4ohm head through an 8ohm cab

[Tractorking]

I have a Silvertone 1483 (1 x 15”, supposedly around 23 watts output) which has stopped working - no idea why - and a Silvertone 1484 (2 x 12”, supposedly 50 watts) that works. I know nothing technical about amps. Both use 2 6L6 power tubes. My understanding is that the 1483 has an 8ohm speaker impedance and the 1484 is 4ohm. Can I run the 1484’s 4ohm 50 watt head through the 1483’s 8ohm cab? Will it blow something up? If it would work, how will it likely affect the tone, distortion, and volume? I acquired both of these to help with getting a very lo-fi fuzzy distortion so I’d be happy if if that were the consequence.

 

[58Bassman]

I have a Silvertone 1483 (1 x 15”, supposedly around 23 watts output) which has stopped working - no idea why - and a Silvertone 1484 (2 x 12”, supposedly 50 watts) that works. I know nothing technical about amps. Both use 2 6L6 power tubes. My understanding is that the 1483 has an 8ohm speaker impedance and the 1484 is 4ohm. Can I run the 1484’s 4ohm 50 watt head through the 1483’s 8ohm cab? Will it blow something up? If it would work, how will it likely affect the tone, distortion, and volume? I acquired both of these to help with getting a very lo-fi fuzzy distortion so I’d be happy if if that were the consequence.

You can use the 8 Ohm cabinet with the 4 Ohm head. How the amp handles the mis-match is up to the design but it wouldn't be the first time someone did this.

It's not going to output full power and it won't be fuzzy just because the speaker impedance is double that of the original, but it won't be as loud, so you may find that the difference in settings creates better/different sounds.

 

[Wally]

Running an amp on a load that is a mismatch to the high side…8 ohm load on a 4 ohm OT….places the stress on the secondary of the OT.
Sometimes damage can occur that might not be immediately noticed by a user. Pierced insulation due to flyback voltage degrades the sonics and decreases output power. As time goes on, this will get worse…until the results will be noticed. This damage is more likely to occur when one is pushing an amp….like trying to get LO-res sonics. An amplifier functions most efficiently and most safely with a matched load. That said, many people run mismatched loads. I choose not to do so.
You say you just acquired these amps? Have they been properly serviced?

 

[Tractorking]

Thanks Bassman and Wally. Both amps are recent purchases and I was told both had been tuned-up and serviced, but I don’t know details. In any event, the 1483 abruptly stopped working. I wasn’t in love with the tone of either one and the 1484 was too loud, so I’ll experiment and do the mismatch-up, mindful of your caution about the OT, and see what happens now that I know that it won’t explode or burst into flames.

 

[Wally]

Thanks Bassman and Wally. Both amps are recent purchases and I was told both had been tuned-up and serviced, but I don’t know details. In any event, the 1483 abruptly stopped working. I wasn’t in love with the tone of either one and the 1484 was too loud, so I’ll experiment and do the mismatch-up, mindful of your caution about the OT, and see what happens now that I know that it won’t explode or burst into flames.

Imho, those amps need to be assessed by a good tech. ‘Tuned up and serviced’ means very little. Operating such an old amp with no understanding of its actual condition…especially since one has failed….
is risky at best. IF the amps have not had the electrolytics replaced, There is a chance you have lost a transformer in that dead amp. Operating an unhealthy amp under less than proscribed
conditions..such as a mismatched load….only increases the risks.
Good luck….

 

[schmee]

With a Fender amp it's usually fine. But who knows with those Silvertone amps?. I'd say if the OT's are large and robust...not skimpy, you're good to go. I ran my Bassman 50 for a couple years gigging with an 8 ohm JBL. I actually liked it, although it was very subtle change that way.

 

[Jakedog]

I think it depends on the amp. Growing up I had loads of amps and stereo equipment both tube and SS that were labeled things like “8 ohm min”. I was taught, and always took that to mean, 8 was the minimum, so don’t go below that, but going over is fine. I’ve never personally hurt a piece of equipment going over on speaker impedance. I have personally seen a large number of amps get blown up by going under. It’s happened a LOT.

In my last “loud” band back in the 90’s, I used a Hot Rod Deluxe. With a 16ohm extension cab. The manual said 8 ohms minimum for the ext jack. I figured it was fine. It always was. I had a couple knock down drag out arguments right on this board with people who claimed I was going to damage the amp. It ran that way 5-6 night per week, loud as hell, for around six years. I sold that amp to gentleman in 2004 who is still using it. With the 16 ohm ext cab. It’s been running that way for over twenty years now and still works like it always did. And many would say that’s not even a very well built amp.

I obviously cant make any guarantees because I have no idea what condition your amps are in, or how they’re made. But I can say I have never in my life hurt an amp by going over on impedance. Only under.

 

[printer2]

What has not been said, or asked, is how you plan on running it. Diming the amp where you have the output stage clipping real hard causes more spikes and stress than if playing clean. Or if using pedals to get dirt, running the output below clipping will be less harmful than pegging the output.

