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Old 19th Sep 2020, 12:43 am   #1
FStephenMasek
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Default PX4 substitutions

The volume from my HMV468 is low. I replaced the capacitors, some resistors, both diodes and the less expensive valves, so that leaves the PX4.... I found some older threads regarding substitutions. What is your current thinking? Of course, if I can find a decent used PX4, that is the easiest approach. I'm not inclined to replace the socket, as this is a somewhat rare radio, although the chassis was used in several different cabinets. This one has the later version of the chassis, with a fuse in the high voltage winder center tap return, a nice feature to save the power transformer. Making an adapter might be a good approach. I do not have any transformer winding equipment, so getting 6.3 volts for the filament of a 6L6 or some such seems impractical.
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Old 19th Sep 2020, 1:56 am   #2
G8UWM-MildMartin
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Default Re: PX4 substitutions

A triode-connected 6L6 (or G/GT) and a semiconductor diode voltage-doubler for the heater (could someone here kindly model silicon versus shottky and different capacitor values for 900mA to get somewhere near enough?) and the cathode connection brought out separately on a wire to go to the centre tap of the original 4-volt winding should do the job.
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Old 19th Sep 2020, 1:57 am   #3
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Default Re: PX4 substitutions

A small mains to 6.3v transformer ought to be findable from somewhere, or Ed Dinning could wind one for you. If you think of a 4v to 6v converter transformer for a valve substitution, it still has to handle the full heater power of the 6L6, and so a mains powered transformer would need to run the same power. You could even use the primary of the main power transformer as an autotransformer to hitch a ride on the mains voltage adjustment.

Is there room to squeeze in an adaptor chassis with the 6L6 and heater transformer? Then the modification would be easily reversible if you ever came across an affordable PX4. If, say, a rumour started that their bass was slow and their midrange granular reducing both 'slam' and 'authority'

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Old 19th Sep 2020, 2:39 am   #4
G8UWM-MildMartin
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Default Re: PX4 substitutions

OK, either of those ideas with a pair of resistors across the original filament supply to create an artificial centre tap for the cathode. I expect it has been done decades ago, but I can't find any info.

I've seen mention of a triode-strapped KT66 having the right characteristics and it being the UK answer to the 6L6, and the very cheap 6P3S (Cyrillic: 6П3С) being the ex-USSR equivalent to that

How's about a forum project to design and test some reasonable PX4 substitutes?

It might help save some decent sets and radiograms.
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Old 19th Sep 2020, 8:00 am   #5
yestertech
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Default Re: PX4 substitutions

Correct me if Iím wrong but arenít the resurrected ĎBRIMARí offering a version of PX4 in their current range of output valves ?

Andy
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Old 19th Sep 2020, 8:32 am   #6
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Default Re: PX4 substitutions

Presumably the resurrected PX4 will be priced for the audiophool market, so it will be out of sensible reach for a task as mundane as repairing the sort of radio the valve type was intended for?

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Old 19th Sep 2020, 8:36 am   #7
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Default Re: PX4 substitutions

Has it actually been ascertained that the existing PX4, or indeed any of the other valves, are actually faulty? For example by taking voltage readings.
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Old 19th Sep 2020, 9:06 am   #8
rontech
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Default Re: PX4 substitutions

Quote:
Originally Posted by yestertech View Post
Correct me if I’m wrong but aren’t the resurrected ‘BRIMAR’ offering a version of PX4 in their current range of output valves ?

Andy
Yes they are. At a cost ( matched pair only ) of £469
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Old 19th Sep 2020, 10:05 am   #9
barrymagrec
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Default Re: PX4 substitutions

In the original Williamson amplifier book there was a version for PX4s as well as the more familiar KT66 version.

They were to all intents and purposes identical. I changed my PX4 version to the KT66 type after an unfortunate accident involving a leaky garage roof and the mains transformer, the replacement had no 4 volt windings.
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Old 19th Sep 2020, 11:24 am   #10
PJL
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Default Re: PX4 substitutions

As Graham says, have you checked voltages to confirm the PX4 is faulty? I would expect the HT to be very high.
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Old 19th Sep 2020, 7:23 pm   #11
barretter
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Default Re: PX4 substitutions

Quote:
Originally Posted by barrymagrec View Post
In the original Williamson amplifier book there was a version for PX4s as well as the more familiar KT66 version.

They were to all intents and purposes identical. I changed my PX4 version to the KT66 type after an unfortunate accident involving a leaky garage roof and the mains transformer, the replacement had no 4 volt windings.
What do you mean by "the original Williamson amplifier book"? DTN Williamson wrote several articles for "Wireless World" which were later gathered together as a pamphlet entitled "The Williamson amplifier" but nowhere in it is there a design for his amplifier using PX4s. He does mention the PX25 but dismisses it in favour of the KT66. There was a design for a push-pull PX4 amplifier in a "Wireless World" article in 1943 but that wasn't by Williamson.
There were quite a few variations on the PX4 by other British valve manufacturers but I doubt if they are any more available or cheaper than the original.
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Old 19th Sep 2020, 7:31 pm   #12
Ed_Dinning
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Default Re: PX4 substitutions

Hi, as the heater power requirements for the valves are similar it would be easy to wind a 4v to 6.3v autotransformer for this set as no isolation would be needed in this position. It would be quite small and easily hidden.

