Kat 6550 - one odd voltage

[Really Aardvark]

I'm new to the electronics game but have completed the build with only one exploding capacitor - those electrolytics don't run well backwards.

All seems well sonically but electrically speaking one voltage is low - g2p (pin 3) on one ECF80 runs at 79v not 95v as specified. Its definitely the valve - swapping the valves over takes the low voltage to the other channel.

The I have questions are
- is this a duff valve?
- does it need to be replaced?
- what effect is it having on the rest of the amp electrically and sonically?

I'm reading Morgan Jones but haven't quite got to grips with it all yet!

Any help, guidance and views would be appreciated.

Thanks in advance

GregS

[JerryT]

Hi
Had the same experience with mine. ECF80s are notorious for variation in voltages. Just swap the valve to check the voltage follows it. If not check your soldering! I wouldn't replace it yet. Run the 6550 and decide what upgrades you want ( I swapped the original ECF80s for NOS Mullard E80CF eventually - and it was a good move.).

Jerry

[NickG]

Yes, I wouldn't worry about that voltage, NOS ones are quiet common, so you could try changing after you have got used to the amp as it stands.

[acorn]

Hi All

Yes I had the same trouble with my ECF80s, But does it really matter if you loose say 10 volts or more, there as to be a margin of error with valves or we would all be in trouble , would any one else care to comment on this subject of differant voltage readings on valve pins, or is it small signal valves only suffer from this voltage varyation.


Acorn

[John Caswell]

Hi all,
Generally voltages on valve pins can and will vary by anything up to +/- 15% without detriment to the operation or gain.
It will only take a variation of say 1mA through the cathode to affect the anode reading quite drastically so in general I would say don't worry about it.
The surefire method of checking is to swap the valve for another of the same type or in the same amplifier to compare conditions. SQ or military valves tend to be better in this respect as they were selected for closer tolerances during manufacture. That having been said don't get paranoid about it and rush out to try to source SQ type valves, the expense and hassle is not worth it as getting the voltages spot on is not a priority and even if you do it is unlikely that it will make the amp sound any better or different.
The conditions for small signal valves are not as critical as for power o/p valves. Here some care needs to be taken to try to ensure that the currents through the op valves are similar. This yields dividends in pp amps with lower (measured) distortion and better balance through the optx primary windings, and in PSE can sounder sweeter.

Regards

John aka Dr John

[Really Aardvark]

Thanks for the help and advice there guys. Have hooked it up to main system. Ye gods - I've never heard so much detail in the midrange - cello bows rasping!!. Overall its a bit strident @ 4hrs old but no doubt will settle down in time.

Cheers

Now onto the Lenco plinth.