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Old 16th March 2012, 12:34 AM   #1
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Default Power Transformer Limitations

Hey everyone, possibly quick/easy question regarding the specs/limitations of a power transformer. Let's say I have an output transformer designed to run 4 EL34s with a plate voltage of 475 Volts at ~100 watts. If I were to put a higher powered tube in (let's say 6550s or KT88s) that could push closer to 150 watts with 475 volts on the plates would I possibly be putting excessive strain on the power transformer? In my mind you cant get more power like that for free and I'm worried that I'd put my power transformer at risk of an early failure. The other (and possibly more likely) outcome that I can think of would be that simply the voltage on the plates will go down considering there's more current running through the tubes and the amplifier would end up putting out 100 watts at ~435 volts just like it was designed to do. As always, thanks for your help!
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Old 16th March 2012, 01:31 AM   #2
tim614 is offline tim614  United States
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i dont think you get 150 watts by just changing from el34 to kt88 at the voltage that you have, its also depend on the bias point that you run the tubes in.
what is the current rating on your PTX?
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Old 16th March 2012, 01:37 AM   #3
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Sorry, I also meant rebiasing the tubes after change out. el34s are running at around 35mA. if that same plate voltage holds and I rebias the kt88s to a safe level (~45 mA) I believe they are capable of doing 140-150 watts. That question of plate voltage staying constant with an increase in current is what I'm "doubting".
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Old 16th March 2012, 02:16 AM   #4
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To Tim's question, you really need to know the maximum current rating of your power transformer HT secondary. Guessing is risky. Any chance the PTX has a manufacturer name or some numbers on it? What amp is it?
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Old 16th March 2012, 03:39 AM   #5
cerrem is offline cerrem  United States
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A couple of things to keep in mind....
The idle voltage means almost NOTHING !!!!
If you are interested in full power output ..then you will need to know the voltage at full power output...both plate and screen voltage at full power output....Typical musical intrument amps can drop about 100V in some cases....if you have screen resistors than you must account for these as well...for example you have 1K screen resistors and the KT88 tubes pull 28 mA screen current at full power output...then you would say Bye-Bye to roughly 30 volts in addition to what has already sagged...
You can put a bigger engine in a car but unless you increase the size of the fuel delivery system...you ain't going to make much more power...
Here are some real numbers from a classic 100W amp with EL34 tubes...
490V on the plates with 485V on the screens at idle....
At full "clean" power output around 98 watts...the screens are at about 400V and the plates at about 430V......
So you get a rough idea...
The 6550 will only suck up more heater current....and the power transformer will be fine....since this is all calculated based on temp rise for copper loss which is usually subjective according to designer....
The plate load and the voltages will determine the power output...
Chris

Last edited by cerrem; 16th March 2012 at 03:57 AM.
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Old 16th March 2012, 12:06 PM   #6
DF96 is offline DF96  England
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Other things being equal (they probably are not!) going from 100W out to 150W out from the same HT voltage means 50% more current draw. 50% more current draw means 125% more heating in the mains transformer secondary. Unless the existing design is very conservative you are likely to overheat the transformer, get insulation breakdown and shorted turns. You may even let out the blue smoke.
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Old 16th March 2012, 02:28 PM   #7
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Ahh, thanks! The bigger engine but same fuel supply = same power makes perfect sense to me...I'm a mere mechanical engineer trying to wrap my mind around Electrical Engineering 101 (circa 1950 I guess)...that's kinda what I was thinking, I just didn't know if the power tubes would try to pull more power than what the transformer was able to supply or if the transformer gave a certain amount of power and "what you get is what you get".

To DF's point - looking at an EL34's datasheet it shows they pull 1.5A @ 6.3V. A KT88 requires 1.6A @ 6.3V. The transformer I have supplies 7A@6.3V. If I have 4 KT88s and 3 12AX7s (300mA a piece) it gives me a grand total of 7.3A...would that indeed make this a bad idea? Initially when I looked at drawing 0.1 amps more per power tube it didn't seem like a big deal but now that I see this puts me over the designed rating I am more worried about "letting out the blue smoke." Is it now obvious that this is not a wise decision? And if so, what about using a power tube that required less filament power (6L6s for example @ 0.9Amps/tube)? Would the fact that I only needed 4.5A of filament supply now put my tubes' life in jeopardy since they require so much less than the supply or would the tubes only take what they need and then pass on the available power. Sorry for all the questions, really appreciate everyone's help.
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Old 16th March 2012, 02:37 PM   #8
DF96 is offline DF96  England
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Heater power is a separate issue. To a first approximation each valve will simply take what it needs, so no problem with using valves which need less.

Bear in mind that to get more audio power simply by changing valves might not work. You might also have to change the impedance presented by the OPT. This may mean changing the OPT. By this stage you almost have a new amp.
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Old 16th March 2012, 02:50 PM   #9
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To get more power out of my old Fender Super Reverb, we (my Dad and I) went from dual 6L6's to dual 6550's, but added a second, higher voltage, B+ PT to supply the 6550's. Managed to get about 120 clean watts out of 2, believe it or not. But as DF96 says, almost a whole new amp at that point. It was a big job.
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Old 16th March 2012, 03:10 PM   #10
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Cool, good to know about excessive filament supply.

So yeah, I actually do have a new output transformer with a wider bandwidth and the primary was better matched with 4 kt88s/6550s rather than EL34s so that was the actual motivation for switching power tubes (not trying to get more power). I was just worried that by switching to higher powered tubes they might try and pull more power and put excess strain on the power transformer. 4 6550s pushing100 watts is A-OK in my book! Guess that information should have been added in my initial question. If the 7 amps of filament supply is "close enough" then I am all set.
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