Surprise today

[Pete_kaye]

I am using this unit with my ‘ 1928 kit radio ‘ which was discussed at length back in 2013/14 but is no longer on here. I had it working quite well from this unit but some instability on LW and some undiagnosed fault with the aerial coil on MW. It got plenty of stations. I have just noticed that I had all the -negatives and GB + connected together to the earth which can’t be right.
There is only one connector on the radio marked LT - which feeds the on /off switch. More to come...isn’t this going to affect the PSU?

[Pete_kaye]

And the circuit I drew at the time. This ties both the HT AND LT NEGS TOGETHER. Won’t this affect the working of the PSU?

[Ed_Dinning]

Hi Pete, this was the standard connection for most sets (think separate batteries).

If your PSU has HT & Lt supplies and the neg lines are commoned, there is no problem, LT + was normally the only switched line, not HT.
GB supplies are negative of common, so GB+ is connected to LT/HT neg.

Do not run the set or even change GB taps with the set powered as iy can cause excessive currents and damage valves and I/V transformers

Ed

[Pete_kaye]

Thanks Ed.

[Pete_kaye]

What concerned me was that the PSU negatives are not common, as the circuit diagram.

[Herald1360]

The filament and HT/GB supplies in the circuit given do have separate 0V but since they're fed from independent floating transformer secondaries, linking them to gether at the set (or at the PSU) will not cause any problems.

Not linking them anywhere might well do so!

[Pete_kaye]

A new question. I have used a 4 ohm speaker with a small output transformer of 320 ohm primary and 0.5 secondary. The output valve is PM 252 which is listed as needing anode load of 1900 ohms. Would this be resistance only or including impedance of the primary? The sound is loud and harsh. I may be damaging the valve. What do you think.

[HamishBoxer]

Not too convinced re valve damage but possibly need one a bit nearer correct impedance for quality I would think.

[peter_sol]

The 350 ohms figure is only the DC resistance of the transformer you are using. you can measure the impedance. have you tried a 0.01 capacitor across the primary?

[Pete_kaye]

What effect would a cap across primary have ?I will try it later.

[peter_sol]

The cap will reduce the harshness.

[Pete_kaye]

Yes it made a very slight difference.

[Cobaltblue]

The reaction isn't set too high is it?

This sort of set does sound horrible when they're on the brink of oscillating.

I believe this might be the set under discussion.

https://www.vintage-radio.net/forum/...ad.php?t=97229

Cheers

Mike T

[ms660]

Originally Posted by Pete_kaye

A new question. I have used a 4 ohm speaker with a small output transformer of 320 ohm primary and 0.5 secondary. The output valve is PM 252 which is listed as needing anode load of 1900 ohms. Would this be resistance only or including impedance of the primary? The sound is loud and harsh. I may be damaging the valve. What do you think.

In the valve data I'm looking at the 1,900 Ohms quoted is the anode impedance not the anode load impedance, the anode load impedance should be considerably higher than the anode impedance for a triode valve, often somewhere around three times the anode impedance, with pentode output valves it's the opposite, the anode load impedance will much less than the anode impedance, often somewhere around a fifth of the anode impedance.

Another way you can tell that 1,900 Ohms is the anode impedance from the valve data is to divide the amplification factor by the mutual conductance, the amplification factor is given as 7 and the mutual conductance is given as 3.7mA/V.....7/0.0037 = 1,892 (Ohms) which is near enough.

Lawrence.

[Herald1360]

And remember that the load impedance seen by the output valve anode is essentially the nominal speaker impedance times the square of the output transformer turns ratio. The actual impedance of the output transformer primary on its own (mostly Henries plus a dash of Ohms) mainly only affects the LF roll off point of the system.