12AX7, voltage?

Eoin · Post by **Eoin** » Wed Mar 09, 2005 6:20 am

Okay, got some spare cash at last, so I'm looking at getting a transformer for this preamp.

I'm planning on leaving the two filter caps in the design, and separating the pre stage from the power after the PI (at the junction of R22 and R25 on schematic 78331).

So what I need is a transformer, a rectifier, and some filtering. The rectifier is going to be a pretty straight forward bridge rectifier. But I'm not sure what voltage and current I need for 3 12AX7s, and I'm not sure how to scale down the filtering to suit. I've looked at the Duncan Amps PSU designer but I don't understand it, tbh.

Can anyone help me?

Billy Batz · Post by **Billy Batz** » Wed Mar 09, 2005 9:55 am

You can use a full wave capaciter input. You can use something from the Hammond classic line like the 270DAX. You have your input filter around 20-40uF and take each of the following filters parallel not series. The other filters can be something like 16 or 20uF.

Eoin · Post by **Eoin** » Wed Mar 09, 2005 3:02 pm

Is there a formula or formulas for calculating the changes in voltage and current caused by filter capacitors in various positions?

What's a fullwave capacitor input?

Billy Batz · Post by **Billy Batz** » Wed Mar 09, 2005 4:08 pm

Yes there is. It depends on what kind of input you have. A full wave is just having each AC secondary rectified by a series diode like a 50w Marshall or a Fender only Fender uses 3 in a row. With a full wave and a 40uF cap input Im pretty sure it would be around 1.55 x the rating of the AC secondary. Maybe somebody can chime in to verify that for certian?

So if you have 240-0-240 it would be 240x1.55=372v. Something in that range. Dont be that crazy about the transformer. Your drawing a small amount of current with a preamp like this so you can drop a lot of voltage before you even dissapate more then 1W. Get one as close as possible but its no problem if its too high or too low by a little bit. Theres a few in that line that would work though obviously one will be closest. All the Hammonds in the classic line that supply that kind of voltage will be rated fine for everything else. Almost all the classics have a 6.3VAC winding with 1.5A or more. Im assuming you know your ohms law to figure out what resistance youll need to drop the voltage you need? What voltges are you shooting for?

Eoin · Post by **Eoin** » Wed Mar 09, 2005 8:28 pm

Eh... I guess about 300 volts on the anode. What would be normal operating condition in a Superlead?

There's a place in Antrim (Northern Ireland) that does custom transformers, and I want to get a quote from them for something as well. I'm in Ireland, so it might work out better for me. The preamp will be 2 channels (so in total 6 12AX7s) and I'm of two minds about whether to use two power transformers or just one. The plan is to make one of the channels optionally powerable from the poweramp instead - the poweramp is going to be a whole separate unit, but with a standard superlead power transformer.

I thought to use a bridge rectifier so I could get greater output from the transformer and ultimately keep the weight and cost down. No prob with the ohms law, but was hoping to get pretty close with the right tranny and filtering arrangement.

'Cept the mathematics are eluding me.

Billy Batz · Post by **Billy Batz** » Wed Mar 09, 2005 9:02 pm

A transformer of that size isnt going to make or break you a few volts this way or that money wise. Its not like an amp PT. It will be very small. About 2.5 inches square. For a Hammond PT its about $30USD. Im not sure how much it would be to order in Ireland or what that custom place would charge you but theres probably Hammond dealers in England or other manufacturers that make similar trannys.

I cant remember exactly off hand but I think the voltage for V3 on a Marshall is typically around 280v-330v. Im really not sure what an optimal voltage is for a PI.

You should have no problem powering 2 of these preamps with the same type PT. Just be sure the 6.3V supply is at least 2A. How will the second preamp be powerable from the poweramp? Are you going to run the B+ accross a patch cable?

