Ekco UL84 Voltage Issue

[Danndad]

I have an ekco zodiac u800 here that I cannot get the grid 2 voltage to be lower than the anode voltage as stated by the ekco sheet.  The voltage instructions state that they should be taken with mains input @210vac into the 200-220 volts tap with the anode on the ul84 at 144v and grid 2 at 142v, those conditions can’t be met here with mains voltage at around 240vac at present.

With mains voltage here at around 240vac the anode =166.8v grid2 =168.2v, the anode voltage of uf89 (IF amplifier) is a bit high at 170v. Resistors have been checked and any wax caps are now gone, I’ve not replaced the big smoothing caps.
I’ve even tried to tak a 470r in parallel with the 1k ht feed resistor to try and bring grid2 voltage down, its lowered the anode voltages on the ul84 and uf89 but increased grid2 on the ul84 to 174.4v.

Any ideas please ??


   

[Danndad]

Forgot to say that I have tried 2 good ul84 and voltage is still higher on grid 2

[ppppenguin]

Are you on the 210V tap? I hope not as you'll be overruning the heaters. The actual HT voltage isn't critical on this sort of set..

[Danndad]

No Jeffrey, Its on the 225-250v tap, I dont have any other radios here with ul84 grid voltage higher than the anode.

[Murphyv310]

You won't get the G2 volts lower in that circuit, it's an accountant that said..... No resistor in that G2. Put the meter away and don't worry, the UL84 will oulast you.

PS putting a 470 ohm in parallel with the 1k will increase the G2 and anode volts further back, are you sure you didn't mean series. Remember the voltage drop on the output transformer to the Anode will be greater than to the G2 and other valves as the UL84 anode will draw more current than the rest of the set.

[ppppenguin]

I'm sure you've already done this, but please makes sure you've have replaced C43 ("that" capacitor) and C45 (tone correction). Failure of either will cause damage. There's a good case for connecting the 0V end of C45 to HT instead. Then if it fails it won't damage the output transformer.

[Diabolical Artificer]

Valves are pretty hardy & in the days when they made UL84's and they cost 3s it didn't matter if the valve was running hard, as there is now a finite supply of UL84's and they can fetch a tenner it may be worth bunging a grid dropper or stopper on g2. Datasheet says Va 170, Ia 70mA, Ig2 5mA, so if we want to drop 5v ish you'd need a 1k between HT and g2, that's if you can get to the base etc to do the work. To do the job properly you'd also put a 100r approx grid stopper after the 1k with a 0.1u - 10u cap de-coupling the 1k to ground, that might be a bit overkill for this case though.

A 1/2w 1k will be sufficiant, if you wanted to drop a tad more a 1k5 or 2k2 should do the job.

Andy.

[Murphyv310]

Hi.
I concur with Jeffrey and to a point with Andy. The UL84 is run fairly easily in these sets, the G2 is also pretty rugged and the valve can stand a fair amount of abuse. There is every likelihood that its the original valve, so its lasted from the late 50s to 2022, so why panic, is the radio going to run 24/7? I doubt it and the UL84 will soldier on for many years, even if the emission drops to 30% I doubt you'd really notice the difference.
Yes change the coupling capacitor to the G1 and replace the tone corrector and move it to be in parallel with the output transformer primary, all sound advice.

I'd also say look at many circuits where the G2 on audio output and frame output valves in TV receivers are directly connected to HT its common practice and doesn't write out the death certificate for valves, that I can assure you.

[Murphyv310]

Incidentally.
The original sheet would use an AVO 7 most likely and that meter could well load up the HT to the G2 and remaining valves enough to give a lower reading, a high impedance DMM will read higher.

[PerdioPal]

Output valves in their time were run at top (or nearly so) ratings for a large advertised output, nothing wrong with upping the cathode resistor to give it an easier time unless you want ear shattering volume. Also saves the electric, so expensive these days. I put a resistor in series with the volume control sometimes to make it more usable too.