I have two STC 205 "Jelly Mould" on the bench at the moment valve line up V1 6J8G, V2 6U7G,V3 6B6G, V4 6V6GT, V5 6X5G, all the correct valves were in place with the exception of Radio 1 which has ECH 33 replacing 6J8.

I have replaced the two original Filter Caps C27 and C28 on both, after completing all the usual safety checks and replacing the power cords I plugged each one in using my Lamp in series set up and they both run quite well, one of them is showing some instability as it occasionally breaks out in a squeal and whistle.

I then checked the voltages on both against the data sheet I downloaded from Radiomuseum.

I found that even though the plate voltages at the 6X5 rectifier were 216 AC using a digital voltmeter,
The plate voltages on both Radios were well below the 200V specified for Plates on V1 and V2 typically 116V -150V the screen voltages were both these valves we about 60V. V3 plates was at about 80V instead of 100V and V4 around 100-130V .

These measurements were around the same range for both Radios. I assumed maybe the multimeter could be the problem, so I checked it with another to eliminate this.

Also the measured voltage drop across the choke is around 16V for Radio 1 and about 20V for Radio 2 where it should be 30V


So my question is assuming these voltage measured are correct, what is impact on the radio long term from running at these lower plate voltages. I had read somewhere that it was not good to run lower plate voltages.

As I said the both radios are running quite well, good volume no distortion other than stability on one which I think is due to bad grounding contact of the shield on the 6U7G?

It won't harm the valves to operate them at lower anode and screen voltages, but performance will suffer.

I find that the only way to cure instability with a shielded 6U7G (or 6D6 etc) is to solder the two halves of the shield together with wire and run a wire soldered to the chassis.

Excessive current draw could be the reason for the low H.T. voltages. This will be certain if you haven't replaced the paper capacitors. Also, the 6X5GTs could be weak.

Cheers.

Thanks Sirwin,
Great suggestion on the shield. I was able to find another 6X5 GT from another of my working radios and swapped it out and the plate voltages were now around 180-190 V. On both Radios, so it appears these two rectifiers will need to be replaced and I have none in my spare valves.

As a rule I do not change all the paper wax capacitors if the radio is performing and voltages look right, but interestingly once I sorted the Plate voltage, I had then went through and changed the 0.01 μF coupling capacitor and my good working Radio with great volume lost its volume. So I went through tring to track down what I might have done wrong or did my 6V6 begin to deteriorate ?
After a couple of hours swapping valves, checking resistors, I then decided it might have been the new yellow poly capacitor that I had put in, after replacing it everything was ok again.

So the lesson I learnt was when something changes after you made a change suspect the last change you made first no matter how unlikely you think it might be. I do not have a Capacitance meter and rarely check new parts as I expect them to work!

So the lesson I learnt was when something changes after you made a change suspect the last change you made first no matter how unlikely you think it might be.

Yep. Cast that in bronze, put it on the workshop wall and shine a light on it.

Waxed paper & old electrolytic caps can present as & cause massive loading. Unused (for abt 2years plus: Mainly plus) Electrolytic caps can present as a short and that does neither transformer, or rectifier any favours at all.

I often find ECH33 /ECH35 and 6J8 interchanged & it is actually something resulting from WWII. Often valves were scarce, as WD loved scarpering off with them. That meant common substitution with Philips & with one HMV I have dealt with the circuit was changed to suit the Philips valve.

Pin 1 ECH33 is the one to watch. It is the metalisation (red conductive coating) and it grounds via pin 1.

Marc
If a short across the Electrolytic caps was present would it not cause the lamp in series I initially use to be brighter. I did change both large 16μF Electros prior to powering up and my 40W lamp was very dim. I had however not changed the only other Electro, the 40V rated 25μF which comes off the Cathode to ground of the 6U7G. Should I ?

So with ECH33 should I expect to find pin 1 going to ground as a modification in the radio or are you suggesting to look if the pin 1 lug is being used as a connection tag?

My normal comment with electrolytic caps is to toss the lot if they are really old. The ones on cathodes die as well. Are the valves in the correct order? I need to look at the circuit. 6B6 & 6V6, I would expect to see electrolytics across Rk not 6U7. The only other "odd ball" one common in Philips, is one across "back bias" CT to chassis. That one is negative to CT positive to chassis.

On a lot of valves like metals, and Philips (not all check valve data) pin 1 is either the shield, or metalisation. That shield / metalisation pin, is grounded.

What can happen & I have seen. ...........

It was made for a glass valve, so pin 1 is not grounded; Someone uses a metal one & it goes haywire.
It was made for a metal tube, so a glass one is used, no shielding: It goes haywire.
Pin 1 is used for a "tag of convenience" someone puts in a metal valve or Philips metalised: It shorts.

I see no value in using a globe in series with B+ as a Barretter: Thermal inertia is all wrong. The "B" voltage often has to be 180V to 200V for the valves to work. I rarely use the Variac. Running the valves at too low a voltage can in some cases poison the cathode.

You could possibly compare Breville 730, it also uses ECH35. 6J8 may be fitted with an external shield / can. 6U7 will go berserk with none.

Looked at the circuit.

ECH35 / 33 are listed as Triode Hexodes: 6J8 Triode Heptode Albeit pin outs are the same except for pin1.

6U7 shares its cathode with 6B6.

The Philips Valve is likely a ring in but should work: The paperwork specifies 6K8, the circuit shows 6J8? I have found mix & match with all 3. 6K8G is also a Triode Hexode. 6K8G is not shielded 6K8 & 6K8GT are via pin 1.

Wonderfully confusing so I would check the wiring of pin 1 of V1.