What happens if you reduce the HV supply voltage?

ROOG

Bit of a geeky question but, what happens to the amp characteristics if you reduce the HV circuit voltage on a tube amp?

That is, If you put an adjustable auto transformer on the mains supply and feed the heaters from a maintained 6.3V AC? 

Would this do anything useful? Kind of curious, that's all!

ecc83

The short and dirty answer is...You would get distortion at progressively lower power levels.

In practice however this might not be ideal since you will be reducing the pre amp and PI headrooms as well as the OP stage and the result might not be optimum or pleasant, MUCH will depend on the topology of any particular amplifier I would think?

The only power control system I am familiar with changes just* the G2 voltage but even that in a far more complex way than a crude "variac" would do it!

*Well, not quite but I can say no more.

Dave.

ICBM

Essentially that's how power scaling works, although it uses electronic control of the HT voltage rather than a variable transformer.

As Dave said, how well it works - especially at much lower settings - depends on a variety of factors in the amp circuit.

I have to say I'm probably unusual in not being a big fan of it - I prefer conventional master volumes and attenuators.

ROOG

Thanks for the replies guys, I did wonder about bringing on distortion earlier, but wasn't sure what the preamp stages would do.

ecc83

ROOG said:

Thanks for the replies guys, I did wonder about bringing on distortion earlier, but wasn't sure what the preamp stages would do.

If you have enough 'tronics smarts to separate out the heater supply then the G2 supply is as easy as a rule (2 Rs go to a cap) Then all you need is a small, few tens of mA variable HT.

Pentodes/tetrodes largely ignore changes in anode voltage but are hugely sensitive to G2 volts. If you dabble make sure you never accidentally remove the anode supply and leave G2 volts in place, pretty near instant valve destruction.

If of course your question was purely academic? Keep pinkies OUT!

Dave.

ICBM

ecc83 said:

Pentodes/tetrodes largely ignore changes in anode voltage but are hugely sensitive to G2 volts. If you dabble make sure you never accidentally remove the anode supply and leave G2 volts in place, pretty near instant valve destruction.

Yes, done that many years ago! I fitted an HT fuse to an AC30 that was causing trouble, but I mistakenly fitted it in the feed to the OT instead of the whole HT supply. I took the fuse out to act as a 'standby switch' while I measured something and blew all four power valves in a couple of seconds...

ecc83

ICBM said:

ecc83 said:

Pentodes/tetrodes largely ignore changes in anode voltage but are hugely sensitive to G2 volts. If you dabble make sure you never accidentally remove the anode supply and leave G2 volts in place, pretty near instant valve destruction.

Yes, done that many years ago! I fitted an HT fuse to an AC30 that was causing trouble, but I mistakenly fitted it in the feed to the OT instead of the whole HT supply. I took the fuse out to act as a 'standby switch' while I measured something and blew all four power valves in a couple of seconds...

Heh! One of the very few amps you will see with an "anode" fuse* is the Series One 200..But! There is an interlock that shuts the G2 volts down if it blows.

*IIRC it is 1A FAST.Don't know WTF they bothered? It blows if a KT88 goes ape... and that usually takes out the 3.15A T mains fuse AND the 13A in the plug as well! Then, punters NEVER have a Fing F ffffuse in the gig bag and they fit any old s**t!

Dave.

ICBM

ecc83 said:

Then, punters NEVER have a Fing F ffffuse in the gig bag and they fit any old s**t!

I'm sure you've come across some real laughs like that! Usually along with a blown PT.

Tinfoil out of a fag packet wrapped around the dead fuse or a bit of guitar G string jammed down inside the holder are the most common I think - or 36A car fuses in the days when both amps and cars took 1-1/4".

Thankfully getting a bit less common now most users are so unwilling to look at their own gear that they normally take it to a tech for a blown power valve and HT fuse...

ecc83

Back in the Day.. A lot of mnfctrs of decent PA and related kit would include a fuse kit and a schematic, often stapled in a brown paper bag inside the gear.

Now everyone wants cheapy cheap-cheap, those days are long gone!

Dave.

ROOG

Thanks for your explanations chaps. What prompted my question was seeing a twin in the classified and wondering if it could be made bedroom friendly! I love big fender amps, it's just that I'm never going to tax it. I take your point about leaving G2, and at the risk of showing my ignorance I assume that by removing the anode supply With G2 energised, G2 becomes the "attractive" current path?

ICBM

A Twin is a surprisingly excellent bedroom amp, and doesn't need power reduction. Of course you will only get a clean sound... but *what* a clean sound! The effortless depth and space that you only get from a big amp running at low volume.

The volume taper is also very home-friendly and doesn't have a step at the low end like many amps do.

ROOG

Oh god, don't say that @ICBM. As much as I like the idea I know someone in our household who might 'struggle to understand' the need!

ICBM

What is this word 'need'?

Unless it's in the context of "I'll never need another amp" - which in the case of a Twin, might actually be true.

ROOG

I guess it might be heavy enough to stop em moving my kit about. Thinks, just one amp,.....hummmm