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Old 10th Apr 2021, 2:43 pm   #1
Diabolical Artificer
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Default PCL86 - Vao meaning of.

Looking at a PCL86 datasheet it gives Va max = 250v but Vao as 550v. Is Vao similar to ICAS, meaning either pulsed IP or not on full whack all day every day/industrial use? Another valves datasheet gives Va(b), so the same parameter but differently phrased?

The problem I have is matching said PCL86's pentode to a less than optimum OPT with a turns ratio. My idea is to up the HT to 275v, here's the sums ... rough Zpri of OPT for SE Class A OP stage = Va^2/Pmax, therefore 250v (Vmax PCL pentode 250v) 250^2/9 =6944 ohms.

OPT Z ratio = 44.8... (44.8^2) x 4 = 8028 ohms, so a tad too high Zpri. However 275^2/9 = 8402 ohms, this tallies with the general rule of higher the HT, the higher the Zpri needed.

The amp once built will only be used occasionally, I doubt an extra 25v HT will harm the valve much, also AFAIK an ECL86 is the same as an ECC83 & EL84, a PCL86 is roughly the same valves but for series heaters. Vmax EL84 is 300v, however given in the same table of limiting values we have Va(b); is this Va(b) the same as Vao?

Looked in my copy of Radio Designers and online, result zip.

TFL, Andy.
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Old 10th Apr 2021, 2:48 pm   #2
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Default Re: PCL86 - Vao meaning of.

Vao = max Vak at cold or during cut off conditions.

Va(b) = Vao so far as I know.

Lawrence.

Last edited by ms660; 10th Apr 2021 at 3:05 pm. Reason: extra info
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Old 10th Apr 2021, 3:05 pm   #3
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Default Re: PCL86 - Vao meaning of.

Quote:
Vao = max Vak at cold or during cut off conditions
Ok, so in a single ended OP stage because the load is reactive, Vak can swing a lot higher than HT, albeit momentarily. Thanks Lawrence. That said is Vao the same as Va(b) ?

Andy.
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Old 10th Apr 2021, 3:06 pm   #4
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Default Re: PCL86 - Vao meaning of.

Yes so far as I know, just added that as an edit to my previous post before I read your last post....ships in the night.

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Old 10th Apr 2021, 3:26 pm   #5
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Default Re: PCL86 - Vao meaning of.

Hello Andy,

For some time now I doubt if 250 V is the true Va max for the PCL86 because this Telefunken datasheet for the ECL86/PCL86 says Va max = 300 V for both. And there are more types of which I have the same doubt: EL81/PL81, EL83/PL83, EL86/PL86/UL86, ECL82/PCL82.

I do not understand what could be the reason for the P- and U-types of these valves having lower Va max and/or Vg2 max. Beside the difference in heater voltage and current, for some the isolation between the heater and cathode is a bit better but how could that make Va max and/or Vg2 max lower? Even the interelectrode capacitances are the same, which points to the same internal structures. And it would not make too much sense to have different production lines for the E- en P-types. So that leaves only the possibility that for some part of the internal structure different materials were used, but to me this seems unlikely.

I suspect that in most datasheets for these valve types some of the maximum values depend on the way the valve type is normally used, even though no valve manual that I know does confirm this.
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File Type: pdf ECL86 - PCL86 Telefunken Röhren-Dokümente.pdf (193.8 KB, 103 views)
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Old 10th Apr 2021, 3:26 pm   #6
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Default Re: PCL86 - Vao meaning of.

It's important in push-pull class B (and AB) power amplifiers. Your HT voltage goes to the centre tap. One valve is pulling current, and had better be below the normal anode voltage rating. See-saw like, the anode voltage is increased on the other valve, which is cut-off. The Vao spec says how far the off side can be allowed to swing.

So that spec sheet allows you to have 250v HT and during peak signals, one anode will therefore go towards 0v (never getting there, but this is rough theory) and the other anode will be driven by the first valve and the signal inversion of the transformer towards 500v (similarly never getting there). This is legit in class B push pull because the valve going high is cut-off. It's nice to see Vg2 has a similar off rating to allow ultra-linear and triode connection to be within spec.

So if you go for 275v, you really are operating the valves out of spec. It's nature's way of telling you to use a bigger valve.

David
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Old 10th Apr 2021, 4:06 pm   #7
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Default Re: PCL86 - Vao meaning of.

