EL84 Acceptible load range (Single Ended operation)

[trevorus]

I have a Valve Junior that I am modding a bit, and it's the one with the 7.5K ohm primary OT. I am wondering if changing the 4 ohm speaker I have in there to the 8 ohm tap will be a problem. It'll bring the primary impedance down to around 3.7K ohm. Is that too low? I know ideal is around 5.5K ohm.

[Chuck_Farlee]

I have a Valve Junior that I am modding a bit, and it's the one with the 7.5K ohm primary OT. I am wondering if changing the 4 ohm speaker I have in there to the 8 ohm tap will be a problem. It'll bring the primary impedance down to around 3.7K ohm. Is that too low? I know ideal is around 5.5K ohm.

If your OT measures 7.5K:4 ohms and you plug in an 8 ohm speaker, you double the impedence to 15K, not halve it.

To halve the impedence you'd have to go the other way, plug a 4 ohm speaker into the 8 ohm tap or an 8 ohm speaker into the 16 ohm tap.

Do you have one of the earlier OTs with only one impedence on the secondary?

[trevorus]

If your OT measures 7.5K:4 ohms and you plug in an 8 ohm speaker, you double the impedence to 15K, not halve it.

To halve the impedence you'd have to go the other way, plug a 4 ohm speaker into the 8 ohm tap or an 8 ohm speaker into the 16 ohm tap.

Do you have one of the earlier OTs with only one impedence on the secondary?

I must have mis-spoke. Plugging in 4 ohm load into the 8 ohm tap. I have a 2 impedance secondary. It's a rebuilt combo. 4 and 8 ohms.

[trevorus]

So, anyone know if ~3.5K ohms of impedance on the primary for an EL84 is safe? I've been running it, because if the amp fries, I'll just rebuild it, and it sounds better like that. I just don't want to HAVE to rebuild it. (AKA re-install the magic smoke...)

[Wakculloch]

The Mullard data sheet on the "franks" page has data on using a 4.5k SE OT, which doesn't give less power than 5.5K, but a rise in overall distortion, tilted mainly towards second harmonic. I suspect this is why it sounds better with a reduced load.
If maximum power transfer was stable from 4.5 to 5.5k, then a reduction to 3.5k doesn't seem that drastic, unless its accompanied by excessive HT voltage, ie over 300V.
This is all theoretical.
Someone out there is bound to have tried it.

[trevorus]

It's in a valve junior, and I can't remember what the plate voltages are... It's PT says 260V, so can't be that high.

[Chuck_Farlee]

It's in a valve junior, and I can't remember what the plate voltages are... It's PT says 260V, so can't be that high.

Depending on your wall voltages, you plate voltage will be anywhere from 320v to 350v, if the amp is unmodded.

The PT is rated for 115v on the primary to 260v on the secondary. After rectification, the peak voltage is 366, if you have 115v coming out of your wall. Peak voltage will be more if your wall voltage is as high as mine @126VAC.

chuck

[Wakculloch]

The person "out there" trying it seems to be you , Mr Trevorus!

[trevorus]

The person "out there" trying it seems to be you , Mr Trevorus!

Yeah, and it seems to be working. I don't think it should be too much of a problem, but it obviously is not perfect ideal. The thing with impedances is that nothing is exact, but there is engineered wiggle room. At different frequencies, a speaker resists signal differently. I will put my cooling fan back in it, probably just to take away excessive heat. Eventually, I will probably get a new OT for it that matches up better. But for now, we'll run this way.