Gilmore Jr: Safe at 20 ohms?

[uFu]

I've built a Gilmore Jr and a 2X10 cabinet to go with it. At the moment I have the cabinet loaded with one 4-ohm and one 16-ohm speaker, wired in series for a combined impedance of 20 ohms. I understand the amp is rated for 4-16 ohms, so my question is whether running it into 20 ohms will cause any trouble. Has anyone done this?

Thanks.

[Jazz P Bass]

I cannot find any info/ specs on the output transformer GGO-88-7.
As you are probably splitting hairs at 1/2 watt output, I would have to say "No", the 20 ohm load will not 'hurt' the output transformer.

Also, why the impedance mismatch? (unless this is what you want)
In a series circuit the 4 ohm speaker will use 0.1 watt while the 16 ohm will use 0.4 watts.
Speaker Impedance

In a parallel circuit, the total impedance will be 3.4 ohms & the power figures will be reversed.
ie: 4 ohm: 0.4 W 16 ohm: 0.1 W.

If you really need to know, I would suggest that you contact the distributor: rich@guytronix.com

guytronics_gilmore_jr_2010_amp_sch.pdf

[Chuck H]

There are three things you should consider. The first is whether or not you are cranking the amp up into clipping. If the amp is rated at 4 to 16 ohms without an impedance switch I'll assume that the designer has already considered the maximum safe load mismatch. You are exceeding that by a tad. Probably not a big deal if you're playing the amp clean, but if you're overdriving it the OT may experience dangerous voltage spikes. For tubes, it's more dangerous running into a too high impedance than a too low impedance, generally. The second thing would be the efficiency of the amplifier. You're not getting your power if you're running into a mismatched load. Since the designer spec'd a load range I'll again assume. The amp is likely nominally rated at 8 ohms with 4 or 16 being the "generally considered safe for tube amps" mismatch. That means that the amp would make the most power at 8 ohms, less at 4 or 16. And you are exceeding even that level of inefficiency. The third thing is the efficiency of the load. With two widely mismatched speakers sharing the load the lower ohm speaker will be dissipating the vast majority of the amps power. What you'll be hearing will be predominantly the 4 ohm speaker. So the efficiency of the cabinet is compromised. With both the amp and it's load operating in an inefficient manor you're going to be sacrificing a lot of potential wattage. If that matters to you. The more important issue is the over 100% mismatch between the amp and the load. If you replace the 16 ohm speaker with a 4 ohm speaker OR replace the 4 ohm speaker with a 16 and rewire them in parallel you could idealize the load for the amp and the efficiency of the cabinet.

EDIT: The above statement in blue is incorrect, as noted by Jazz P Bass. For a parallel circuit the lower impedance transducer will use more power. For a series circuit (like yours) it would be the higher impedance transducer.

[Jazz P Bass]

[QUOTE=Chuck H;374586] With two widely mismatched speakers sharing the load the lower ohm speaker will be dissipating the vast majority of the amps power. What you'll be hearing will be predominantly the 4 ohm speaker. QUOTE]

Not to get pedantic but it is a series circuit.
The 16 ohm speaker will consume the most wattage.
Again, at 1/2 watt output, it's splitting hairs.

[olddawg]

I would just disconnect the 4 ohm or the 16 ohm speaker (one or the other) and play it that way until you can get another matched speaker and have a balanced load at 8 ohms. I doubt you will hear a difference. Especially if you plan to mic the cab.

[Chuck H]

[QUOTE=Jazz P Bass;374640]

With two widely mismatched speakers sharing the load the lower ohm speaker will be dissipating the vast majority of the amps power. What you'll be hearing will be predominantly the 4 ohm speaker. QUOTE]

Not to get pedantic but it is a series circuit.
The 16 ohm speaker will consume the most wattage.
Again, at 1/2 watt output, it's splitting hairs.

Pedantic is fine as long as my mistakes don't get taken for correct accidentally. I don't at all mind being corrected when I make a mistake. One of my pet peeves are people that get angry at others they became embarrassed in front of. There's a lot of that here already so I'm not doing it. Thanks for having my back.

[Jazz P Bass]

[J M Fahey]

It might have been WORSE:



yea, I know I'm giving my age away