4x EL84 BASS AMP BUILD

[unparalleled]

Hi,
My first thread!
I used to do valve servicing in the 1970s and I am getting back into it again.
I have several projects on the bench at the moment, this is the latest plan:
I have a hefty OP Transformer from a Hammond P4 organ chassis which was originally driving 2 15" speakers @ 2ohms and was driven by pp EL84s.
What would the reflected impedance effects be if I was to drive it with 4 parallel pp EL84s?
What speaker load impedance would then be required for good matching?

Alan.
Auckland, New Zealand.

[6267]

I'm assuming it was originally 2 el84 --> 2 ohms. Normally EL84 transformers for two tubes are spec'd around 6-10k, generally 8k (close enough to fudge it, anyway). So if you were to raise the speaker impedance to 4 ohms, you have 16k or higher as load, which is no good. For 4 el84 in push-pull parallel (ppp) you would need a load impedance of roughly 4k (say 3-5k). That would represent a load of 1 ohm on your current transformer. Eek! The good news is that plenty of transformers are available for El34/6L6GC outputs, which are the correct load impedance and wattage ratings. 5k to 8 ohm would be about right, 4k into 8 ohm would probably be better (assuming a relatively usual configuration, i.e. not pure class B or something).

[loudthud]

The reflected impedance to the plates is always going to be the load times the turns ratio squared. You say originally there were two EL84s and now you want to run four? Assuming the power supply is up for the task, you could connect a load of half the original impedance. Are you sure there isn't a typo in your post?

[unparalleled]

The reflected impedance to the plates is always going to be the load times the turns ratio squared. You say originally there were two EL84s and now you want to run four? Assuming the power supply is up for the task, you could connect a load of half the original impedance. Are you sure there isn't a typo in your post?

No typo, Two EL84s were driving, load was 2ohms (2x 15" Jensen alnico in parallel).
There is only one very heavy output winding. Transformer has separate CT UL feedback winding which I may use.
This Iron was from specific bass channel.
Two smaller Xformers were driven by pairs of EL84s for what they called treble channels one of which had a "dangly spring" reverb driving it.

[unparalleled]

I'm assuming it was originally 2 el84 --> 2 ohms. Normally EL84 transformers for two tubes are spec'd around 6-10k, generally 8k (close enough to fudge it, anyway). So if you were to raise the speaker impedance to 4 ohms, you have 16k or higher as load, which is no good. For 4 el84 in push-pull parallel (ppp) you would need a load impedance of roughly 4k (say 3-5k). That would represent a load of 1 ohm on your current transformer. Eek! The good news is that plenty of transformers are available for El34/6L6GC outputs, which are the correct load impedance and wattage ratings. 5k to 8 ohm would be about right, 4k into 8 ohm would probably be better (assuming a relatively usual configuration, i.e. not pure class B or something).

I am just trying to make the best use of a very nice, very large transformer, by the size of it I think it is good for a lot more than 20Watts and the LF response will be excellent.

[tubeswell]

Hi Alan

FWIW for 4 x EL84s, I 've got somewhere in my notes that the Matchless 30W was a 4k reflected load from the OT, but then the Matchless 15W for 2 x EL84s was also a 4k reflected load. The vox 15W was 8k and the 30W was 4k. The datasheet attached suggests 8k for a pair in PP Class AB1, so that would make 4k for a quad.

In reality for power beam tetrodes and pentodes, as long as the load resistance is between 1/6th and 1/8th of the TUBE's dynamic plate resistance ( rp ) value, you will be safe, although the harmonic distortion you get will be less with a lower zed and the output power will be higher with a higher zed.

So maybe the tranny may have been from some other amp, and the builder merely may have organised it to give approximately the correct reflected load for what they thought a low powered nice bassy sound would be (or maybe they didn't). Perhaps it is a tweed bassman type OT (4k/2R), or maybe it is an 8k/4R OT that they were trying to make into a 4k reflected load by running 2R off it. As loudthud already implied, the only way to work out the reflected load is to power up the secondary with a variac and measure the VAC on the primary and then square the result etc. (But you know all this stuff)

(Nice to have another k1w1 around BTW)

[tubeswell]

Forgot the attachment

Ooops!

[6267]

I used to hang a 4-ohm speaker off the 8-ohm tap on my 8k 2xEL84 transformer. Sounded pretty good to me, lotsa grit. Wire a voltmeter to the secondaries (so you don't have to mess around with it) and plug the primaries into wall power. Divide wall voltage by secondary voltage and square it. Multiply that number by whatever speaker load you are going to use and that will be the approximate load impedance for the tubes. Careful, it's better if you can use say a 12v AC supply instead of the mains.