high gain amp design

[beinz]

i apologize in advance if this thread is not placed correctly, i was looking for a beginner thread..., couldnt find..., anyhoo

im working on a 1 channel a/b push/pull modeled after the soldano slo100 high gain o.d. stage for the pre,
married with a more or less jcm 800 2204 power stage.

it'll be a 50w w/ 2 x 6550's and 4 x 12ax7/7025's

based on the tube data sheets and known speaker ohms i would be using, i have determined ideal o.t. primary to be around 3.3k @ 8ohms with
a plate voltage of 400v

im ok to this point.

i should have no trouble selecting the ideal o.t,

my question is how does the p.t. affect the plate voltage.
perhaps i am going about this backwards, but if i have an idea of the desired plate voltage i would like to end up at, how do i go about selecting the ideal p.t.?

for similar builds and similar amps, i have seen quite a variance in p.t. specs, (some 175-0-175, some 320-0-320) im just trying to understand how these specs translate to values such as power tube plate voltages.

josh

[big_teee]

Multiply 1.4 times the Primary Voltage.
At 120 Volts AC Wall Voltage.
So if you have a Primary Voltage of 320-0-320.
1.4x320=448V DC.
I built a 2204 With 345-0-345.
My Local voltage runs 123-125V AC.
I'm running 495V.
That's Hotter than I like.
I recommend you measure your Local AC.
Then you can calculate what your secondary voltage is going to be.
I wish I had gone with the 320-0-320.
Good Luck,
Terry

[diagrammatiks]

the slo transformer should be 360-0-360 with a loaded plate voltage of right around 500vdc.

2 6550s with 400vdc at the plates? Seems a little low to me but if you did the math then it should work out.

you'll want about 290-300 unloaded so either at 300-0-300 / 600vct or a 300vac.

[beinz]

thanks guys, i figured it was pretty simple, but found everything except from scouring the interwebz.
i appreciate the help!

[beinz]

hopefully this image attached,

i have been using this schem as a reference point for the power section, it has voltages listed at different points along the way with exactly 120v mains applied.
it has the b+ listed at 468v with a 320-0-320 drake p.t.
468/320=1.4625 this is close to your 1.4, but heading towards 1.5.
is this "1.4" rule a ballpark?
are there other factors inside or outside the amp that could make this number vary?
im assuming that the difference in between 1.4 and 1.46 x is minor

and yes, i think you are right about 400v being a bit on the cold side, i will likely head upwards of 450v... maybe to 500v
ive not used 6550's before, but as b.t. is saying he finds it a bit hot, so maybe 450v to stay on the safe side...

thanks guys

[defaced]

Rule, yes. It's technically correct per the theory. Theory is not reality, so it does not consider all the variables of a system. 320 might be what the PT was designed to deliver, but considering winding variations, it could vary that much. What's the measured PT secondary voltage?

Theory: http://en.wikipedia.org/wiki/Rectifier

[pdf64]

Yes, the specified voltage will usually be at the max rated current. Whereas the schematic voltage measurements will normally be given at static, no-signal conditions.

[beinz]

pdf64) ok that makes sense, defaced) as per the schem the 320-0-320 was at 120v mains, and it says "exactly" 120v, as far as measured.. who knows. i was just using the schem as a reference point.

thx guys

[defaced]

Did you measure exactly 320-0-320 on the secondary?

[beinz]

this is just a design to this point, nothing to measure yet. i was having trouble understanding how to end up at desired plate voltage, and then found this (468v w/320v b+ @ 120v mains)
in the 2204 schem ive been using as a reference and noticed it was a bit more than 1.4. just wanted to double check it!

Did you measure exactly 320-0-320 on the secondary?

[defaced]

Ah-ha. I thought you had iron in hand and was taking measurements.

[beinz]

not yet, hoping to avoid too much messing around by getting it right the first time!

[defaced]

I've got an amp I've been working on for three years that has the same goal. It's on its second prototype, I fully understand where you're at.

[big_teee]

I've got an amp I've been working on for three years that has the same goal. It's on its second prototype, I fully understand where you're at.

So What is it, and What is the Hold Up! lol
T

[defaced]

That's the thing, it's so flexible it's whatever I want it to be. Generically, it's a high gain channel switching amp. In the end, it'll have three or four channels (4 stage high gain, 3 stage high gain, clean and maybe a crunch channel), series/parallel FX loop, programmable options for each channel, foot switch, quad bias, and the list goes one. I should really just finish it up and show everyone.

It's taken alot of time because I've had to design alot of stuff for it, and when I started the project, I knew little more than how to read a tube schematic and *nothing* about MCUs. Here's the tally so far: MCU channel switcher code/boards, relay boards, output impedance selector board, quad bias supply/board, DC coupled MOSFET followers to drive the 4 x KT-88s (still have to test if the MOSFET drivers are worth keeping), effects loop (that's the current task, it's on it's third design, and almost done being prototyped). Then I have to to see if adjusting the cathodyne PI is needed/desirable, design a clean channel (hopefully that will be easy), then button up the chassis design (90% done, but I have to talk to the sheet metal shop around the corner and see if they can make the bends I've specified, otherwise it's off for re-design - again) and have it burnt. I'm almost to the point of just building the damn thing and adjusting stuff later. The design supports that option, but I'm stubborn.

And yes, even with all this fancy shit, it will still sound good. Hence the prototypes.