Is this valve defective? - UK Vintage Radio Repair and Restoration Discussion Forum

Skywave · 9th Apr 2021, 8:05 pm

The valve is a PL83. Although this valve is usually found in television sets, for this post and the reason for it, I'm regarding it as a component.

Test set-up.
I built myself a simple valve tester. I can adjust the anode voltage, Va, & screen grid voltage, Vg2, (if such a grid exists) over the range 110v. to 375v. and monitor the resultant currents; I can adjust and monitor the control grid bias Vg1 over the range 0v. to -40v; I can set the heater voltage to common values.
The cathode and suppressor grid (if fitted) are wired to the common 0v. / chassis. There are no resistors between any valve electrodes and their associated sources of voltages.

Data and evidence.
With this particular valve, I suspect I have a control grid to screen grid internal leak. With the electrode voltages set as appropriate for a PL83 (Va = 170v.; Vg2 = 170 v.; Vg1 = -2.3 v.), the anode current is excessive at 100 mA or more (should be 36 mA). Moreover, that Vg1 voltage keeps drifting around by itself over the range -1.0 v. to -4.5 v. (with Ia changing accordingly). Also, when I adjust Vg2, Vg1 changes accordingly: if Vg2 increases, so does Vg1. The valve glass envelope does get very, very hot.

Now there is not an internal valve connection between grid 1 and grid 2 (if the valve is not defective), and I cannot see why varying Vg2 should bring about a change in Vg1 - apart from an internal leak, g1 to g2.

I'd appreciate the thoughts & options of other members here of this matter, please.
Thank you.
Al. / Apr. 9th.

ms660 · 9th Apr 2021, 8:48 pm

Make sure the circuit hasn't turned into an oscillator, if it hasn't I would say the valve is faulty.

Lawrence.

radiomobile · 9th Apr 2021, 9:00 pm

2 possibilities
You do not mention g3 (pin 6) Have you left it floating? It should of course be connected to cathode in your set-up.
The PL83 is a high slope valve with a gm of 10 mA/v. It could be oscillating...(going in to business on its own!) which always gives chaotic readings on a valve tester. In that case a few ferrite beads or grid and anode stopper resistors on the wiring close to the valveholder could solve the problem.

Skywave · 9th Apr 2021, 9:05 pm

Thanks for the replies.
The suppressor grid, g3, is wired to the common 0v. / chassis - as I stated.
I hadn't considered the possibility of self-oscillating: easy to check with the 'scope: will do it now.

Al.

Sideband · 9th Apr 2021, 9:12 pm

Quote:

Originally Posted by ms660

Make sure the circuit hasn't turned into an oscillator,

Lawrence.

Yes be careful with that. Long leads at the valveholder can cause all sorts of problems. I once made up a B9A 'extender' socket for my Tayler valve tester (no longer have it). Some valves tested fine, others gave very odd results. When I ended up with a heavy negative voltage on a grid, I scoped it and it was oscillating at well over 60Khz. I had to redesign the B9A adaptor.

Skywave · 9th Apr 2021, 10:00 pm

Fixed it: the valve was oscillating at approx. 175 MHz. The addition of a ferrite bead at pin 7 plus a 150 pF mica cap. from pin 7 to chassis / 0v. stopped the osc.
With Va, Vg2 and Vg1 are the appropriate voltages (refer to post #1), Ia is now 33mA (the 'book' value is 36 mA). Moreover, varying Vg2 produces no change in Vg1 at all.
I will just mention that this tester does not use any a.c. signal voltages: only d.c. voltages and currents are utilized. Therefore, that 150 pF cap. will not be attenuating any a.c. test signal waveforms.

A point arising.
This home-made valve tester is furnished with many different types of valve sockets. Consequently, there is great deal of under-chassis wiring. I need to bear in mind what I have learnt from this problem & its fix: I may meet this problem again with other valves and other sockets at a later date.

Again, thanks for the assistance / advice.
Al./ Apr. 9th.

ms660 · 10th Apr 2021, 10:02 am

Here's what Avo did, page 12 in the manual, 2nd para:

https://frank.pocnet.net/instruments...VCM_Mk-III.pdf

Lawrence.

Skywave · 10th Apr 2021, 6:56 pm

I read through that manual. I can appreciate that AVO have taken account of the large variations, different types of valves, different valve sockets, different things to measure, etc., with the aim of making this instrument as all-serving & as comprehensive as possible and overall and in summary,yes, it is an impressive result. But would I want to own one? No thanks. I very much doubt that I could understand how to use it sufficiently without running the risk of either damaging the valve under test, the instrument, or both.

