Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on February 15, 2014, 04:18:33 am
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I have an Allesandro working Dog amplifier on the bench and I am trying to figure out a better way to have a master volume control in this circuit. It has been a while since I modified any amps so I am not fresh on what to do here yet (been busy building).
This amp has a "final power output" control that is supposed to vary the output of the amp from 50% to 100% and it probably does just that only the owner wants more control of the volume. As is this control doesn't make much difference in volume, maybe 3-6dB difference.
I was going to recommend installing a VVR but after thinking it over, I don't want to drill in the chassis.
So what I think may best the best option is to convert what is already in this circuit to a PPIMV.
Since I haven't seen this particular power output design before I am unsure how to proceed.
See attached pic:
The 0.1 uF PI caps each connect to the right side of the dual ganged pot, the wipers of the dual pot connect to 1k5 resistors and then to pin 5 of the power tube sockets, the left side of the pots are connected to the outer "v" 47k resistors that are connected to the bias circuit.
My thoughts are to connect the outer "v" points of the 47k resistors to the 0.1 caps and then also over to the pin 5's of the power tube sockets, just like most fender circuits do. At this point I can then install a PPIMV and be done.
My question is…which PPIMV do you think would be best suited in this particular amp?
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The LarMar is a pretty good master volume.
(Very popular mod for Marshall flavored amps. Great paired with a 2203 front end into 68 style PI and power amp, ie (RIP)Rockstah's "Mod5")
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Larmar Master is really nice. Use shielded cable and I ground at both at the pot. They are going to the grid replacing the resistors.
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The 0.1 uF PI caps each connect to the right side of the dual ganged pot, the wipers of the dual pot connect to 1k5 resistors and then to pin 5 of the power tube sockets, the left side of the pots are connected to the outer "v" 47k resistors that are connected to the bias circuit.
This IS the Larmar PPIMV,,, with a slight modification. Rather than connecting the left side of the pots directly to bias like the Larmar, your MV connects thru the 47Ks to bias. This prevents you from turning the volume to zero. (Many people don't care for the sound with this type MV turned down really low.)
If your dual pots are 250K or thereabouts, just replace the 47K resistors with a jumper wire. You may also want to put the 2.2M resistors across the wiper and left lug to protect against loss of bias in the event the wiper loses contact with the resistor element. Doing these two things will make your circuit exactly like the pictured Larmar PPIMV and give your customer total volume control.
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Thanks for the help and advice guys. As always you helped me sort things out.
Steve, yes when I looked and compared the Larmar I was surprised how much similarities there are.
I added the 2M2 resistors, change the 47k's to jumpers, installed a 33k grid stopper on v1 and added a tube shield to block rfi (which the owner complained about in his house) and next I will replace the Eminence Patriot Screamin Eagle speaker with a new Weber 12A150. The Weber arrives this coming week.
That should get the tone to where the customer wants it to be.
The Larmar works wonderfully in this amp too. I know the owner is going to really love and appreciate this mod, and so will his wife haha!
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Thanks for the help and advice guys. As always you helped me sort things out.
Steve, yes when I looked and compared the Larmar I was surprised how much similarities there are.
I added the 2M2 resistors, change the 47k's to jumpers, installed a 33k grid stopper on v1 and added a tube shield to block rfi (which the owner complained about in his house) and next I will replace the Eminence Patriot Screamin Eagle speaker with a new Weber 12A150. The Weber arrives this coming week.
That should get the tone to where the customer wants it to be.
The Larmar works wonderfully in this amp too. I know the owner is going to really love and appreciate this mod, and so will his wife haha!
I really love this in a Marshall Plexi type. 2, EL34's can really get loud. This enables me to CAREFULLY run lower wattage speakers to get air movement at lower volumes which I know all us old rockers dig. :icon_biggrin:
I said carefully as I just had to recone a Celestion Blue 15 watt, but man did it sound great. Great crunch and nothing harsh. I now know these speakers will blow quite easily when being pushed. Wish I knew how much they can take. The golds are not the same, but will hold up better.