Philco 333 help

[Norman Raeburn]

Hi Folks, Thanks for the replies. Ken. thanks for the pics, I will study them in some detail tomorrow. The sound is ok taken from the grid and fed to an amp. I don't know if something else has happened, the waveform on the grid looks ok but on the anode it is severely clipped it was lacking the positive swing before. The speaker and output transformer have been substituted and didn't sound any better although they may be mismatched as well. Norman

[PJL]

Have you replaced C9, the tone correction capacitor? Replace it again just to be sure as any leakage here will cause your symptoms.

The ERT sheet seems to confirm the anode current is only 2.5mA. To get any useful output from such a low anode current, the anode impedance needs to be very high, 15-20,000 ohms. If it has a non-original output transformer, it may not be suitable.

It's possible the KT24 is not happy working at these low currents but the problem is that more anode current may saturate the output transformer. You can increase the anode current by reducing R8 but you would also have to reduce R9 or V1 will be cut-off.

If you have a scope and a signal generator, what peak to peak swing do you get on the grid and anode? Try substituting the transformer primary with a 10K resistor and measure the peak to peak.

[Boater Sam]

I'm sure the KT24 will need more current than 2.5mA as you have found, it gets better with more. But you say it goes off again if you take it up too far, suggesting that it is not in a good condition. The whole thing about the KT24 is that it is an unknown quantity, it may be low emission, and we know that it is not the correct valve.
The PEN23 is not listed anywhere and it certainly is unusual working with so little anode current. As a thought, what screen current is it taking?

If the audio is good on the grid, you may have to consider a redesign of the output stage using a valve that you know.

[Norman Raeburn]

Hi, C9 has been replaced, I also ran the set with C9 disconnected. I tried a different transformer and speaker and it was a bit better than the original. I will need to look at the suggested alternatives for the output valve. I don't have anything suitable so will need to look for something. I have wasted ridiculous amounts of time on this and will have to bring it to a close soon. Norman

[PJL]

The test you did to increase anode current would not have worked as the increased HT current would have increased the bias to V1 and cut it off. If you decrease R8 to increase the anode current, you must also decrease R9 to set V1 bias to about 1.5V.

[ms660]

Might be worth configuring it for a more readily available valve type and take it from there.

Lawrence.

[Boater Sam]

My thinking too Lawrence, but what? His requirements are B5 base, 2v direct filament (unless he goes indirect and sets the bias on the cathode, which may be better), low anode current, 120v HT, 15Kohm anode load.
Gets a bit difficult finding a valve then. Or stuff a modern miniature inside an old one or underneath but then it needs to find the LT for it, 6.3v DC off a doubler?

[ms660]

PEN231, PEN220, PM22a possibly, all designed for high impedance transformer, haven't checked the valve holders but the holder could be changed if needed.

Lawrence.

[electrogram]

KL35 ? 19k anode load, but has an octal base

[PJL]

Need to check the output transformer is definitely the original part before redesigning for a 15-20K anode impedance. Measure the primary resistance which according to all 3 sets of service data should be 850 ohms.

[Herald1360]

Hiding a DL94 or similar in a tube on a B5 base would be fairly easy. The filament voltage can be taken care of with a series silicon diode (2V > 1.4V). Main HT could be dropped to 90V or a decoupled resistor used in series with the OP TX primary.

[Boater Sam]

Clever, I like that idea. DL94 is a known circuit. I would stuff it inside a B5 valve by cutting the pinch out, no one need ever know.

[PJL]

Does anyone own one of these or a similar battery set like the Ultra 150? I assume the sound quality is abysmal as anything but quiet volume and the output will be into class AB. The only battery set I have is a Murphy and that uses PP and is still pretty terrible.

[Norman Raeburn]

Hi Folks, I will try to answer some of the questions. When I increased the current through the output valve V1 still had it's original bias, I only tagged the variable supply onto V3. It has the original output transformer fitted which has 18k stamped on it and it reads 850 ohms. I will check out the suggested valves for base compatibility. I don't really want to change the base as it's not my radio. Thanks for the help, I am sure we will resolve this. Norman

[peter_sol]

The output transformer may have shorted turns.
If you have a radio using a DL96 you could substitute its transformer to prove.
Resistance measurements are not going to help there.

[PJL]

Norman, the bias for V1 is determined by the total HT current so increasing the anode current in the KT24 will automatically increase the V1 bias.

Good to see the txfmr is correct so everything points to the wrong valve. You could try an entry in the wanted section for the list Lawrence suggested above?

PS: Try Peter's suggestion first!

[Herald1360]

Originally Posted by PJL

Does anyone own one of these or a similar battery set like the Ultra 150? I assume the sound quality is abysmal as anything but quiet volume and the output will be into class AB.

Dunno about these early sets, but the sound quality with a couple of hundred milliwatts in class A from a DL94 or the like in 1950s sets is normally pretty good.

[Norman Raeburn]

Hi Folks, I am demented with this now. I have taken the plunge and purchased the manufacturers data as it has more voltages quoted and a nos PEN23. The sound is better but still distorted. The voltages I have on the valves are as follows. V1 hf amp, anode 120v reads 121v, screen 67.5 reads 53v, grid -2.5 reads-1.2v. V3 detector anode, 12.5 reads 18, screen 40v reads 28v, grid not quoted although reads 0v. V2 output, anode and screen 120v reads 121v, grid manual quotes -6.5 at c2 tag 2 measures -3v and there is -2.9v on the grid.

The ht current is quoted as 6ma which it is, this has the early version of R1/2 fitted which are 300 ohm and 200 ohm. I tried fitting the later resistors which then quotes ht current of 8.5ma which it is, I have fitted the originals back in.

I found by playing with the output stage that the max signal you can put into the grid and get clean signal at the anode is 2v p/p this gives 100v p/p out.

I have tried again taking the audio out from the grid to an amplifier and it sounds fine.

I don't know what to do with this next, I do know it will be coming off the bench soon and won't be going back on.

I can't understand why the audio is ok on another amp and how the output stage works ok using the signal generator.

I really don't know where to go with this now. I look forward as always to the replies. Norman

[ms660]

I might be wrong but I wouldn't have thought you would get more than 100 volts peak to peak undistorted at the anode with the HT rail as designed, it might be worth trying another loudspeaker/output transformer again seeing as you now have a NOS valve fitted.

Lawrence.

[Norman Raeburn]

Hi Lawrence, I have just tried another transformer and speaker, I think it's probably a mismatch but the max clean signal I can get at the anode is 35v p/p with 3v on the grid. Norman