Philco 333 help

[Norman Raeburn]

Hi, Yes I am running 120v and 67v, I did see the other thread. The anode current on this is just under 4ma, I don't know if this is where the problem lies. I wonder if the KT24 is faulty or if it's the operating conditions that are wrong. Norman

[Boater Sam]

Looking at the diagram it strikes me that the G2 voltage on the output valve is going to be higher than the anode by a goodly amount. Have you tried a resistor in the G2 to drop the voltage below the anode?

What result do you get by taking the audio off at the G1 and feeding it into an amp? If the distortion disappears, I would find another 2v output valve.

Finding a valve with enough slope at 10mA anode current is going to be a challenge unless you are prepared to up the current and use up more batteries.

[Boater Sam]

At 4mA you are so far down the curve that it will not work properly. Why so low? is it the G2 taking current?
KT24 was designed for anode up to 150v.

[julie_m]

The cathode bias resistor is actually formed of R1 + R2 (manufacturers' service data) in series; the bias voltage is developed across this pair. The tapping point can be ignored to begin with. Get V2 biased right first, with a variable resistance in parallel with R1 + R2, then calculate the individual values of R1 and R2 so as to supply -2.5 V to the grid of V1 (you can assume no current flows into or out of a control grid, especially compared to the current through R1 + R2). Let this current be I; now Vbias = I * (R1 + R2) and Vtap = I * R2, so you can calculate R1 knowing what (R1 + R2) should be.

EDIT: It's common in battery sets to apply the full HT voltage to G2. Don't worry about this.

[Boater Sam]

Julie, I agree with that for V2, can we assume that V1 bias is not -2.5v at present, I don't know if Norman has measured it. We seem to be considering a redesign.
The V3 stage is puzzling me, firstly is is designed for a high slope valve for which we have scant detail apart from 10mA anode current but we have only 4mA, and the g1 is fed audio via a 490k ohm resistor with a g2 far higher than the anode.
How is that going to produce gain? The output is supposed to be approaching 650mW.

The fact that this set has not worked well since restoration by person unknown is worrying, has Norman or anyone checked it against either another set or the circuit diagram for errors? I think they were never "good" performers.

[julie_m]

Originally Posted by Boater Sam

Julie, I agree with that for V2, can we assume that V1 bias is not -2.5v at present, I don't know if Norman has measured it. We seem to be considering a redesign.

Well, the redesign is required by the substitution of a KT24 for V2. Unless a PEN23 can be found at a sensible price, R1 and R2 will have to be changed to suit the KT24. And according to this page, the slope of the curve changes around 5 mA; so no wonder the output is asymmetric.

We can't go much further than that without some more figures. Norman -- what are the voltages at V2 pin 2 and the junction of R1 and R2? What values have been fitted in these positions? And have you got a 1 kΩ wirewound pot, or (better) a decade resistance box? Connect that across the series combination R1 + R2 slider and anticlockwise end, if using a pot; start with it fully clockwise. If using a decade box, start at 1 kΩ.

Thinking out loud, V1 is going to want to conduct harder with the smaller negative bias voltage that is inevitably going to get during this test. Perhaps unplug it for now and use a signal generator coupled to the junction of R3, R4 and C5 via a 10 nF, 400 V capacitor.

Adjust the bias resistance to get 10 mA idle current through V2, then calculate R1 and R2 values to give -2.5V at the junction for V1.

[Norman Raeburn]

Hi Folks, Many thanks for the help so far, I am away for the weekend and won't get back to this till Monday. I am going to thoroughly check the circuit against what is fitted to the set. I really appreciate the time and effort being put in. I feel I am learning as we go along. All the best for now. Norman

[Boater Sam]

Reading the Trader sheet, run 2 sets had 8.5mA HT current, with altered R8 R9, that's R1 R2 on the makers sheet. They were reduced to 180 & 120 ohms respectively. Have you read the Trader sheet?
If you follow what Julie has said, you should aim for at least this HT current to get the bias correct on the first 2 valves, which for some reason are V1 & V3 on makers sheet.
PLUS
The original Pen23 ran at 2.4mA, Ia , the replacement KT24 will not work at this figure, you are going to have to push more than 5mA Ia through it. I don't know of any other valve you could try in this position, the suggestion that it is the same as PEN231 is interesting. The battery consumption will rise, no way round that.
Fascinating problem Norman, thanks for sharing it.

[Silicon]

Instead of using resistors to set the bias on the 2 valves, would it be possible to use a string of forward biased diodes?

The bias would be less dependent on the HT current but you would lose the 'automatic bias' effect provided by resistors.

Perhaps a combination of diodes and resistors would help in the fault-finding stage.

Production run 1 used 300 Ohms and 200 Ohms respectively for R1 and R2. That is significantly different to the 180 and 120 Ohms used on production run 2.

[Boater Sam]

Yes, and that resistors change significantly increased the HT current but probably not enough to make this replacement valve work properly. It was still only 8.5mA for the whole HT.
I don't think Norman will want to go down the semiconductor route but its a novel thought.

