Hoffman Plexi 50W Grid Stoppers on V1

[mike123]

Hi all,

I've just finished a Marshall Plexi 50W build using Hoffman's schematic. It's been "warm biased" at 60%. All is sweet apart from a feint but annoying high pitched squeal from the Hi Treble channel when using the hi-input jack. It is not audibly present if I use the low-input jack on this channel or if I unplug from the jack.

It's a 4 input design using shielded cable grounded only at the jack end to connect from the jacks to V1a & V1b grids. I'm using 4 x Cliff style jacks & all the resistors (including V1a & V1b grid stoppers) are mounted on the jacks. I've already reduced the squeal by shortening some runs & re-positioning some wires (lead dress). But an annoying feint squeal remains. If I turn the Hi Treble volume up, the squeal gets louder.

I'm thinking that if the V1b grid stopper was mounted directly on pin 7, the hi pitch noise will stop (I'm thinking it's RF). I don't want to rewire the jacks, so can I add a grid stop resistor attached directly to V1b pin 7 without changing the resistors on the input jack? And if I can, what value should I use?

Thanks, Mike.

[sluckey]

so can I add a grid stop resistor attached directly to V1b pin 7 without changing the resistors on the input jack? And if I can, what value should I use?

Give it a try. Won't hurt anything. I'd use 33K. If it fixes your squeal consider rewiring the jacks and move the stoppers to the socket as Hoffman's layout shows.

[mike123]

Thanks, I'll give it a go & let you know the results.

[mike123]

OK, the 33K placed at the grid has stopped the squeal! For my knowledge, why did you choose 33K? And would making it a smaller value of resistance reduce the hiss? If yes, what size R would you suggest? Thanks, Mike.

[sluckey]

For my knowledge, why did you choose 33K?

Because that's what Hoffman used. Hoffman probably chose 33K because the original Plexi with 4 input jacks used 68K, but when you plug into the high jack two 68Ks will be in parallel for a total of 34K.

And would making it a smaller value of resistance reduce the hiss?

Not likely, but try it and see. You won't hurt anything but you may get your squeal back if you go too small.

[mike123]

I replaced the additional 33K resistor with a 1K metal oxide resistor. The high pitched squeal (parasitic oscillation??) is still eliminated & to my ear (as you suggested), the hiss did NOT reduce. But I am happy with the outcome, thanks for your advice.