC mains) on the plates of my EL34s, so I can run them at 50mA with a reasonable level of power dissipation in the plates that I would think should result in a reasonably long lifespan for the EL34s.

    If other amps like Jolida and others nowadays are running their bias lower, I suspect it is also to reduce the likelihood of warranty claims.  But, we’d also need to know how their plate voltages run to get an idea of the kind of stress their tubes are actually getting.  For instance, my Cary SLI-80 sets the 6550/KT88 to about 35mA apiece, but, from what they have told me, the plates operate somewhere in the vicinity of 550 volts.  Even at that voltage, it seems like there should be room to turn the bias up some...

    Tubes4ever

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    Location : Star, Idaho

    Re: ST 70 Power out versus EL34 Bias point

    Post by Tubes4ever on Wed Aug 26, 2015 1:11 pm

    I have a VTA st70 from Bob that is about 3 years old. With the bias set at 50mA and the input voltage set at exactly 120V using a variac, I get 410V B+ which is exactly what the original 1950s st70 schematic says I should get. This confirms that the new PT is wired for 120V as Bob has stated. I am using the 5AR4 rectifier. Also, the EL34 heater voltage is very close to 6.3V.

    I agree with Peter in that we need to know what amps like Jolida and Rogue are running their B+ at.
    If they are running the same B+ as the ST70, distortion is going to be much higher at 35mA idle current. There's a reason that a bias point is chosen other than seeing how long we can make the tubes last.

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