Fooling an OT...

[skeksis]

Just wondering if there's any good reason to use a low value resistor to match OT & speaker impedance?

I've got a little 8 ohm, 8" speaker that I found going through my stuff that I'm thinking to drop into a Champion 600 that I cut a new baffle for... I know the mismatch is probably ok in this case, but would the amp be happier if I put a little 3 or 4 ohm resistor in parallel with the speaker?

(note: I corrected this below, but I meant a 7 or 8 ohm resistor - just wasn't thinking straight)

There's probably a good reason I've not heard of people doing this sorta thing more often - maybe someone can explain it to me. thx.

[kevinlekiller]

Well I know some people use resistors between the speaker and transformer to reduce the power going to the speaker, I think its called L pad. So I wouldn't think that what you want to do is bad.

[jefrs]

You can run a slightly higher impedance load off a valve output transformer, but not a lower one - they do not like to be shorted.

Adding a resistor as you describe - it would have to be a big one (at least 5 watts), not a little one, and 8 ohm, not 3 ohm - it is going to get very hot. Adding an 8 ohm resistor in parallel with another 8 ohm gives a 4 ohm load. If you use a 4 ohm resistor the load will be 2.7 ohm = too low (and a 3 ohm resistor even less). The D.C. resistance on an 8 ohm speaker is about 7 ohms because impedance is not quite the same thing as resistance.

The point being that you can probably get away with running an 8 ohm speaker off a 4 ohm outlet (Laney actually advise putting a 6 ohm load into the 4 ohm socket). Guitar amps being what they are, they are fairly tolerant of speaker impedance mis-match.

[JohnnyCrash]

You mean running a resistor in parallel to the speaker?

I'm not sure if the resistance of a speaker and a parallel resistor will "fool" an OT. I'm doubting it'd work, since a speaker is more than just a simple resister - though I don't know. Perhaps some of the more technically minded TDPRI'ers can chime in with real info... that's a question I've wondered myself.

[skeksis]

First - If I'd have been thinking, I would have meant a 7 or 8 ohm resistor... doh!

Second - Yeah, I'd think to solder the resistor across the lugs of the speaker... reduce the resistance of the pair to roughly 4 ohms, closer to what the OT wants to see.

If not, why not?

[jh45gun]

I have done it a long time ago and it worked but I think that resistor like was said has to be a heavy wattage AND I think it soaks up a lot of power I think you would be better just going with a pair of speakers to get your 4 ohm load or buy a 4 ohm speaker.

[4 Strings]

Interesting question...

In order to get away with using resistors of a lower power rating, you should be able to use two 16 ohm resistors in parallell with the speaker instead of using a single 8 ohm resistor.

[muchxs]

Originally Posted by skeksis

I've got a little 8 ohm, 8" speaker that I found going through my stuff that I'm thinking to drop into a Champion 600 that I cut a new baffle for... I know the mismatch is probably ok in this case, but would the amp be happier if I put a little 3 or 4 ohm resistor in parallel with the speaker?

What you're describing is a quick and dirty attenuator. If you wire an 4 ohm resistor in parallel with an 8 ohm speaker and your electrons always prefer the path of least resistance in this case the resistor. Most of your sound (power) would go through the resistor.

Let's just make it a straight voltage divider for the sake of discussion. 8 ohm speaker, 8 ohm resistor. Your power in theory would divide equally between the speaker and the resistor, 50% to the speaker, 50% to the resistor. I'd still use a 5 watt resistor, 10 watts would be better. Look inside an attenuator some time... lots of 25 watt resistors or larger in there. They're converting power to heat. Higher wattage resistors will convert more power into heat without cooking.

Impedance mismatch: 8 ohm speaker on a 4 ohm transformer? Don't worry about it.

[markw51]

A 8 ohm resistor in parallel with a 8 ohm speaker will give a total impedance of about 4 ohms but waste about HALF the power going to the speaker. The other half will be disipated as heat by the resistor. Use a 4 ohm speaker and it will SOUND better IMO. A proper match is always best.

[Wally]

That amp wants to see a 4 ohm load and the best way as noted above is to get a 4 ohm speaker load whether with a single 4 ohm or two 8 ohm speakers in parallel.

