K5881 one valve running cool.

[shadders44david]

Hi there, I have yet another problem with my aged K5881 mk2 and wonder if someone can help. The right channel is slightly lower output than the left and the bass notes distort at anything slightly above normal listening levels, the left channel is fine. Also, valve number 4 - far right, does not get anywhere near as hot as the other 3. I have tried swapping the valves, checking the heater circuits, changed the cathode resistor bypass cap`s (again), checked all resistor values, swapped C3 and C7 with the left channel and checked and rechecked continuity of all solder joints all to no avail. Has anyone come across this fault before ? Best regards, David.

[John Caswell]

Hi David,
The one you apparently have not done is check the voltages. The fact that one 5881 is running cool implies that it is not drawing enough current, so... Please measure voltages on the 5881s in the channel in the poor channel and report them back here we will than see what can be done.

John

[Richard]

Yes agreed with John, check HT voltage at pin 3 looking for 440V and cathode voltage at pin 8 for 34V.

[shadders44david]

Hi there,
Thankyou for your suggestions, I have checked the cathode and anode voltages as you advise. Here are the results; (left to right 1-4) V4 cool one.

valve 1 valve 2 valve 3 valve 4

pin 3 450v 450v see below 14v


pin 8 34v 33v 29v 14v


when trying to measure voltage at pin 3 valve 3 the multimeter goes overlimit and there is a worrying blue spark and a "crack" when attempting to connect the probe.

Don`t know if this has anything to do with the problem but a couple of years ago i was suffering intermittent static "clicks" and after referring to the forum i was advised to remove the insulation between both OPTs so the body of each was connected to the chassis which relieved the problem.

I look forward to your reply but I fear I need a new R/H OPT, any ideas on sourcing one ?

Best regards, David.

[Richard]

Hi David, with everything switched off and the HTV discharged (check with voltmeter) do a resistance check on the OPTX primary windings.

Put a probe on centre tap CT of the OPTX primary (this is where the HT connects) and the other on Pin 3 of a 5881 looking for around 100R. Repeat for the other 5881 of the pair looking for similar 100R.

You can also do this on the known good channel to confirm what the resistance should be and that your testing method is good. Good luck and let us know!

[shadders44david]

Hi Richard,

Thankyou for your suggestion. I have carried out the tests and the results are as follows; Valve 1 (far left) pin 3 to CT - 156R, Valve 2 - 156R, Valve3 - 157R, Valve 4 (cool one) No reading - open circuit.

I look forward to your reply.

Best regards, David.

[Richard]

Hi David,

Re-check the wiring to pin 3. If the OPTXs have tags you can meter straight across them to eliminate any bad joints but it looks like you need a new OPTX. Certainly this agrees with the problem, but do check as a broken wire or joint would cause similar.

If the worst is confirmed give us the exact dimensions of the existing TX in mm inc the mounting holes.

Looking at the data sheet the valves wil be quite happy with 6K6 which is easily available for 5-20 designs in all levels of quality. Suitable TXs are easy to find and it will be much easier and neater to use one (or a pair) wound on the same size lams you have at present. Most will have UL taps which you could then try or ignore and just use the full winding for pentode as at present. Also to consider, do you use the 4R or 16R taps to speakers or are 8R only fine?

[John Caswell]

Hi all,
Hmmmm not good news, however these tx were made by SP Wound in Poole and I believe Geoff does still make them.

John

[shadders44david]

Hi Richard and John,

I have now rechecked the OPTXs via their tags with the valves removed. on the good left TX I get 157R between CT and Anode 1, and 158R between CT and Anode 2. However on the faulty right channel I got 157R between CT and Anode 2, but no reading - open circuit between CT and Anode 1. This surely confirms that the r/h OPTX is at fault. Can you shed any light on why it has failed, I have not experienced a cherry red glow which I understand burns out OPTXs.

I have contacted Geoff at SP Wound who advises he can make a new TX in about 3 weeks @ £115. Would you recommend changing both the TXs ? I may contact World Audio to enquire if they have a replacement but it seems a long shot after 14 years !

I must say a big thankyou to both gentlemen who have guided me through the checks to confirm the problem. It really is good to know there are people on this forum capable of diagnosing valve related problems. Only issue now is doing without the amp` for 3 long weeks !!!

Best regards, David.

[david counter]

Originally Posted by shadders44david

I may contact World Audio to enquire if they have a replacement but it seems a long shot after 14 years !
.

I seem to remember that when WAD went bang everything got either lost or disposed off, as you've discovered SP wound are your best bet,