Quote:
So if you go for 275v, you really are operating the valves out of spec. It's nature's way of telling you to use a bigger valve.
It's a single ended OP stage David, 275v HT is a bit much? Your right I suppose an EL84 would be a better fit, I've no EL84's though in the Animal Zoo of OP valves in my stash. will have to have a think.

Andy.
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Old 10th Apr 2021, 4:20 pm   #8
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Default Re: PCL86 - Vao meaning of.

I don't think an extra 25 volts is going to kill it providing Wa Wg2 and Ik aren't exceeded.

Lawrence.
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Old 10th Apr 2021, 4:25 pm   #9
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Default Re: PCL86 - Vao meaning of.

In addition to my earlier post, I like to present some evidence that justifies my doubts about the true/absolute meaning of some maximum values in some valve datasheets. This example is not about Vamax and not about a power valve. But still.

In this Philips datasheet for the EF86 Vg2max is given as 200V (see page 4). Now look at the data for the EF86 triode connected for Vb = 400 V and Ra = 47K (see page 3). Ia here means: Ia + Ig2 (see the curves on pages E and F).

So in this situation Va and Vg2 both are:

Vb – (Ia x Rk) – (Ia x Ra) = 400 – (0,0037 x 1200) – (0,0037 x 47000) = 400 – 4,44 – 173,9 = 221V.

So under certain circumstances Vg2 apparently is allowed to go above Vg2max = 200V eventhough the same datasheet states that Vg2max = 200V.

Additional: Maybe the P-types have somewhat lower Vamax and/or Vg2max because normally they are part of a series heater chain, which can cause higher voltage differences between some pins than in the situation with E-type valves. The latter normally have their heaters grounded. If this is true, than earthing the heaters of the PCL86's would allow for some higher Vamax.
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Old 10th Apr 2021, 4:51 pm   #10
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Default Re: PCL86 - Vao meaning of.

Here's a couple of measured examples:

http://www.r-type.org/articles/art-003f.htm

https://www.radios-tv.co.uk/wp-conte...K70-Audio2.png

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Old 10th Apr 2021, 7:12 pm   #11
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Default Re: PCL86 - Vao meaning of.

Va0 of 550V does sound like maximum anode voltage.

There has to be a limit: excessive voltage causes flashover between the pins of a B9A valveholder. Also, if there is any residual gas in the valve, an excessively high anode voltage accelerates ions to such a velocity that they can cause excessive cathode bombardment damage.

Quote:
Originally Posted by Diabolical Artificer View Post
AFAIK an ECL86 is the same as an ECC83 & EL84, a PCL86 is roughly the same valves but for series heaters.
I don't think it is, actually - EL84 is definitely a pentode, but ECL86 is a triode and (I'm 75% certain) a beam tetrode. If you look at the heater alone, the EL84 guzzles 0.76A, but the ECL86, with more in the bottle, is 0.7A. If it were really an EL84 plus half an ECC83, you'd expect it to take 0.76 + 0.15 A = 0.91A.
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Old 10th Apr 2021, 8:21 pm   #12
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Default Re: PCL86 - Vao meaning of.

I am pretty sure that the ECL86/PCL86 contains a beam tetrode. Although the EL41 is a power pentode (?), the data and curves of the L-part in the ECL86/PCL86 and the EL41 are almost identical.
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Old 12th Apr 2021, 9:51 am   #13
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Default Re: PCL86 - Vao meaning of.

Quote:
In addition to my earlier post, I like to present some evidence that justifies my doubts about the true/absolute meaning of some maximum values in some valve datasheets.
Your example is a good one,you'll find similar disparities of the Vg2max/triode strapped with the 807 and quite a few other valves.

Thanks for those Lawrence, having looked at a few ECL/PCL86 amp schematics they all have a small cap from pentode/BT anode to g1P/BT to stop oscillation. I had a hell of a lot of trouble getting the PCL86 OP stage to stop oscillating,it was happily singing away at 25-50khz as soon as I connected the triode OP to g1. In the end I used a 1n across the OPT primary,all other solutions resulted in attenuation at HF.

The ECC83 and PCL86 triodes aren't quite the same, see att. They both have the same gain, IE 100 but the PCL triode looks to have a higher ra. The pentode/BT is about the same as an EL84, but has lower Pmax, I guess because the anode etc is smaller.the differences in thetriode could be down tothe same thing, IE physical limitations.

Andy.
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