Eoin · Post by **Eoin** » Thu Mar 10, 2005 6:24 am

Basically, yeah. There'll be two cables, one containing the B+ and earth, the other containing the 2 PI outputs, the negative feedback, and earth. Might include a lift for the earth in the second cable.

Just looking at the Hammond, most of those only have a US voltage primary.

We have 220-230v here. I'll look at their other series' and see if something else suits.

If I go with something along the lines of 260-0-260 VAC, what kind of minimum current should that have for all 6 valves? Getting a 6.3v tap with 2A or more seems common enough anyway.

Billy Batz · Post by **Billy Batz** » Thu Mar 10, 2005 8:17 am

Your gonna have a B+ of 400+v with a 260-0-260. Thats a little too big. You dont wanna have to drop 100v.

Any transformer that supplies this much voltage is probably going to be fine with current. I would estimate it around 20mA. If you look at that Hammond classic line they dont even make one that supplies less then 50-60. Im sure other manufacturers would be similar so you'd be fine there.

So you want to get a PT that would be fine for both just in case, even though you plan on using it for one at this point?

Basically, yeah. There'll be two cables, one containing the B+ and earth, the other containing the 2 PI outputs, the negative feedback, and earth. Might include a lift for the earth in the second cable.

I really dont know but Id find out about complications with noise doing something like that. Im not only talking about your quasi-balanced PI output setup. That cable carrying the B+ would be a bit dangerous. I would use something like a IEC plugs and connectors going back and forth or those twist lock ends used on pro audio speaker cable.

Eoin · Post by **Eoin** » Thu Mar 10, 2005 8:49 am

Well, I intend to use the preamp for other applications, but when using it with the amp I want it to run off the same power as the amp so the system functions closer to normal Superlead parameters. The potential danger of the power cable has occurred to me alright. I'll probably use something industrial for it.

As for noise etc, it did occur to me also. I guess I'll build it and see. As I see it, worst case scenario is that the parts turn into a regular SLP clone instead.

The 270DAX you mentioned is a 260-0-260, what should I be shooting for instead then? I'm still confused by the effects of different filter caps. Could I could build an exact replica of the standard Marshall power supply, but scale down the capacitance values and exclude the choke? For the above reason I want to leave the last two 50x50uF caps in the preamp/PI stage.

Billy Batz · Post by **Billy Batz** » Thu Mar 10, 2005 9:30 am

I was just giving an example I didnt know the speca of that tranny. The secondary you need would probably be closer to 220-0-220. You cant go wrong with that. If you order it from another manufacturer just tell them your specs:

- Primary of 220
- HV Secondary is full wave(or bridge) rectified, capaciter input to yield 300vdc
- 6.3VAC winding

You could use 50 for the preamp but you dont need to. It wont radically effect the tone. For this application 20 would be fine Im sure.

Agian someone else please feel free to chime in.......

myker · Post by **myker** » Thu Mar 10, 2005 9:58 am

you can use 2 transformers like we are with our SLO preamps. i believe i gave the model numbers from mouser in that thread. it is a three tube preamp...
553-FP230-25 - 230VAC @ 25mA (series)
553-FP12-475 - 6.3VAC @ 950mA (parallel)
they are about $12 each.
mike

Billy Batz · Post by **Billy Batz** » Thu Mar 10, 2005 10:04 am

You could do that. They would be perfect. Id get a filament tranny with a higher current rating though if your gonna use it for 6 tubes.

Eoin · Post by **Eoin** » Fri Mar 11, 2005 1:23 pm

Thanks for the advice so far. I've taken a few readings from my Superlead and studied the schematics some more. I think what I want to do is impossible.

When the 12AX7's are in, the voltage at the point I want to insert the new power supply is 356VDC. With the 12AX7 valves out, it's 464VDC. There are 7 resistors (in various places after my drop-in point) whose purpose is to reduce the voltage hitting the 12AX7 plates. I guessing that if I drop in 300 volts dc, it will be reduced to something unusuably low. If I remove or change the resistors to accomodate a low voltage, the preamp will fry if I try switching to the amp power.

