Motorola 3 Channel Amplifier...Can I connect all 3 channels to 1 Pot?

[HiFi-Stereo]

Hi, just picked up a Motorola Amp from a SK35W console...
It obviously has the two transformers for high and mids, and one for the Bass...
Using it as a stand alone, Is it possible to hook up a 100K stereo pot to all three channels or am I not thinking right?

 

[century tek]

Hi, the center or bass channel derives its signal from the left and right inputs. If you were to add a pot, it would have to be at the inputs and since there are only two inputs, you could either use two individual pots or a single double stacked pot. Mind you that on these 3 channel amps, if you are going to use separate pots, when turning up the left channel, the bass will also come up. This is also true of the right channel. With these amps, you cannot control just the bass unless you know how the circuit works and where to insert a single pot.

FYI, you will want to use this amp as intended with all 3 channels. Some can be converted to 2 channel but it is a big hassle unless you really know what you are doing and can read a schematic and pull & replace parts where needed.

The mid & high transformers do not produce any of the bass. Only the bass channel does this with these amplifiers.

 

[HiFi-Stereo]

Thanks for the info... So there is one 12AX7 and 2-6BQ5's for the left and right channels, then another 12AX7 and 2-6BQ5's for the bass channel...
could I actually connect one stereo pot to the input pins of both 12AX7's?

 

[cademan]

The left and right channels are single ended (meaning one 6BQ5 per speaker), and the bass channel is push/pull mono (meaning two 6BQ5's driving the one bass speaker).

Aside from adding a stereo pot at the inputs, I think you could add one at the input pin of the 12AX7 for the bass channel for a total of either 3 individual pots, or a single stereo (double stacked) and one single.

As mentioned, you will want to use this as the intended 3 channel set up.

 

[gadget73]

There should only be 2 inputs for this, so a stereo pot will do the job fine. The third channel is made up from the two inputs summed together, so if you cut those it would cut the third channel at the same time.

and yeah, it wants all 3 channels or its going to have no bottom end.

 

[century tek]

Yes, these amps have only two inputs, and I think it would work fine for you if you wanted to have a bass control pot connected between pin #2 of the bass channel 12AX7 tube, but simply adjusting the bass control on whichever preamp you happen to be using would accomplish the same thing wouldn't it??

 

[gadget73]

it should. The trick will probably be matching the speaker efficiencies to maintain the original balance. I can see where having level pots may help that out.

 

[HiFi-Stereo]

Thanks for all the help...
Is it correct that the pots 3 terminals connect to the: ground, input, and a pin of the 12ax7? 2 pins of the 12ax7 for stereo? If so, could i simply connect both the 12ax7's pins together?
I don't have a preamp, so was hoping to connect the bass channel with the others, unless I'm still not thinking right...

 

[Leland47]

I’ve thought these amps would be good for bi-amping....with modifications.

You’d have to find a matching amp someplace....

Just speculating....

HiFi-Stereo, I don’t think your original premise is correct that you have a hi, mid and base transformers....

I believe it is SE left and SE right channels ( combined mid and hi, stereo)
And a PP shared mono base.

 

[RWood]

You can connect the 12AX7 grids together but then the output would be dual mono.

I’ve owned 4-5 of these amps and I like them. But even runnng them with their intended preamps, the bass level never seemed to be right once you pull it from the cabinet and use different speakers. I found that a bass cut pot was a big help, and I would fine tune it for each recording.

So for you to run it without a preamp, what I’d do is put a stereo dual ganged pot on the input channels AND a bass cut pot on that channel. You could start with just the stereo dual pot and see if it does the job, great. But if the bass is too strong add the cut pot.

Post your schematic if you want specifics on where to tie in.