The Official Dual Rail Thread

[Wilder Amplification]

Well since my OT Plate Load thread kinda morphed into that, I figured I'd start a separate thread dedicated to dual rail technology. I'll be posting up schematics of my build once I come up with a preamp that I'm happy with for it...the power supply and power amp are pretty much a given.

For those of you who have experience with dual rail amps, please post up! This will hopefully be a very informative thread for those who are interested in learning about dual rail amps. I'm very interested to know things like what phase inverter would work best with them (paraphase, cathodyne/buffered cathodyne, LTP) as well as valve life expectancy in a dual rail amonst other things.

Let the posting begin!

[Enzo]

I have to admit, I have no idea what this means. Are we talking about split rail supplies as used in solid state circuits? Or are we talking tubes with the cathodes going to some negative voltage supply instead of ground? Or are we talking about high B+ for power stage plates, and low B+ for screens and preamp? Or something else?

[Bruce / Mission Amps]

Other then using the old 6146B or 6550, Ampeg SVT B+ rail circuit... this is all I've ever messed with....

[Wilder Amplification]

I have to admit, I have no idea what this means. Are we talking about split rail supplies as used in solid state circuits? Or are we talking tubes with the cathodes going to some negative voltage supply instead of ground? Or are we talking about high B+ for power stage plates, and low B+ for screens and preamp? Or something else?

High plate B+/low screen B+.

Bruce...the SVT was the amp that actually got me curious about dual rail technology. That's one amp I never hear about people having problems with in regards to eating tubes, blowing OTs and whatnot. One badass amp.

[defaced]

I've got my MOSFET buffered cathodyne phase inverter/driver up and running. If you have any questions/want me to take any measurements, let me know.

Over complicated schem because I want to play with individual bias.
http://music-electronics-forum.com/t16854/#post148551

[Joey Voltage]

Other then using the old 6146B or 6550, Ampeg SVT B+ rail circuit... this is all I've ever messed with....

Yeah! that will do it! the valve rectifier may not be a bad idea since you squash any chance of silicon hash getting injected into the B+ line. If you parallel them, and maybe throw in some resistive balancing, you will still have punch and possibly some give to still retain a decisively tubey nature.

[Joey Voltage]

I've got my MOSFET buffered cathodyne phase inverter/driver up and running. If you have any questions/want me to take any measurements, let me know.

Over complicated schem because I want to play with individual bias.
http://music-electronics-forum.com/t16854/#post148551

Mike,

I have a good design that I'm working on as well in the same design concept of loud punchy an clean. It is a bit more complex than the split plate/screen design as there is some active circuitry, but when I have it up and running, and working the way I want it to. I will shoot it over to you If you don't plan to use it commercially. In fact I may even let you look it over if you are really curious.

[Joey Voltage]

I have to admit, I have no idea what this means. Are we talking about split rail supplies as used in solid state circuits? Or are we talking tubes with the cathodes going to some negative voltage supply instead of ground? Or are we talking about high B+ for power stage plates, and low B+ for screens and preamp? Or something else?

Enzo,

it's a a bit of a misnomer. Indeed Dual rail suggests a dual polarity supply. what he is talking about is just partitioning the plate and screen supplies in order to maximize headroom and stiff, clean output power under very large signal conditions.

From my own experience this has never been an issue for me, I have played heavier music, and have never needed anything rated over 50W to get a punchy articulate sound at high signal levels, and I have played some very large venues in my day.

[imaradiostar]

I think it'd be wise to copy over Steve's post from the plate z thread- the thing about selecting plate voltage and impedance to meet your power goals then setting screen voltage to limit current consumption.

Thanks to Bruce for posting the picture- I've had good results with that kind of design so far!

jamie

[imaradiostar]

I also find the "dual rail" description misleading. It makes me think of a world with complementary symmetry tubes- where we could heat the plates to make them emit electrons and we'd be able to build tube amps that would look largely like bipolar solid state amps...and the hifi nerds would rejoice!

In all seriousness- is there a name for the difference between normal FMV operation (screen at or near plates) and the operation this thread is describing- screens at something less than 90 percent of plate supply voltage.

If anyone has the time and tools it'd be interesting to see comparisons of a given tube operating both different ways and the differences in power output and current draw. I know this info is in data sheets but some real world guitar amp examples would be pretty cool.

sorry about the double post.

jamie

[Joey Voltage]

I also find the "dual rail" description misleading. It makes me think of a world with complementary symmetry tubes- where we could heat the plates to make them emit electrons and we'd be able to build tube amps that would look largely like bipolar solid state amps...and the hifi nerds would rejoice!

well yeah, It shouldn't be called "dual rail", and I think of the exact same thing..... complementary source followers

In all seriousness- is there a name for the difference between normal FMV operation (screen at or near plates) and the operation this thread is describing- screens at something less than 90 percent of plate supply voltage.

unfortunately not to my knowledge, it is all under the umbrella of "normal" operation. Allthough glancing at the spec sheet for most kinkless tetrodes will almost imply the supply partitioning. Of course you have to understand misuse to achieve tonal coloration was never a driving force behind the development of the device. I think you even mentioned this.

