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Old 2nd September 2005, 04:17 PM   #11
mr mojo is offline mr mojo  United States
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I should clarify by stating that the 0A3 can never be used to keep an exact ratio since the 75v drop is a constant. What I was wondering if there would be some OTHER way of keeping the ratio exactly at 1:1.9 ratio.
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Old 2nd September 2005, 04:38 PM   #12
ilimzn is offline ilimzn  Croatia
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Yes, resistive divider with the aforementioned ratio, feeding a current gain stage of your choice (cathode, source or emitter follower)...
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Old 7th September 2005, 06:59 AM   #13
PRR is offline PRR  United States
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SY is of course right. And if you are graphic-brained, you can graph the voltages and see that G2 will fall to zero while Vp is still ~75V; or if Vp rose to 2,000V(!) then G2 would be 1,925V, "same-as" 2,000V for any practical purpose.

The misregulation is not a problem in normal operation. I'm being picky about inelegance, and excess bottles. But then, excess bottles is not always bad. And not everyone can enjoy gas-tube glow.

> for example true screen regulation held at a steady 405v. If B+ is at 475v and for some strange reason were to dip to 400v, then the grid will conduct current rather than the plate.

No. In Pentodes (including the Power Tetrodes), the G2 current is a small fraction of plate current for any decent plate voltage. "Indecent" plate voltage would be like under 50V: as the plate current curve drops to zero, the G2 current goes high. But this is also where you can't get much power from the plate. So we pick a load-line that hits near the knee of the plate curves, not over where they are near vertical. Then the G2 current is still small.

Exact ratio of P/G2 DC voltage is easily done with a resistor divider. To ensure no sag as G2 current changes, divider current must be much larger than G2 current. G2 current is much less, but not much-much-less, than plate current. So the simple resistor divider is eating as much current as the plates, which means total chassis heat is doubled. SY suggests a smaller divider with a buffer: I've done that, but it is an inelegant can of worms. Not so bad with a MOSFET buffer, but you still wonder if you really want a Ratio or a Fixed Voltage. Use a tube buffer, and it is a semi-power tube with a cathode voltage so high it needs its own heater winding.

If the transformer is low-sag, but utility power wanders, a tapped power transformer gives fixed ratio of Plate and G2 supply coltage.

But considering the wide range of speaker impedances, I don't see how any plan to hold G2 within 10% of "goal" is justified. "8Ω" can be 6Ω or 12Ω, and will be 50Ω at bass-bump; what matters some 1.7:1 or 1.8:1 "ratio error"? Keep G2 high enough to deliver plenty of load current, or as much as you can afford, first-cost or replacement-cost (guitar amps run high G2 voltage to improve power/weight, at the expense of more tube replacement, but a guitarist is presumably getting PAID and can afford the cost of doing business).

I can find a lot of Old Men commercial designs that used a G2 voltage divider, especially in radios where there is a large current in the RF/IF tubes to swamp the output's G2 current variation. Also a lot of amps where G2 is fed direct from plate supply, or through a small dropping resistor. This forces the plate to run not much higher than the G2 rating: this fact seems to have driven tube design.

For maximum power with easy drive, G2 should be fairly low while Plate voltage should be high. This leads to tubes like 6550 and 8417, where the max-power condition is roughly Vp=600V, G2=300V. Note that a simple ceter-tapped transformer, or a voltage-doubler, can produce solid 300V and 600V with low parts count.

Most of the 7591 designs I recall ran the plate a little but not a lot higher than the G2 rating. Probably around 425V for both, or 450V to the plate and a dropper resistor to give 425V-400V at G2.

> the Westinghouse .... article is geared toward drumming up interest in the then "new" 7591.

I remember.

And they cranked it up to the very biggest numbers you can get from a 7591 without breaking the warranty. What does it claim, 50 watts? A few years before, 36 watts from a pair of larger harder to drive 6L6 was "great", so 50W from small sensitive 7591 is amazing. But most real-world designers tended to get 35W-40W from a set of 7591. Gas tube sales are good for Westinghouse, but bad for profit. (I have seen, here, a PA amp which used the gas-tube dropper trick, and in that case I think it was so they could push the ratings to the very edge, yet have the amp punk-out benignly when abused.)

FWIW: the 7027 has a 100 watts/pair rating; nobody ever got more than 35W-70W out of them. The 100W rating requires regulated 800V: if it goes higher, the base burns up, any lower and you don't get 100W. G2 voltage is also dead-critical and must be regulated. Of course in tube days, a regulator was essentially an audio amp with a DC signal: the same parts used AS an audio amp would double the power. So 100W with 7027 is really done with four 7027 and a 500V-600V raw supply, not two 7027 and a regulated 800V.

The original 7027 100W rating might have made sense in somebody's AM radio transmitter. An 800V regulated supply might have been available; might have aided compliance with regulations; anyway the cost vanishes in 10,000 watt output stages and a 1,000 foot tower.
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Old 8th September 2005, 11:44 AM   #14
mr mojo is offline mr mojo  United States
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PRR,

I really appreciate your time and detailed, thoughtful responses-I've learned a great deal from this thread.

It's forums like this and folks like yourself that make the internet such a great thing-an exchange such as this simply wouldn't have been possible 15 years ago.

All the screen regulation info you've given me will be put to use on the next amp-nobody can build just one!

I'm with you on this particular solution-it's not the most accurate, it's not the most elegant-but when they're not sputtering and arcing, they sure are the prettiest.

Thank you for walking me through all the reasons and implementations for and against regulated screens and regulated plate and screen ratios. This is the kind of little stuff that, built over time leads to a holistic understanding of exactly what's going on in a tube amp and why-and it's these little things, and their implications, which aren't covered in detail in the books I've been grinding my way through.

As for the amps output, the voltages on the circuit are given under no signal. Elsewhere in the article it states the 7591 is capable of 50wpc, but at the cost of higher distortion, less tube life and more elaborate power supply. Artilce has the plates at 450 and gives a 35wpc estimate. Pretty durned close to your approximations!

Today, with the help of our only local tube tech, I hope to bring it on-line with a variac, multiple meters and a signal generator. If this thing actually works I hope to post some pictures this weekend.

Thanks again PRR, I really do appreciate your time and help.

Best,
mr mojo
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