Al. / Apr. 10

9th Apr 2021, 8:48 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Is this valve defective? Make sure the circuit hasn't turned into an oscillator, if it hasn't I would say the valve is faulty. Lawrence.

9th Apr 2021, 9:00 pm	#3
radiomobile Pentode Join Date: Nov 2020 Location: Bristol, UK. Posts: 132	Re: Is this valve defective? 2 possibilities You do not mention g3 (pin 6) Have you left it floating? It should of course be connected to cathode in your set-up. The PL83 is a high slope valve with a gm of 10 mA/v. It could be oscillating...(going in to business on its own!) which always gives chaotic readings on a valve tester. In that case a few ferrite beads or grid and anode stopper resistors on the wiring close to the valveholder could solve the problem.

9th Apr 2021, 9:05 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Is this valve defective? Thanks for the replies. The suppressor grid, g3, is wired to the common 0v. / chassis - as I stated. I hadn't considered the possibility of self-oscillating: easy to check with the 'scope: will do it now. Al.

9th Apr 2021, 10:00 pm	#6
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Is this valve defective? Fixed it: the valve was oscillating at approx. 175 MHz. The addition of a ferrite bead at pin 7 plus a 150 pF mica cap. from pin 7 to chassis / 0v. stopped the osc. With Va, Vg2 and Vg1 are the appropriate voltages (refer to post #1), Ia is now 33mA (the 'book' value is 36 mA). Moreover, varying Vg2 produces no change in Vg1 at all. I will just mention that this tester does not use any a.c. signal voltages: only d.c. voltages and currents are utilized. Therefore, that 150 pF cap. will not be attenuating any a.c. test signal waveforms. A point arising. This home-made valve tester is furnished with many different types of valve sockets. Consequently, there is great deal of under-chassis wiring. I need to bear in mind what I have learnt from this problem & its fix: I may meet this problem again with other valves and other sockets at a later date. Again, thanks for the assistance / advice. Al./ Apr. 9th.

10th Apr 2021, 10:02 am	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Is this valve defective? Here's what Avo did, page 12 in the manual, 2nd para: https://frank.pocnet.net/instruments...VCM_Mk-III.pdf Lawrence.

9th Apr 2021, 8:05 pm	#1
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Is this valve defective? The valve is a PL83. Although this valve is usually found in television sets, for this post and the reason for it, I'm regarding it as a component. Test set-up. I built myself a simple valve tester. I can adjust the anode voltage, Va, & screen grid voltage, Vg2, (if such a grid exists) over the range 110v. to 375v. and monitor the resultant currents; I can adjust and monitor the control grid bias Vg1 over the range 0v. to -40v; I can set the heater voltage to common values. The cathode and suppressor grid (if fitted) are wired to the common 0v. / chassis. There are no resistors between any valve electrodes and their associated sources of voltages. Data and evidence. With this particular valve, I suspect I have a control grid to screen grid internal leak. With the electrode voltages set as appropriate for a PL83 (Va = 170v.; Vg2 = 170 v.; Vg1 = -2.3 v.), the anode current is excessive at 100 mA or more (should be 36 mA). Moreover, that Vg1 voltage keeps drifting around by itself over the range -1.0 v. to -4.5 v. (with Ia changing accordingly). Also, when I adjust Vg2, Vg1 changes accordingly: if Vg2 increases, so does Vg1. The valve glass envelope does get very, very hot. Now there is not an internal valve connection between grid 1 and grid 2 (if the valve is not defective), and I cannot see why varying Vg2 should bring about a change in Vg1 - apart from an internal leak, g1 to g2. I'd appreciate the thoughts & options of other members here of this matter, please. Thank you. Al. / Apr. 9th.

10th Apr 2021, 6:56 pm	#8
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Is this valve defective? I read through that manual. I can appreciate that AVO have taken account of the large variations, different types of valves, different valve sockets, different things to measure, etc., with the aim of making this instrument as all-serving & as comprehensive as possible and overall and in summary,yes, it is an impressive result. But would I want to own one? No thanks. I very much doubt that I could understand how to use it sufficiently without running the risk of either damaging the valve under test, the instrument, or both. Al. / Apr. 10