Power transformer in ac15 BUILD

[Oddvar R]

Hi. I am just about to build a AC15 type amp from parts I have lying around.

I have a Weber w022855 40 watt Output Transformer, 125A9A I was thinking of trying out. I have a 25 watt PT upfront.

What would be the downside of using this combination?

[pompeiisneaks]

That weber transformer is setup for a set of dual 6L6's that want about 4-5k impedance on the secondary side. (they're often about 4.2k I think?) and the ac15 wants more like 6k and are often setup about that way. So you're going to mismatch impedance a bit. That means the tubes may be stressed a bit, or may just not sound optimal...

As for the PT, saying it's a 25w pt doesn't speak to how much heater current it can handle or how much B+ current etc. So there's no way to know what it can 'do' without knowing what it can 'do'

Edit: typo, replaced 'speaker' with the correct word 'transformer' lol
~Phil

[Stevem]

The original Vox OT is 6200, so if you use that Webber OT you had better have the plate and screen voltage some 15 to 20 percen down from where Vox runs things if you do not want to have very short lived output tubes.

[Oddvar R]

So the Weber is for the closet then.

Ihave a Marshall type 18w Ot, would that be suitable, combined withe the PT on the pic?

[pompeiisneaks]

Yes I think that type of 18W build is designed for EL84 tubes so that should work. As for the PT, if I'm doing the math right, for a classic AC15 (with no other schematic to go on other than your note of 'ac15') it has an EF86, 4x12AX7's and 2xEL84's... the current for these tubes, if my math is right, is

EL84 = 760mA x2 = 1.52A
12AX7 = 300mA X 4 = 1.2A
EF86 = 400mA

That's 3.12 A and the PT is specced at 2.5A so you're over on current ratings. As for the B+ voltages, I think you might end up a little high for the AC15's expected 315VDC at the anodes of the power tubes with a 325/325 output if sent through solid state rectification... Also, that PT has 5V tap for the rectifier but the AC15 used the GZ81 (6CA4) rectifier which is expecting 6V for the heaters not 5 like yours has, so you'd have to either go solid state, or use a different tube rectifier... and adapt from there.

Basically you're deviating a LOT from what an AC15 wants, and without more detail on the exact circuit you want to build, I can't even guess farther than the variations I'm supposing above.

I'm guessing if you want to use the transformers, you might want to adjust the amp to have a bit less on the preamp side, maybe not have the trem channel? that might bring your heater current down, tube rectification with something like a 5Y3GT on the 5V winding might put the voltages closer to what an AC15 expects... 325 * 1.1 = 357.5VDC - voltage drop of 20V gives 337 VDC... approximate of course.

OR stick with solid state and use a mosfet or other kind of voltage dropper in the circuit. (some just use a zener diode to drop the voltage...)

These are the 'problems' you run into while trying to throw random transformers at a project, and it's something I'm still learning... so above might even be a bit off somewhere...

Hope that makes sense... do you have a specific schematic you're going to build from?

~Phil

[Oddvar R]

There's a guy called Stephen Grosveneor who ha written a book called "A service engeneer's guide to the Vox AC30 valve amplifier". It aslo include info on ac15.

I have enclosed the layout in his book.

[pompeiisneaks]

So that's not a schematic, BUT it does seem to mirror the expected AC15 I was referring to, which does imply you have problems with the PT. BTW I've built an AC30 that's the exact same as this amp except for 4xEL84 tubes and a different OT. I recommend you look for a more appropriate power transformer, as the one you've got doesn't have the heater capacity for the 5 preamp tubes and the power section, as my math denoted. If you're building the one I think you can see the schematic from sluckey's website: https://sluckeyamps.com/VAC15/ac15.htm

Mine was basically redone by him to add the additional power tubes etc, so mine ended up almost identical to his. Love the amp... Just think you might want to either:

1. buy the right transformer(s)
2. choose to build a different amp.

Up to you.

~Phil

[Oddvar R]

I will follow your advise, wiseley as allways.

[Oddvar R]

Here is the schematics.

[pompeiisneaks]

Yeah that looks, upon a quick review, to be the same schematic Sluckey and I have used, but just redrawn. I've not done a component by component comparison.

Again, the transformers you have currently can't take the heater draw that amp requires. The OT is a decent match but not an exact match. As you mentioned it's designed for the 18W output of a marshall style amp, and probably is biasing the tubes differently than they are done in the AC15.

It should still work, but if you've already got to buy the right power transformer, it wouldn't be a horrible idea to buy the right OT. But of course that does add significant cost.

~Phil