Heater Voltage & Todays Tube Recto's

[Amp Kat]

Most of the older tranny's were designed for 110 line VAC and it seems today that the standard wall voltages are from 122 to 126 VAC which causes the preamp tubes to run in the 7.2 range and it doesn't seem to bother them. But how about 5 volt Rectifiers ? These voltages that were 5 are now 6 and sometimes 7. Do you guys modify these circuits to run them at 5 volts or does it really make that much difference ? I'm sure theres gotta be some effects of maximum voltage and thermionic emmisions somehow.

[Tom Phillips]

One comment: A 126V line voltage is 14.5% higher than a 110V line. If you are getting 7V on a loaded 5V winding then I suspect that something is wrong with the transformer because that would be a 40% increase. Did you perhaps measure the open circuit voltage when you got the high reading?
If the loaded winding voltage is within 20% of 5V (6V max) then I think you will be OK.

[Amp Kat]

Thanks Tom, and yes that is unloaded not regarding the voltage drops across the heaters. I'll have to load em down and see the results. Was trying to get a threshold of the max voltage and what would happen or some of the effects if it was given overvoltage.

[MWJB]

Running heaters significantly over voltage, or under voltage, may lead to reduced tube life (I remember finding a graph on the net somewhere). By how much...I've no idea. I wouldn't panic if your heaters are just a shade over 7VAC, this is a typical voltage for some 60's Fenders. I have deliberately run heaters at 7.5VAC in the past, as part of a B+ experiment, no sudden catastrophic failures (but I figured I'd rather have the right PT and subbed it out after a few weeks).

Bear in mind that the number & type of tubes will have a bearing, e.g. if you have a vintage 100W PT and you are just running a pair of 6L6s, or even 6V6s, & a few preamp tubes (less current draw), your voltages will be a little up.

Bear in mind that shorted PT primaries may also push up heater voltages, this isn't always easy to determine if just a few turns are bad & some amps rattle on like that for years without anyone spotting it. I guess if B+ & other voltages are all WAY over 12% (or the 14.5% as Tom states) beyond vintage ratings (with the PT specified on schematic - be aware mfrs sometimes changed specs leaving no clues) then that COULD be a symptom, along with a PT that runs abnormally hot FOR THAT MODEL OF AMP - just in case everyone with an old amp that gets warm suddenly panics and decides to sub out their PERFECTLY FINE PT. Some amps do just run hotter than other models, always have done & always will so you must compare like with like. There's not really any useful info to be gathered by touching components on your amp, those that typiclly run hot are often too hot to touch, so please don't.

I don't usually like to type in CAPS but this can be a close call and takes a bit of judgement & experience to spot, just need to stress that.

[Bruce / Mission Amps]

I sure would not run them at 7vac Kerry but do check them under load.
I've wondered if running a tube's filament voltage a little under it's listed voltage is bad for the tube. Personally I don't think so.

By the way, for all you cheapskate DIY'rs out there, I've run many 6V6 tube amps with a solid state recitifer with the 6V6s on the 5v0 filament supply and many NOS $3.00 to $5.00 5V6GT tubes on the 6v3 supply with a couple back to back high current Si power supply diodes in series.
However, MWJB's comments are well stated and the sole reason I spent thousands of dollars on having my 5E3 transformers made and remade until I finally got the secondary voltages where I like them with respect to the average 122-126vac line voltages we now see.

[R.G.]

Most of the older tranny's were designed for 110 line VAC and it seems today that the standard wall voltages are from 122 to 126 VAC which causes the preamp tubes to run in the 7.2 range and it doesn't seem to bother them. But how about 5 volt Rectifiers ? These voltages that were 5 are now 6 and sometimes 7. Do you guys modify these circuits to run them at 5 volts or does it really make that much difference ? I'm sure theres gotta be some effects of maximum voltage and thermionic emmisions somehow.

Not a reader of Geofex, I see.

Have a read at the Vintage Voltage Adapter, which is my overly optimistic name for a bucking transformer device to correct the line voltage back down the design voltage.
Vintage Voltage Adapter

There are a bunch of other tips there that tube amp hackers should know. See Tube Amp Tech Pages

[Amp Kat]

Damn RG, you know I forgot all about that gadgit and even have it printed out all-ready. Thank all of you fellows for the input and I knew there was notes about this but my memory ain't what it used to be. I'm pretty anal about running trannys and tubes where they should be and this is no exception so the advice will come in real handy. I send your website to many a people RG but I just dropped the ball on this one.