Mullard 5-20 PSU

After the great sucesss of the stereo Mullard 3-3 I have built https://www.vintage-radio.net/forum/...cb+mullard+3+3 I am now building a stereo Mullard 5-20 with a semiconductor power supply. The question is, is it worth doing a delayed HT or just let the valves cope with full HT from the start? It won't be much of a problem as I am using a couple of big MOSFETs to reduce the ripple anyway.

The most amazing bit of the build is I can get brand new EL34s, EF86s and ECC83s from Rapid Electronics for a good price (they have quite a range http://www.rapidonline.com/SearchRes...?kw=thermionic ), Most likely a lot less in real terms than in the late 50's when this ampliifier was designed. I must say the EL34's do look quite impressive sticking out of the chassis.

[G8HQP Dave]

My 5-20 (ish) amp has silicon rectifiers. I do have an inrush limiter, but that doesn;t make much difference to the HT voltage. I used NOS valves, which may be more robust than modern ones.

[maninashed]

You could just use a 'standby' switch as many guitar amps did, which is also very cheap. Alternatively use a couple of GZ34s which would warm up at the same rate as the valves, and the ht would rise slowly. Presuming of course that you have a mains transformer with 5v secondary. Personally, I have a Dynavox VR70 (ST70 clone) with EL34s and ECF82 preamp valves, that too has a solid state ht rectifier and it gets full ht from start-up and I havent experienced any problems with the valves under these conditions.
Bill

[ukcol]

Hi MM

Given the fact that you are using MOSFETs for active smoothing it would be relatively easy to design a slow start HT circuit rather than an HT switch-on time delay.

This would have the same advantages as a delayed HT switch on i.e.:-

1] Protection of the output valves while the cathodes are coming up to temperature.

2] Protection of the HT electrolytics from no load overvoltage.

- but with the additional advantage of having no thump in the speakers that is likely to happen when switching to full HT after a time delay.

Colin M

Thank you all for the reassurance. As the amp will only be turned on once a day and as new valves are not that expensive I think I will go along with maniashed.

I will start a new thread with pictures etc..

[G8HQP Dave]

So-called 'standby' switches can make things worse. Best avoided. Guitar amps have them because musicians expect to find them.

[maninashed]

Guitar amps have them because the valves are run very hard (because musicians like that sound), and the standby switch is there to prolong valve life as the amp can be idle for long periods. It also enables a quick start up when the artist starts to play.
This isnt really relevant to this thread. Apologies to the op
Bill

[G8HQP Dave]

'Idling' with full heaters but no HT can shorten valve life, as it can cause cathode interface to develop. I know one guitarist who insists on letting his amp cool down on 'standby' after playing before switching off, as he thinks that extends valve life. Untrue, but myths can be persistent!

Anyway, this area has been covered in several threads both here and elsewhere. Normal domestic equipment with relatively low HT voltages does not need any special arrangements.

[qualityten]

Dave's comment is interesting. When I had my Maplin Newton phono preamp rebuilt by a professional hifi restorer a few years ago, he added a separate switch for the valve heaters and advised me to let the heaters warm up before adding HT. Was this unnecessary?

Looking forward to the new build thread too.

[Sideband]

Originally Posted by qualityten

he added a separate switch for the valve heaters and advised me to let the heaters warm up before adding HT. Was this unnecessary?

Probably. Domestic equipment has relatively low HT so standby switches are not really required. Think of old radios that have solid state rectifiers be they silicon or selenium. Many of these sets are still working with original valves, 60 years old or more. They do not appear to have suffered any adverse effects having HT supplied instantly before heaters have warmed up.

Probably very different for high-powered equipment that may use HT in excess of 500V.

Rich.

[qualityten]

Thanks Rich. I think he added this feature just to make use of a spare switch that was already fitted to the front of the chassis! I was not the original builder of the kit.

[PJL]

What is the purpose of a delayed HT . With the low forward voltage of a solid state rectifier and large reservoir capacitors the surge voltage can be kept to a minimum. In any case, an output valve must be able to handle twice the HT voltage or it would breakdown when at full power. There will be no thump as the valve takes a long time to reach full operating temperature during which the conductivity will steadily increase.

Delaying the HT seems to be exactly what you don't want to do as you are guaranteed a currrent surge when you introduce HT to an already hot valve.

What is the purpose of a delayed HT

The very question asked, I was a little concerned about ion damage to the cathode, I don't know if the new valves manufactured are any better/worse than 'old' ones. However the consensus is I don't need it at all. Less components = more reliability = cheaper build, win win win!

As an apetite whetter a picture...

[PJL]

Ion damage, not mentioned so far but I see the concerns. I am not sure if this is the same as the dangers of leaving the heater on and no HT. In this case I beleive the problem is ions created within the electron cloud around the cathode, without electron bombardment I am not sure you would have ions.

PJL, what was I thinking of course you are right.

[G8HQP Dave]

Ion damage to the cathode tends to happen when the valve is running in temperature-limited condition: cathode cooler than normal, but still hot enough to emit electrons. Under this condition there is no protective space charge around the cathode, as emitted electrons get immediately grabbed by the anode.

You can generally reckon that modern valves will be no better than decent NOS samples, and perhaps worse in some areas such as heater-cathode leakage, grid current, service life and sample variation.

It was going so well... (one day I will draw a circuit diagram and get it right first time) decided to go for fixed bias, the transformer has a 18-0-18 winding so bridge rectified thats's 50V (perfect). Made same, measured volts... +50V, not much good for negative bias, having a beer now, which from reading the label, has malted barley in it, well I never.

[jjl]

Apologies if I'm teaching you to suck eggs, but you need to connect the positive side of your rectified and smoothed bias supply to the 0V rail of the amp. That is how the bias supply becomes a negative supply.
Is there a particular reason why you want to use a fixed bias supply for a 5-20? The original circuit works very well with cathode bias for the output valves and avoids the extra complexity and potential hazards of fixed bias.

John

[maninashed]

I can see why you want to use a fixed bias, especially if you are to include an adjustable bias supply. You can then load each o/p valve so that they run as close to their specification as possible and balance both pairs. I like that arrangement too, and Im not an audiofool! BUT it does make a big difference to the sound of a well sorted stereo amp when everything is in tune-just try it for yourself. Nothing wrong with experimenting with the sound and adjusting things to your liking. Its different though with a guitar amp which has no or little NFB to dealing with a hifi amp for quality reproduction which may incorporate a lot of neg feedback
Bill

but you need to connect the positive side of your rectified and smoothed bias supply to the 0V rail of the amp

So that's what the problem is (!!!) no need to appoligise, it is quite often the big bits one misses. I am using fixed bias for the very reasons Bill said and having that extra winding on the transformer means I can use more of the HT.