Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Miyagi_83 on December 15, 2020, 10:09:05 am
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Hello everyone,
On Merlin's website I found info that for a Class A amplifier the B+ must never exceed half the permissible peak plate voltage, and the reasons for that are clear to me. Now, some tube datasheets tell you exactly what value that is, but others don't, unfortunately. Is there a rule of thumb to determine the maximum peak plate voltage?
Another thing that I've been trying to figure out with regard to the above is whether the max B+ peak for an audio amplifier is the same thing as for a vertical deflection amplifier because, e.g. the datasheet for the 6AQ5 provides such info for a VDA, but there's nothing about it in the audio amplifier section.
Thanks in advance for the help,
Maciek
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Not familiar with the 50% B+ rule for Class A. Can you cite to that?
"max B+ peak" is confusing me. "Peak" refers to AC, but B+ is DC. Max DC is just max DC. I think it shouldn't matter what purpose the tube is used for.
The tube manufacturer should state the max B+ plate voltage (among other max specs).
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If the *DC* plate rating is, say, 300V, and you work it there, and the tube is good, and the amplifier is also, and signals are symmetric, then the peak plate voltage will be almost twice the DC plate voltage.
Symmetry is true for Sine and all normal speech/music signals over the life of the tube. Male speech and a few other sound signals have short-term asymmetry but not long enough to bother the tube.
TV sweep signals can be VERY asymmetric, with some TV set performance parameters getting better with more symmetry, and then they have to tell you those limits. But general audio is NOT Sweep or Switch work.
And with few exceptions, you will burst your output transformer insulation before the tube. That's the Herzog; run an amp OT with no load, beat the heck out of the tube, the switching fly-back kicks go from 300V DC to 550V normal peak to 1,500V spikes.
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Thanks for the replies, gentlemen.
Not familiar with the 50% B+ rule for Class A. Can you cite to that?
Sure thing. The info comes from
http://valvewizard.co.uk/se.html
"If you are wondering why it appears that the signal voltage can now swing higher than the HT voltage, it is because this is exactly what happens! Inductances abhor changes in current. When current through the transformer increases it stores energy, which is released when the current falls again, allowing up to twice the HT voltage to be developed. Because of this, the HT in a Class A amp must never be more than half the maximum peak anode voltage rating of the valve, given on the data sheet. For the EL34 this is 2000V so we are well within safe limits!"
Just now I had a look at a few EL34 datasheets. The 2 kV limit Merlin refers to is marked as Va(b) max by Mullard and Ua0 by JJ, so that must be the supply voltage when switching on from cold. I could swear, though, that I saw a tube datasheet which states Vp0 max only 50 V higher than Vp max. I'll report back as soon as I find it.
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If the *DC* plate rating is, say, 300V, and you work it there, and the tube is good, and the amplifier is also, and signals are symmetric, then the peak plate voltage will be almost twice the DC plate voltage.
Thank you, PRR. I guess that answers my question.
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I could swear, though, that I saw a tube datasheet which states Vp0 max only 50 V higher than Vp max. I'll report back as soon as I find it.
Found it. The Russian 6F3P (6ф3п) ratings for the pentode section are:
Vp max = 275 V
Vp0 max = 300 V - so that's only 25 V more than the maximum operating voltage
But they also provide
Vp impulse = +2.5 kV / - 200 V
I'm a little confused right now, to be honest. :w2:
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look for a duty cycle spec. I hammered 500w tubes at 25kv all day long with a 10% duty cycle and temp restrictions, not audio though :icon_biggrin:
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> (6ф3п) ratings for the pentode
Doesn't seem to be aimed at POWER delivery; choke/transformer coupling.
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Thanks a lot for all the hints and information.