Plexi or jtm 45 PPIMV: could it fry output tubes?
Moderators: pompeiisneaks, Colossal
Re: Plexi or jtm 45 PPIMV: could it fry output tubes?
Sorry guys, just fiew questions: after this PPIMV mod (despite previous) need to rebias the output tubes? In order to install 6550 output tubes we got to swap the 220k bias resistor with 150k, in this mod there's not more 220k resistor but 2M2, could I install without trouble 6550?
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martin manning
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Re: Plexi or jtm 45 PPIMV: could it fry output tubes?
The MV install shouldn't affect the bias much, if any, but no harm in checking.
For the 6550's, you should want no more than 150k from the grid to the bias supply. As it is with the MV turned up all the way you have 2M2 in parallel with the pot's 250k element, so 224k.
When the MV is turned down, the resistance is the parallel combination of the 2M2 and the part of the pot's element that is between the wiper and the bias supply, so it will be somewhere between 224k and zero.
Since 150k pots are not common, I suggest replacing the 2M2 with 370k so the maximum resistance will be 149k. Since the total resistance from the top of the pot (CW lug) to the bias voltage will increase as the MV is turned down (up to 250k when the volume is at zero), the frequency roll-off out of the PI will change a bit with the volume setting, but it goes from around 6 Hz to 10Hz, so it should not matter.
For the 6550's, you should want no more than 150k from the grid to the bias supply. As it is with the MV turned up all the way you have 2M2 in parallel with the pot's 250k element, so 224k.
When the MV is turned down, the resistance is the parallel combination of the 2M2 and the part of the pot's element that is between the wiper and the bias supply, so it will be somewhere between 224k and zero.
Since 150k pots are not common, I suggest replacing the 2M2 with 370k so the maximum resistance will be 149k. Since the total resistance from the top of the pot (CW lug) to the bias voltage will increase as the MV is turned down (up to 250k when the volume is at zero), the frequency roll-off out of the PI will change a bit with the volume setting, but it goes from around 6 Hz to 10Hz, so it should not matter.
Re: Plexi or jtm 45 PPIMV: could it fry output tubes?
Great explanation Martin, but I prefer to use a KT66. This change in resistors maybe I will do in my newest bild: the 2two inputs JMP 1987 lead with your/LaMar PPIMV. Talking about biasing, no change in sound I noticed, if I found 2 minutes I send you 2 different clip with the oldest and new MV
Thanks
Thanks