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Old 15th June 2013, 08:13 AM   #11
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Thanks for your replies, I appreciate the time you have spent in looking at this.
I will get back with voltages in a day or two.
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Old 15th June 2013, 09:43 AM   #12
AJT is offline AJT  Philippines
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if you say that your builds are working fine, then all is well....
just remember that tubes have specs that need to be observed,
exceeding voltage, power dissipation ratings
are a sure way for tubes to die early...
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Old 15th June 2013, 12:29 PM   #13
Enzo is offline Enzo  United States
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Guitar amps routinely run tubes outside the published specs. We expect to have to change tubes a lot more often than we did for dad's old table radio or TV. Think of it like race cars. They wear out a lot faster than if we drove politely down the street. But if you do that, you are no longer racing.
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Old 15th June 2013, 08:13 PM   #14
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Quote:
Originally Posted by AJT View Post
that's correct, g2 is a mesh wire in a pure pentode, beam formersnin others like the 6L6, a plate is well, a plate, but did you know that the g2 can be used as a plate?
You're talking about g3, which is designed to prevent secondary emission of electrons from the plate from flowing to the screen grid. G2 (the screen) is always a mesh wire grid that is aligned with the control grid. Sorry to nitpick
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Old 15th June 2013, 08:41 PM   #15
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If you look at plate and screen voltages with a scope you will see nice steady DC on the screen, but probably 10 Volts peak to peak or more on the plate. Many times the screen voltage will be between the positive and negative peaks of the plate voltage. Now if you look at the published plate curves of a power tube you will see that the plate voltage can move atleast 100V up or down and the plate current won't change by more than about 5%. So it just doesn't matter if the screen is 5V or 10V higher than the plate when the tube is at idle.
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Old 15th June 2013, 10:29 PM   #16
AJT is offline AJT  Philippines
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Quote:
Originally Posted by AmpKiller66 View Post
You're talking about g3, which is designed to prevent secondary emission of electrons from the plate from flowing to the screen grid. G2 (the screen) is always a mesh wire grid that is aligned with the control grid. Sorry to nitpick
yes, sorry my bad, beam formers are indeed g3 in beam tetrodes...
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Old 17th June 2013, 02:52 PM   #17
Struth is offline Struth  Canada
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Hi Guys

Poor Phil has not received an answer to his original question. So, here it is:

The voltages listed on the schematic are at idle. With a standard DC voltmeter of any type you will see voltages approximately what are listed, or at least in similar proportion.

Measuring the plate voltage under load with anything other than an oscilloscope is pointless. The plate can pull the OT end almost to ground - usually within 40-80V, anyway. At the opposite end of the signal swing, the plate voltage soars above B+ by the same signal peak voltage due to the flyback action of the OT. This is all normal. One might expect that the DC meter reading here would simply give an average - which it tries to do but it will be inaccurate, and again rather pointless.

Keep in mind that every power supply sags a bit under load. Guitar amps are built economically, so a 10% sag is expected and 20% is rather typical. If you measure the plate voltage at the point with your DC meter, you will note that it is much lower than the schematic lists, but so too will be screen voltage and every other voltage in the amp.

Screen voltage in an of itself is not important except that it is what grid control voltage relates to - not plate voltage. Screen resistors drop a tiny voltage that is unimportant overall in the operation of the tube, particulalrly with respect to protecting the tube while clipping. That protection comes from the value of this resistor. The 100R value commonly used with EL-84 is a joke - it should be 1k-1W minimum and better as 2k2-1W. The 470-2Ws Fender uses for 6L6s - which everyone copies - should be 1k-5W. All screen Rs should be flame-proof types - not carbons that burst into flames. 'The Ultimate Tone' (TUT) series books explain all of this in detail, along with tonnes of other guitar amp circuitry.

With respect to guitar amp designs "violating" tube ratings - this does not happen. Tube specs changed in the 1950s or so even though the tubes did not. A more important facet is that the _applications_ do not imply limitations. And still another facet is the blissful ignorance that those apps and the RDH bible follow tube hifi rules, not tube safety rules. Again, our FAQ and books explain this.

Have fun
Kevin O'Connor
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Last edited by Struth; 17th June 2013 at 02:56 PM.
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Old 20th June 2013, 09:08 AM   #18
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Thanks Kevin.

And I am sure there is magic to be found in the screen voltages...

Cheers
JimG
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