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  #31  
Old 10th August 2011, 10:53 PM
ssaliba ssaliba is offline
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Default Re: WAD Kel84 Glowing EL84

Hi Forum colleagues

Just to give you an update so far.

I received the Soniq Sax coupling capacitors and replaced the Jensen's.I also replaced the Cathode Bias resistors r24/25/26/27 with metal film and installed the 2W version.

I also replaced R10/R11 with 2W metal film and R12/13/14/15 with 1W metal film. (as suggested by the FAQ Team)

Still though waiting for my Russian Power EL84 to arrive.

Volatges on the Power Valve grid g1 now range between 10mV and 0V. They are not stable, they constantly change. I guess these values are normal.

One issue which I have though is that the aP and the gT of one ECF80 slot are a bit higher then spec irrespective of which ecf80 I use.

The readings are 97V for both the aP and the gT , when they should read 75V. Any ideas why this is happening ? Could this be something with the step network components R34/C18 ?

Looking forward for your reply

Stefan
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  #32  
Old 10th August 2011, 11:39 PM
Richard Richard is offline
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Default Re: WAD Kel84 Glowing EL84

Hi Stefan,

Yes I wouldn't worry about 10mV on the el84 g1. You probably know but do be careful metering to keep meter cables tidy and away from magnetic influences as induction can sometimes cause fluctuations in the reading at these small levels near mains TXs etc.

The small valve voltages might be worth investigating whilst you're waiting for the el84s.

First off it will most likely make little or no difference to the sound, however,

it won't be the step network as that is an ac path and we're looking at dc conditions

if it is definitely a similar result even with the valves swapped over then it will be the anode and/or cathode resistors of the pentode section,

accurately meter and compare across channels; R10/11, R32/33, R6/7 and whilst your at it, R12/13 and R14/15. All can be checked with power off and valves out and a probe either side of the resistor without having to disconnect it. If you don't find anything significant try and lengthen the meter path to include tracks and solder joints along the way, so put a probe on pin 7 and the other on signal ground looking for R32/33 750R, for instance, which will check the whole path for perhaps a dry joint causing a high value.

Do be aware though that these small pentodes are well known for varying voltages which do not make much difference to their sound so don't go chasing ghosts and be sure it's not just valve differences.
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  #33  
Old 11th August 2011, 01:41 PM
John Caswell John Caswell is offline
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Default Re: WAD Kel84 Glowing EL84

Hi Stefan,
Good to hear that your problems are almost solved.
The ECF80 valves are pretty notorious for wildly different voltages on the various anodes and screens, and I did a lot of work comparing about 20 of them and came to the conclusion that this is normal for this valve. (See FAQ).
I have absolutely no solution to this problem, but it does not appear to matter as gain and distortion figures remained very constant so I would not worry about it at all.
I would also suggest changing the cathode bias capacitors on the EL84s as they may have suffered over voltage due to the leaky capacitors and /or faulty valves.

John
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  #34  
Old 11th August 2011, 08:39 PM
ssaliba ssaliba is offline
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Default Re: WAD Kel84 Glowing EL84

Hi John and Richard

Thanks for your replies. Richard, I checked all resistors and no dry joints found, and values were to spec. I guess as you both said, it is pretty normal that these valves so nothing to worry about.

With regards to the cathode bias capacitors, I did not notice any buldging which is why I did not replace them. I have BlackGates installed. Do you still suggest replacing even if there is no buldging ? Is there a way I can confirm if they need replacing using measurment ?


Stefan
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  #35  
Old 12th August 2011, 11:41 AM
Richard Richard is offline
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Default Re: WAD Kel84 Glowing EL84

Hi Stefan,

Yes ecf80 often show variance in the voltages. It's always a good idea to have spares so I'd buy a couple of Mullard ecf80, they're not expensive or difficult, which are usually more consistant than most and see how they read.

The BG cathode bypass will usually show signs of leaking/bulging if there's a problem. Try metering ohms from pin 3 to ground looking for 270R. If the caps have gone short this will show as less ohms.

Then, if you have a cap meter, you can check capacitance. The low value cathode resistor is across the cap so you need to lift a cap leg to get an accurate reading but I often check them in situe by doing a comparison check to see that they're all reading the same as it's unlikely they'll all fail the same at the same time. Expect a reading to settle around 1/2 cap value (perhaps depending on your meter) and to be similar for all 4.

If so, the final check is to fit the el84s, power up, and check cathode voltage looking for 11V.

The voltages in a valve amp (comparison across channels and against the schematic) are the best and easiest way to see all is well but do allow a little leeway and those small valves in partic are well known for discrepency
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  #36  
Old 12th August 2011, 12:14 PM
ssaliba ssaliba is offline
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Default Re: WAD Kel84 Glowing EL84

Hi All

I just received the EL84's Actually while you were writing the reply Richard I was testing them out. These are the results

V6
g1= playing between 10 and 20mv
K = 10.1V
A = 307V
G2 = 310V

V4
g1= playing between 10 and 20mv
K = 10.5V
A = 308V
G2 = 311V


V6 and V4 are matched pairs

V5
g1= playing between 10 and 20mv
K = 9.6V
A = 308V
G2 = 310V

V3
g1= playing between 10 and 20mv
K = 9.6V
A = 310V
G2 = 311V

V5 and V3 are matched pairs

Values are well in spec (at least that is what in think ). Richard I will now let the amp cool down and check the capacitors using your method. Will update.
accordingly.

Stefan
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  #37  
Old 13th August 2011, 04:38 PM
ssaliba ssaliba is offline
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Default Re: WAD Kel84 Glowing EL84

I hooked up the amp to the rest of the equipment and the sound is great. My Kel84 is back to life again . The sound will of course get better once the EL84's and all new components break in.

A big thanks goes to all those who contributed in this thread. Special thanks goes to John and Richard.

Thanks

Stefan
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