Master Volume : Fx loop 1987x

[FabGTO01]

Hello everybody,

I want to built a volume box to use with the fx loop of a marshall 1987x as master volume. I thought using just a 100k (or under) log pot. What do you think about this value and do you have some advices for this ?

Fab

[Blackboarcult]

An A100K pot should do the trick AFAIK, but most of the volume pedals I've seen go up to 500K.

However... what would be the purpose of this MV? If it's to act as an attenuator, bear in mind that your tone will be affected, because you wouldn't be pushing the power tubes as hard as without it. Yes, you'd be able to crank up your gains without going deaf, but some of the power section overdrive will be lost in the process, methinks.

[HamishR]

As someone who has built quite a few Marshall 50W style amps I have tried most of the popular master volume designs.  And I'm sorry to have to say that a volume pot in the FX loop won't be much of a master volume!  Sorry!

Honestly the best MV I have tried on a Marshall is the Marshall MV as used in the 2203 and 2204 MV amps.  It's not very difficult to convert a 4-hole 1987 style amp to a 2204, and not difficult to convert it back again either.  The more old-fashioned the amp is the easier it is to do of course!  If the amp is SMD style parts on a modern PCB I wouldn't do it, but if it's an older style PCB with through-hole components it's not so difficult to do.

Trouble is that all of these amps sound best just turned up loud.  You might be better off trying one of the recent 20W Marshalls which are designed to sound like an old amp at lower volume.

[FabGTO01]

Thank you both for your advices, I know how it will sound, i tried a volume pedal in the loop. It's not the best master volume but it's an easy tonal option (clean phase invertor and lower volume). I'll chek and different pots.

I'm not an electronician, so to mod my amp i'll ask to a technician in my town.
My amp is a reissue not collectible so why not mod it. I tought about a switchable post phase invertor master volume. HamishR advice a 2203 MV that i don't understand the difference with the fx loop placed before phase invertor (i'm not electronician...). Maybe could you explain me how it works.

I tried the marshall 20W (JCM800 and plexi) and they are really cool but 20w at max is loud too. To be perfect the plexi needs (IMO) a way to manage the output volume, the JCM800 sounds as my 2203 reissue 100w.

Fab

[HamishR]

Well it's a bit difficult to explain the details of pre- VS post-phase inverter master volumes in a short space!  I guess (to simplify greatly) a POST PI MV uses the preamp gain to generate all of the distortion and amplify it with the power tubes (in my experience it sounds thin and fizzy until very loud, by which time you may as well just turn the amp up!).  A PRE PI MV does a similar thing but uses one of the first gain stages as a clipping stage.

The Marshall Super Leads and similar circuits are all based on the 5F6-A Fender '59 Bassman.  It has four inputs with two "channels".  Each channel uses a single triode of the first (dual triode) preamp tube before they are mixed together at the second gain stage - the second tube.  The 2204/2203 runs the first two triodes in series with one of them being biased in such a way as to clip and create distortion.  If you plug into the Low input on one of these amps you bypass that triode so get a much cleaner sound, at quite a reduced volume.

It's all quite simple when you get your head around it!  And with a few tweaks a MV Marshall can sound pretty good.  A non-master will sound better cranked but as we know that is deafening. A MV Marshall is more of a gigging proposition.

Or just use a really good pedal.  Into a Marshall to get Marshall dirt sounds I really like the Rockett 45 Caliber and the Skreddy Screwdriver.  In fact I use these pedals with my home-brew tweed amps and get a very old-school Marshall kind of tone.

[FabGTO01]

Thank you HamishR for your explanations, i'll keep your advice in my mind.