How important is power supp voltage for tube pre amps??

[gary o]

HI there thats my main question How important is the right voltage for the HT & even the heatersin home made tube kit ?

Ive been making some Pre amps & compressors from schematics found here & else where on the web, RCA BA1A BA21A BC2B BA6A V72 Altec pultec LA2A for examples, with little knowledge really, especially at first.

What ive noticed in my my experiments is that they work within a wide range of the HT voltage, with in the ball park.

I havent done extensive listening tests but doesnt seem to effect the sound just the sound level.

The RCA BC2B is meant to run at 250V I think I read & yet some chap runs his at 363V for a better sound, obviously you cant go too high.

Also will it hurt to under power heaters ? I did this on my V72 to quieten hum, it also took the output level down a bit too but in my case I need to loose some level from that unit anyway.

Any thoughts would welcomb I have much to learn thanks for reading.

Gary O.

 

[NewYorkDave]

The short answer is that most of this stuff didn't run on regulated supplies in the first place, and it usually works fine as long as the voltages are "in the ballpark." A typical B+ supply for studio equipment was well-filtered but not "regulated" in the sense we usually think of today. They did have "swinging chokes" and voltage regulator tubes, but they weren't always used. Active regulation did exist but was usually reserved for laboratory equipment. Unless your mains voltage is varying over an unusually wide range, it's probably nothing you need to worry about.

Running a preamp intended for 250V on 363V is reckless, though.

 

[gary o]

Hi Dave & anyone,yeah I have couple of power supplies that I made, not regulated couple of 47uf caps a choke & some resistors to get me the various HTs, 155V 230V 250V 270v & so on, they run all my bits ,but not all at once

Just wanted to no that so that if I make a new bit of kit I might be able to run it on a HT voltage I already have thats not too far away from what it says on the schematic.

That BC2B pre thing came from the guy the who originally put the schematic on the web Larry Seyer I think ,I think thats where we all got that from while back, he said he ran his useing a variac, I havent tried 363V hence im typing this haha.

Cheers Gary O.

 

[mcs]

The heater voltage is the most critical. You should normally stay within the +/-10% stated in the data sheets. If the voltage is too high, you will shorten the tube life. But with many tubes undervoltage will shorten the life also.

B+ voltage is normally not very critical. One circuit I made (designed for about 340V) worked fine all the way down to 70-80V...

Best regards,

Mikkel C. Simonsen

 

[Analogique]

The transfer characteristics of simple triode circuits are non-linear curves spaning the range of input voltage (total headroom). As supply voltage drops so does the operating "input-width" of these circuits - meaning the curve gets compressed horizontally with diminishing dynamic range - also resulting in "more curvature per input volt." The resulting difference in curvature will vary depending on the biasing resistor values.

The generation of "added" harmonics is intimately tied with this curvature-per-input-volt profile. So with lower operating voltages, and less input headroom, you'll get more harmonics per volt of signal added onto it for a same amplitude input signal - how much depends on biasing arrangement.

The other thing too is the inter-nodal parasitic capacitance of triodes varies inversely to supply voltage and so they are less loaded by these at higher voltages and therefore sound a little brighter - inversely with dropped voltage.

Some people intentionally run their 6.3vac filament circuits down to around 6.0-5.9 vac (just above the shut-off limit) in order to reduce noise in the triodes. As far as I know there is no damage as long as you don't stay too close to that limit or else you can start stripping the cathodes - especially if the B+ is high.

 

[gary o]

Very interesting I suppose when these circuits were designed they didn,t want harmonics & distortion & now days we seem to like certain amounts it could be worth playing around with the B+ in some of these circuits

I better check the heater voltage in my V72 then, I think its running too low tho the tube glow very healthily.

Also I run my tubes in the 12.6V DC mode couple of my the supplies are regulated via LM317 & others are just 1A wall wart type things, anything wrong with doin things this way, my V72 seems to hum whatever I give it, unless I lower the voltage,I,ll have to do some investigating I think.

Thanks guys??Gary O.