Distortion when using reverb on MK 3 Simulclass - need fix?

Promethean · Dec 30, 2013

I have had my Purple Stripe Boogie from new since the mid 80's - Mk3 Simul, Rev, EQ, EV

I have never modded it till just recently. This last week i have added the R2 level adjust mod using a coax pot with the direct out level which has proved perfect!! It's a real 3 channel amp now!

I have also boosted the reverb with a 10K resistor to the fx return and this has also worked out really great too. Loads of deep, lovely reverb if I want now.

The final problem i can't seem to fix is a slight buzzy distortion that appears whenever I use the reverb. This has always been a problem with the amp from new. Some times it is worse than others and I have just used rack fx instead. I would love to solve it.

I have so far tried:

- several replacement valves (V4 position) - some valves do better than others but none fix it.

- checking cables and obvious earths

- the reverb mod recommended to me by mesa boogie to put a 120k resistor on the red wire from the reverb transformer to earth. From what i could see on this forum, I was hoping this one would fix it but it has not and in fact made it seem worse so i have undone it.

If anyone has any ideas or managed to fix this issue, i would love any insight.

megavoice · Dec 30, 2013

Before you go on with the electronics remove the reverb can from the leather sack, disconnect the cables and clean all the connectors and jacks with an agressive cleaner that eliminates the oxidations beeing very likely there after so many years. Do this as well at the other end on the amp inputs for the reverb cables. BUT USE THAT CLEANER ONLY FOR THIS AND ANYTHING ELSE !!!!!

Goog luck,

Promethean · Dec 30, 2013

Thanks. That seems to have helped. There was some oxidation there - probably from its early days stored in a damp cellar.

Will run it for a while and see if it comes back.

I'll work my way through all the other pots/connectors and clean them too. Its worked hard for nearly 30yrs and needs some TLC - full preamp revalve next.

Thanks again.

megavoice · Dec 31, 2013

Promethean said:
I'll work my way through all the other pots/connectors and clean them too. Its worked hard for nearly 30yrs and needs some TLC - full preamp revalve next.

Thanks again.

That's a very good idea. Especially when the pots are scratchy. When they'll scratch after a while again you'll have to replace them as the carbon surface is very likely worn down after that long time of heavy using as you've mentioned above.
Note: Your amp will never sound better as the pots do work. Before you haven't done this you need not to begin to swap tubes or replace caps, etc. :wink:

terickson · Dec 31, 2013

Mr B. went through my Purple stripe a few months ago, and along with all the other magic he worked he cleaned out and replaced the old foam rubber from the reverb unit. Huge improvement.