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    Will VTA Mk III with octal boards run EL34?

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    Dingojazz

    Posts : 39
    Join date : 2010-09-03

    Will VTA Mk III with octal boards run EL34?

    Post by Dingojazz on Thu Dec 04, 2014 12:23 am

    Will these amps run EL34s as well as 6L6, 6550, and KT88?
    If so, is it just a bias change to 40ma.?

    Thanks,

    (Sorry if redundant - couldn't find answer by search..)

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Will VTA Mk III with octal boards run EL34?

    Post by Bob Latino on Thu Dec 04, 2014 8:45 am

    Dingojazz wrote:Will these amps run EL34s as well as 6L6, 6550, and KT88?
    If so, is it just a bias change to 40ma.?

    Thanks,

    (Sorry if redundant - couldn't find answer by search..)

    You probably could run EL34's in your Mark III but ..

    1. The amp should bias properly with EL34's in there in place of 6550 but they may bias over to one side of the bias pots range or one tube of the pair may not bias at all down to 40 ma. If that does happen you may have to increase the value of R39 (the board's main bias resistor) to alter the range of the bias pots.

    2. The EL34 output tube is the traditional output tube for the ST-70. The ST-70 runs a B+ of about 430 to 440 VDC and with todays higher line voltages maybe even a little higher. A Mark III will run a B+ of 475 to near 500 VDC. (with today's higher line voltages) Modern EL34's are not as well built and "robust" as the original Mullard EL34's of the 1950's and 1960's. I have a feeling that today's EL34's will not stand up to the higher voltages in a Mark III as long as a KT88 or 6550 output tube will. Read that as shorter tube life .. People like to use EL34's in a Mark III because EL34's are about 1/2 the price of KT88's. Personally, I don't recommend the use of EL34's in a Mark III.

    You also mentioned 6L6 output tubes .. I would not use 6L6's in a Mark III either for the same two reasons listed above ..

    With all that said .. If you use a 5U4 rectifier in place of a 5AR4 rectifier in your Mark III, the 5U4 will drop the B+ voltages 30+ volts and place the Mark III's in range where you probably could use EL34 output tubes. The only minor issue you would have now is that your Mark III's will not give their full output. You would probably be limited to somewhere around a 45 to 50 watt max output from your Mark III's.

    Bob

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Will VTA Mk III with octal boards run EL34?

    Post by GP49 on Thu Dec 04, 2014 11:59 am

    The stock Dynaco Mark III is the same basic amplifier as the stock Mark II, except for power supply differences and output tubes.  The Mark II used the EL34 output tube, a 5U4 rectifier, and a 50Ω, 10 watt dropping resistor instead of the filter choke.  It had different values in the resistor string for bias voltage, as Bob said.  

    If you can get the bias down to 40mA (or 50mA, the original Dynaco specification) and have genuine original Mullard EL34s, which were rated at 800V maximum for plate voltage, you'll be safe running them at Mk III B+ voltages.  But the current reissue imitation Mullards, not so!

    However it is always best to remember the following, which I got from an Internet source years ago (and which original source I can't find, now:

    Selecting a bias point for output tubes

    MAXIMUM DISSIPATION:

    There are other factors in selecting a bias point, but the tube MUST be operating under its dissipation limits. Dissipation is measured in watts, and it relates to how much heat the internal elements can take. The formula is:

    Dissipation (watts) = (Plate voltage - cathode voltage) x cathode current. Keep in mind that the cathode current is the plate and screen currents combined...

    If you have a tube passing 50 ma (.05 amps) at idle and for purposes of illustration you have 450 volts across the tube, then the dissipation will be .05 x 450, or 22.5 watts. The 6CA7 EH has a plate dissipation limit of 25 watts, and a screen limit of 8 watts, there is no combined dissipation listed on the EH datasheet. So I like to use the plate dissipation as the maximum. 22.5 watts is about 90% of the plate limit, and that's about my limit for audio applications.

    There is more to all this, but for tube replacement this should help. And as was posted elsewhere - BE SURE to dial the bias current down before installing the new tubes!! The bias setting for the old, tired tubes will likely be WAY off for the new tubes - and can cause damage. And remember - whenever you change output or rectifier tubes you must set the bias!!

    Dingojazz

    Posts : 39
    Join date : 2010-09-03

    Okay good to know, thanks for replies. One question;

    Post by Dingojazz on Thu Dec 04, 2014 12:29 pm

    So, Bob are you saying that with the rectifier change (5AR4 to 5U4) no other mod.s needed to run EL34? I.e., just rect. change and re-bias?

    Thanks,

    Kevin

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