planning an AC4 + soldano pre as second channel. power doubts

[pietro_moog]

hi guys
i'm planning to build a AC4 with diode rectifier.
i'd like to add a soldano preamp as a second channel. my dilemma is the power supply.
i have 3 12ax7 in the soldano pre, 12ax7 + el34 + ef86 in the vox.
i was wondering about the mA needed in the secondary.
do you guys have an idea?

thanks

[loudthud]

Three 12AX7s is probably only 6 to 10mA. On the heater supply, (6.3VAC) you'll need an extra 0.9A unless you are using some kind of DC for low hum.

[pietro_moog]

thanks
so an Hammond 369jx would be more than adequate, maybe? (250-0-250, 60mA DC, 6.3 vac @2.5A)

[NorCalTuna]

Hey pietro, neat project! I know you didn't mean the EL(34). That's a 1.5a load all by itself! Now, you said an AC4, so I'm going to guess you meant el84 : ^) Now you're lookin' gooood. BTW check out the atomic 16 schematic, theres a lotta slo feel there with 75% less stuff! Also, i think those amps get a lot of their swagger from the phase inverter and the push-pull power section (cuts even harmonics) and their big power, so... i hope your slo/4 ends up edgier than that JCM/Champ i once built!

So, to recap: thud +++ you need an additional .9 for the heaters. Lets add up the heater loads!

EF86: .2
3x12ax7: .9
EL84: .75

So, you need yourself 1.85a of heater current.

2.5a of current and you're on the team.

As for the plates, your el84 (right?) wants 30-35ma class a at 350v ((250+250)x.707=353.3v+-) or so, and the soldano's gonna want 10ma or so but- somebody jump in here in case i'm all shades of crazy- I think each ef86 draws about 5-6ma. Thats 50ish, so....

http://www.hammondmfg.com/300series.htm

That transformer- which is 69mA, I think- seems like it should be a good choice and could make something like 6 watts fully cookin. It also will allow some playing around.

[pietro_moog]

hi. thanks, sometimes when i'm tired i get confused. yeah, i mean el84.
thanks for your help.
the transformer i mentioned is a good choice for me because is cheap at Tube Town (EU). cool.
the Slo/4 (good name btw, i'm gonna steal it) gotta be 5 watts because i mean to use it indoor, in my house mainly.
i thought about the 15W idea, maybe something like tiny terror with triode/pentode switch, but i think it's gonna be too loud.
i can break balls to my neighbors still with my 5W princeton tweed.

so. i have a few questions about the power supply design.
my idea was connecting the secondaries to a 4 1n4007 bridge. from here start 2 rails, one for the AC4 circuit, and the other for the soldano preamp.
i think i have to adjust the resistors values in the RC filter segments to match the needed voltages. (ohm's law)
i think the capacitors original values are fine.
i think i should mind the mA absorbed by the tubes in every step of the rail in the math
is all of that correct?

do you think i should use a 15 watt OT to prevent core saturation? ( i mean, is that a good move?)
maybe it's not important and i should use a 5W OT ?
i'm not sure. i thought a 3 tube preamp in a SE power amp could be a little nasty.., but maybe it's not a bad thing..
do you guys have similar experience?


thanks guys

[NorCalTuna]

I'd figure out wether I wanted a push pull amp or a se one. You could build a micro push pull with two halves of a 12at7 or the like if you wanted very low wattage push pull. Also, small amps attenuate well- I have a -10db speaker cable made of a few big resistors in an inline box- and that's not as bad as trying to tame a big amp- so there's that route too. Then you don't get some unusably small amp out of the deal, either.

As for the power supply, I can see why you'd want to run the EF86 off a separate rail, but the stages will probably stack just fine with some small changes. Don't sweat the milliamps drawn so much, you'll probably be limited to a certain amount of fudge by resistor selection anyhow. If you clone the power supply sections you want, as long as there is sufficient voltage where you tap it, ie equal or greater than what the original design had, the only value that probably needs much work is the resistor that couples the cloned section to the source. The rest of the resistors will likely be ok or just need a little adjustment, as will 16 or 20 uf caps or whatever mikey put in there. Don't sweat those values as much as layout and having a firm- but flexible- plan to start from. There's some things you KNOW will likely stay, they're gospel- like the 100k plate resistors or something. The power supply resistors then get adjusted to get everybody on the team.

It's time to draw a schematic. It's not hard, just copy the grafts. Post it and then we can get moving on this. The OT will be determined by your power amp architecture decisions (don't forget to re-evaluate your filament current!) but there are some- like the Hammond 125- that can pretty much cover any base ok- and some really, really well.

[pietro_moog]

thanks.
so, i think i wanna build a SE amp.

this is the schematic i'd like to use.
i like the idea of separate rails, i could just cut out the soldano pre in the case i change my mind, without affecting the vox. (am i missing something?)
i have to do the math about the values. so you are saying i should just adjust the first resistor of both rails? the others should be fine?
i think i still have to change the values of the soldano pre rail's ones, as i cut off 2 other channels. (am i correct?)
so caps are fine.

do you think a hammond 125CSE could be a good choice?

[pietro_moog]

now pics are fine.
i would just use the channel rhythm of the soldano.

i didn't notice that: a few post before you said you hope this thing to be edgier than your tweed/jcm800
did you mean that your amp sounds muddy? is that the SE structure?
how do your amp react to the jcm preamp?

[pietro_moog]

hi guys. i've done almost all the math.
i need the last info. how can i calculate how much mA my output transformer (Hammond 125 CSE) absorbs?
it's connected to the power rail of the ac4 and i need to find the value of the last resistor

thanks