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Empee 30th July 2007 04:26 PM

OPT impedance question
Hi all,

I've got this OPT from which I do not know the specs,
all I know is that this OPT was in a Parallel Push Pull monoblok using EL34's.

So two EL34's per side of the OPT, four in total in PushPull configuration.
what primairy impedance should this OPT be ?

What would happen if I tried this OPT with three EL84's per side ?
so six in total for PushPull.....

How does one calculate the total impedance of parallelling tubes ?



Yvesm 30th July 2007 04:48 PM

Hi Empee,

A quick look at EL34 data shows that a PP implementation for 2 tubes can be done as well with a 3K4 than with a 11K OPT, this being related to the plate voltage (300 to 800V) and expected output power (30 to 100W).

If you know the plate voltage, you can narrow this range.

Anyway, all other things being equal, doubling the number of tubes calls for reducing the impedance by two.
You obtain twice the output power at the expense of twice the input power.


John Caswell 30th July 2007 04:57 PM

Output impedance
Hi Empee,
Just having an output tx with no information is a bit pointless, so I make one assumption that the secondary winding is for 8R impedance, so........
With the Tx on the bench, place a DMM set on high ac (600V or so) across the primary and then feed into the secondary 5v ac accurately measured, then measure the voltage on the primary.
Primary volts/secondary volts = turns ratio, and turns ratio squared = impedance ratio.
Once you know the impedance ratio you can calculate the primary impedance from the secondary impedance
e.g. primary volts = 200, secondary volts = 5, so turns ratio = 200/5 = 40:1 therefore impedance ratio = (40x40):1 = 1600:1.
As we have assumed the secondary impedance to be 8R then primary impedance = 1600x8 = 12,800R.

For valves in parallel if the anode load for one valve is given as 5K then for two in parallel it is 5K/2, for three in parallel it is 5K/3, and so on

Note that it does not matter if the output stage is single ended or push pull
as manufacturers impedances for SE are given as anode load, and for Push Pull given as anode to anode load.
Note of course that the current rating of the primary will need to be doubled/tripled etc for 2/3 valves in parallel

John Caswell

Empee 30th July 2007 07:40 PM

Hi !

thank you for your replies.
I will measure the OPT using John's method.

But as I understand, the parallelled impedance for three EL84's will be 5K / 3 = 1666 ohm.

How bad is it if the OPT has a higher impedance ? or lower ?

Thanks for your knowledge............


Colt45 30th July 2007 09:43 PM

EL84 PP is more often ~10k, not 5k..

Empee 31st July 2007 06:54 AM

Thank you,

but as the datasheet states: Ra-a = 10K

So that's from anode to anode, so through the complete primairy winding, so past and beyond the centretap.

so "as per valve", it's still 5K isn't it ?

PLEASE correct me if I'm wrong,
I'm learning here :)



cerrem 31st July 2007 07:55 AM

With respect to the Center-Tap....
Each side of the Push-Pull OPT primary winding is at 1/4 the plate load.... When you get more advanced this is a bit more involved than simply 1/4 ....But for first order analysis it is fine for now...

Don't make the mistake of associating a plate-load based on tube type...
The operating conditions such as plate voltage and screen voltage..ect determine a proper plate load.....


Empee 31st July 2007 08:13 AM

I get the idea....

the idea is that I got a lot to learn :D

But just to double-check;

When I measure a P-P OPT using the method John described,
I should measure the primairy from end-to-end, and ignore the centre tap.....

check !


ilimzn 31st July 2007 11:21 AM

Keep in mind that the number you get (and you should be looking at end to end, ignore the CT, as this is what plate-to-plate means in the datasheet), may not be entirely the one you expect. The P-P impedance that the tubes worked into depends not only on the number of tubes and voltage, but also on the operating point chosen, namely class A, AB or B. In your case it is possible you may be looking at a class B amp (low bias), for which a larger P-P impedance (up to 2x) is required.

Empee 8th August 2007 12:49 PM

Hi !

I've finally measured the OPT I was talking about a while back.
We are taking about a PushPull OPT

It has a Ra-a of 1K7

with secundairy taps at:
0,25 ohm
1,5 ohm
3 ohm
6 ohm
12 ohm
24 ohm

Now I want to make a push-pull amp with 6 EL84's per channel,
so 3 EL84's on each end of the OPT.

I think I need a Ra-a of 3K4 or slightly higher for that

Now if I connect a 8 ohm speaker to the 3 ohm secundairy tap,
will the reflected load in the primairy become 2,66 times 1K7, e.g. 4K5 ?

is that ok for six EL84's in PP ?



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