Randall combo amp wiring query

[ots]

Hi folks, I've bought an old Randall combo that my son has been using. I know nothing about amps, but he's put it up for sale and it's become apparent that only one of the speakers are wired up.

So I'm trying to work out how it should be wired so that I can sort it.

The 2 pictures here show how it is at present, and the 2 options for wiring it properly.

 

Option 1 - Single amp output with connection between speakers using blade and spade connectors (in red).

Option 2 - Use both amp outputs – 1 for each speaker. No connection between speakers (in blue)

 

Any thoughts on which it should be and anything else I need to consider? I'm ok with a soldering iron and crimping kit, but I don't want to damage anything.

 

Thanks in advance.

 

 

 

 

[BigRedX]

The second jack socket is for a second cab. The two speakers in the combo should be wired together do give an impedance of 8Ω and attached to a single socket on the amp. The when you plug in a second 8Ω cab you get the full output (although with a valve amp for guitar that shouldn't be necessary as it will be plenty loud enough without).

 

I suspect that only one speaker is currently wired up because they were originally in series and the one that is now unused has failed. Are there any impedance makings on the speakers? That would give an indication of how they were originally connected (4Ω in series, 16Ω in parallel).

[ots]

Thanks @BigRedX  I'll have a look for impedance markings and switch the wiring over to check if both speakers do actually work. 

Edited July 22, 2022 by ots

[Matt P]

from the wires visible i'd guess they were in series and either one has been disconnected at an attempt to cut the volume, or one has failed (which would cut all sound when they were in series) and the wiring changed so that it gives some output.

 

but as @BigRedXhas already said it will be much clearer if you can find the impedence value marked on the speaker (clear photos of any writing or markings should help.)

 

if you have access to a multimeter then it's possible to test the speaker, or it's also possible to test with a small battery (AA or AAA)

 

Matt

[ots]

Thanks again both, I've just swapped the wires and the other speaker is working fine - I'll look for the impedance markings and order some spade and blade connectors to make up a replacement wire. 

[Matt P]

if the other speaker is working then i suspect that you don't need any more wires, have a good look for the impedance markings, i suspect they are 4Ohm speakers designed to be in series, In that case all the wires you need are there already, I'll scribble out a diagram to show the wiring and post it below.

 

matt

[ots]

3 minutes ago, Matt P said:

if the other speaker is working then i suspect that you don't need any more wires, have a good look for the impedance markings, i suspect they are 4Ohm speakers designed to be in series, In that case all the wires you need are there already, I'll scribble out a diagram to show the wiring and post it below.

 

matt

Thanks, that would be much appreciated.

Edited July 22, 2022 by ots

[Matt P]

I suspect that this is the wiring that you need. The black wire on the lower left hand connection should be moved to the top right speaker connector

[ots]

Thanks - I'll give that a try!

 

Edit - that seems to work great. I'm no guitar player though, so I'll need my son to properly try it out later and check that the speaker is performing as it should.

 

Thanks again!

Edited July 22, 2022 by ots

[BigRedX]

If you'd wired it up wrong, you'd get a very thin sound as the speakers would be out of phase and attempting to cancel each other out.

[BassmanPaul]

It's usual for a two speaker combo to have the drivers wired in parallel for a 4Ω load. The output jacks are labeled 4Ω too,. Wired in series a break in any of the wiring will silence the unit.

[BassmanPaul]

The battery test is best performed with a 9V battery. it's easier to make contact with the terminals of the driver(s). When you have fixed the wiring applying the battery across the jack plug should make both coned move in the same direction. This checks the polarity is correct.

[Matt P]

1 hour ago, BassmanPaul said:

It's usual for a two speaker combo to have the drivers wired in parallel for a 4Ω load. The output jacks are labeled 4Ω too,. Wired in series a break in any of the wiring will silence the unit.

 

there was another amp the same listed on Reverb, it has the speakers wired in series as well, It's not common but I think this one is supposed to be series.

Reverb

 

I suggested the 1.5v battery as it's a little kinder on the speaker coil and i've always found it to be enough for testing, it's not too hard to do especially if you have a short length of wire like the jumper wire on this amp.

Edited July 22, 2022 by Matt P
added link

[BassmanPaul]

Is there a model number for this beast? I can see if I have the schematic. That should settle the matter.

[BassmanPaul]

If the OP has a multimeter he could measure the DCR of the voice coils which will indicate what the impedance may be.

[Matt P]

I think it's a  rg120-210, aka a commander 120, I believe that Kurt Cobain used the head version about the time of bleach. 

[ots]

Thanks again all, especially @Matt P the boy is home and has been putting it through its paces and all seems well as per the wiring diagram.

It is the rg120-210 and is a great bit of kit, but surplus to requirements as he's just bought a Vox AC30 C2. 

 

[BassmanPaul]

I'm glad you got it sorted.

 

I have a soft spot for the AC30. The top boost AC30's were arguably one of the best sounding amps for guitar at the time.

 

In 1968 I used a borrowed AC30B at a gig at the Silver Blades Ice Rink in Liverpool. It was a sloped stage and the VOX was wandering down the stage but it handled everything my Precision threw at it.

 

Memories! LOL

[Bill Fitzmaurice]

1 hour ago, BassmanPaul said:

The top boost AC30's were arguably one of the best sounding amps for guitar at the time.

They still are. 😉