Taming The Caswell #39

[Jana]

I've seen the shield connected to the plate on production amps. The insides of an amp are supposed to be off limits to people who don't know what they are doing--hence the sticker: "Danger, High Voltage--No user serviceable parts inside."

[Cliff Schecht]

The only time that it's bad practice to connect the shield to high voltage is when you do it on the first stage of an amp and run the hot shielded wire to the input jack (seen'd it before!). Other than that, as long as you use properly rated coax and shield the hot braid from shorting to anything it's not anymore unsafe than selling amplifiers that run at 450V+..

[roberto]

On metroamp there is a safer mod, simply an equivalent cap between plate and grid.

EG
http://img197.imageshack.us/img197/4344/34v3.jpg
http://i443.photobucket.com/albums/qq15 ... G_1069.jpg
http://img814.imageshack.us/img814/348/36modmk11.png

[Cosmo]

I just took a look at the Caswell 39 thread on Metro forum. There's almost 5,900 posts! Rather than read through that, I decided to scrap the internet schematic and draw my own. I've started wiring and will report back once I have something that works.

[Cosmo]

Well I spent quite a bit of time playing with this over the last couple of days. I scrapped the #39 schema I found on the web and started over. I have it working and the successive gain stages are cooperating with each other. Wicked gain by the time the last one is engaged. No more hissy mess, just lots of gain.

But I'm left with two problems. The first is that there is too much noise/hiss when everything is dimed. I'm sure it would be fine in a live stage type venue, but I find it unacceptable while sitting here at my bench listening to it. The second problem is excessive microphonics in the signal path wires as the gain stages are engaged. I've tried everything, but no joy. I suspect this is a result of so much gain amplification, and I not sure there's a solution. Like the excessive ambient noise, I suspect it's just something you accept with this sort of amp.

I can't bring myself to design/build an amp with these inherent problems. I come at this from the hifi world and am fixated on quiet amps. I think I'll abandon this design and take a look at what Dumble did.

Thanks guys for all you help!

[Reeltarded]

Volume at the master decreases hiss tremendously when the gain is moderated for volume. If you play this amp at low volumes with everything else cranked it's all hiss. Crank the volume, and roll back the tones some. In a similar thing my bass is about 4, mids 6-7, treble 5-6, presence can be almost anywhere. I like it on 10 for the more forward thing.

What guitar are you using? These high gain things favor a very resonant body. Gibsons are kind of too hard to sound like wood. (imo)

[Roe]

reduce the grid resistor on the first gainstage for less noise. try adding grid resistors later

[Cosmo]

Volume at the master decreases hiss tremendously when the gain is moderated for volume. If you play this amp at low volumes with everything else cranked it's all hiss. Crank the volume, and roll back the tones some. In a similar thing my bass is about 4, mids 6-7, treble 5-6, presence can be almost anywhere. I like it on 10 for the more forward thing.

What guitar are you using? These high gain things favor a very resonant body. Gibsons are kind of too hard to sound like wood. (imo)

I'm using my Les Paul with '57 PAF pups. The noise is all coming from the first gain stage and then getting boosted as it travel through subsequent stages. Backing off the volume does clean it up significantly. The tone is fine since I redid the schema. There were just too many bright caps. Removing most of them and adjusting the gain of each stage fixed all the hissy problems. It's just the ambient noise with everything cranked and nothing plugged in, that I can't accept.

[Reeltarded]

Aha, yeah, I always install bright caps and peakers last, then quickly remove them, or trade in an odd peaker here and there. I never like the stated values.

Roe has a good idea there. He has a lot of experience with these kinds of amps.. so when I get back from the grocery store, my neighbors better fear me. lol