Magnavox 175: Adding a volume pot

JohnMac

Addicted Member
Howdy all,

I'm finishing up a Maggie 175 and could use some help with correctly adding a volume pot. Do I simply interrupt the signal between the input jack and the tube? I assume the the 470K to ground gets removed and the 47K and input cap go between the pot and tube? Also, is it best to put the input on the wiper or an outer leg? I've read conflicting info on this. Any help will be much appreciated!

John
Maggie 175 Scheme.JPG
 
I put one on my Maggie 9303. Rca jack to outer leg, wiper to input, other outer leg to ground. And I do remember removing the 470k resistors with the wiper connecting after that.
 
You are correct Lavane. I tried clipping the input to the center lug and that didn't work. I switched it around and removed the 470K Rs and it works fine.
 
The 470K resistor acts as a bias resistor for the control grid. If you want to do this correctly you should make the input 1 circuit identical to the input 2 circuit.
- Remove the 470k resistor.
- Replace with 500k pot wired as discussed above.
- Add a capacitor similar to C13 across the new wiper to the high side of the pot.

Good luck and happy listening!
 
volvogv, what's the advantage of adding that small value cap across the wiper and high side of the (100k) pot? It looks like that was part of the balance circuit, which I removed. C13 is a 220 mmf, which I don't have. Just to do it I clipped in a .001uf. It increased the gain somewhat and made the highs harsh.
 
Not in that position. It helps keep you from losing treble at lower volume settings. Its basically a Miller effect compensation thing. Big resistance ahead of a tube forms a low pass filter.

Ideally you'd want to experiment with the value and tweak it for best frequency response at your normal listening volume position.
 
When I removed the balance circuit I added a 470k resistor in it's place and matched the feedback circuit to the other channel but didn't consider that cap. Sounds like it needs to be there with or without a volume pot. Or, maybe it's not that important?
 
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A large resistor between the signal input jack and the grid will lose some high frequency without a small value cap across it to compensate. The magic value cap smarter people than me could probably explain how to calculate, or you can try some until you find the right one. It won't be very large though, guessing its probably somewhere around 500pf, possibly smaller.

The grid to ground resistor does not want a cap across it, or it will cause high frequency loss.
 
OK, I'll order an assortment of smaller caps and experiment.

Another question. I ran twisted the ground and positive wires from the RCA jacks and ran them from the back of the chassis to the volume pot at the front. The jacks are grounded at the chassis so maybe I should have eliminated the ground wire that goes from the jack to the pot and grounded the pot separately. I guess it does the same thing but would have been cleaner with less wire.

006.JPG 005.JPG
 
The 470K resistor acts as a bias resistor for the control grid. If you want to do this correctly you should make the input 1 circuit identical to the input 2 circuit.
- Remove the 470k resistor.
- Replace with 500k pot wired as discussed above.
- Add a capacitor similar to C13 across the new wiper to the high side of the pot.

Good luck and happy listening!

With the 470K resistors removed, is the 100K pot that I used acceptable?
 
I used two single 100k ohm alps pots on my 175. If I recall correctly, I removed the cap and 470k resistor and attached a wire from the rca input directly to the input of the pot and a wire from the output of the pot to the junction of the 47k resistor the feeds the tube. I grounded the pot directly to the chassis via a wire.
 

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The lower the value you use on the pot, the less problems with HF roll-off at lower volume you'll have. It also lowers the input impedance, which could possibly be a problem depending on your source. Most things will drive 100K no problem, some old preamps won't like it.
 
I used two single 100k ohm alps pots on my 175. If I recall correctly, I removed the cap and 470k resistor and attached a wire from the rca input directly to the input of the pot and a wire from the output of the pot to the junction of the 47k resistor the feeds the tube. I grounded the pot directly to the chassis via a wire.
What power tubes are you using?
 
The 175 takes four 6V6GT's, two 12AX7's, and depending on what suffix (67, EE, FF etc) the rectifier would be either two 5Y3's or a single 5U4GB.
 
The 175 takes four 6V6GT's, two 12AX7's, and depending on what suffix (67, EE, FF etc) the rectifier would be either two 5Y3's or a single 5U4GB.
Ya, I have a cople 175's. The one In my avatar is next up. Im wanting to go with 6l6's or kt77's.
 
Output transformer won't be the right impedance, and I have a feeling the heater supply isn't up to the task either. A 6L6 wants double the heater current a 6V6 does. 6L6 tends to want something around the 6.5K range, 6V6 is probably closer to 10K. B+ supply may or may not be up to the task.

If it has a second heater winding, that could take care of that much of it. You'd have to change the output transformers to fix the mismatch, and you'd still have to see if the B+ winding can actually deliver enough grunt to make 6L6 tubes work reasonably. Basically it would be a near complete re-work of the amplifier to make that work out.
 
The lower the value you use on the pot, the less problems with HF roll-off at lower volume you'll have. It also lowers the input impedance, which could possibly be a problem depending on your source. Most things will drive 100K no problem, some old preamps won't like it.

Would it make sense to add some resistance to compensate for the difference between the 470Ks that were removed and the 100K pot? Maybe 330Ks.
 
6L6's will work as the 175 had to drive its tuner/preamp off of the amplifiers power transformer. Without the tuner tubes, using 6L6's will work fine however, the 6L6 won't give you any more power unless you up the plate voltage for them and the stock 175 power transformer just isn't up to the task.
 
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