Hello, I've had a Marshall Class 5 for over 10 years, and I installed a VVR kit on it (and I've made many other modifications in those 10 years...).
I can play at pretty much any volume I want, it's fine.
I just found a Marshall 1974x, and I'd like to be able to get a crunchy sound from it at low volumes (without using an overdrive pedal).
Apparently, it's possible to install a Lar/Mar PPIMV on this amp?
If anyone has a detailed procedure (as well as an opinion on this modification)... Thanks!
P.S.: I have a Bad Cat Unleash attenuator, but I find that it alters the sound way too much (Class 5 and 1974x).
Donations
Marshall 1974x Lar/Mar PPIMV?
Moderators: colossal, zaphod_phil, CurtissRobin, Daviedawg, Graydon
-
Crabman
- Newbie
- Posts: 86
- Joined: Thu 10/28/10 2:00 am
- Location: Sudbury, Ontario
Re: Marshall 1974x Lar/Mar PPIMV?
I haven't tried a PPIMV so just an opinion here.
From what I can see the Lar/Mar is intended for use with fixed bias circuits where there are limitations on the size of the grid leak resistors. This mod uses a 250K dual pot and replaces the existing grid leaks (typically 220 K) in an EL34 power section with 2.2M grid leaks. Why? If you do the math you will see that the parallel combination of 250K and 2.2M will give you about a 225K grid leak path at full volume .. same as before the mod. Furthermore, the load on the phase inverter remains fairly constant regardless of volume setting .. same as before the mod. Also, the taper on the pot would not be altered much.
Very clever except for one thing that I can think of. If one of the tracks on the pot were to fail then one of the power tubes may see a 2.2M grid leak path which far exceeds specification for fixed bias … thermal run away may then take place. Yikes … not so good.
Your 1974 is most likely cathode biased unless it has been modded. Could you add this configuration to a cathode biased EL84 amp?. I think so. Of course you don't have a fixed bias supply so your pots would be connected to ground. You would now be using a 220K grid leak path instead of 470K. So a different configuration than what you started with and some attenuation of the signal. The grid leak spec limit for El84 in cathode biased is 1M which is unusually high for a power tube. However if the pot fails then this limit will still be exceeded, so some risk involved.
An alternative approach is to use the PPIMV mod for the 18 watt circuit that you will find in the download section here. In this case a 1M dual pot and 1M grid leaks are used. If you do the math this parallel combination will give you a grid leak of 500K which is close to the existing 470K. However, the loading of the phase inverter will not be constant as the volume pot is adjusted. So not quite the same as before the mod. The interaction of the coupling cap and this changing load will have an effect on frequency.
On a positive note, if a track on the pot were to fail then the EL84 now sees a grid leak of 1M which is at the limit for cathode biased operation.So technically still within spec.
Anyway, my 2 cents worth
Dave
From what I can see the Lar/Mar is intended for use with fixed bias circuits where there are limitations on the size of the grid leak resistors. This mod uses a 250K dual pot and replaces the existing grid leaks (typically 220 K) in an EL34 power section with 2.2M grid leaks. Why? If you do the math you will see that the parallel combination of 250K and 2.2M will give you about a 225K grid leak path at full volume .. same as before the mod. Furthermore, the load on the phase inverter remains fairly constant regardless of volume setting .. same as before the mod. Also, the taper on the pot would not be altered much.
Very clever except for one thing that I can think of. If one of the tracks on the pot were to fail then one of the power tubes may see a 2.2M grid leak path which far exceeds specification for fixed bias … thermal run away may then take place. Yikes … not so good.
Your 1974 is most likely cathode biased unless it has been modded. Could you add this configuration to a cathode biased EL84 amp?. I think so. Of course you don't have a fixed bias supply so your pots would be connected to ground. You would now be using a 220K grid leak path instead of 470K. So a different configuration than what you started with and some attenuation of the signal. The grid leak spec limit for El84 in cathode biased is 1M which is unusually high for a power tube. However if the pot fails then this limit will still be exceeded, so some risk involved.
An alternative approach is to use the PPIMV mod for the 18 watt circuit that you will find in the download section here. In this case a 1M dual pot and 1M grid leaks are used. If you do the math this parallel combination will give you a grid leak of 500K which is close to the existing 470K. However, the loading of the phase inverter will not be constant as the volume pot is adjusted. So not quite the same as before the mod. The interaction of the coupling cap and this changing load will have an effect on frequency.
On a positive note, if a track on the pot were to fail then the EL84 now sees a grid leak of 1M which is at the limit for cathode biased operation.So technically still within spec.
Anyway, my 2 cents worth
Dave
1 x
-
Crabman
- Newbie
- Posts: 86
- Joined: Thu 10/28/10 2:00 am
- Location: Sudbury, Ontario
Re: Marshall 1974x Lar/Mar PPIMV?
I don’t have the parts for the LAR/MAR but did get around to trying out the type 3 PPIMV. This is the simple phase cancelling type that Ken Fisher came up with. I used a 500 K pot rather than 1M to improve the range on the pot at low settings.
Like I expected, you get some extra distortion from the phase inverter but its terrible sounding at low volume (bedroom setting). I think any time you monkey with the signal intensity in the power section in a tube amp you are killing the mojo generated from the interaction of tubes-OT-speaker.
It’s a step above the scale control method which alters the tail resistor in the PI, but I am now convinced that removing the power tube vibe from the equation is kind of pointless. The videos I have seen for the LAR/MAR at low bedroom type settings sounds about the same … think bees in a can.
Where it is useful is at mid volume where you can pull back a bit when cranked and remove a bit of flub from the power section. It helped a bit during my last jam session with taming the full-on scenario. Also, it’s a bit better suited to the TMB than normal channel for obvious reasons.
Well, I guess its now onto voltage regulators and attenuators.
Like I expected, you get some extra distortion from the phase inverter but its terrible sounding at low volume (bedroom setting). I think any time you monkey with the signal intensity in the power section in a tube amp you are killing the mojo generated from the interaction of tubes-OT-speaker.
It’s a step above the scale control method which alters the tail resistor in the PI, but I am now convinced that removing the power tube vibe from the equation is kind of pointless. The videos I have seen for the LAR/MAR at low bedroom type settings sounds about the same … think bees in a can.
Where it is useful is at mid volume where you can pull back a bit when cranked and remove a bit of flub from the power section. It helped a bit during my last jam session with taming the full-on scenario. Also, it’s a bit better suited to the TMB than normal channel for obvious reasons.
Well, I guess its now onto voltage regulators and attenuators.
2 x