4ohm head into an 8ohm cab?

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1,027
I have an 8 ohm Avatar cab, and I've been thinking about picking up an old Silverface Bassman 50 watt. Those heads are 4ohms, though. Is there any kind of a device that I can place between the amp out and the cab to convert it to the proper load, or is there some other way to rig it?
 

4styx

Member
Messages
1,269
Two 8ohm speakers in parallel is your best option.I know I'm stating the obvious.
 

4styx

Member
Messages
1,269
Why don't you try it at that mismatch, it might even sound better that way.
I've heard those output trannies can't take it,maybe if you keep the bass low.I know someone that uses an outboard variable load device for his Super,but it was custom made.
 

matt5150

Either U Know Or U Don't
Silver Supporting Member
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1,583
I don't think it will make a difference and neither does Kevin O'Connor from London Power:


Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.
As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors. The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

Matt
 

alivegy

Member
Messages
1,176
If they're $100 you might as well look into one of his attenuators as well just to have the extra functionality for a bit more.
 




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