DAC90A capacitor question

[David G4EBT]

Originally Posted by Voxophone

It’ s a bit of a long shot, but is there a way to energise the heater of V3 with DC rather than AC to see if the hum disappears? Obviously I would replace it in the heater chain with a suitable resistor.

That's a novel idea - nice bit of lateral thinking!

I know that some high end hi-fi amps use DC for heaters.

The UBC41 has a heater voltage of 14V and draws 0.1A, so to keep the continuity of the heater chain, and attain the same voltage drop that V3 would present, that would be 14/.1 = 140 Ohms, and 14 x .1 = 1.4 Watts, so to make up 140 Ohms (non standard value) you could use a 120 Ohm and 22 Ohm in series, at say 2 Watts to bridge the heater chain. Then you'd need a DC voltage source for the V3 heater.

[Voxophone]

Thanks. I was thinking it would be too lateral to work but your reply is encouraging!

One problem. I'm very low on high wattage resistors, but I've got a huge supply of assorted volume pots (not vintage ones unfortunately). Most of them are probably 10k or 100k, but I suppose I might be able to turn them down to 140R. I've got no idea what the wattage of these is, but I would think they're bound to be more robust than all the 1/4W resistors I have.

[kalee20]

I reckon Voxophone is on the right lines. If grounding the grid of the UL41 kills the hum, then it's not due to the ripple on the 33uF capacitor (HT line).

It could be internal heater-grid leakage in the UL41 though, so try disconnecting the coupling capacitor from the UBC41 anode to see if it stays the same.

If it goes, then you have proved that it's coming from the previous stage. It may be internal heater-grid leakage in the UBC41. Or it may be leakage across the valveholder or wiring.

Feeding the UBC41 heater with DC would definitely prove that, but a bit less jiggery-pokery would be to just connect the UBC41 grid to chassis. The volume control SHOULD do this at minimum, but with an old control don't take this for granted!

If you do want to try DC on the UBC41 heater, then since the heater voltage is 12.6V, just connect a car battery across it. No need to disconnect anything! The other heaters will be marginally over-run, but 12.6V in 240 is only 5% so not a problem. The battery will absorb the 100mA AC with no problem either.

[David G4EBT]

Originally Posted by kalee20

If you do want to try DC on the UBC41 heater, then since the heater voltage is 12.6V, just connect a car battery across it. No need to disconnect anything! The other heaters will be marginally over-run, but 12.6V in 240 is only 5% so not a problem. The battery will absorb the 100mA AC with no problem either.

According to the valve data the UBC41 heater is 14V (AC or DC) - not 12V6:

Datasheet is here:

http://www.r-type.org/exhib/aaa0245.htm

[Herald1360]

It'll still work on 12V, though, and the car battery is an inspired bodge!

[Voxophone]

I've tried grounding the grid of V3, which did not reduce the hum. Removing C18, which feeds the grid of V4, also did not reduce the hum, so I think we can be pretty certain it has something to do with the output stage.

I spent a while measuring the resistances between the pins of the UL41 and the heater to pin 5 was as low as 1.36M. I zapped the valve with a sparking cigarette lighter between pin 5 and the anode (I wasn't sure whether this would damage the heater). Afterwards the resistance was >20M, but no change to the hum!

[kalee20]

Definitely sounds like leakage between heater and grid. Zapping with a cigarette lighter shouldn't hurt at all, but it may nit be sufficiently high energy.

[Boater Sam]

Try zapping with a charged capacitor or if you have ignition on a gas hob use that.
Connect all the pins on the valve except grid 1 together by wrapping a bit of bare wire round them.

Then.
Do this with care.......................
Charge an electrolytic cap. one of the same as the smoothing caps in the radio by putting some long leads on it and put it across the HT in the set the correct polarity. Fire the set up with the valves in. Remove the cap before switching off so it stays charged and don't short the leads or touch them!
Touch the bare ends to the grid 1 and the joined pins, it may spark, should clear any short in the valve.
A bit tricky because you need the valves in the set to get HT so you may have to charge the cap before you remove the UL41 and wire the pins with a charged cap lying about! Be careful, it will give you a jolt if you touch the bare wires! If you had a spare valve it would be simpler.
or
connect across the ignition on the gas hob to the wired up valve the same way and operate the ignition for a few seconds.

