New Power Tubes for Club 40

Discussion - HT Venue amplifiers. Inspiration from Studio to Stage.
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DreamTheater
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Tue Jan 05, 2016 11:05 pm

I currently have stock Ruby EL34HTB, something like that. They claim that they handle higher plate voltage. Question is can I use JJ EL34 Power Tubes safely, and where should the bias voltage be?

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thephantum
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Tue Jan 05, 2016 11:21 pm

Marketing hype. Maximum plate voltage for an EL34 tube is 800v. That's the build spec. Ruby doesn't make tubes. They just buy tubes, test and re-brand them. So they've tested them to max plate voltage. Woohoo. The rest is hype.

Besides, the HT-40 will never, EVER get any higher than 450v at the plates...so it's a moot point anyway. :geek:

As far as bias, there is no one size fits all setting. Either learn how to measure and set it correctly or take it to a tech. More details here: viewtopic.php?f=32&t=5117&p=24742#p24747

DreamTheater
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Wed Jan 06, 2016 12:01 am

Sweet....I'll just buy a matched pair of Regular JJ EL34 then. Do I need to worry about plate voltage, or can I just make the value across the two test points 50-55 mv and call it good?

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thephantum
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Wed Jan 06, 2016 12:05 am

From the link I posted above:
thephantum wrote:There is no "one size fits all" bias setting. Every amp is a little different and every tube is a little different. You need to measure the plate voltage, do the math to calculate desired bias current, measure and set the bias/balance, then rinse and repeat. The reason for this is as you adjust bias, the plate voltage will change slightly. So each time you adjust, you have to measure and adjust again until you get it dialed in.

DreamTheater
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Wed Jan 06, 2016 12:13 am

So if I understand correctly, it's not really the bias voltage that I should be so concerned with, but more so with the plate voltage. Changing the bias effects the plate voltage. If this is correct, what plate voltage am I shooting for.

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thephantum
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Wed Jan 06, 2016 12:32 am

Plate voltage is what it is. It's part of the amps design and it's based on output power. In theory, an amp putting out X amount of wattage should have a plate voltage of Y. However, in practice; resistors, capacitors, etc all have tolerances. In addition, tubes are not perfect...each one is a little different. So actual plate voltage will vary from amp to amp...even those that were manufactured right next to each other. That's why it needs to be measured.

You then use that plate voltage to calculate what the bias current should be based on the tubes being used (EL34's in this case). However: as you adjust bias, the plate voltage will change slightly (Ohms law). Since the plate voltage has changed, you now need to recalculate and reset bias. Hence, you need to repeat the process a few times and is why they call it "dialing in an amp".

What you are actually doing when setting bias is adjusting the idle current flowing from the plate (anode) to the cathode. So bias is a current adjustment, not voltage. However, by measuring it across a 1 ohm resistor between the cathode and ground, you can read it as a voltage.

DreamTheater
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Wed Jan 06, 2016 2:17 am

Ok...I understand the theory now. But how do I measure the plate voltage.

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thephantum
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Wed Jan 06, 2016 3:51 am

You need to measure the voltage drop between the plate (anode) and cathode. For an EL34, that's pins 3 and 8.

DreamTheater
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Wed Jan 06, 2016 4:05 am

Is the polarity of the multimeter important? Is the plate + or -

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thephantum
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Wed Jan 06, 2016 4:14 am

By definition, the plate (anode) is positive. Electrons flow out the plate (anode) to the cathode, hence the term "plate voltage". If you have the meter reversed though, it won't hurt anything. The meter will just read a negative voltage, but the number itself will be the same.

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