Montgomery Ward GIM 9151a thread mods and stereo output jacks expert help needed

[tonefiend]

Hello Everyone,

scored a rare guitar amp head, (actually im sure its a bass amp) that I want to mod for guitar use.. I would like this to be a mod thread for this pretty rare amplifier, so that it may benefit others who come across this rare bird. Its a montgomery ward GIM 9151a, it has 4 6l6 output tubes, and has 2 output transformers, looking at the amp the output jacks to speakers are wired in some kind of stereo configuration,
although on the back of the amp the output jacks are labeled "main" and "extension". My question would be it seems the way the output jacks are wired that each output jack has to go to a separate cabinet? could I safely just run to just one cabinet? Does it need some special stereo output cable? Ive never seen a stereo output configuration like this, so please pardon my ignorance of the matter. Would like to mod so that I can just run to one cabinet safely (im getting old)

The "main speaker" stereo output jack has on one side a black wire(or dark green, I cant tell) going from tip to one of the output transformers, and another black wire on the other side connecting tip to the other ouput transformer, then a blue wire and red wire that connects from tip of "main speaker" jack to each side of the "EXT speaker" jack tip... not sure this makes sense to the reader so Ive included pictures . Picture 1 shows the "main" stereo output jack, picture 2 shows the "Ext" stereo output jack.

airline-gim-9151a-amplifier-schematic.pdf






Also, would like to mod the input jacks to look more like a fender, so anyhelp would be much appreciated there as well

Here are some pictures of the amp including gut shots, and the schematic

[52 Bill]

My question would be it seems the way the output jacks are wired that each output jack has to go to a separate cabinet? could I safely just run to just one cabinet? Does it need some special stereo output cable? Ive never seen a stereo output configuration like this, so please pardon my ignorance of the matter. Would like to mod so that I can just run to one cabinet safely (im getting old)

Welcome to the place. I am old, so I don't like carrying lots of stuff around anymore either.

The amp head originally came with a single cabinet that was wired in stereo and was connected with a three conductor stereo speaker cable. You could add an additional stereo cabinet via the extension speaker jack.

I always assumed that these amps were designed this way to make use of cheaper output transformers, but there could be other issues as well.

If you are going to use a normal mono speaker cabinet, you can remove two output tubes and use it that way, or completely redesign the output stage to use a single transformer.

[tonefiend]

If you are going to use a normal mono speaker cabinet, you can remove two output tubes and use it that way, or completely redesign the output stage to use a single transformer.

I would rather just pull 2 tubes, than redesign the output section... Does it matter which 2 tubes to pull out? and does the stereo output jack need to be replaced with a mono output jack?

Thanks very much for the help

[52 Bill]

Does it matter which 2 tubes to pull out? and does the stereo output jack need to be replaced with a mono output jack?

Tubes V6 and V7 should be pulled as these are the two tubes that use the ring conductor of the main speaker jack. If the tubes are not marked, you should start by figuring out which output transformer is connected to the ring terminal of the main speaker jack. Then follow the transformer wires to the two tube sockets that connect with that transformer. These are the two tubes to pull.

The jacks can be left as is, but if the extension jack is wired as it is shown in the schematic where the tip and ring connections are swapped at the two jacks, you will need to either rewire the extension jack or just not use it.

[tonefiend]

Tubes V6 and V7 should be pulled as these are the two tubes that use the ring conductor of the main speaker jack. If the tubes are not marked, you should start by figuring out which output transformer is connected to the ring terminal of the main speaker jack. Then follow the transformer wires to the two tube sockets that connect with that transformer. These are the two tubes to pull.

The jacks can be left as is, but if the extension jack is wired as it is shown in the schematic where the tip and ring connections are swapped at the two jacks, you will need to either rewire the extension jack or just not use it.

thanks! figured out which tubes to pull, Is it ok just to disconnect extension jack all together?

Also, would I connect to an 8 ohm speaker? Im going to build my own cabinet..

[52 Bill]

[QUOTE=tonefiend;335230]Is it ok just to disconnect extension jack all together?

Also, would I connect to an 8 ohm speaker?[QUOTE]
Yes, you can cut the wires going to the ext. jack if you want. An eight ohm load should be fine.

[olddawg]

I wouldn't rewire the head at all. No need. I would just wire a 2x12" or a 4x12" cab in stereo, use the correct plug make a special cable for connecting to the cab. You could always put stereo /mono switching on the cab so you could use it as a mono cab with another amp. Modern Marshall 1960 cabs are already set up this way. Seems like a no brainier to me rather than pulling tubes, butchering the wiring, etc.

[tonefiend]

Thanks for any and all replies... Ok now its on to "mods" looking for help turning the inputs into more of a fender bassman ab165 type input..

here is a link to the fender schematic http://www.turretboards.com/layouts_...sman_ab165.gif

If you look at the Montgomery ward input say for "channel 2" could I just change the two 47k resistors to 68k, and snip out c1 cap and r3 resistor?

Also, I dont see a bypass cap to the cathode, where the fender has a 25/25 cap on the cathode? Does the montgomery ward share c1 and use it
as a bypass cap, as well as a lowpass filter? Im hoping not, because I want to get rid of it...

Also the fender input jacks have a 1meg resistor, but im not sure how I would connect it on my input jacks on the montgomery ward?

again, thanks for all the help

[52 Bill]

The first question is why do you want to change the inputs to be like a Fender?

The original jacks do not have a shorting contact like the Fenders use, so you will need to replace them. The 1Meg resistor is wired directly on the input jack. So are the two 68K resistors. If you search you will probably be able to find photos showing the way they are connected.

There is no cathode bypass cap in the original circuit. C1 is there to shunt high frequency signals like radio waves, to ground so they are not amplified by the input circuit.

[tonefiend]

The first question is why do you want to change the inputs to be like a Fender?

I think it has to do with pickup loading? which i know nothing about.... trying to get it to sound more like a guitar amp than
a bass amp...


The original jacks do not have a shorting contact like the Fenders use, so you will need to replace them. The 1Meg resistor is wired directly on the input jack. So are the two 68K resistors. If you search you will probably be able to find photos showing the way they are connected.

There is no cathode bypass cap in the original circuit. C1 is there to shunt high frequency signals like radio waves, to ground so they are not amplified by the input circuit.

I see so if i understand correctly that cap does not operate in the audio frequency, so it should be ok, to just leave it in?

[52 Bill]

You can leave it or remove it, depends upon what you are trying to do.

It could be in the audible range of frequencies.

[Sweetredtele]

I know the thread is a bit old, but thanks for posting this. I have the Airline 62-9054. 9054 is the precursor amp. There aren't any schematics I can find for the 9054, but luckily the 9151a is actually the same amp as the 9054 (minus the lights and the external jack). I'm recapping and fixing mine to use as a bass amp in the studio.