Dumb tube questions

[gbono]

Okay why do most tube schematics hide the heater circuit and when do you use DC for a heater (preamp section)? I'm going to be working on a Sunn Sorado

sunn_sorado.pdf

[Enzo]

Hide? Here in the Sorado drawing, heaters are drawn off the power transformer.

I looked at a Fender AB763 Bandmaster and the PT winding notes " to all 6.3v heaters and pilot light."

A Marshall 1959 says to all valve filaments 6.3v.

AMpeg VT22 - spelled out.

What would be some examples? There won't be many details, since the wiring is simple and assumed. Like no one would supply a schematic for a guitar cord either.

And where DC? I repair amps, so I run into it wherever they use it. if you are building an amp, the purpose of DC heaters is to eliminate one source of hum - heater to cathode electron flow. In a high gain input stage, this can add some hum. It will not affect any other sort of hum though. SO one would use it for input stages where the potential for such hum exists.

[überfuzz]

Is there a problem with the heater circuit?

[tubeswell]

when do you use DC for a heater (preamp section)?

If you have octal pre-amp tubes, they can benefit from DC heater circuits. But with 12A_7 tubes, there's no need. The filaments in that Sunn amp are just wired in the plain old way - as shown in the shematic. (i.e.; AC heater circuit)

[Ronsonic]

Okay why do most tube schematics hide the heater circuit and when do you use DC for a heater (preamp section)? I'm going to be working on a Sunn Sorado

[ATTACH]10795[/ATTACH]

The heater circuit is usually drawn as a side bar. Same as the supplies in most IC circuits. It's assumed you know you need one and the only question is which pins so why let it clutter up the drawing.

DC heaters are useful for high gain preamp circuits or where you're using tubes with known noise issues.

[Diablo]

It's done for the same reason that doctors describe your disease in latin. It's part of the secret ritual to keep the amateurs guessing.......

[Euthymia]

Funny this should come up just now, because I worked on an amp last week whose heater circuitry had my head spinning, and I never did figure out why the designer would do it the way he did.

It was a Peavey Valve King 100, and I couldn't find the heater arrangement spelled out on the schematic, and it had a configuration I'd never seen before. What this amp does is run its power tubes' heaters in series from a single 32VDC supply. Like it's Christmas 1940 and we have a series light string running on DC.

How I found this out was that after I fixed the stuff that was broken about the amp, I put in just two of the power tubes to see if everything was all right. Dark tubes, no filament. Much staring at schematic and probing of tube pins before I traced the PCB and figured out what was going on.

All I could think of was that the designer must have been bored and wanted to come up with something weird. I'm not real big on DC heaters to begin with, as I believe that if your circuit is so touchy that DC filaments seem necessary, you might want to rethink things. To use DC AND a series connection on the power tubes in a push-pull seems almost sadistic in terms of the likely effect on innocent technicians.

[nashvillebill]

Peavey has several amps that run the heaters in series. Classic 50, and Delta Blues if I remember correctly.

[Enzo]

Valveking 100 - look on the page with the power tubes, upper left corner. There is the heater wiring. it is not hidden. And as bill points out, PV has used series heaters on many models for at least the last 15 years. As soon as you don;t see a 6v winding on the PT, you can be pretty sure what to expect.

On many amp models from various makers, the high current 6v heaters have been a connector killer. Molex pins burn up, companies tell us to just solder the wires to the pc board, eliminating the connectors. And many amps these days need more than B+ and heater voltages. They have op amps in switching or reverb circuits or they have relay supplies, etc. So by running the heaters in series off a higher voltage rail, they reduce the current dramatically, and in the bargain remove the 6VAC potential for noise, AND they get the bonus of using that "extra" low voltage winding for the purpose.