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Old 27th Apr 2018, 1:07 pm   #1
Levente
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Default RCA 6X4/EZ90 vs EZ91

Hello People...

I have a unit which takes an RCA 6X4 full wave rectifier according to the manual....can this be replaced or substitued with a UK version of EZ91 full wave rectifier?

The only difference between:

RCA 6X4 Filament Vf 6.3 Volts / If 0.6 Ampere

EZ91 Filament Vf 6.3 Volts / If 0.95 Ampere

Please if any of you could advise on this would really appreciate it.

Thanks!
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Old 27th Apr 2018, 1:28 pm   #2
'LIVEWIRE?'
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Default Re: RCA 6X4/EZ90 vs EZ91

The only problem of which I know is that the higher current drain of the EZ91's heater might be too much for the mains transformer's heater winding. ( I presume the unit in question uses a conventional mains transformer with a 6.3v heater winding?) May I ask why you would want to change a 6X4 (EZ90) for an EZ91 anyway? I first read the numbers as EZ80 & EZ81, the common B9A based rectifier valves, didn't know there was an EZ91 (B7G Base), but the above comment still applies.
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Old 27th Apr 2018, 1:43 pm   #3
cathoderay57
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Default Re: RCA 6X4/EZ90 vs EZ91

Unfortunately most mains transformers (assuming your equipment does have a mains transformer fitted) are not clearly marked with their AC current capacity, so it is difficult to check for sure. If your equipment doesn't have a mains transformer and the valve heaters are wired in series with a mains dropper resistor then the 6X4 and EZ91 would not be interchangeable because either the rectifier or one of the other valves in series could burn out. If you do have a mains transformer fitted then check if the rectifier has its own heater winding or whether the rectifier heater is wired in parallel with other valves. In the former case, personally I would risk it for a test and check the transformer at regular intervals for signs of overheating. In the latter case (all heaters in parallel) I wouldn't risk it as there is probably more risk of overloading the transformer. In that case, if you need to replace the rectifier, try to get the 6X4 (or EZ90 which is an equivalent). Hope that helps.
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Old 27th Apr 2018, 2:32 pm   #4
Levente
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Default Re: RCA 6X4/EZ90 vs EZ91

Thanks guys for the info and suggestions.

The only reason I was asking was I have a vintage Mullard EZ91 and thought, IF it is interchangable, I don't need to spend an extra 50 USD and that Mullard is pretty old from the 50's

Attached is a picture of the power transformer and the missing tube socket for the 6X4.

I will take your advice, will purchase a 6X4 rather than killing this precious machine.

BTW this is a vintage studio tape recorder RCA SRT-1.

Cheers guys.

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Old 27th Apr 2018, 2:34 pm   #5
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Default Re: RCA 6X4/EZ90 vs EZ91

If your "set" is a parallel heater chain @ 6.3V and all the heaters are on one winding, you may have enough redundancy in the current demand, after all it's only a bit more than the average scale lamp demands. Depends on how many valves there are, the more the better chance that the transformer was designed with a bit to spare.
If it's a dedicated winding I would not worry at all. It's unlikely that the winding was designed for exactly 0.6A but we are looking at a 55% overload.
But, I would carefully test it for overheating of the transformer.
If your mains voltage is normally lower than 240V and you are on the 250V tapping, it gives you a bit more headroom.
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Old 27th Apr 2018, 3:47 pm   #6
Levente
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Default Re: RCA 6X4/EZ90 vs EZ91

Hey!

What we have here is a US made machine 110V/90W.

On the chain we have one 12AU7, a 12AX7 and next to the motor we also have another 12AU7.

I do not have the schematics for this unfortunately and I can not find anything on the web with this type of RCA tape deck..

pics attached..

at the outputs there are two RCA's one is HI and the other is LO...wondere what Ohm's are these...hmmmm....would be so great to have a schematics...
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Old 27th Apr 2018, 4:23 pm   #7
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Default Re: RCA 6X4/EZ90 vs EZ91

Nice vintage 6X4s can be had for a lot less than 50 USD, even once tax and postage have been added e.g.

https://www.watfordvalves.com/produc...il.asp?id=5005
https://www.langrex.co.uk/products/6...nia-nos-boxed/

Cheers,

GJ
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Old 27th Apr 2018, 4:32 pm   #8
Boater Sam
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Default Re: RCA 6X4/EZ90 vs EZ91

New boxed 6X4 £9.70 plus postage to you, no idea what that would be.
Sam.


PS slow post is £4.40 Fast and tracked is £9.00

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Old 28th Apr 2018, 12:16 am   #9
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Default Re: RCA 6X4/EZ90 vs EZ91

I can't see how any such transformer could be practically tested for 'over heating' based on an increase in loading of resistive 2.2VA - just saying, that would require a laboratory style controlled environment and temperature sensors and many hours of testing for stabilisation (and preferably with the valve in question relocated so that it doesn't thermally distort the transformer's temperature).

And even then, you'd have to rationalise what the rise would actually mean as far as transformer insulation stress.
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Old 28th Apr 2018, 2:18 am   #10
G8UWM-MildMartin
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Default Re: RCA 6X4/EZ90 vs EZ91

I'd risk it if one is to hand, otherwise a pair of 1N4007s and a suitable dropping resistor in series with the comon cathode output or in series with either each anode or cathode (probably several hundred ohms.)

Last edited by G8UWM-MildMartin; 28th Apr 2018 at 2:25 am.
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Old 28th Apr 2018, 11:25 am   #11
'LIVEWIRE?'
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Default Re: RCA 6X4/EZ90 vs EZ91

They say you learn something new every day - I never knew there was such a valve as the EZ90, as my comment in post # 2 shows.
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