Dummy Load

[79lp]

Hi,
I have a Twin Reverb that I am currently running on only two tubes. Now I realise that this causes an impedance imbalance. If I understand correctly the amp wants to see a 4 ohm load but is now looking at 8 ohms. I've heard that the Twin is ok with this kind of thing. I've also heard that I can disconnect one speaker to rebalance, but I like the two speakers. What I'm wondering is, can I connect an 8 ohm resistor in series with each speaker, thereby creating a 16 0hm load which when connected in parallel with the second speaker would effectively give a load of 8 ohms? Can anybody tell me if that sounds right please? Thanks

[Tom Phillips]

Everything you said is correct. Including that it isn't really necessary to go to the trouble of adding the resistors to make a equivalent 8 Ohm load and still use the two speakers. On the other hand, it won't hurt anything if you would like to try and you want to spend the time & money. If you do add the resistors as you describe there will be two changes.
1) The acoustic power output will be attenuated so the amp will be perceived as just a little quieter.
2) The tone will change a little since the impedance of the resistors over the frequency band is fixed. That's different than a real speaker's impedance which varies quite a bit as the frequency changes.

[79lp]

Everything you said is correct. Including that it isn't really necessary to go to the trouble of adding the resistors to make a equivalent 8 Ohm load and still use the two speakers. On the other hand, it won't hurt anything if you would like to try and you want to spend the time & money. If you do add the resistors as you describe there will be two changes.
1) The acoustic power output will be attenuated so the amp will be perceived as just a little quieter.
2) The tone will change a little since the impedance of the resistors over the frequency band is fixed. That's different than a real speaker's impedance which varies quite a bit as the frequency changes.

Thanks Tom. Worth a try and resistors are cheap.

[stokes]

Another option is to just leave the speakers alone and run the output into the 4 ohm load.The OT can handle the mismatch just fine.I did some work on a SF Dual Showman Reverb ,basically the same circuit as the Twin.Running this type of mismatch causes the tubes to run a bit hotter,so I biased the tubes a bit colder,dont know if it even makes a difference,but he's been using it for 2 years like this with no problem.

[79lp]

Another option is to just leave the speakers alone and run the output into the 4 ohm load.The OT can handle the mismatch just fine.I did some work on a SF Dual Showman Reverb ,basically the same circuit as the Twin.Running this type of mismatch causes the tubes to run a bit hotter,so I biased the tubes a bit colder,dont know if it even makes a difference,but he's been using it for 2 years like this with no problem.

Yes, that's basically what I've been doing. The tubes are biased a bit cool, thanks to Tom's advice, and I've finally got it sounding just like I want. My main concern is overloading the OT. But if I can leave it the way it is I will. It has been an experience, a steep learning curve, and a lot of work. It's also been a lot of fun and I've learned a lot thanks, in large part, to people like you guys. Now I want to build a Deluxe from scratch. This is always what happens...