valve preamp power transformer testing question

[9 volts]

Hey there, I have a transformer I want to use in a valve preamp. It's a proper valve (old) one. It has three wires on the secondary. It either doubles the voltage or equals the voltage thats put into it depending on which wire combination is used, so 240 =240 or 240= 480. These tests are done without a load of any kind (transformer removed). Can I make a nice valve preamp on 240, the preamps I've been look at seem to run at a higher voltage than this. Going to use a diode bridge, just a bit confused as to how to bring 480 volt down to a more useable preamp voltage eg a resistor. equation voltage /resistor equals amps. Just seems a big drop to 300. Thanks

[eggman6]

I did something similar a while back when i built a SLO preamp, used capacitors in series (most caps wont be any higher than 450v) with resistors across the caps for the higher voltage , this helps even out the voltage over the capacitors, and disscharges the capacitors  when you turn off the preamp, but it doesn't happen instantly always be sure to get your multimeter out and check before you mess around with it. I then dropped the voltage with a high wattage resistor. If you want an example let me know.

You can indeed run the preamp on 240v, but it will sound different, the lower the voltage the more gain the cirucit will produce.

EVH used lower voltages in his amps to produce more overdrive using a variac which will ruin your amp, as you need the heater voltages to be correct. Also metroamp sells a transformer that lets you switch between the secondaries to a lower voltage to get more gain out of the amp without cranking.

But if you are building a high gain circuit, you really need those higher voltages.

[g.]

240 VAC on the secondary is quite enough for a preamp !
most of guitar preamp circuits will run fine on 250DC

[eggman6]

If you wanted to build a marshall preamp then yes that will be about right, now i think about it (i always have high gain in mind when it comes to preamps).

But fender, or high gain you may want to use this


The unmarked resistors you will want to adjust to suit the preamp, you could run it all from the first 22uf cap, but this will allow you to adjust the voltages since most preamps have different voltages at each stage.

The 2w will need to be adjusted to get he voltage down to what you need, maybe start with a 20k and work from there.

[9 volts]

Hey thanks, from my reading I gather that a higher voltage equal more headroom. Maybe I'll start with the lower (which is obviously still high). My confusion came about because when looking at amp schems, those power tubes place a larger load  on the transformer and need the high voltage. I was also looking at a jcm800 pre which had a 280 plate voltage in the pre amp stage...  Looking at your schem, if you built up the power supply (as shown) do you notice much of a drop once its connected up to the valves? Thanks for the advice.

[eggman6]

I didn't take note of the voltages when diconnected from the preamp. but i wouldn't imagine the drop to be that significant. This schematic is pretty universal, if you just wanted to use a full wave rectifer and change the 2w reistor to 100r so you dont drop all that voltage, then that should work fine, you can then experiment and see what you like. Also the cap values dont have to be exact.

[9 volts]

Time to start building, I'll keep you posted, thanks again

[sean k]

a choke offa the rectifier is good 'cause that cuts you down to rms as opposed to peak to peak which you'll get offa cap inputs. The other way is to put zeners between the centre tap and ground and those pull it off the peaks. I've never done that myself. Over at Ampage they do it lots but I can remember that you work out current load on the amp then times that by the drop you need on the zeners. I'm pretty sure they go in anode to anode, cathode to cathode etc, in a chain, so you'll have 3 x 47V one way and 3 x 47V the other way to drop 150VAC. But check over at Amage before you take it as gospel from me.
Are you taking into consideration that the 240 might be an rms measure which means you'll get 240 x 1.4 which is 336 with a cap input.

[9 volts]

a choke offa the rectifier is good 'cause that cuts you down to rms as opposed to peak to peak which you'll get offa cap inputs. The other way is to put zeners between the centre tap and ground and those pull it off the peaks. I've never done that myself. Over at Ampage they do it lots but I can remember that you work out current load on the amp then times that by the drop you need on the zeners. I'm pretty sure they go in anode to anode, cathode to cathode etc, in a chain, so you'll have 3 x 47V one way and 3 x 47V the other way to drop 150VAC. But check over at Amage before you take it as gospel from me.
Are you taking into consideration that the 240 might be an rms measure which means you'll get 240 x 1.4 which is 336 with a cap input.

Thanks. I'll look into this. I've decided to go with the hotbox design as it seems pretty well documented. To be honest this quote goes over my head so I'll be doing a little more reading. Does peak to peak relate to DC? I though that was a measurement for ac

[sean k]

Your power at the wall is 120VAC. But this is a root mean square (rms) measure which means it  is the average voltage as it goes through its cycle from ground up to peak and back to ground so the peak voltage is 1.4 times the average and because the capacitor is a storage device it means it can store the rectified positive swing on the AC voltage at its peak as +DC voltage. But to add to this the AC also goes negative as much as it goes positive so the potential voltage swing accross earth is +120Vrms(168 peaks) and -120Vrms(168 peaks) which + peak to - peak is 336VAC. Wire coils circling past magnets with negative and positive ends.

Anyways, what that means is that the AC coming from the transformer is rectified through diodes, the negative (in this instance) we don't want so it's earthed, but the postive is still a big hump from ground to the peaks. The capacitor input filter will store the full voltage to peak, rms x 1.4, and this is what you get as DC. So lets say we have a 12V output. 12 x 1.4 = 16.8 V.

What that means is that from a 12VAC output you can get a 33VDC swing from negative through earth to positive.

Sorry if it's a little confusing but once you get it all you're home free.

[9 volts]

Thanks for the explanation Sean it's making more sense, though I'm in Australia where we have a 230 volt ac coming out of the wall. Hence doubling that is gonna get up there. So maybe it's best to take it from the middle centre tap.

[sean k]

I've always just used the old duo 12's for Valve preamps, thank God yer an Aussie 'cuz Ima Kiwi an we got 230 at the wall as well and I can call them valves instead of that misbegotten term tubes.
I just went an photo'd a preamp with a 12AX7 but my PC doesn't wanna see the USB so I'll try and attach the photo tomorrow morning for you. Two Jaycar 230 - 12 mm2013 tranformers back to back.

              Wall Voltage---- 230/12-------12/230---Rectifier/caps

Run your heaters, AC or DC, from the 12 volts between the two transformers and then get about 320VDC to run yer valves from the other side. EZ!
The mm2013 is rated at 500mA and will easy run one 12AX7 and most probably run two at only 16% above specified load. Don't even worry about the Valves as its only a few milliamps. All blow and no snot, I like valves!

[sean k]

Here you go

This is a bass preamp with a 12AX7 and the schematic for the power is for all to see. The voltage divider accross the end of the B+ is set for about 50 volts or so and that goes to the 150/150 ohm divider accross the filaments (two of in the schemo), 4 - 9 - 5, with 9 being the centre tap. This lifts the filament voltage above where the signal is happening. Oops, its been so long and I can't remember if  you can raise a DC voltage with another DC. Shit! I think you can only do it with AC.

Somebody put me straight on this...

So to be safe you can, and it helps to quiet hum from the fils, run the fils on 12VAC( top schemo in picture below) and use the raised earth from the B+ divider to pin 9 and its divider accross 4 and 5 or follow the  bottom schemo and rectify, smooth, and regulate for 12VDC  for pin 4 and earth pin 5.

Like I said, it's been a while and I may not be remembering all I should be, I'd be more conscientious if I had High voltages in front of me and I was doing it myself... so check it out at ampage maybe. I'm saying all this 'cause when you gotta tube amp power transformer you should build a tube amp.