philips: PM5320 (PM 5320); AM/FM Generator E80F substitu

Good evening Forum Readers,

I herited some time ago a defective PM5320 generator, and I am currently repairing it.

This set is equipped with a E80F Special Quality tube, used as output stage/AM modulator.

I tested this tube with my AVO VCM 163 tube tester and there is a severe C/H Insulation loss.

This means that there is a problem with the isolation between the cathode and the heater.

This tube has similar characteristics as the EF86. Now my question: can I substitute the defective E80F by a EF86?

 

Thank You

 

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Hello Georges,

The E80F was a high quality version of the EF86 and has the same characteristics as the EF86. I can see no reason why you could not substitute an EF86 for it in your application.

William.

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hello georges , i have the mullard reference book giving original and alternatives [tubes]and no mention of either of the valves , are in it , are u sure that these valves in  your set are the original recomended tubes by philips , have you got a schematic of your model confirming that these are in fact the recomended tubes [valves ], respectfully maitiu,

 

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Dear Georges,

If you cannot find one, I can probably send you one.

Regards,

Roy

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The only thing which directly substitutes for an E80F is ... an E80F.

It isn't quite an EF86, even though some people try to charge a premium for E80Fs by claiming this is the case. The E80F is physically taller and takes 300mA of heater current as opposed to 200mA for the EF86. The anode curves are similar if you correct the screen grid voltage. The capacitances are similar and the two tubes shares the same internal structure with the electrostatic shield for the heater, contributing to the low hum levels for both types.

g1g2 transconductance differs by nearly 60%, powerlevels likewise (the E80F is the toughest).

But you can plug an EF86 into a socket intended for an E80F, the pinouts are the same. It may even work after a fashion. Keep an eye on power dissipation.

An EF86 works well at RF. It has no built in frequency counter and will amplify whatever is presented to it if the operating conditions are choosen appropriately.

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Dear All,

in the meantime, I replaced the bad E80F by a good EF86, and the PM5320 seems to operate normally. I only made a test with relative low frequencies. (AM IF frequencies). I will go on later with FM IF frequencies.

I am aware that an E80F is not an EF86. The E80F is indeed taller, and has golden plated pins, and some specs are indeed different.

I am wondering why Philips used for this application the E80F instead of the (probably cheaper) EF86.

I am also wondering why only this "Special Quality" tube went defective, all the other (standard quality) are OK...

All the other tubes in the PM5320 are standard quality ones, so why use only one tube that has a life of 10000 hours?

PS. Roy, thank you for the offer. I will try to find a replacement E80F here in Belgium.

 

Georges

 

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Hi Georges,

After having looked at the circuit I would hazard a guess and say that the Philips engineers were looking for the screen grid dissipation of the E80F, combined with its other particular properties.

In the PM5320 the E80F is run with a very large combined cathode resistor, 560 + 5K6 ohm, so the supressor grid, g3, is quite negative with respect to the cathode. This makes it very easy to make good 'grid AM modulation', because there is little risk of running g3 positive due to 'overmodulation'. You alread mentioned being a modulator was one of the purposes of the output buffer stage.

But running g3 very negative means that the screen grid will - on average - see the brunt of the cathode current. The normal division between screen - and anode current is of course only valid for zero volts on g3.

I haven't done the math, but I would find it interesting to hear if you made any voltage measurements on the circuit with the good EF86 in place. That would allow you to compute the screen and anode currents, and thus the dissipation in each. At 400mW the E80F has double the screen dissipation rating of the EF86.

The reason why your particular E80F has gone bad while the others seems OK could be that the other common valves has been replaced, while the perhaps original E80F was left in place due to the lack of a suitable replacement.

Hope this helps a bit.

Frank.

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