 

[Wally]

Fwiw, Jake…running that 16ohm speaker on the extension Jack with the internal load of 8 ohms yields a 5.33 ohm load…..less of a load than the 8 ohm by itself, right? So, you have run an amp at less than the ‘minimum’, have you not….long and hard by your testimony. (;^)
Fender provided for a mismatch to the low side. I don’t use those ext jacks, but they are there and yield a 50% mismatch to the low side.
That mismatch to the low side puts the stress on the primary side of the OT…mainly on the power tubes. If one wants to run such a mismatch, one would want to be assured that the power tubes are in good condition and that the biasing is ‘proper’. It can be that a power tube failure could take out an OT.
Imho, discussion of impedance mismatches with tube amps should never be had with any commentary about SS amps. In almost all cases, Solid state amps can tolerate any impedance at or above the minimum advised by the manufacturer….including an ‘open’….infinite impedance….no load at all. Impedances below that stated minimum start to look like shorts, and solid state amps do not tolerate shorted loads. Whereas tube amps can tolerate shorted loads for some time period. That is what the shorting Jack on the amplifier’s output shows the OT when one forgets to plug in a speaker….in most tube amps. Some amps do not provide for this safety measure…and one would want to have a speaker plugged into such amps. Vintage Marshalls, many if not all Boogies….
Speaking of Marshalls, with vintage Marshalls it is definitely advisable to match the load.
Imho, it is best to match the load to the OT in any tube audio amp. This provides for the most efficiency and safest operation. Ommv…..and that is fine. I don’t loan my amps out, so I don’t have to worry about what others do.

 

[ChicknPickn]

I think it depends on the amp. Growing up I had loads of amps and stereo equipment both tube and SS that were labeled things like “8 ohm min”. I was taught, and always took that to mean, 8 was the minimum, so don’t go below that, but going over is fine. I’ve never personally hurt a piece of equipment going over on speaker impedance. I have personally seen a large number of amps get blown up by going under. It’s happened a LOT.

Yeah! I have actually witnessed this!

 

[Tractorking]

I’m hearing there is some risk and then again maybe not, depending on the head’s specs and general condition. Printer2, I will be playing it all the way up or nearly so, but with no pedals. Also, I just play at home so I’m only playing it for 30-60 minutes at a time, maybe 3-4 days a week. I’m going to go ahead with it, and I’ll let you all know how it’s working after a week or so of playing. If I blow something so be it, and maybe I’ll be fortunate like Jake was. Thank you all for these replies, this has been very helpful.

 

[griffbones]

You will not hurt the output transformer by putting 8 ohms on it, but do not put less than 4 ohms on it.

 

[printer2]

You will not hurt the output transformer by putting 8 ohms on it, but do not put less than 4 ohms on it.

Some of us are more concerned with high voltage spikes punching a hole through the insulation due to transits rather than overheating the winding.

That said it might be fine, I would not think much of it running an 8 ohm load but then again I have no problem replacing a transformer if needed.

 

[Tim S]

Speaking of transformers, having one of these around can be handy

Z-Matcher 100w Impedance Matcher – Weber Speakers

www.tedweber.com

 

[BradKM]

I'd say if the OT's are large and robust...not skimpy, you're good to go.

The OTs in those two amps are not large and robust.


The Z-Matcher mentioned above is a great tool to have on hand once you start amassing amps and cabinets.

 

[telekaster1999]

I've been doing that for a few months without any issues.

 

[2L man]

Higher than designed loudspeaker impedance can increase Pentode/Tetrode Screen current and if there is not sufficient series resistance in Screen circuit it can destroy power tube(s).

Also OT has higher danger to destroy because of Flyback Voltage phenomenon where very high voltage pulses appear in OT primary circuit. Simplified mechanism is when output clip the anode voltage stop coming lower but anode current can keep flowing thru OT primary. This store energy to the inductance OT has. Then when anode current direction is abruptly changed too high loudspeaker impedance can not "use" the current and it turn to a higher voltage in OT primary.

Using lower that designed loudspeaker load drop output power and increase power tube anode peak current. Anode voltage swing stay high and when power = voltage x current this increase anode power dissipation. If these power peaks are brief this should be safer against sudden breakups but most likely will shorten power tube life. Tube rectifier increase High Voltage sag and this can be seen as protective mechanism.

Loadline is great to test both mechanisms using this links excellent tool. High impedance change the loadline angle low and power persentage which Push Pull amp produce operating A-class increase. Anode voltage swing go much farther to the right.

Universal Loadline Calculator

This calculator allows you to trace the anode characteristics graph, and to compute and tune loadline and operating point of various vacuum tubes.

www.vtadiy.com

 

[zook]

I may be wrong, but I got an original OT for a 1484 to replace the aftermarket one. When checking it I think it had both 4Ω and 8Ω taps.