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I did this type of mod on many marshall 1987/1959 type heads over the past years! Works great!
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Moderators: pompeiisneaks, Colossal
Bias voltage is AC ground in that grid circuit, and that is the reason for the shield going to Vbias. Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that. Reeltard, did you do one that way? Did it work out?sepulchre wrote:That's the diagram I have too. I wired it exactly as it shows but I can see how the shielding could be used to convey the bias voltage to the pots. However, since I gave the bias its own wire would it matter if I hooked the shielding to ground instead of to the bias V?
The beauty of this MV is that when it is dimed, the circuit is 100% back to the original. The odds are that your oscillation is due to some other lead dress issue. For low volume settings I suppose you could increase the NFB with a smaller FB resistor.sepulchre wrote:Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?
It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.dorrisant wrote:While on the Lar/Mar subject....
Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?
Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.martin manning wrote: It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.
Martin is right and it's worth the wait!dorrisant wrote:Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.martin manning wrote: It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.
Usually I don't use shielded wires by putting in a PPIMV into an amp, I just twist the cables and additional I'm running the twisted bundles close to the chassis - and never have had PO problems.martin manning wrote:Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that.