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Old 10th Feb 2015, 7:55 pm   #1
oldgeezer
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Default PX4 AC044 cheap equivalent or similar?

Well I've just started looking at a Clarke's Atlas set that should have an AC044 output valve, but its gone and someone put a PEN220 in its socket. (Wish I'd noticed that before I bought it.)

The PEN220 is 2v battery valve max 150v on the anode so I thought I'd just get a PX4 / AC044 and put the right valve in. (4v heater and 250v)

Hahahaha have you seen the price of them!

Any ideas for a suitable cheap equivalent please?

thanks
Keith
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Old 10th Feb 2015, 8:07 pm   #2
Alan Stepney
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Default Re: PX4 AC044 cheap equivalent or similar?

I dont think there is one.

Or to be accurate, anything that will substitute for a PX4, will have been discovered by the audio brigade and will fetch high prices.
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Old 10th Feb 2015, 8:08 pm   #3
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Default Re: PX4 AC044 cheap equivalent or similar?

Yup

Triode connected KT66 or EL34 are not bad subs, but heater volts are wrong and they're not cheap either though at least available.

Probably easiest to find any 4V output pentode and rewire accordingly. It won't be original but it'll work.
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Old 10th Feb 2015, 8:21 pm   #4
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Default Re: PX4 AC044 cheap equivalent or similar?

I'm wondering if you could strap up something like a Cossor 42OT

http://www.r-type.org/exhib/abb0009.htm

as a replacement? it's got something like the right bulb-profile to look vaguely original?

I've got one here which I plan to fashion into a single-channel audio amp [with a SP41 as the preamp] now that I've got a 4V 3A transformer.
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Old 10th Feb 2015, 8:22 pm   #5
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Default Re: PX4 AC044 cheap equivalent or similar?

You could change the base and fit a AC2PEN.
john
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Old 10th Feb 2015, 8:25 pm   #6
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Many thanks.
The AC044 is a triode so if I get a 420T should i wire the other grids as usual or leave them dangling?
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Old 10th Feb 2015, 8:27 pm   #7
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Well the AC2PEN is a bit more affordable.

Not too worried about originality (its already been well bodged!)
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Old 10th Feb 2015, 9:39 pm   #8
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Default Re: PX4 AC044 cheap equivalent or similar?

If you wind a 4V-6.3V autotransformer you can fit most octal output pentodes. Just wire them as a triode. You'll obviously need to change the base though, and it's certainly a bit of a bodge.
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Old 10th Feb 2015, 9:50 pm   #9
Hoveradio
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Default Re: PX4 AC044 cheap equivalent or similar?

Hi,Just use a PM24M and either tie the sceen to the anode for triode mode or feed the screen grid from HT.
John
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Old 10th Feb 2015, 11:39 pm   #10
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Excellent suggestions! - Many thanks.

Off to junk valve box, then ebay to see what I can find.

regards
Keith
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Old 11th Feb 2015, 12:17 am   #11
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Default Re: PX4 AC044 cheap equivalent or similar?

I guess this is the set with the speaker in the corner? Nice simple art deco design similar to the Murphy A8 that I have always wanted.

I would agree with John, the PM24M is affordable, 4V 1.1A filament, 30mA anode current and looks very similar to the AC044, and if your extremely lucky will fit in the existing socket or if not it only requires a hole to be drilled. The triode option should give the best impedance match, bias conditions 250V anode/-20V grid/approx. 30mA anode+screen current so a cathode resistor of 560/680 should do.

Better be quick as the price will rocket now the cat is out of the bag.
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Old 11th Feb 2015, 12:38 am   #12
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Many thanks for extra info.

Yes its the set with the speaker at an angle and chrome and ebony. Very art deco and even my wife likes it!

I did look at the impedance match, so will do as you suggest and triode it!.

The valves go for around £10 so I'll wait and see when one turns up about that price.

Again, thanks very much.
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Old 12th Feb 2015, 1:02 pm   #13
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Default Re: PX4 AC044 cheap equivalent or similar?

I've just found a spare 5pin with side contact MPT4 - can this be put to use?
Its a pentode. The cathode is indirect so easy to sort, but
What should I do with the screen grid. Tie it to anode or bias it below anode volts.

I would like to lower its output impedance like a triode so should I strap it to the anode? or would that upset the suppressor grid?

Not really sure...?
Thanks
Keith
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Old 12th Feb 2015, 4:25 pm   #14
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Default Re: PX4 AC044 cheap equivalent or similar?

Screen grid to anode, suppressor grid to cathode.
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Old 12th Feb 2015, 5:50 pm   #15
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Default Re: PX4 AC044 cheap equivalent or similar?

People sometimes strap both G2 and G3 to the anode when strapping for triode characteristics.

David
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Old 12th Feb 2015, 6:14 pm   #16
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Default Re: PX4 AC044 cheap equivalent or similar?

The MPT4 will be fine and sound exactly the same as a PX4. A Mazda AC/PEN is also an excellent replacement. J.
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Old 12th Feb 2015, 6:18 pm   #17
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Default Re: PX4 AC044 cheap equivalent or similar?

Quote:
The MPT4 will be fine and sound exactly the same as a PX4
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Old 12th Feb 2015, 6:35 pm   #18
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Many thanks again.

I'll sort out the valve holder and I'll be very pleased with it I'm sure.

Its a shame it has been got at in the past as it looks like spaghetti junction in there. Most of the resistors are miles out too - so I'll change the most outrageous ones!

I've noticed the output transformer has probably been changed from the original too. It has a label from Gardners Radio Ltd, Christchurch. It has primary taps on it for 50:1, 70:1, 85:1, 100:1. So I'll have some fun fiddling with that too!

regards
Keith
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Old 13th Feb 2015, 11:03 am   #19
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Default Re: PX4 AC044 cheap equivalent or similar?

Mmmmm. For an output triode you'ld need to go between the 50-70 or 70-100 taps to get a low enough load R.

I wonder if any QPP double pentodes have 4v heaters? That would be possible using the 50:1 point as a centre tap, and an interestig bodge as well.
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Last edited by Herald1360; 13th Feb 2015 at 11:13 am.
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Old 13th Feb 2015, 12:09 pm   #20
oldgeezer
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Default Re: PX4 AC044 cheap equivalent or similar?

Interesting.. Think I'll skip on the double pentodes as that's a bodge too far! (I don't know of any 4v heater ones anyway).

The transformer does have a centre tap and instructions for use on it. I spent last night looking at many trader sheets for sets using the MPT4 as output valve. With the transformer I've got, I'm going to use it as a normal output pentode and replace the 4 pin valve socket with a 5 pin. SG comes out the side.

That should give the best result..........(we'll see!)
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