You will, with luck, see sparking in the valve with either method. That should clear the short without wrecking the valve.

Be careful. Not likely to harm you but you could get a jolt.

Sam.

[G8UWM-MildMartin]

And if that doesn't work, it's worth trying borrowing a 10P13 or a good UL41, or sending ypur UL41 to someone for testing, or changing the valveholder to a B9A and fitting a UCL82 or UL84 with apropriate mods.

[Electron Mick]

Unfortunately V3 shares a common cathode resistor with V2. Even if you directly short the grid of V3 at its base pin to chassis there is still the possibility of hum from the cathode of V2 being transferred via the common cathode resistor. Before you try anything else it might be worth checking the common cathode decoupling capacitor ( O.5uF) If OK try shunting it with say 8 or 16uF capacitor to see if this makes a difference. If this reduces the hum it could be a possible solution to hum that is being created from AC disturbance of the heaters within the cathodes of these aging valves.

Using DC on the heaters would be a way of confirming this. For UBC41 you would need to replace the valve heater with 190 ohm resistor and 126 ohm for UF41.

If the UY41 heater voltage is correct at nom 31v and the value of its anode resistor correct then the low HT voltage across the first reservoir capacitor is almost certainly due to poor cathode emission on this valve suggesting its time for replacement

[kalee20]

That would be true Electron Nick, if there was leakage heater-cathode,though it would have to be quite bad to cause hum.

Leakage heater-grid os more likely, as the impedance of the UBC41 grid circuit is much higher - so a few microamps will make its presence much more obvious!

Shunting the common cathode resistor with a big capacitor is a good suggestion - though I'd say 1,000uF or something equally huge. If in doubt, try something extreme - the cathode will then be as hum-free as it gets.

[Voxophone]

Since I tried disconnecting the coupling capacitor between V3 and V4 and still had hum, would this not indicate that the hum is coming from the output valve?

[kalee20]

Sorry, yes, you are right and we have established that!

[Voxophone]

A quick update on the DAC90A. I managed to get hold of a job lot of used valves from Ebay which included a UL41. Substituting this for the original reduced the hum considerably. Hum is still present, but it is not uncomfortable at normal listening volumes.

I still have the wax tone corrector to replace, and I'll probably reinstall the original smoothing caps too, since these have turned out not to be the cause of the hum. Apart from this all that should be needed is a good clean-up of the cabinet.

One other thing I’ve been considering is replacing the existing cathode resistor with a higher value one to reduce the UL41 anode current and hopefully get a longer life out of the valve. I get far more volume out of the radio than is really necessary, so this shouldn’t be an issue. Can anyone recommend a suitable replacement value? I think the original is 150 Ohms, so I was considering 200.

Thanks

[beery]

Hi Voxophone,

I would not bother increasing the cathode resistor as this will change the bias point and to some extent the anode impedance.
The best thing you can do for the life of the UL41 is to re-locate the components that are on either pins 3 and 4 of the UL41 socket. Bush seemed to think at the time that IC (internal connection) meant a spare pin. It was Gerry Wells that first pointed out to me that this caused the valve to develop internal leakage and hum. Sometimes moving these components will noticbly reduce hum too.

Cheers
Andy

[Nickthedentist]

What's the voltage on the control grid of the UL41? Do measure this before assuming the valve's OK.

Have you tried the Pin 4 mod? Try Ed's suggestion (post 8) if mine (post 9) seems too involved.

N.

[Voxophone]

I already disconnected pin 4 (by removing the pin from the valve holder) while attempting to figure out the source of the hum. I'm not sure what's connected to pin 3, but I'll look into it.

Thanks

Liam