[julie_m]

Different values, but in the same proportion; as you would expect for a potential divider.

200 / (200 + 300) = 120 / (120 / 180), so the tap-off voltage will not be changed. The current through the chain is increased, which suggests maybe a reliability enhancement; making V1's bias voltage less affected by currents flowing where we assumed they would not.

Anyway, first determine experimentally a value for (R1 + R2) that will give you 10 mA through V2 with no signal; then use the fact that R1 = 0.6 * (R1 + R2) and R2 = 0.4 * (R1 + R2) to give you the individual values to get the correct bias voltage for V1. 0

[Silicon]

I don't think that we can use the same ratio of R1 to R2 now that the output valve has changed.

The original bias design doesn't make much sense to me when I do the calculations.

The Philco service instructions say that the bias voltage across R1 & R2 is -6.5V.
That requires an HT current of 13mA on production run 1 sets.

It may be better to start with the valve characteristics and design a new bias network.

The VP21 RF valve probably requires a grid bias voltage of -1V to get the 2mA anode current and 0.5mA screen current specified on the service sheets.

I can't find any data on the Pen23 or KT24 output valve yet.
If Norman rigged up a circuit to plot the anode current of the KT24 at various grid voltages we might be able to suggest a suitable bias network.

Double check the polarity of the electrolytic capacitor across the bias network. The Positive terminal should be connected to chassis.

[julie_m]

No, we can't use the same ratio of R1 and R2 with a different output valve. We have to work out what the new bias voltage will have to be to give 10 mA through V2, which defines the total resistance of R1 and R2 in series; and then select R1 and R2 so as to give the correct bias voltage for V1.

The only data I can find online is for the KT21, an obsolete type for which the KT24 was reckoned to be a replacement; and even that is not too comprehensive, though it does suggest that the present operating point is sub-optimal, as confirmed by observation.

We really need Norman to supply some figures now (see post #26), otherwise we are just going to be going around in circles.

[PJL]

To measure the output valve anode current just disconnect the anode and put a meter in series set to DC current, it should read around 2-3mA. Using the sum of all the currents from the trader, the grid bias across R8+R9 is about -3V. The KT24 looks like it will give similar anode current to the PEN23 so hopefully no changes required.

The critical valve is the leaky grid detector, V2, and the 210SPT is listed as suitable for this function. However, the datasheet suggests the anode current at Vg=0V and Vsg=40V may be higher than the original SP21 which might explain the distortion. You may need to adjust V2 anode load R2 and screen grid feed R3 to get the DC conditions right. The trader says the anode volts=15V and SG=40V which is probably set low to keep the battery consumption down but I just wonder if the 210SPT is still linear at those voltages. Ideally you would find a similar set that uses this valve for a detector and copy the values.

Lastly, you should check the anode current of the 210VPT which is marked as around 2.1mA. Looking at the datasheet for the 210VPT, the grid would need to be about -1V and it might be a little lower than that but near enough.

Don't expect it to be distortion free using this type of detector and manufactures built their designs around the characteristic of a particular valve so 'equivalents' are not guaranteed to be a direct replacement.

[kenneth]

Norman, I have a very rusted Philco 333 chassis which shows some signs of have been worked on slightly in the past (ie each of the 3 large Philco paper capacitors have been clipped at one end). If you think pictures of the underside may be useful I'd be happy to oblige. Ken.

[Norman Raeburn]

Hi Folks, Back to this radio after a weekend away, I am working off the trader sheet. I did take a feed off the o/p valve grid and fed it to an amplifier, it sounds ok. The grid bias resistors R8/R9 are the earlier version 300 and 200 ohm. The grid bias applied to V1 is -1.25. The voltages on the output valve are anode 118v, screen 121v and grid -3.5v which gives 4ma anode current. I tried removing the bias resistors to the o/p valve and substituted a variable power supply in there. It gives most volume around -3.5v on the grid. I varied the voltage to the grid and -0.5v gives approx 9ma anode current. The sound is clearer at this but has much lower volume. I am trying to stay with you on this but it's getting over my head. I have had a look at my valve stock and don't have anything to substitute the o/p valve with. I really don't know what to try next but I am hopeful of a solution. Hi Ken, a few pics of the underside may be useful although I don't think there is a wiring fault. All the best and many thanks, Norman

[kenneth]

A few pics.

[kenneth]

......and a few more. Ken.

[PJL]

There seems to be quite a discrepancy between the manufacturer and trader sheets on anode current. The trader says it is 2.4mA but the manufacturers sheet shows -6.5V (probably rev 2) on the grid which requires a total HT current of 13mA with maybe 8mA for the output valve.

I would therefore say your 4mA anode current is about right. Are you confident the sound was good taken from the grid? If so then maybe there is a problem with the speaker?

[electrogram]

The anode load requirements of the pen231 - 19k - looks too different from the kt24 -10k - for the latter to give satisfactory results when used with the original output transformer