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In Reply to: RE: 7189 and 6BQ5 - Plate Voltage and Dissipation posted by FlaCharlie on July 29, 2016 at 10:57:33
I'd say you're probably right, that the plates of some well-made old stock 6BQ5/EL84's could withstand higher voltages. However, I would still be weary about running a 6BQ5/EL84 into that range (forget new production tubes). Unless otherwise specified through rigorous (published) testing, I would respect the data sheet.
And, it's not the be-all end-all.
The 7189 designation (and others like it) indicates some level of quality control and military/industrial purposing (like the 5881, 7025, Red Bank tubes, etc). Looking at the data, I *think* the 7189 was intended for use in ultra-linear circuits or setups with regulated screen supplies.
Also, we should consider the *7189A*. The 7189A by GE is much more interesting than a regular 7189. Plate goes up to 440V. The screen can operate at 400V. The maximum plate and screen dissipation are up a little. And, there are extra internal pin connections (1+2 for control grid, and 6+9 for the screen).
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May your tubes be lively, warm, and long-lasting. Holy be thy heater.
Follow Ups:
If arcing does not occur, voltage (potential difference) is of little consequence. What really matters is power dissipation by the plate and, especially, the screen grid. Fundamentally, power is defined as I 2 R, but it can be expressed, via Ohm's Law, in other terms.Exceeding power dissipation limits, not mere volts, is what wrecks tubes.
See my remark about OS tubes labeled 6BQ5 being "truly" 7189.
You are making an "apples and oranges" error in comparing 7189A to 7189. The GE 7189A datasheet uses the Design Maximum system, while the TungSol 7189 datasheet uses the Design Center system. Increase Design Center numbers by approx. 10% to get their Design Maximum equivalents.
Fact , the current production EL84M, which is really Russian 6p14p-ev, is both very tough and good sounding. Perhaps the SCARCE and costly NOS is slightly better, but the reasonable price of well performing current production tubes (IMO) ends the discussion.
Eli D.
Edits: 08/03/16
Exactly! You cannot compare tube specs unless you know what rating system was used:
1. Design-center - the most conservative and most commonly used; the most tube-friendly of the three systems
2. Design-maximum - middle level, but needs to be heeded. Look at the specs for a 7591A for instance, they are design-maximum specs on the excellent Tung-Sol datasheet.
3. Absolute-maximum - rarely used, but I can promise you that running a tube close to the absolute maximum is almost always going to be trouble.
Also, the JJ EL84s are pretty stout tubes! They certainly have not trouble with 400 volts and they seem to work well and survive (I'm not a JJ fan, but this is a good tube!) though nothing approaches the EL-84M/6p14p-EV for ability to take a pounding. It is every bit as durable as any NOS IMHO.
So there are indeed current production tubes that are quite happy at 400 plus volts.
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