I have produced these in the past to "shift" the CRT heater voltages when different voltage tubes were used

Ed
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Old 19th Sep 2020, 8:12 pm   #13
barretter
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Default Re: PX4 substitutions

The problem with all the suggestions about using a triode-connected pentode or beam tetrode is that Mr Masek says he wants to retain the original socket and I doubt that there are any adapter sockets unless some enterprising Chinese manufacturer has made one.
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Old 19th Sep 2020, 8:21 pm   #14
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Default Re: PX4 substitutions

Quote:
Originally Posted by barretter View Post
I doubt that there are any adapter sockets
Yes there are - smash an old dud 4 pin valve and use the base to make an adapter.

Or, find one of those EverReady 90+1.5 volt battery radios and chop the plug off....Yes, I know, that's a terrible suggestion!
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Old 19th Sep 2020, 8:28 pm   #15
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Default Re: PX4 substitutions

There was a discussion in this thread which came up with some solid state possibilities:

https://www.vintage-radio.net/forum/...d.php?t=155129

No mods to the set but you'll need a B4 plug or a suitable sacrificial toob
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Old 19th Sep 2020, 9:16 pm   #16
barrymagrec
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Default Re: PX4 substitutions

Quote:
Originally Posted by barretter View Post
Quote:
Originally Posted by barrymagrec View Post
In the original Williamson amplifier book there was a version for PX4s as well as the more familiar KT66 version.

They were to all intents and purposes identical. I changed my PX4 version to the KT66 type after an unfortunate accident involving a leaky garage roof and the mains transformer, the replacement had no 4 volt windings.
What do you mean by "the original Williamson amplifier book"? DTN Williamson wrote several articles for "Wireless World" which were later gathered together as a pamphlet entitled "The Williamson amplifier" but nowhere in it is there a design for his amplifier using PX4s. He does mention the PX25 but dismisses it in favour of the KT66. There was a design for a push-pull PX4 amplifier in a "Wireless World" article in 1943 but that wasn't by Williamson.
There were quite a few variations on the PX4 by other British valve manufacturers but I doubt if they are any more available or cheaper than the original.
There is an earlier book than the one usually seen - I have a copy in my loft somewhere but I can`t find it - I seem to remember it was called something like "High quality sound reproduction" by DTN Williamson and published by GEC.
It contains , from memory, PX4 and KT66 versions of the classic amplifer, an AC/DC unit using KT33c, Tuners for both the AM and FM experimental VHF broadcasts, a preamp with a huge number of different disc EQs and a puff piece about the merits of the GEC matal cone loudspeaker.

I was given the book and amplifier by my Uncle, who used it to build the PX4 amplifier in the early fifties, possibly because of better availibility of PX4s at the GPO.
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Old 19th Sep 2020, 9:24 pm   #17
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Default Re: PX4 substitutions

Contemplating the sacrifice of any veteran toob may be painful, but the 7475/CV188 is fairly commonplace and not terribly in demand with a typically compact B4 base- and the device itself could then be treated as a wire-in reference if you don't like to contemplate discarding it! As any deviation from the original PX4 type is "unoriginal" anyway, perhaps a substitute consisting of B4 base to wired adapter bracket with small autotransformer and a B9a valve like a triode-connected 6BW6 could be fitted in somewhere unobtrusively. Functional, if hardly concours.
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Old 19th Sep 2020, 9:36 pm   #18
Joe_Lorenz
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Default Re: PX4 substitutions

Hello,

my favourite substitute would be the AL 4 valve in triode hookup. It has Uf=4 V, all you need to make is a base adaptor. AL4 is easily obtained via ebay

Regards, Joe
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Old 19th Sep 2020, 9:50 pm   #19
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Default Re: PX4 substitutions

Unfortunately its not just PX4's the humble 2P are being ripped out along with 41MP's the thirst for the "Power triode" sound knows no bounds.

Unlike PX4's these valves are not being reproduced.

While there is a market those interested in a profit will take a chance on pretty much anything.

I once bought a Cossor set the pictures showed it was complete I had to carry out a 300+ mile round trip to pick it up when I arrived it was without triode. I had asked before purchase if the pictures were a true representation and all parts shown were included.

Not a pleasant experience for me or the seller who had been convinced by a smooth talker the buyer wouldn't mind it was a easy to replace part.

Now I always make sure before setting out that nothing has been removed.

Cheers

Mike T
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Old 19th Sep 2020, 9:53 pm   #20
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Default Re: PX4 substitutions

Quote:
Originally Posted by turretslug View Post
Contemplating the sacrifice of any veteran toob may be painful, but the 7475/CV188 is fairly commonplace and not terribly in demand with a typically compact B4 base- and the device itself could then be treated as a wire-in reference if you don't like to contemplate discarding it!
Dead no emission B4 base rectifiers are pretty common and will supply the base.

Cheers

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