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Old 17th Jan 2019, 12:36 am   #63
joebog1
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Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: 120W PP EL34 amplifier re-born.

Hi to both Andy's.
Andy biker has said some very sensible things. 6 off EL34/6CA7 are hard enough to get running in known circuits, and without raising the ire of too many people, the eastern bloc versions are not a patch on those from Mullard, and I have had MULTIPLE failures with them, unless you just trickle them along. You can get 100 watts/pair, but they wont last long. To biker Andy's point1: cathode bias is always pretty safe!! Its built in protection to some degree
Point 2: running modern EL34/6CA7 @ 500 volts is the absolute top end limit!! 400 volts or so will still fetch better than 60 watts in pentode mode, and a very pleasant sounding 35 watts in UL.
Dont get too dissappointed with your results so far !!! You have done a masterful job even completing the project!! AND to such a degree of visual perfection.

To my transformers: They are SPECIFICALLY wound to suit 4 off EL34 in Ultra Linear mode with around 400 - 450 volts maximum. They will produce 100 watts @50 Hz with little distortion. They were designed to run around 60-70 watts @ 30 Hz. I did use hard biassing in my design!! around 30-35 volts, depending on your exact HT.
I did incorporate 1K5 G2 resistors between the UL taps and the commoned G2's for each side of the HT.

This brings me to G3 or the suppressor grid. Mullard did a wonderful thing by bringing this grid out so it could be used. Instead of tying it to the cathode as 99% of designs do, tie it to the raw negative bias voltage, that way its impossible to exceed maximum ratings of the valve. Traynor guitar amplifiers ( Canadian firm) used this technique, and are famous for their reliability, even when using Russian valves. They did produce some pretty big amps BTW.

Try a "standard" design, forgetting the CCS thingies! About the only modification I would suggest is to use a largish valve for driving 4 output bottles. 12BH7 comes to mind, also 6CG7.
12 volt heaters in series for the output valves is very easy to implement using single twisted pair heater runs, and couples straight to your 12**7 voltage amplifier, phase splitter/driver or separate phase splitter and then driver. Although for 4 off EL34 cathode followers are not really necessary.
I can send you more theory on all the above mentioned suggestions, especially the work done by Traynor.
I think its time you downed tools for a time and forget the amp. I always find that when I have forgotten the ONE specific aspect of an amp I am trying to improve, my head clears and I can come back to it with an open mind, rather than more running around in circles.

It is a new year and that means a new project.
I have started my pre amp/phono stage. Watch this space.

Joe
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