If anyone has the time and tools it'd be interesting to see comparisons of a given tube operating both different ways and the differences in power output and current draw. I know this info is in data sheets but some real world guitar amp examples would be pretty cool.

sorry about the double post.

jamie

unfortunately this also takes money

[black_labb]

i guess the issue is really the dynamic effects on the power supply. if you are running the plates at high voltages, you will need to have the screens running lower. you can put in a big resistor and drop the voltage, but when the screens draw more current the voltage will sag. if the preamp is running off of this suppy then the pre will be effected by the voltage changing alot. dropping the voltage uses up power.

using resistors and chokes are fine for voltages used in alot of taditional amp styles, but what about an amp running 6550's or kt88's at 700v? are you going to use a resistor to drop the voltage to 350-400v to get the screens to a suitable voltage? thats alot of resistance in the supply and the idle voltage will be wildly different to the voltage at full power. you are also putting alot of power into resistors in the form of heat. the same power going to the screen supply is going into your resistors at 350v.

another option is to use a regulator. a simple one could be zener diode(s) dropping the voltage. this will keep the voltage between the plate and the screen supply at a fixed value. this works pretty well. the voltage is pretty constant, and only sags as much as the plates. you are still wasting alot of power in the same way as a resistor, but the power supply to the screens and following preamp is at a pretty consistant level. dropping 350v like in the example above will require a number of zeners of reasonable power ratings (zeners over 50v if that are hard to find as far as i know).
there are more complex regulators that you can use using transistors (i think mosfets are what people have mentioned, but i'm not sure). these will keep the voltage at a fixed value and are pretty good. again they still use power and create heat.

using separate supplies for the plate and the screens avoids all these problems. you just need a second rectifier. one winding is rectified to feed to the plates, the other is rectified and fed to the grids and the rest of the circuit which doesnt run at such high voltages. you can also float one winding ontop of the other or use a voltage doubler and take the grid supply from the midpoint and the plate supply from the top.

the things to consider are
- your grid supply usually has been filtered before the grid stage by the plate supply filter cap, so you need to up the value of the filter caps to filter to the same level. this is easy to do with modern caps.
- ensure that the way you fuse the power supply is done in a way that the grid supply will be shut off if the plate supply is. if the grid supply is on when the plate supply is off you will blow your output tubes very quickly. not cheap when you have 3 pairs of 6550's or similar in a monster amp.

it can be hard sourcing transformers unless you use 2 power transformers (unless you get them custom wound). for a moderate sized guitar amp its not hard to get a small 30-50vac transformer with 2 secondary windings. thats 80-140v that can float over the usual supply. using 6550's or kt88's a voltage doubler is a great way to do things. 700v isnt too much for the plates when the grids are at 350v.

i used "dual rails" as described here in my big bass amp build using a 450vac tranny for the plates and a 250vac tranny for the grids for a 630vdc/350vdc supplies. works very well imo.

[Wilder Amplification]

For the record, the term "dual rail" can be used to refer to any sort of dual plate supply config regardless of polarity (excluding the bias supply since it's not part of the normal "current carrying" supply).

"Sliding screen" is the term I use to refer to the standard single rail supply that references the screens a few volts behind the plate since the screen voltage slides up and down with signal current.

i used "dual rails" as described here in my big bass amp build using a 450vac tranny for the plates and a 250vac tranny for the grids for a 630vdc/350vdc supplies. works very well imo.

This is essentially what I'll have for my dual rail build...450VAC and a 225VAC for a 630/315 setup. I just got the plate tranny wound by Heyboer...nice thing about them is that a custom wound PT from them doesn't cost much different than an off the shelf one, and they're LOADS cheaper than off the shelf stuff at MM.

The OT will have a 5K Zp-p. With the math I did on this load for peak power (5000/4 = Zp-ct = 1250...630 / 1250 = 505mA Ipk) I had them rate the plate tranny at 450VAC @ 500mA so I'm thinkin' there'll be very minimum to no sag at all depending on the % regulation of the PT.

Now according to the plate characteristics chart on the datasheet, for an idle bias at 40mA it tells me I should have roughly around -35 volts, which I'm sure is +/- a few volts. Does this seem about right?

[Joey Voltage]

For the record, the term "dual rail" can be used to refer to any sort of dual plate supply config regardless of polarity (excluding the bias supply since it's not part of the normal "current carrying" supply).

for your record maybe. but the term dual rail is most commonly used to refer to or illustrate a single supply with dual polarity (which is about every supply),more commonly in the SS world. it just depends on where the word dual is emphasized.

"Sliding screen" is the term I use to refer to the standard single rail supply that references the screens a few volts behind the plate since the screen voltage slides up and down with signal current.

I have learned that you can kinda run into problems using terms figuratively. (especially here!)

[Wilder Amplification]

I have learned that you can kinda run into problems using terms figuratively. (especially here!)

Kinda like KO'C and his self-derived constants?