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Old 14th Dec 2018, 11:02 pm   #1
keith61
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Default Ediswan 6P26 valve alternative?

Hi. I am restoring an old British Thomson Huston 301 projector, the output valves are Ediswan 6P26. Is there any other valve I could use as they seem hard to find and would any one have a copy of the schematic of the amplifier I could borrow?

Thanks, Keith.
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Old 14th Dec 2018, 11:20 pm   #2
paulsherwin
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Default Re: Ediswan 6P26 valve alternative?

The 6P26 is a fairly obscure Mazda IO beam tetrode and I'm not aware of a direct equivalent. You could probably modify the amp to use more common valves such as the 6V6 or 6L6 but it's difficult to give specific advice without seeing the circuit. The pinout is the same.

https://www.radiomuseum.org/tubes/tube_6p26.html
https://www.radiomuseum.org/tubes/tube_6v6g.html
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Old 15th Dec 2018, 6:56 pm   #3
keith61
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Default Re: Ediswan 6P26 valve alternative?

Thanks very much for the info paul
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Old 15th Dec 2018, 9:30 pm   #4
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Default Re: Ediswan 6P26 valve alternative?

6P25 are common though I have not checked whether suitable.
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Old 15th Dec 2018, 9:40 pm   #5
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Default Re: Ediswan 6P26 valve alternative?

6P25 does have quite a high heater current requirement, though, at 1.1A- some mains transformers might be OK with it but I'd err on the side of caution rather than risk a puddle of wax from a veteran transformer, or worse. I like the idea of investigating whether appropriately biased 6V6s would fit the bill, for a respectable output valve they have a notably low 0.45A heater current. Not to mention being commonplace.
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Old 15th Dec 2018, 9:41 pm   #6
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Default Re: Ediswan 6P26 valve alternative?

6P26 valve data, book page 28:

https://frank.pocnet.net/other/Edisw...ataBooklet.pdf

Lawrence.
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Old 15th Dec 2018, 11:14 pm   #7
paulsherwin
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Default Re: Ediswan 6P26 valve alternative?

I think the 6P25 was the Mazda sort-of 6L6 equivalent and the 6P26 their 6V6. They are all pin compatible. It would be easy to swap them around in a class A stage simply by adjusting the cathode resistor (if the heater supply was up to it), but things are more complex in push pull.
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Old 16th Dec 2018, 5:15 pm   #8
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Default Re: Ediswan 6P26 valve alternative?

I found the attached in an old Ediswan data book.
Looks like a KT61 or possibly a 6V6 may do the job Ok. Heater requirements are not too demanding, anyway. Pin compatible as well. May require a little juggling with the bias, but looks like it will do the job. Tony.
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Old 17th Dec 2018, 12:37 am   #9
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Default Re: Ediswan 6P26 valve alternative?

The information in the Ediswan data book is incomplete.
There is no data on the G2 current or the gm (mutual conductance).

The Mazda 6P26 valve data has similarities with the Mazda 6P1 beam tetrode.
If they are similar, the G2 current may be 7.5mA, and the gm may be 8.8mA/V.
This suggests that the cathode bias resistor (if there is one) of the 6P26 may be 180 Ohm.

If you were to try a 6V6 valve you should increase the cathode bias resistor to at least 250 Ohms.
You could try 390 Ohms to be on the safe side.

This is partly guesswork. Other factors such as anode and g2 supply voltages will change the calculations.
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Old 17th Dec 2018, 3:24 am   #10
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Default Re: Ediswan 6P26 valve alternative?

Why do you want new valves, are they missing?
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Old 17th Dec 2018, 10:55 am   #11
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Default Re: Ediswan 6P26 valve alternative?

Quote:
Originally Posted by Silicon View Post
The information in the Ediswan data book is incomplete.
There is no data on the G2 current or the gm (mutual conductance).
Hi Silicon,
The data sheet was presented simply because no other data seemed to be forthcoming.
Better than nowt, as they say round here.
Also, if you look more carefully, you will find that the screen current (Ig2) is actually given.
I agree that gm is not quoted, but it is reasonable to assume it will not be some outlandish value, and the value of Rk can be found empirically by monitoring the anode current with various values of Rk. Tony.
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Old 17th Dec 2018, 11:25 am   #12
Silicon
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Default Re: Ediswan 6P26 valve alternative?

Hi Tony,
My previous post was not meant to be a personal criticism. I was trying to point out that there is limited information on this valve. It is missing from several data books and web sites.

I incorrectly assumed that your data was the same as the one in my Mazda data booklet from 1968 which does not include the g2 current. My mistake.
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Old 17th Dec 2018, 11:38 am   #13
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Default Re: Ediswan 6P26 valve alternative?

Avo valve data book, 6P26....8.8mA/V, Vg1=-8.5V, Va=250V, Vg2=250V, Ia=40mA.

Lawrence.
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Old 17th Dec 2018, 12:33 pm   #14
boxdoctor
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Default Re: Ediswan 6P26 valve alternative?

Quote:
Originally Posted by Silicon View Post
Hi Tony,
My previous post was not meant to be a personal criticism. I was trying to point out that there is limited information on this valve. It is missing from several data books and web sites.

I incorrectly assumed that your data was the same as the one in my Mazda data booklet from 1968 which does not include the g2 current. My mistake.
That's Ok Silicon, no offence taken.
The catalogue that that data was taken from was an Ediswan catalogue of 1960 vintage. The valve would have been more widely of interest at that earlier time and thue warranted more detailed info. Also the catalogue was aimed more at the industrial valve users.
I would still have expected the gm to be given, as it is, as your comment implies, an important parameter. Tony.
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Old 18th Dec 2018, 1:20 am   #15
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Default Re: Ediswan 6P26 valve alternative?

Hi,
Mazda Data Booklet 1970 has a substitution section on page 134 and lists obsolescent 6P25 as a plug in replacement together with a few additional notes, some of which have already been mentioned.

1. Bulb is larger with maximum diameter 45mm and seated height 109mm.
2. Heater current is 0.5A higher. Check mains transformer for overheating and Vh drop.
3. Connect metallising to earth via pin no. 1

Rich
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Old 18th Dec 2018, 4:16 pm   #16
keith61
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Default Re: Ediswan 6P26 valve alternative?

Hi. The original valves are missing so I am going to use some 6V6's as I have some already.

Thanks to everyone for their input and comments.
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