#1
Hey, I'm Adam and I just joined this board tonight. It seems like there is quite a wealth of information here!

I am trying to build a copy of the Sovtek Mig 100 H guitar amp. I am learning a LOT about electronics and am trying to do my research before going out and buying a bunch of parts. First, I am trying to figure out what kind of power transformer I need. I have read that you can estimate the total current requirement for your amp by adding up the total plate current used by each tube in the amp. Here is my problem. Please look at the schematic for this amp,

http://www.blueguitar.org/new/schem/sovtek/mig-100h.gif

I count 10 tubes. 4 12AX7's, 4 5881's and two unknowns. These are labelled V3.1 and V3.2. Does anyone know what these are or might be?
I also have questions about choosing a choke and output transformer. But one step at a time. I appreciate any help or comments! Thanks,
#2
V3.1, and V3.2 refers to each side of the 12ax7. The 12ax7 is a dual triode, so it is kind of like 2 tubes in 1.
#3
Ahhh, ok I see now. Wikipedia has a great article, http://en.wikipedia.org/wiki/12AX7

So I take it this means that V1.1 and V1.2 are the same tube, V2.1 and 2.2 are the same tube, and V3.1 and V3.2 are the same tube, and therefore there are 3 12AX7s in this schematic? For a total of 3 12AX7s and 4 5881s?
#4
Yes. 2 of those 12AX7s are in the preamp, and one of them is being used as a phase inverter for the power amp.
#5
Ok great, thanks. So now figuring out the total current requirement

12AX7s are 300mA each
(according to http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12AX7.pdf)

6L6 5881s are 900mA each
(according to http://www.mclink.it/com/audiomatica/tubes/6l6.htm)

Total current requirement
(3 x 300mA) + (4 x 900mA) = 4.50A

But I may want to sub in some EL34s later, in that case
EL34s are 1.5A
(according to http://www.mclink.it/com/audiomatica/tubes/el34.htm)

Total current requirement (with EL34s)
(3 x 300mA) + (4 x 1.5A) = 6.9A


Alright so with that knowledge in hand I've been looking at PTs here, http://www.hammondmfg.com/1608.htm

Now I am wondering how to go about choosing the right one? I guess what I don't understand is how the current value I just calculated can be matched to a particular transformer. I am consulting this, http://ken-gilbert.com/building-your-own-tube-amp. The description of how to select a power transformer goes from figuring out total current straight to recommendations for transformers without describing HOW to pick the right one. Or maybe I just don't understand (more likely!)
#6
Quote by BBBQ
Ok great, thanks. So now figuring out the total current requirement

12AX7s are 300mA each
(according to http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12AX7.pdf)

6L6 5881s are 900mA each
(according to http://www.mclink.it/com/audiomatica/tubes/6l6.htm)

Total current requirement
(3 x 300mA) + (4 x 900mA) = 4.50A

But I may want to sub in some EL34s later, in that case
EL34s are 1.5A
(according to http://www.mclink.it/com/audiomatica/tubes/el34.htm)

Total current requirement (with EL34s)
(3 x 300mA) + (4 x 1.5A) = 6.9A


Alright so with that knowledge in hand I've been looking at PTs here, http://www.hammondmfg.com/1608.htm

Now I am wondering how to go about choosing the right one? I guess what I don't understand is how the current value I just calculated can be matched to a particular transformer. I am consulting this, http://ken-gilbert.com/building-your-own-tube-amp. The description of how to select a power transformer goes from figuring out total current straight to recommendations for transformers without describing HOW to pick the right one. Or maybe I just don't understand (more likely!)
you definitely don't understand.

You're taking the maximum current the devices can handle and adding them all up. so your current estimate are excessively high.

there's NO WAY you would ever need to supply 300 mA to a 12AX7 used in a preamp. it will NEVER see that kind of current. the plate resistor for each triode will limit the maximum current possible in the circuit. using a 300V supply (kinda high in the preamp) and a 100K plate resistor, the maximum current in that triode can never be more than 3 mA. NEVER. and that would only be for a brief moment it each cycle. and the next and previous stages would be approaching cutoff while that stage hits saturation. so their currents would be at minimum.

bottom line: use supply voltage / plate resistor to get the max current in the circuit and use that to calculate your budget. it will be more than enough.

next, lets look at your calculations for the output tubes:

two of the tubes will be in saturation (max current), while the other two are in cutoff (no current) so even if you calculate according to device capacity, you need only consider the current in 2 tubes, not 4.

and you are not gonna see 900 mA in each 5581 with a serious voltage on the PT, else you'll redplate the tubes and shiz will start failing, big-time. something like 200 mA would be about right for the peak current.

a reasonable estimate would be 100W / 450V = 222 mA. if we calculate a horrible OT efficiency of 50% that brings you up to 450 mA required. There is no reason you would need more than 0.5A to be supplied by this transformer. 750 mA would give you plenty of headroom. 6.9A is not only ridiculous, it would cost a fortune.
Meadows
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#7
Hey. Kinda set this on the back burner for the last few weeks but now I'm back at it. I want to build myself a copy of the Sovtek Mig 100H 100W guitar amp. A schematic can be found here,

http://www.blueguitar.org/new/schem/sovtek/mig-100h.gif

Still working on how to select the proper power transformer, output transformer and choke. I'll try to break this up so it's a little more organized,

So I found a source where the B+ for the output tubes before the screen resistors was measured at 485VDC. I've posted the link to this info below,
http://archive.ampage.org/articles/0/ampdiy/012448/Review_of_The_Sovtek_Mig_100-H.html

And from the schematic the plate resistors are 750 ohms, so 485V/750ohms=646mA
This still seems high though. There are four tubes and four 750ohm resistors, one for each tube, I'm probably not accounting for something...

Choke selection:
485V/750ohms=646mA
This is the information I need to select my choke, right? So I took a look at Hammond chokes found here,

http://hammondmfg.com/153.htm

It would look like part number 159Y would be ideal. here are the specs,
Max. DC voltage: 500V
DC current: 750mA
Resistance: 11ohms

Output transformer selection:
My selection here seems a little too simple. This is a 100W amp. Again I went to Hammonds website and their selection of classic push pull tube amp output transformers,

http://www.hammondmfg.com/1608.htm

It would appear that the 1650T would be what I need
Audio Watts (RMS): 120
It's what they suggest for 4 or 6 5881 tubes

Alright, am I good so far? Have I learned anything lol? Ok then, last but not least,

Power transformer selection:
Dear god. Ok so first the filament heaters, there are two. The first one is 6.3VAC and it heats the 4 5881's and 2 12AX7s. So ther requirements there should be 1/2(4*900mA)+2(300mA)=2.4A @ 6.3VAC

The second filament heater is only for the first 12AX7, so for that part 300mA @ 6.3VAC

About the filament heaters. Will each filament only draw as much current as it requires? Like for the 5881s and 12AX7s that run off the same heater, what stops the 12AX7's from taking in too much current??? Or if I use a filament heater power supply that is rated for 3A and the tubes need only at max 2.4A, would that be a bad thing? If not, why?

Then the high voltage secondary. So from what I figured out before (B+ is 485VDC, 646mA), this is the info I need for secondary selection right?

And finally the bias tap (I believe thats what it's called). I'm kind of stumped there, haven't thought much about that one yet...

Thanks for all the previous help! I feel like Ralph Wiggum when he says "I'm learn-ding".
#8
wait, obviously the grid voltages are what controls the amount of current going to the filament heaters