jefrs wrote: "You can run a slightly higher impedance load off a valve output transformer, but not a lower one - they do not like to be shorted."
This is not accurate. Fender tube amps have provided for a mismatch since 1961, with the excepetion of the 5-watters,w hich want to see 3.2(4) ohms only. Even the Champ will tolerate the 8 ohms as long as the power tube's bias pint is considered...but you lose a lot of power.
The larger Fender tube amps provide for the mismatch in the direction of less of a load...The 4 ohm amps provide an extension speaker for another 4 ohm load in parallel for a load of 2 ohms. Even the Super REverb, whcih is a 2 ohm amp, provides for another 2 ohm load. That is one 'mismatch' that I will never try because it does indeed get very close to that 'short', but Fender provides for it and some folks have done it. These amps will tolerate a 100% mismatch in the other direction, also, but with a loss of power.
Some amps are designed so that a mismatch is not advisable. All tube amps function more efficiently at their recommended load.
IT simply is not accurate to say that only a mismatch in the direction of increased load is 'proper' or 'doable'. The OT do not like to be shorted...but a load is not a short. Within certain restrictions...generally 100% in either direction with a FEnder amp....a mismatch is not soemthign to worry about. Outside of those parameters, the OT is put into a dangerous situation. The sonics are challenged at the leaset. At the worst, it fails due to the gross mismatch. IF your statement above is true, then Fender's amps from 1961 on were badly designed??? I think not. Too many of them have functioned well throughout the years in the provided for mismatched scenarios for your statement to be true.

[jh45gun]

So wally what about tweed amps and their clones will they take a mismatch? I am not worried about my 5F4 Super Clone as the OT I have is massive in it it was a Peavey OT that came out of a 100 watt tube amp. I have it wired so the OT is using the 4 ohm tap so it thinks it is an 8 ohm load due to only using two tubes instead of 4. However as heavy as the OT is I figure I can mismatch it and it will not hurt the OT since it is what it is. But back to regular Tweeds and Clones can they take a mismatch?

[muchxs]

Originally Posted by jh45gun

So wally what about tweed amps and their clones will they take a mismatch? I am not worried about my 5F4 Super Clone as the OT I have is massive in it it was a Peavey OT that came out of a 100 watt tube amp. I have it wired so the OT is using the 4 ohm tap so it thinks it is an 8 ohm load due to only using two tubes instead of 4. However as heavy as the OT is I figure I can mismatch it and it will not hurt the OT since it is what it is. But back to regular Tweeds and Clones can they take a mismatch?

Your Peavey transformer: It's designed for 4x6L6GC, right? Your Peavey transformer is only handling half the current it was designed for. You'll have to get really creative to damage it in your application.

One click on either side of ideal isn't going to hurt anything. Say you have an 8 ohm OT. You can usually go 4 ohms or 16 ohms. Many tweeds aren't demanding circuits. If you have an amp with low voltage and cathode bias the tubes and output transformer aren't under a lot of stress.

[jh45gun]

Originally Posted by muchxs

Your Peavey transformer: It's designed for 4x6L6GC, right? Your Peavey transformer is only handling half the current it was designed for. You'll have to get really creative to damage it in your application.

One click on either side of ideal isn't going to hurt anything. Say you have an 8 ohm OT. You can usually go 4 ohms or 16 ohms. Many tweeds aren't demanding circuits. If you have an amp with low voltage and cathode bias the tubes and output transformer aren't under a lot of stress.

Thanks for the info yea this came out of a quad 6l6 amp. Actually I have two of them I bought one from the local Music Go Round for ten bucks then bought a broken Peavey that had the circuit board punched out some one took it apart and put the chassis back in and the bolts are longer in the back then the front they reversed them and damaged the board beyond worth fixing. So I bought the amp for parts for 50 bucks got the tubes and a good 2x12 cab ect. Transformers ect so I have an extra OT if I ever need it.

[jefrs]

I think what I was getting at was that you can safely put 8 ohm speaker into a 4 ohm socket but not a 4 ohm speaker into an 8 ohm one. No, it won't sound the same.

Imo the Fender will accept 2 ohm speaker in the 4 because that is only 50% mis-match. I don't like the idea of a 2ohm load on a valve amp, ideally they prefer a higher load like 25 ohms (as in hi-fi/transcription use). Guitar amps are by-and-large very tolerant because they are designed to be bullet-proof for us idiot guitarists. If you put too low a load onto the circuit, it will draw too much current - this will probably not hurt the transformer but may red-plate the valves.

Putting a resistor across the speaker in parallel (as described by our OP) is a good way of getting a very hot resistor, it must dissipate the heat, and it may get hot enough to melt solder. In terms of load, the amp cannot tell the difference between a resistor and a speaker (ok, speaker back-emf).

On a transformer with multiple taps 4, 8, 16, I find I get the best sound from the 16 ohm tap using impedance matched speakers (the same ones re-configured) probably because the full 16 ohm winding is most efficient.

Oh, and the transformer primary must be matched to the valves, ripping two bottles out is bad news for those remaining valves.
Take one bottle, it wants 2X ohms load - the full primary winding.
Two bottles share so they want X ohms load - half the winding = centre tap.
We can therefore run four bottles into a centre tapped primary of 2X total load, or one bottle across the lot (hence the 5/30 watt options on some amps).