How much mains ripple is OK on a valve set?

[rjmusto]

Ok - will look at replacing the bridge rectifier.

I'm pretty sure I checked the grid resistors (they are all good quality 5% jobs in this set), but will double check to be sure.

...will be tomorrow now, 11:30 here....

Thanks for all the posts so far.

Ralph

[rjmusto]

Have changed the bridge rectifier. Had a classic smoke demon moment; the first rectifier I fitted was a duff one - no idea how that could be, it was new and unused, but there you are - and the transformer of course overheated..... jings. Seems ok now, but who knows what damage was done and how long it will last. Ah well.

Also realised that I am chasing my tail with this mains hum issue. Even a bigger cap, when fitted nice and tidy still left me with the same levels.
So have put in caps to the correct spec level and refitted everything into the cabinet.

With it sitting on a shelf playing an FM station, I now think I was being over-critical of the hum....or maybe the hum is a bit lower and it just likes life better with everything back in the correct place.

So thanks all for the advice and help with this one. I intend to give it back to the owner tomorrow.

As a PS: I noticed the ECL86 runs pretty hot. But looking at the shielding that is fitted and the discolouration on the pcb, my guess is it is meant to be that way. I don't have volt and current info for this set, but the volts look ok to me and the output stage is running at around 34mA. For a single-ended output (in Class A?) my thinking is that is probably right?

[Nuvistor]

The manual for the 3040ph is ECL86 pentode, Anode 28.7ma/26ma G2 4.4/3.9, so your 34ma looks about right.

[rjmusto]

Hi Nuvistor - thanks for that.
But if the manual says it should be 28mA, then 34mA is a bit high. The resistors all check out, so does that mean it's the valve? Should I increase the cathode resistor?

I've read that the ECL86 max rating is 36mA, so it is running awful close to limits....

[paulsherwin]

It will do no harm to increase the cathode resistor a bit. It will make the output valve last longer too, and though maximum output volume will be reduced, you probably won't be able to tell the difference.

With sets of this era, there shouldn't be obvious hum when you are listening to it normally. If you stick your ear by the speaker with the volume turned right down you will hear some hum from just about any domestic set.

[Nuvistor]

I was thinking your 34ma was the stage, i.e. Anode and screen together. Increase the cathode resistor as per Paul's post.
It will sound so good on your shelf you will probably want to get one of these for yourself!

[Sideband]

Measure the valve current either by disconnecting one end of the cathode resistor and measuring the current through it (it will be the total anode and screen grid current) or measure the voltage across the cathode resistor and work out the current by Ohms Law. (I = V/R). I =(current in amps) V (voltage) divided by R (resistance in ohms)

Note that most output valves run too hot to touch

Rich

[rjmusto]

I've added another 56ohms to the cathode resistor and the current is now about 28mA, so think I'm happier with that. Haven't noticed any difference on the audio, so that's fine.

.... and, yes, you're right, the set does have a great sound. Long live the valve amp!

Ralph

Hum can be almost zero on a well designed set, my EKCO U354, FM only AC/DC set, has a very low hum level only just audible right next to the 'speaker when the room is very quiet.

[Alistair D]

This program should be able to answer your how much ripple question.

http://www.duncanamps.com/psud2/

Al

[rjmusto]

Thanks for the tip on psud. Handy bit of software....runs quite happily under Wine on my Linux laptop too.