Valves, I think I'm starting to understand what the fuss is about.

[Ali Tait]

I had some Valvo e80cf's and I bought a matched set of Philips NOS from Langrex, which I thought were very good without paying silly money for Mullard etc.

[anthonyTD]

Not saying you are wrong, but how does that occur with an indirectly heated valve?

6.3v was originally the heater voltage for DC heated valves after all (3 Lead Acid cells 2.1 * 3 = 6.3v)

hi Nick,
i was refering to indirectly heated cathode valves the types that are in the amp in question.
anyway,,,here is one theory why it is a good idea to have the option of reversing the polarity of DC supplied heaters.

There have been never-ending arguments for valve heater filaments (including output valves) to be supplied with DC rather than AC. High current DC has a disadvantage similar to the Electrolytic corrosion caused by vehicle batteries. Electrolytic corrosion effects cable, connectors and valve filaments resulting in reduced filament life. However, Cathode emission life is less than the reduced life caused by DC corrosion of the filament. Therefore, DC for valve filaments is not a real problem. A simple solution to minimise DC electrolytic corrosion is to switch (reverse) the polarity of the DC to the filaments approx each month

I also have a theory that constantly running a valve heater on one polarity [+ or -] can cause a similar effect as leaving the valve heaters on full power for long intervals without any HT voltage on the anode/plates,[standby mode] ie; a coating is formed that prevents electron flow hence reduced emision.
just my ramblings though.

As for directly heated triodes...
Where the cathode is directly heated (the filament has the oxide coatings directly applied), DC operation is mandatory, or hum would be inevitable .however directly heated cathodes will always emit electrons unevenly, because of the voltage gradient across the filament. The most positive end will have the greatest emission, since the negative voltage between cathode and grid is lowest.

Anthony,TD...

[Clive]

The easy way if you have 2 identical valves is to wire them in opposite polarities and swap the valves every so often.

[lurcher]

There have been never-ending arguments for valve heater filaments (including output valves) to be supplied with DC rather than AC. High current DC has a disadvantage similar to the Electrolytic corrosion caused by vehicle batteries. Electrolytic corrosion effects cable, connectors and valve filaments resulting in reduced filament life. However, Cathode emission life is less than the reduced life caused by DC corrosion of the filament. Therefore, DC for valve filaments is not a real problem. A simple solution to minimise DC electrolytic corrosion is to switch (reverse) the polarity of the DC to the filaments approx each month

I am not sure we are reading that the same way. To me that seems to be saying that DC heater power will reduce the life of the heater, but it will still be less than the life of the cathode so it doesn't matter. Not how you seem to be reading it as the DC heater supply will reduce the life of the cathode.

[anthonyTD]

I am not sure we are reading that the same way. To me that seems to be saying that DC heater power will reduce the life of the heater, but it will still be less than the life of the cathode so it doesn't matter. Not how you seem to be reading it as the DC heater supply will reduce the life of the cathode.

hi Nick,
i was refering to indirectly heated cathode valves the types that are in the amp in question.
anyway,,,here is one theory why it is a good idea to have the option of reversing the polarity of DC supplied heaters

Anthony, TD...

[anthonyTD]

hi Dave,
sorry for deviating [slightly] off topic something that rarely happens here on AOS :eyebrows
anyway, i think what we are all trying to say is, concentrate on improving on the the things that are imediate, ie; power supply caps, Diodes, coupling caps, bypass caps etc, the AC heaters in this amp should be sufficient as far hum and noise are concerned as long as they are either center tapped or have one end fixed to earth/chassis. better valves are always a choice of the end user, NOS valves [on the whole] do tend to be made better than most of the current production, however, original old stock valves like EL84 etc are becoming very scarce now so dont be surprised if you see for eg Mullard versions fetching upwards of £60-00 each!
hope this helps.
Anthony,TD...

[Ali Tait]

These might be worth a try, they are cheap and I've read they are good-

http://cgi.ebay.co.uk/EL84-7189-6BQ5...#ht_506wt_1139

[HighFidelityGuy]

Thanks for all the input and suggestions chaps. You've come up with some very interesting points.

From what's been said I'll definitely stick with AC heaters on the output valves as the standard supply is centre tapped and I can't hear any hum what so ever through the speakers.

On the subject of the input valve DC heaters; I'll definitely upgrade the diodes on this. I've seen a good guide that shows how to do this including a pi filter to keep the correct 6.3V output.

Would there be any benefit to using a regulated supply here instead of the above?

Cheers.

[lurcher]

I dont think there is enough headroom for a regulator. Maybe if you use low Vf diodes in the bridge.

[HighFidelityGuy]

I was wondering if this would be suitable: http://www.tentlabs.com/Components/T...ter/index.html

It can can supply 1.5A and the two ECF80's only need <500mA each. So I should have over 500mA headroom.

Do you think this would work and more importantly could it provide an audible benefit over the standard non-regulated DC heater supply?

Cheers.