Question about "Grid Stoppers"

[davidjon_99]

I bought a well used Mesa Boogie Son Of the Original Boogie (SOB) 60 watt 1x12 combo a couple of months ago for $300.

It had multiple problems. I sorted most of them out but find it interesting that the amp (and schematic) does not have grid stoppers between the input jacks and the first preamp tube grid. Is this unusual? Why would it not have these?

I'm still learning how to repair and build amps and have a lot to learn. I greatly appreciate your help.

The attached schematic is the only one I can find for the 60 watt version of this amp. It is supposedly from Mesa/Boogie.

[Leo_Gnardo]

the amp (and schematic) does not have grid stoppers between the input jacks and the first preamp tube grid. Is this unusual? Why would it not have these?

No reason you can't add grid stoppers, carbon comp half watt R's about 33K are commonly used, but anything from 10K to 100K will likely be fine. Why Mesa left 'em out, you'll have to ask the brain trust there. To save a buck maybe? Preamps tend to run more stable with them, also helps prevent the volume control in your guitar from "scratching" should it receive a small DC charge from the preamp tube grid.

Nice find on the SOB too, good for you!

[davidjon_99]

Thanks Leo.

The volume pot does scratch a bit and I enjoyed listening to a local radio station through the amp the other day. I was using a cheap guitar cable, but from what I understand grid stoppers are supposed to prevent RF signals from entering the preamp.

The amp sure didn't look or sound like it was worth $300 when I got it. The first picture was taken by the guy selling it. The second is after I put many hours into restoring it. This particular amp was painted by Mesa Boogie with a textured paint rather than having tolex. It looked like the original paint was white or cream. The previous owner painted it a gloss black. I sanded it down and repainted it with a product called DuraTex from Parts Express. It ended up looking pretty good.

[Leo_Gnardo]

Even more impressive in the "after" pic. Shows you what some brains & elbow grease can do. If you're still plagued by radio after the stopper's in place there's some remedies to try. You'll want to test that amp at night for unwanted radio, that's when reception is best for hams, worst for amps. Doesn't hurt to make sure input & output jacks are tightened down securely, also pot mounting nuts. That's step #1 in beating radio.

You want your music on the radio, not vice versa.

[davidjon_99]

Thanks again Leo.

[R.G.]

It's not a grid stopper (at least not a really effective one) unless the body of the resistor is as nearly jammed up to the terminal on the tube socket as it can get.

The point of a grid stopper is to form a (1) dissipative (2) low-pass network with the capacitances at the grid. It drops the RF response and usually kills off RF parasitic oscillations.

The 68K/68K resistor networks at the input jacks of most amps are usually there for making a high-gain/lower-gain input from two jacks. Different purpose. That's not to say they don't kill off some incoming RF, but that's probably not what they were put there to do.

[schaublin65]

Hi,

I had a Mesa Nomad that would pick up a South Carolina "holy roller" radio station on the crunch channel if it was in one place in my house.

My house is on the other side of the Atlantic Ocean

take care..

John

[R.G.]

My house is on the other side of the Atlantic Ocean

Wow, that's impressive!

There's got to be something about your input wiring that accidentally tunes in that station.

I think you can kill it by bypassing the signal terminal of the input jack(s) to the chassis right at the jack itself with 100pF or so ceramic capacitors.

If it doesn't work, maybe it will tune a different station!

[davidjon_99]

It's not a grid stopper (at least not a really effective one) unless the body of the resistor is as nearly jammed up to the terminal on the tube socket as it can get.

The point of a grid stopper is to form a (1) dissipative (2) low-pass network with the capacitances at the grid. It drops the RF response and usually kills off RF parasitic oscillations.

The 68K/68K resistor networks at the input jacks of most amps are usually there for making a high-gain/lower-gain input from two jacks. Different purpose. That's not to say they don't kill off some incoming RF, but that's probably not what they were put there to do.

Interesting, thanks R.G.

[davidjon_99]

Hi,

I had a Mesa Nomad that would pick up a South Carolina "holy roller" radio station on the crunch channel if it was in one place in my house.

My house is on the other side of the Atlantic Ocean

take care..

John

I used to be into Ham radio many, many years ago. It can be pretty impressive how far some radio signals can go given the right conditions.

[ampmike]

I remember calling Mesa and asking them why my tremoverb combo get radio stations.No mention of grid stoppers,Thanks Guys,Good info

[Reeltarded]

That is the best Boogie, ever.

Great deal!

[Leo_Gnardo]

I remember calling Mesa and asking them why my tremoverb combo get radio stations.No mention of grid stoppers

Don't tell Randy Smith, he'll patent 'em.

[davidjon_99]

I remember calling Mesa and asking them why my tremoverb combo get radio stations.No mention of grid stoppers,Thanks Guys,Good info

What did they say about the radio reception?

I emailed Mesa Boogie about the SOB. There's not a lot of info around on it and they never even made an owner's manual for it. The next day I got a call from someone at Mesa Boogie. He was really nice and talked to me for a pretty good while. He was in Dripping Springs, Texas which is close to where my wife and I plan to retire. So we talked about that area for a while. He didn't have a lot to say about the amp other than it was kind of the "red-headed step-child" in the Boogie family. Mesa could have just sent me a generic email, but I was surprised and pleased that an actual human called me to talk about an 80's era amp they built.

[davidjon_99]

That is the best Boogie, ever.

Great deal!

I actually could have gotten it for a little less, but I bought it from one of my wife's coworkers and they need the money. I told them their $300 asking price was way low, but they were in a hurry and already had several people look at the amp and not buy it. It did look and sound pretty bad when I bought it. Snap, crackle and pop. But I knew I could make it better and it was an interesting project.

I still have a bit of work to do on it. One thing that is confusing me is why did they use a TCR5305 (constant current diode) off of the cathodes on V3?