Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 18, 2005, 05:55:15 am
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> 4 x EL34 in A class
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Leevi
Junior tube assistant
Posts: 128
(3/31/04 8:33 am)
4 x EL34 in A class
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I'm planning an A class power amp with 4 EL34s. I think the plate voltage will be on the level 250-300V and idle current less than 100mA. My question is: Should I use a common cathode resistor together with a cap for all four power tubes or should each power tube have a separate cathode resistor with cap. What is a good value for the cathode resistor(s).
/Leevi
jc
I only work on Fender's
Posts: 694
(3/31/04 5:42 pm)
Re: 4 x EL34 in A class
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I would at least use 2 cathode resistors so that you can remove a pair of tubes to run the amp with less power. Plus you will be able to use 10 watt resistors. If you use 1 resistor for each pair of EL34 I would start with a 150ohm 10 watt.
Jackson Ampworks
Junior tube assistant
Posts: 2
(3/31/04 11:44 pm)
Re: 4 x EL34 in A class
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The spec sheets say that at 250V on the plates that you should bias the tubes at 100mA. Anything higher than 250V I would think it would be best to bias them at 90% of max plate dissipation.
Brad Jackson
Jackson Ampworks
Leevi
Junior tube assistant
Posts: 129
(4/1/04 12:42 am)
Re: 4 x EL34 in A class
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The data sheet below says that on 250V plate voltage and on -14,4V grid 1 voltage the grid 2 serial resistor is 2000 Ohm. Respectively on -13,5V grid voltage the grid 2 resistor is 0.
Does the latter case really mean that I should connect the B+ directly to the grid 2 (without any resistors).
The power provided by the tubes is with that setup 11W which mean that the max power of the amp will be ~45W.
The total load impedance what the tubes see is according to my calculation ~2 Kohm. Do you agree to my assumptions?
www.triodeel.com/6ca7ap2.gif
/Leevi
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