adamamp said:
The 2 mA. idle current was a seat of the pants guess. I wanted a Class "AB" operating point fairly close to Class "A" operation.
R = V/I
R = 6/.004
R = 1.5 KOhms
BTW, the idle current dissipation in the shared cathode resistor is 0.024 W., as P = I^2R. A 1/2 W. rated part should easily accomodate full power heating.
The max peak to peak drive is 12 V. That "equates" to 4.24 VRMS. Increase the grid bias voltage to accomodate higher drive levels. The "downside" of increased grid bias is reduced idle current and an operating point that is closer to Class "B". Max power O/P increases as you approach Class "B" operation. You want to be sure you do not exceed the power capability of the O/P traffo.
6360?
12AT7 has Rp around 10k at the lowest, so you'll need like two triodes in parallel for your 22k OPT. (Where'd you get something like that anyway? Universal OPT? Freq. response can't be terribly great at that impedance...) I'd recommend a low-Z input source so you can overdrive some grid current, but that's probably not going to be possible. AT just doesn't really have as much current as I recall, so good luck on getting 1W or so. Have fun!
Tim
12AT7 has Rp around 10k at the lowest, so you'll need like two triodes in parallel for your 22k OPT. (Where'd you get something like that anyway? Universal OPT? Freq. response can't be terribly great at that impedance...) I'd recommend a low-Z input source so you can overdrive some grid current, but that's probably not going to be possible. AT just doesn't really have as much current as I recall, so good luck on getting 1W or so. Have fun!
Tim
Sch3mat1c said:
Lorenz ELL80 (essentially 2 EL95s in 1 envelope) -- good for 2 1/2 > 3 1/2 W / channel
dave
Eli Duttman said:
Eli you're da Man.
Thank you for bearing with my poor math and spelling. Not sure what the ^ means in your power eq. Is it the same as
P= I squared x R?
Why is reduced idle current a bad thing? It seems to sound colder at lower volume settings but is there something else😕
planet10 said:
Nice Dave, where was it made? Those OP tranformers look familar. I have a small Phillips stereo that runs SE EL95, nice tube for low power.
Yeah, P = I^2R means P = I squared x R.
The closer to Class "A" you run, the less worrying you do about crossover distortion. I also tried to guess an operating point at which clipping would occur before the O/P trafo core gets saturated.
The closer to Class "A" you run, the less worrying you do about crossover distortion. I also tried to guess an operating point at which clipping would occur before the O/P trafo core gets saturated.
Sch3mat1c said:
Right on, thanks for that Tim.
Hammond has been selling them for years. Several models to choose from, 3 to 15 watts but definately not HIFI. Fq bandwidth is about 150 to 12000 HZ. Good enough for guitar and experiments. They do make a special line for SE but not sure if they have an air gap or if the Fq is any better.
I have decided to bread board a couple of poweramp options before building it into a cab with a preamp.
Tell me more(if you dont mind) about your ideal input source. Are you talking bout a cathode follower before the driver/PI or 2 followers after the PI, one to feed each side of the OP tubes?
PS, Man I love this place,
THANKS
diyAUDIO
🙂
Eli Duttman said:
That explains that colder sound(crossover dist) I experienced when biasing for maximum efficantcy due to power supply restrictions. The one thing I noticed about class A is the large idle current really heats up your power trafo, but I do prefer the sound, even in a low fi flee flicker giter amp 🙂
So when you pick a operating point for this type of class A(but drive it AB if needed) is it like a simple % of the maximum current or does it change with other variables?
Please Sir, I want some more
Yeah, ideally you want a low-impedance signal source to man-handle the wimpy grids and get that tube going. It could be done with a preamp and IST, or cap-coupled CF with a grid choke to sink the current. Any plate follower or resistance coupled topology won't cut it though.
Tim
Tim
adamamp said:
It is out of a Normende hifi made in West Germany -- the OPTs are often called Telefunken, but i suspect they just come from the same OEM... most of the German OPTs are, at least externally, much better looking than the typical north american fare.
dave
Sch3mat1c said:
Would I get any benifit using a simular CF to the one in the SEP amp above? Extra stages are unfortunately getting away from my idea to use a twin triode for a low current drain poweramp. If this is just for guitar and I use 10K grid R and say 1K grid stoppers will the OP tubes survive
Eli Duttman said:
Can I bug you for a wee bit more Eli?
🙂
A. So if I have built SE amps this way are they saturating the trafo even at lower volume? Could not early OP SAT in a guitar amp be a good thing
?😕?
B. Please don't laugh 😀 How do you determine cutoff
?😕?
😎 Please and thanks ahead 😎
planet10 said:
Do not know who made the OPTs for my Philips but they are real nice. Not enough power for bass without K horns, just a little floppy with them😉
adamamp said:
Philips is big enuff (and wide enuff in scope) that they likely built them themselves. The Philips OPTs i have seen are distincly different from the sameness of the ones found in Normended, Lowe-Opta, Imperial, Grundig, Telefunken, and more obscure German makes.
The most impressive German OPTs i've got are out of an East German console -- EL84 SE the size of the OPTs in a Dynaco Mk III -- they totally dwarf anything else i have. Another of my favorites are some diminuative EL95 OPTs that were used in a "2x Sat/Sub" combo for mid-tweeter use only... almost jewel-like ... they should make a nice little tweeter amp.
dave
>>How do you determine cutoff?<<
From the spec sheet. A tube biased to cutoff is not conducting. As a practical matter, Ib of a few micro-Amperes can be regarded as cutoff.
BTW, it's allowable to bias a tube past cutoff; in which case, the tube conducts less than 1/2 the time. This state of affairs is Class "C" operation. Class "C" is very efficient, but it's non-linear. Class "C" operation is fine in a RF power oscillator.
>>Could not early OP SAT in a guitar amp be a good thing?<<
I don't know, as I'm not a guitar amp guy. Since guitar amps are controlled distortion machines, perhaps OPT saturation is good for "tone".
From the spec sheet. A tube biased to cutoff is not conducting. As a practical matter, Ib of a few micro-Amperes can be regarded as cutoff.
BTW, it's allowable to bias a tube past cutoff; in which case, the tube conducts less than 1/2 the time. This state of affairs is Class "C" operation. Class "C" is very efficient, but it's non-linear. Class "C" operation is fine in a RF power oscillator.
>>Could not early OP SAT in a guitar amp be a good thing?<<
I don't know, as I'm not a guitar amp guy. Since guitar amps are controlled distortion machines, perhaps OPT saturation is good for "tone".
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