impedance selection - best compromise?

[Justin Thomas]

Hello all,

I've got a Hammond 125E with 1.2k-25k primary, 1.5-15ohm secondary. I'm using this in a 2xEL84 build, and would like to have impedance selection for 4,8, & 16 ohms. This OT has 6 terminals on the secondary. I need an 8k:4-8-16 according to data sheet I have...

I need to choose one of the terminals to be common (shared), correct? I would use either #1 or #2. So after crunching the numbers with the chart on the OT box, I get the following percentages off of "ideal." I've listed them in order of 4ohm, 8ohm, 16ohm:

Terminal 1 shared: -28%, -30%, -30%
Terminal 2 shared: +40%, +3%, 0%

Terminal 1 is off by the same amount each time, and Terminal 2 is perfect except for that 4ohm setting. To be honest, I'm not sure how many 4ohm cabs I'll run into, except at my house! (I like Fenders)
My question is, is either of these choices better or worse, and why? I've also heard not to overthink it, so feel free to say "don't worry about it."

Thanks, and hope it can help someone else! I kinda like these "universal" OTs!

Justin

[Steve Conner]

If you use a double-pole impedance selector switch, you can change both terminals, you don't need a common one.

4P3T rotary switches are widely available, and you can (should?) strap two sets of contacts in parallel to make a DP3T switch with better current handling.

[Justin Thomas]

Hey Steve,

Thanks for the response! I'll keep it in mind gor future builds, maybe that bass combo a friend wants. I've already got a switch, it's that big honker Bruce has been using, rated at 15A or something like that. I like big switches... this one's just a bitty amp, and I'm trying not to get any more parts for it... Given that, would you go one way over another? Thanks again!

Justin

P.S. using the one I have now, I <DO> need a common, right?

[Steve Conner]

If it's a single pole switch, you need a common. "Bruce's big honker" means nothing to me.

[Justin Thomas]

Steve,

It's a physically large, about 1 1/2" at the back, high power switch, not worried about arcing it in this small amp. Don't have a way to post a picture, maybe this link will work:

Three Position Rotary Switch

Okay, it's 6A. Worked in previous amp, but that only had 2 secondary wires and no choices...

Any opinion on which terminal I should use for common, 1 or 2? For best tube/OT health...

Justin

[MWJB]

2&4 gives you ~11K:4ohms

2&5 gives you 8.2K:8ohms

2&6 gives you 8K:15ohms

Terminal 1 doesn't give you 8K at any of the output impedances you want.

[Justin Thomas]

Thanks MWJB,

Would using that 11K at home cause any damage? Most of the cabs in our house are 4ohm because we have lots of Fenders around. I know that out in the real world, most cabs are 8 or 16. So just go with terminal 2 and live with the mismatch? Or is it just as okay to run ~30% under all the way across? Just don't want to fry anything, even if it IS at home! I'm still learning how all this works out, impedance matching & such... sorry for my ignorance!

Justin

[MWJB]

A mismatch up (regarding speaker load) of +38% (8K to 11K) doesn't really qualify as "mismatch" as such...it's roughly what your 8K primary would see at 1000Hz anyway. 30% under (8K to 5.6K) is tolerable too. But using tap #2 as the common gives you the spot-on primary Z at 8 & 15ohms, whereas using tap #1 as the common means you are 30% adrift, downwards, at all loads...

In reality though you could wire up 1&4 and run 5.8K at 4ohms, 11.6K at 8ohms and ignore 16ohms?

Speaker's impedances rise with frequency in normal use, so the tubes are more used to seeing a mismatch up, with respect to speaker load.

[Justin Thomas]

Cool, thanks, that's what I was wondering. Now I can get some reading done oncthe topic, too! I'm also looking at max versatility walk into any situation and use what's on hand; that's why I'd like to keep 16ohms, too. And I just realized that terminals 2&4 with a 2ohm load would be 5.6K, not great, but it just might work with 4x10 Fender cabs?

Justin

[MWJB]

Cool, thanks, that's what I was wondering. Now I can get some reading done oncthe topic, too! I'm also looking at max versatility walk into any situation and use what's on hand; that's why I'd like to keep 16ohms, too. And I just realized that terminals 2&4 with a 2ohm load would be 5.6K, not great, but it just might work with 4x10 Fender cabs?

Justin

5.6K with a 2ohm cab would work well enough...& with a 3x10" Fender style cab @ 2.67ohms you are back at 8K-ish.

[Enzo]

The only thing impedance mismatch will damage is the quality of the sound. It won;t hurt the tube or certainly not the transformer. It simply reflects the impedance back to the tube in a manner that shoves operation down the curve somewhere that doesnlt sound as good. None of this is rocket surgery, these are not precision circuits.

[MWJB]

The only thing impedance mismatch will damage is the quality of the sound. It won;t hurt the tube or certainly not the transformer. It simply reflects the impedance back to the tube in a manner that shoves operation down the curve somewhere that doesnlt sound as good. None of this is rocket surgery, these are not precision circuits.

Indeed & certainly not at the degrees of mismatch we have been discussing on this thread, although extreme mismatches will cause failure of tubes & possibly OTs is a very short period of time. I'm talking in the region of, say a pair of 6V6/EL84 running into a 2K primary Z. An additional problem, that scuppers any "rule of thumb" is that not all manufacturers use the same datum, so running a pair of 6L6s into a Fender OT at 2K (half the datum) will be OK, but some other amps start at 2.5K/2.7K, some 2x6V6 amps at 4.5K & halving the primary Z here is not a good idea. Nor is halving the primary Z of an amp with a low speaker load to start with, like running a 2ohm, 4x10" Bassman/Concert/Super Reverb at 1ohm speaker load.

[Justin Thomas]

Thanks to Steve, MWJB, & Enzo!

Glad to know it won't (shouldn't?) melt down on me! I trust this Hammond to put up with some abuse... I'll rig it up to 2 common-4-5-6 & try it out on evrrything I can find!

As an aside, I try to match the Fenders as exact as possible or go up no more than double. Those replacements are a little more expensive... If I use two cabs, I try to double each to keep it stock.

Justin

[Steve Conner]

The only thing impedance mismatch will damage is the quality of the sound. It won;t hurt the tube or certainly not the transformer.

That's not strictly true. With true pentode tubes like the EL34 and EL84, a too-high load impedance can melt the screen grids. The amount of mismatch that can be tolerated depends on the screen supply voltage and screen resistor value.

I don't have much of a feel for the size of the effect, but I'd sooner have 5.6k than 11k. The impedance of speakers tends to rise with frequency, so if you start out too low, you buy yourself more headroom against screen destruction.