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I've been experimenting with dropping B+ voltage significantly in my ancient Field Coil speaker amps (Valco Supro and National) and have had a great degree of success in lowering the Audio output by dropping B+ voltage, without disturbing the tonality or response too much.
The volume on Field coil speaker amps drops a lot if you reduce the B+ voltage significantly (as in 100vdc or more), because you not only reduce the output of the tubes, but the Field coil strength at the same time, so it's a good way to get all the distortion of the Preamp, PI, and Power tube sections at a much lower volume. (Pubs don't like too loud a music these days). I've tried many master volume arrangements, including post PI tube, but they never sound good without the Power section being driven a bit. I usually play my amps flat out or close, playing hard rock.
First thing I tried was using a Variac to drop wall voltage, but as everyone here knows you need to drop a lot of AC voltage to attenuate audio output, and that also drops the filament voltages and is not healthy for the tubes, but furthermore, I don't like the "Brown" sound too much in a vintage amp, of for that matter a Marshall.
Next up I used 17 x 12V 5W Zener diodes on a tag strip in series to nock off quite a bit of voltage. That worked out well, and the diodes never get seriously hot at around 180 Degrees Fahrenheit Peak, and that dropped the power tube plates from 350vdc to around 200vdc (going by memory here). Problem is the volume drop is fixed, unless I want to put taps in at various points on two tag strips.
Next I tried a Big 10k Ohm 100W Rheostat. This seems to work the best, even though it seems to introduce more compression, and perhaps more sag. The volume can be brought down just a little, or way down and anything in between, and everything seems to function nicely and sound just lower (the amp runs seriously cooler). There is some drop off of Bass, as you might suspect from the radical shift in voltage and operation, but not too much on the amp in question to make it unusable, and it still sounds much thicker than with a master volume.
Sorry for the long beginning, but currently I have the 10k Ohm Rheostat mounted inside a vented wooden project box, and it never get's above 130deg Fahrenheit, so I am happy with that. The problem is I have it tethered to the amp with 16 gauge speaker wire, and this is less than optimal.
Question : I would like to use a better cable system for the high voltage out of the amp to the Rheostat box, and have the ability to disconnect the Rheostat when I move the amp from gig to home.
I don't care if the amp is not functioning without the big Rheostat, as I will always run it connected. My goal is to have no serious shock hazard, and some means to disconnect the box rapidly without a soldering iron involved.
I could just mount the Rheostat in a small box inside the amp, but the cabinet is small, and doesn't have much room, and I like the concept of an external box that can be on top of the amp and dialed in without going into the back of the amp.
Thanks for any Ideas !
The volume on Field coil speaker amps drops a lot if you reduce the B+ voltage significantly (as in 100vdc or more), because you not only reduce the output of the tubes, but the Field coil strength at the same time, so it's a good way to get all the distortion of the Preamp, PI, and Power tube sections at a much lower volume. (Pubs don't like too loud a music these days). I've tried many master volume arrangements, including post PI tube, but they never sound good without the Power section being driven a bit. I usually play my amps flat out or close, playing hard rock.
First thing I tried was using a Variac to drop wall voltage, but as everyone here knows you need to drop a lot of AC voltage to attenuate audio output, and that also drops the filament voltages and is not healthy for the tubes, but furthermore, I don't like the "Brown" sound too much in a vintage amp, of for that matter a Marshall.
Next up I used 17 x 12V 5W Zener diodes on a tag strip in series to nock off quite a bit of voltage. That worked out well, and the diodes never get seriously hot at around 180 Degrees Fahrenheit Peak, and that dropped the power tube plates from 350vdc to around 200vdc (going by memory here). Problem is the volume drop is fixed, unless I want to put taps in at various points on two tag strips.
Next I tried a Big 10k Ohm 100W Rheostat. This seems to work the best, even though it seems to introduce more compression, and perhaps more sag. The volume can be brought down just a little, or way down and anything in between, and everything seems to function nicely and sound just lower (the amp runs seriously cooler). There is some drop off of Bass, as you might suspect from the radical shift in voltage and operation, but not too much on the amp in question to make it unusable, and it still sounds much thicker than with a master volume.
Sorry for the long beginning, but currently I have the 10k Ohm Rheostat mounted inside a vented wooden project box, and it never get's above 130deg Fahrenheit, so I am happy with that. The problem is I have it tethered to the amp with 16 gauge speaker wire, and this is less than optimal.
Question : I would like to use a better cable system for the high voltage out of the amp to the Rheostat box, and have the ability to disconnect the Rheostat when I move the amp from gig to home.
I don't care if the amp is not functioning without the big Rheostat, as I will always run it connected. My goal is to have no serious shock hazard, and some means to disconnect the box rapidly without a soldering iron involved.
I could just mount the Rheostat in a small box inside the amp, but the cabinet is small, and doesn't have much room, and I like the concept of an external box that can be on top of the amp and dialed in without going into the back of the amp.
Thanks for any Ideas !
They would likely have some pointers.
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