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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Aug 2018, 8:23 pm   #1
Biggles
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Default EL84 heater cathode V max?

Just a quick one. Does anyone know what the maximum voltage spec is heater/cathode for an EL84? My data books don't seem to have this info. Thanks in advance.
Alan.
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Old 25th Aug 2018, 9:11 pm   #2
pmmunro
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Default Re: EL84 heater cathode V max?

See http://www.r-type.org/exhib/aaa0028.htm (Mullard EL84)

On page D3, this gives Vk - h max as 100v but does not state whether ac or dc.

The RCA 6BQ5 data sheet gives the same value.

PMM
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Old 25th Aug 2018, 9:22 pm   #3
Biggles
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Default Re: EL84 heater cathode V max?

Ok, thanks for that. I will have to factor that in to my design for a regulated HT PSU, and run the EL84 heater from a separate transformer winding. Cheers.
Alan.
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Old 25th Aug 2018, 10:02 pm   #4
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Default Re: EL84 heater cathode V max?

As a general rule valve-regulated HT psus do need a separate heater winding for the pass valve and may need another one for the error amplifier too (if you're using one). By the way, have you considered using an EL86 instead of an '84 ? They were designed to do this job and if you're looking at vintage valves they are a great deal cheaper than '84s. And their Vhk max is 200V.

Cheers,

GJ
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Last edited by GrimJosef; 25th Aug 2018 at 10:08 pm.
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Old 26th Aug 2018, 1:08 am   #5
Biggles
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Default Re: EL84 heater cathode V max?

I have found a transformer with enough windings on it to do the job GJ but it's worth considering the EL86 too. I have never seen one before. The PSU is just a low powered job to supply about 30 or 40 mA and uses an LM317 as the regulator driving an EL84. I did have a couple of blow ups initially, and found that a noisy control pot was the cause. Adding a slug resistor in parallel so that the pot never went truly o/c cured that. The circuit is from a magazine article years ago.
Alan.
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Old 26th Aug 2018, 11:03 am   #6
GrimJosef
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Default Re: EL84 heater cathode V max?

Ah, OK. I thought you might be designing it yourself. The EL86 is not a drop-in replacement for the '84 of course and if you have an existing design then it'll be based around the characteristics of the latter rather than the former.

I sympathise over the blow-ups. In the event of a hiccup valves can often be strong enough to take it but in high-voltage circuits solid-state devices can be a lot more fragile.

Cheers,

GJ
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Old 26th Aug 2018, 2:13 pm   #7
Biggles
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Default Re: EL84 heater cathode V max?

Yes, the LM317 went with a mighty splat a couple of times, but everything else just groaned and sat up again. To be honest I think the original circuit design was probably running near the edge, so the slightest thing would blow the semi. Nice simple little circuit that seems to work ok once I ironed out the teething problems.
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Old 26th Aug 2018, 3:30 pm   #8
GrimJosef
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Default Re: EL84 heater cathode V max?

Quote:
Originally Posted by Biggles View Post
... the slightest thing would blow the semi ...
Pretty much the story of most valve/semiconductor hybrids I've ever come across. They're fine. And then they're not. The variant that uses a big MOSFET as the pass element, which always fails short-circuit, can take the downstream valve out too if the unregulated HT has been specified too extravagantly.

Cheers,

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