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Old 20th Aug 2017, 8:01 am   #1
thyratron
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Default PX25 fault?

I have a push-pull pair of px25 triodes in my radiogram and one of them looks as if only the centre of the heater is glowing. It doesn't look as if the little springs are relaxed or that the filament is broken. Has anyone else seen this? Perhaps I should give it a good tap on the side of the bench!
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Old 20th Aug 2017, 8:07 am   #2
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Default Re: PX25 fault?

Its a long time since I've seen one of them! I would suggest that you check the resistance of the filaments between the two, if the one not lighting up properly is higher you have lost part of it.

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Old 20th Aug 2017, 9:16 am   #3
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Default Re: PX25 fault?

This is a very common fault with PX25 valves. I'm not sure of the cause fault but it may be a spot welding fault between the two filaments.

It is not often noticed, as surprisingly it has little effect on the actual sound quality unless running flat out. I have been operating an HMV 800 with a pair of half filaments for decades! I believe the PX4 can suffer the same fate.

A sub chassis with a pair of KT66s connected as triodes makes a really excellent replacement in grams. John.
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Old 20th Aug 2017, 9:19 am   #4
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Default Re: PX25 fault?

I have a few photographs :
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Old 20th Aug 2017, 9:23 am   #5
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Default Re: PX25 fault?

and some more:
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Old 20th Aug 2017, 9:40 am   #6
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Default Re: PX25 fault?

Looks like I need a replacement. I don't want to use strapped tetrodes as the other PX25 is in great shape. The problem is that the HT current through the output transformer is no longer balanced, even though I can achieve some kind of dynamic balance by increasing the bias on the good valve. The valve line up in this amplifier is L63, ML4, AC/P, AC/P, PX25, PX25 with fixed bias. Thanks for all the comments so far.
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Old 20th Aug 2017, 1:57 pm   #7
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Default Re: PX25 fault?

You've not said how well it sounds? If OK, leave well alone. The valve line up suggests there is plenty of gain.
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Old 20th Aug 2017, 6:19 pm   #8
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Default Re: PX25 fault?

It drives extension loudspeakers so it needs to be in good order. There's no distortion with ordinary listening but I really need to get it working properly.

Last edited by Station X; 20th Aug 2017 at 6:24 pm. Reason: Forum rule compliance.
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Old 21st Aug 2017, 12:58 pm   #9
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Default Re: PX25 fault?

Your choice.....


Bias back the good PX25 for dc balance and carry on regardless if the sound is OK with whatever speaker combinations you use.


Shell out mega£ on a couple of NOS PX25s.


Shell out <mega£ on one good used PX25.


Interesting dilemma.
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Old 21st Aug 2017, 2:09 pm   #10
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Default Re: PX25 fault?

Mega£ is correct, I have just looked at some examples. Didn't realise they sell for so much.

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Old 21st Aug 2017, 4:11 pm   #11
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Default Re: PX25 fault?

Mega bucks indeed, I just had a look, rather shocking prices. I would go for the triode strapping and sell the remaining good one and make a healthy profit.
 
Old 21st Aug 2017, 5:54 pm   #12
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Default Re: PX25 fault?

I drive several extension speakers from my 800. Believe me it sounds very good. This can happen with PX25s at any time. The idea of a pair of KT66s strapped as triodes is very viable. The output transformer will match a PX25 to a KT66 triode connected and give excellent results. Chinese KT66s are available at a fair price and work well if not overloaded.

If you build it on a sub chassis with a 6.3v heater transfromer, you can retain the original B4 bases. The sub unit is easily replaced when a good PX25 is available.

They are dodgy valves to buy second hand. Hope you get it sorted. John.
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Old 21st Aug 2017, 6:29 pm   #13
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Default Re: PX25 fault?

Thanks for the valuable tips. I wouldn't think it a good idea to post valves with these filaments, especially when they're sure to become more fragile with age? I have an output transformer with two separate primary windings and have brought the HT ends to separate one ohm resistors and decoupled. A 50uA meter is now across at this point and, being centre zero type, I can achieve perfect DC balance. It's interesting to play a record and turn up the wick as the needle deflects one way as the troubled valve starts to go into grid current and then, with even more signal, the meter goes the other way as the valve with more cathode emission takes control. Then we're into bad crossover distortion as the pull is greater than the push! I definitely want to find a good s/h valve and will probably drive to fetch it!
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Old 21st Aug 2017, 6:43 pm   #14
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Default Re: PX25 fault?

I should have asked where are you able to see the cost of these triodes? Are they made in China like the KT types?
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Old 21st Aug 2017, 8:03 pm   #15
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Default Re: PX25 fault?

I just did an eBay search, and quickly looked away at over £700 for a pair of NOS.

Frank
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Old 21st Aug 2017, 8:36 pm   #16
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Default Re: PX25 fault?

Triode connected PT15's are said to be/sound very similar to PX25's without the hassle of providing a new heater supply and sub-chassis for KT66's.
I guess it all depends how much you value the originality.
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Old 21st Aug 2017, 10:49 pm   #17
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Default Re: PX25 fault?

Will the PT15 be happy at only 4V on the filament? Does it have enough emission at less than half its normal filament power?

Looking at r-type.org suggests that the PT15 forerunner PT5 might be a better bet, though how easy to find?
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Old 22nd Aug 2017, 7:40 am   #18
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Default Re: PX25 fault?

At least with a big set is there's room to add a filament transformer if the replacement valves need a different heater voltage.

Triode-strapped KT66 or 6L6 would need 6.3v, anyway.

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Old 22nd Aug 2017, 8:53 pm   #19
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Default Re: PX25 fault?

Hi Gents, simple solution is to use a filament auto transformer. Much more compact than a mains unit, very low turns count and can easily be converted back if funds ever allow.
I've made some to use on CRT's to convert between 2v and 4v.

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Old 22nd Aug 2017, 9:01 pm   #20
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Default Re: PX25 fault?

The PT15 has a six volt filament and the impedance is likely to be a good deal higher with a requirement for a few hundreds more volts HT to get some watts out of it. I wonder if anyone has tried parallel strapping PT15s in push-pull to make loading easier?
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