Ekco UL84 Voltage Issue

[ppppenguin]

I agree with PerdioPal. It certainly won't hurt to increase R23 from 180R to 220R. Needs to be at least a 1 watt part. Or you can put a 47R resistor in series with R23. I assume you have replaced C44, the cathode bypass. 47uF, at least 25V is suitable.

[Murphyv310]

Hi.
Indeed a good idea to up the cathode resistor which inevitably gradually drops in value anyway in these sets. This will increase the HT a little and increase the anode volts, no harm will happen so don't worry if you see an increase in HT volts.

I have a RAP 646 and the RF amp (6K7) has an anode volts of nearly 400!! Which is way over its 250v rating, as far as I can tell its the original valve.

[Danndad]

Thanks for all the replies, I have put a 1k resistor on grid 2 as suggested but have some sort of instability issues as it's now motorboating, wish I'd left it as it was.
More replies later.

[Murphyv310]

Hi.
Yes you could get motorboating with a resistor to G2, you need to decouple the resistor /G2 to ground anything from a 4.7uf to 10uf rated at least 250v.

Leaving it as was is certainly what I'd have done.

[Murphyv310]

Hi.
The UL84 AFAIK has a higher slope than its EL84 cousin so is more prone to instability, there could be reasons the G2 is run at near or at full HT potential, the Philips set I have with a UL84 the G2 Is actually 11v higher than its anode! It's my set I use virtually every day on Radio 2, never missed a beat in 10 years.

[Mike Watterson]

It's quite common for o/p valve g2 to be direct to an HT rail. Otherwise it certainly needs decoupled.

[Danndad]

I have sorted the instability issue now, it didn’t need the de-coupling capacitor, it was a minor wiring error by me…

The grid 2 voltage is better now at around 163v, anode voltage is still around 170v so I might have to look at r22 again, I did mean in series Trevor. I have already replaced c45 but might look at moving it, if upping the cathode resistor increases the anode voltage then I will leave it, its nearly up to its limit already. I haven’t replaced the grid coupler as yet as it a mullard mustard type, ive not come across a bad 1 yet,  voltage on grid 1 is +12mv, all voltages are taken with the DMM.  C44 has been replaced.
   

[Diabolical Artificer]

Anode voltage at 170v is fine, with the screen grid at 163v it's kushti. Lot's of guitar amps don't bother with g2 de-coupling, though in hifi amps it's normal. Vg1 at 12mV is about right though it might be worth checking the grid leak resistor - 1M ? The higher values of those Erie ceramic resistors are more liable to drift high. Have you checked the cathode resistor and cap? If the former has gone high and the latter is a tad leaky Vk might be higher. Whilst most seasoned techs wouldn't change anything on the amp, mucking about and changing things does teach one a great deal.

[Murphyv310]

Low ohmic value resistors tend to go lower over the years while high values drift higher, although there are exceptions.

Seems like a good result, I just wonder though if a few volts lower on the G2 would really make any difference to the valves reliability or life, IMHO I somehow doubt it, nice exercise in seeing how it was done in the day.

The interesting thing is ultra linear P-P amps. Looking at the circuit you'll see the G2 trappings are closer to the HT point, their low current requirements will always mean the G2 is always at a higher potential to the Anodes as they are further from the HT feed. Most of these circuits have a direct connection from G2 to the transformer, I have though seen limiting resistors in series with the G2s but from what I have seen this negates the whole reason of UL "apparently".

[Danndad]

The modifications suggested aren't going well.
I have moved chassis end of c45 to 1 end of r22 and radio is making very nasty rasping noises.