So I'm thinking it's one or the other - Or else I need a transformer with about the voltage as the full Superlead transformer, but needing to put out much less current. Thoughts?

My next question is, relating to the simpler alternative: does a standard 100 watt Superlead power transformer have enough current to accomodate another three 12AX7's? Getting a little tran like the one above for the extra filaments would be easy enough anyway.

Billy Batz · Post by **Billy Batz** » Fri Mar 11, 2005 1:56 pm

Eoin wrote:When the 12AX7's are in, the voltage at the point I want to insert the new power supply is 356VDC. With the 12AX7 valves out, it's 464VDC. There are 7 resistors (in various places after my drop-in point) whose purpose is to reduce the voltage hitting the 12AX7 plates. I guessing that if I drop in 300 volts dc, it will be reduced to something unusuably low. If I remove or change the resistors to accomodate a low voltage, the preamp will fry if I try switching to the amp power.

That would be your main concern. Those 3 12AX7s draw current and when you take them out you have to recalculate the voltage drop accross each resister in the poweramps B+ rail. It takes a higher resistance to drop voltage when you lower current draw so your voltages go up. Quite a lot from your measurments. You can try to average it out somewhat but youll probably be dealing with a less invasive preamp if you just have it powered independently.

You wanted to keep the power supplies tied together so it wouldnt alter the tone as much right? I dont see any other reason for doing it but to accomplish something that probably doesnt have as major an effetct on tone as you think, you may have to do something that will change the specs of the amp a lot more. Can you tell me why you want to do it this way? By that I mean how is it you intend to use this thing that you want to have it powered by the power amps transformer & still have them stand alone?

I guess Im still a bit confused how it is you want to use it. I wasnt really giving it though when I answered your other questions before. I cant see a good reason that would justify all the hassle. If you make a seperate preamp and poweramp whose power supply is optimised for being powered together, then you just have a single amp housed in 2 different enclosures that wouldnt be of much use as stand alone units because the voltages would be off greatly. On the other hand making 2 independant units would not only be easy, its not a bad idea. How different the tone is from a single complete amp is something I dont know.

If you really want a preamp then you should just make an independent preamp. Hell, if you settle for that then you could still use the dual preamp design you talked about. Have a normal 1959 on one side and a JCM 800 on the other. Or a modded JCM800. That would be interesting.

Eoin · Post by **Eoin** » Sat Mar 12, 2005 11:50 am

Breaking it down into a modular system in line with pro-audio standards appeals to me for some reason, even though I can't honestly say I'd have any use for a guitar preamp on it's own.

Tbh, I may dump the stereo idea anyway. I can't see an advantage of having two of the same preamp - if so inclined I could just mod the amp to have a switchable set of volume pots. What I was planning to do (and definitely still am) was make the Super Bass preamp as well. I think I can mod the power amp stage to be switchable from the guitar to the bass configuration - as far as I can see, there's only three resistors in the difference. So, even if my modular preamp is being powered off the amp, it still has the advantage that I can switch it for the bass preamp.

A large part of this is a learning experience for me. I will more than likely make this and then play with component values for months trying to see what difference various things make. I want to clone a Super Lead because in my three years of studio recording (though mostly home studio, alas) and live engineering, it is the best sounding guitar amp I've ever heard. Most of my modding ideas that haven't been thought of before - like having variable input impedance - are probably crap or useless. If they weren't, someone would have done them already. But I want to know why they're crap. Some knowledge I won't take for granted.

I noticed (you also mentioned it) that there's a voltage increase across a filter capacitor - I observed about +10v when there was no current being drawn. Is the increase dependant on the capacitance, and/or the position in the circuit? Is it proportional to the voltage? And is there a formula for calculating it? Better still, do you know the formula?

Anyway, thanks a million for your help so far, Billy Batz.