Output Load Line

[Steve Conner]

Thank you for the explanation, JM. I agree with your method and result, I also thought 1500 ohms was a bit too low.

As you increase the B+ supply voltage to the tubes, you should find that your calculated load impedance goes up. Reason: the maximum output current of the tubes is a more or less constant limit, set by the screen voltage.

R = V/I. V went up, I stayed the same, so more R.

If you intended to run the KT88s at low plate voltage and high screen voltage, the way 6L6s are used in a guitar amp, then 1500 ohms would be more like it, but power would only be about 70W per tube pair.

[kg]

personally i'd go with the hammond 1650w output tranny and connect it in ultralinear. perhaps i'm a bit biased that way, though.

[J M Fahey]

"hot" biased or "cold" biased?
It's not the same, you know

[Tage]

Yeah with a 4k load I get better results, so I would be looking for a 2k or a bit more OT Primary impedance.

also I didn't realize that were the two main filter caps meet in series that there is half the HT voltage, that would work realy well as a spot to get around 300V for the screens, are there any amps out there that use this method?

Another question if one tube needs the

Delta current=400-55mA=345mA=0.345A

then will 4 tube draw four times that? around 1.4 amps?

I want to thank everyone for doing my designing for me

[J M Fahey]

are there any amps out there that use this method?

Out of the top of my head, without checking schematics, Music Man and the higher power HiWatts (which incidentally used KT88).
Also did, for another reason, those kranky yet good sounding Aussie Goldentones and similar ones, using 6DQ6 as power tubes, which can take thousands of volts on plates but only around 170V on screens.
There was also a classic "100W" Philips design, with only 2 x EL34 , 800V on plates, which my fuzzy memory says *must* have had 400V on screens. Transformer primary impedance was 11K ohms.

Another question if one tube needs the
Delta current=400-55mA=345mA=0.345A
then will 4 tube draw four times that? around 1.4 amps?
Without delving into differential equations which i *hate* with passion (specially now because my son is starting Engineering so I'm forced to refresh them to check his homework and help him) let me explain it in a practical way:
One tube passes 400mA Peak current.
Forget nice sweet sinewaves, we in the MI amplification must consider that the amp *will* be overdriven (ok, maybe not all the time in Blues and such) so our waveform of choice is the easy to math squarewave.
It means our tube will be passing 400mA "continuous" , during *half* the time (it's push-pull) which is equivalent to 200 mA *all* the time.
4 tubes=800 mA equivalent load for your PSU.
If you doubt it, consider two tubes (1/2 the push pull) drawing 800mA "continuous" during half the cycle, then the other two doing the same on the other half; the PSU couldn't care less, it will "see" an 800 mA continuous demand.
With sinewaves on the verge of clipping, it will see 0.7 times that, but remember the MI environement.
With *no* signal applied it will see 4 x 55mA continuous=220mA , which given the 600V idle voltage mean 132W; add heater power plus preamp power, you will easily be surpassing 160W heat dissipation into that tiny enclosed chassis.
Ever wonder why tube amps are so hot?
Better design a PSU which can supply comfortably 1A @ 600VDC nominal, dropping to no less than 520VDC while doing so.
Mind you, it's a tall order !!!
It also shows clearly that the power amplifier is just that funny thing sucking juice from a PSU and passing it to some speaker, duly audio modulated.
One reason for my buying a winder early in my career.
It's payed itself 1000 times along the way.

[kg]

to go along with what JM wisely points out...

typical efficiency for a tube AB1 push pull amp will be somewhere around 40%.

that means to make 200w output power, you'll need some ~500w plate input power.

a watt is a volt*amp, so that means 1a @ 500vdc under load.

that also means 500w pip (plate input power) minus 200w pout (power output) leaves 300w or so lost as heat in the output stage.

there's a reason i hooked up two fans on the BAGA.

[Tage]

Yeah this amp will have adequate ventilation, probably two fans like yours. I have decided to use the PSU from a Highwatt 200watt schematic I have, from the Mark Huss collection. It shows an unloaded HT of 650, so that should sit around 600 under load, heat is gonna be an issue though that I am needing to account for.

[Tage]

So if I will have a 12AX7 and a 12AT7 can some one help me calculate what "dropping" resistors I should use to bring down me 410V to a more preamp friendly voltage? If you want to know what schematic I am working from I got it here

http://hiwatt.org/Schematics/DR_200wPS.pdf

[kg]

the schemo linked is just a power supply, so i can't tell you what the at and ax are pulling for plate current...

for the sake of a discussion let's say your average is probably 2-5ma each

so total 4-10ma

if you wanted a 300vdc supply you need to drop ~100v

v=ir,
100v = 7ma (i just took an average) * r
r = ~14k

now keep in mind this is an iterative process, since as you change the operating point of the preamp tubes, the supply voltage will change too. it's not something to worry a lot about... tubes are remarkably forgiving of voltage stress. if you had the operating points well known then it is possible to get it within a few volts, but you really don't have to.

[Tage]

Yeah I figured that a 10k-22k would get me to a comfortable voltage ie. 300V

Sounds good I got my schematic all drawn. the only thing I'm wondering is what kind of bypass capacitor to use on my first gain stage, I want to pass good bass frequencies so a big cap would be good
HT=250
Ra=220k
Rk=2.2k
Rg=68k
Co=0.068uf

[J M Fahey]

1) You posted the power supply schematic, not the actual preamp.
2) If you don't know Ohm's law, you should not be building a high power high voltage tube amplifier.
Even less so if you are not building a known one but taking little bits here and there and mixing them.
Sorry.

[Tage]

I know ohms law, really just wasn't sure if each tube drew 5ma or each triode, I suppose that would have been the better question to ask. I posted the power amp schem to show were I was getting my voltage, that's all, I suppose showing how my preamp tubes are biased and their corresponding setups would have helped too. I just needed a rough confirmation. this is not my first high power build, in fact I have never built anything below 50 watts, but they have always been a simple clone, this one is taking a lot more thought and testing my knowledge a lot, thanks for the help though.

[kg]

i think juan just wants you to be careful.

i got hit with 450 about a decade ago. knocked me off my stool. didn't touch an amp for a month.

your likelihood at getting a second chance with >600v is pretty slim.