The Steel Guitar Forum

I have a old Christmas Tree Amp that has a M-31 JBL 8 ohm speaker in it.
How can I find out if it would hurt if replaced it with a D-130 JBL 4 ohm speaker

Larry

You ought to be OK, Larry. Impedance mismatch doesn't matter as much for solid-state amps.
Miss you, and love you.

I don't know if the Xmas tree amp would like a 4 ohm speaker, usually solid state amps have a safe minimum load. Sometimes that's 8 ohms. If the factory put in 8 ohm speakers, it's likely that is the minimum speaker impedance the amp can safely tolerate without overheating and damaging the output transistors. That's my guess anyway..Jerry

I had a Sho-Bud "Christmas tree" that I bought from Larry Sasser - it had a D-130 JBL (8ohm) speaker. In a weak moment I soldit and I sure wish I had it back!

I agree with Jerry! For what it's worth, I had always held the view that the tube amps were generally more forgiving to an impedance mismatch due to the massive power output transformers that are present; whereas, the solid state amps don't have that same luxury of large transformers in the output stage. An impedance mismatch of lower impedance than what the transistors were designed to safely run at could result in them frying themselves if pushed too hard! A higher impedance value than what the amp was originally designed to see would probably not damage the amp at all, but would just limit the available wattage output; i.e. 200 watt rating @ 4 ohms would probably drop to arount 120 watts or so into a 8 ohm load. I'm certainly not an amp technician, but before I would do this of going into a lower impedance speaker under the assumption that everything would be OK, I would check with an amp guru like Ken Fox or someone of his caliber. Hope this helps! Good luck, Roger.

With tube amps less load is always better than more. Most all that have a speaker out jack use the shorting type so the output is never open,(infinite resistance). With SS amps you have to be careful how little load you feed the output. Most tolerate an open in the speaker load, (at least at idle).

Does anyone have a schematic for this amp? If I had a schematic I could answer Larry's question. If this amp uses the old germaniun output transistors, I would almost bet it was designed for 8 ohm speakers. Back when this amp was built 4 ohm speakers were not very common.

The schematic shows the output transistor as 40636, which cross referenced 2N3055, si NPN.

So which speaker would work best?
would it indanger the D-130-4ohm?

M-31-8ohm
D-130-4ohm
Larry

The schematic I saw shows the output transistor on your amp as a 2N3055. I don't believe it will harm the amp if you use the 4 ohm speaker. However, due to the impedance mismatch the transistor would run warmer.

Personally, I would leave the original JBL 8 ohm speaker in the amp. The amp was designed to have an 8 ohm load as far as I know, damage may result if you go with a lower impedance. You are dealing with a forty year old amp here...Jerry

Hope this is legible: