Dansette major rectifier

[Nuvistor]

The UL84 is deigned to be able to be used at a lower HT, intended for AC/DC circuits, check the link in post 37, it gives operating parameters of 100v and 170v.

Still I agree a higher voltage would be better, a circuit for this model of Dansette seems unavailable which would make things a bit easier.

The valve is drawing just over 41ma (5v 120 ohm), which is reasonable.

The output voltage from the transformer is not high, but if the transformer was faulty I would have expected that to show with overheating or very poor regulation.

Interesting thread.

[Station X]

What's the HT voltage under no load conditions ie with the valve removed?

My understanding is that in a half wave rectified smoothed circuit it should be 1.4 times the AC input voltage. AC voltage from the mains transformer has been quoted as 147V.

[leslie5555]

Graham , without the valve it measures 198v

[Station X]

Interesting. You're dropping 86V somewhere. For the 41mA of combined anode and screen current that represents a resistance of 2097R.

You mention a 10k resistor. Is that the screen grid dropper resistor? Are there any other high value resistors around the output stage or PSU?

[leslie5555]

That 10k is connected to the cathode of the diode and yes the screen grid, there are no other high value resistors

[Herald1360]

Don't forget the resistance of the HT winding and the on load sag in the half wave rectified dc.

If the HT winding on the TX is an isolated one, you could disconnect the 0V end from chassis and use it to feed a bridge rectifier. Take the bridge - to where the TX 0V went and + to the reservoir capacitor.

That'll give you less dc sag and give the TX an easier ride too.

[leslie5555]

Thanks for your reply Herald , but unfortunately all that is a bit over my head lol.

[Station X]

Originally Posted by leslie5555

That 10k is connected to the cathode of the diode and yes the screen grid, there are no other high value resistors

That fits in with the representative diagram in post #31.

What is the resistance of the mains transformer's secondary HT winding?

[Heatercathodeshort]

I can't see a UL84 working without a preamp with only 112v on the anode. Is the reservoir capacitor O/C? There is only .005v on the cathode..J.

[Station X]

Originally Posted by Heatercathodeshort

There is only .005v on the cathode..J.

That confused me too. There is no decimal point and they're redundant leading zeroes, so 5V. At least I hope that's what's meant!

It might be worth connecting a known good cap of similar value in parallel with the reservoir cap (32uF according to the representative diagram) to see what happens.

[Nuvistor]

I would have expected lots of hum with the reservoir cap o/c, but it’s worth checking, the 005v made me think but then took it as 5v, could be wrong though.

[leslie5555]

Sorry my mistake I rechecked and the cathode measures 5.6 volts the tx sec is 120 ohms ?

[Station X]

The transformer resistance would drop the HT volts by about 5V which is insignificant.

Are you sure the resistor in series with the diode is 100R? If that's OK the only thing left is the reservoir cap. If that's OK I guess the HT voltage was meant to be low.

[leslie5555]

The original one was coloured brown black and orange and I and I believe its been in from new, the replacement I measured before fitting and it is 10k the only difference is the new one is 2w and I think the original one would have been 1/4w I don't suppose that makes any difference?
On the service sheet it shows 4.7k but there are a few differences with this amp and I just changed like for like which is what I have been advised to do in the past

[Station X]

I was referring to the 10OR resistor used to compensate for the lower forward volt drop of a semiconductor diode when compared with the metal rectifier.

If you didn't fit one there's no need to bother.

[leslie5555]

Sorry , in this amp its 120 and didn't need changing.

[Station X]

That's the cathode resistor.

I'm going to lash up a similar PSU circuit and see what sort of HT I get.

[leslie5555]

The one connected to the diode is 10k

[Chris55000]

Hi!

Using Duncan's PSU Designer, assuming a Silicon Rectifier and a 32uF reservoir capacitor, with the transformer details given, you can expect the rectified H.T. to give an average reading of about 155V d.c. with about 27.5V peak-t0-peak sawtooth 50Hz ripple.

A bridge-rectifier unit (one of the small square ones with a single fixing-hole in the middle) will give an average rectified H.T. of about 172-175V with about 10V p-p sawtooth ripple at 100Hz.

For a half-wave rectifier, the screen-grid voltage is about 135V with about 0.5V p-p ripple half-wave, and about 150V with about 0.1V p-p ripple with a bridge rectifier, based on a typical screen-grid current of 5mA and an anode current of 50mA.

I based the calculation on a load of 2.7k which approximates to the current drawn by a UL84 valve operated at that voltage.

Chris Williams

[Station X]

Thanks Chris.

Is that with a load of 41mA?

The HT has been variously quoted as 150V in post #1, 147V in post #14 and 112V in post #39. The final reading is actually an anode voltage, but I wouldn't expect the resistance of the OP transformer primary to make much difference.