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Discussion in 'Shock Brother's DIY Amps' started by Dbsoundman, Aug 1, 2020.
Had a bit of a mad scientist moment...
I recently picked up this Peavey Audition 20 to use up some trade in credit at a local guitar shop, only to find out the tone circuit is either really bad in this amp, or it simply doesn't work in this particular amp. I mulled over taking it back, but I really don't want to give the shop a hard time. Then I realized...this is the perfect chassis for this amp build. The metal chassis is about 12"x5", and the cabinet is already cut out for an 8" speaker. The front baffle is particle board, but the rest of the cab is plywood, and it's in nice shape. I'm thinking I'll reuse everything, even the knobs. Just have to find a push-pull potentiometer for the high tone control, which will engage the bright circuit when pulled out. I'll put small toggle switches on the underside of the amp chassis near the back for the negative feedback and bypass controls.
I did some rudimentary speaker research and it looks like the G8C gets good reviews. Weber obviously offers a lot of good ones, but they're more risky to me; lots of variables and about twice the price. Any thoughts?
One more question: is the 100k resistor from B+2 to the MOSFET's source terminal appropriate? Based on the MOSFET follies link I assumed that the B+ limiting resistor was still appropriate, and the value was largely not that important and could be the same as a tube doing the same job, so I went with 100k as the other tubes in the circuit are also using 100k.
We're looking at different circuits, there are different follower configurations and some with variations. I'm not very familiar with what you see on the MOSFET follies page. I've been referring to the AC coupled CF on Merlin's site only. In that circuit the 100k load resistor is from the junction Rk/Rg to ground and B+ is directly on the plate (or drain in this case).
That looks like a great speaker for the money. The Weber Signature 8 inch would be the main alternative if you want to try an alnico, at $58. Plus there are these Champ style ones for $50 on eBay.
Here's the latest schematic, please critique!
That looks right to me!
So regarding your power supply, if you are indeed dropping 100V over 22k that's 4.55mA. We've added some current at B+2. We biased the LND150 to 1.5mA. So 6mA will drop 132 volts across 22k, that drops B+2 down to about 260.
You can get that back up to 290VDC by changing the dropping resistor to 15-18k (16.7k to be precise).
Right! It’s been a while since I studied up on this stuff and admittedly I haven’t “walked” the voltages as I kind of just went with what Rob spec’d. I totally forgot that the LND would have its own bias voltage and therefore would affect B+2. I’ll change out that resistor.
Yeah if you can't be sure of all the inputs, maybe just have a few values on hand. It's also OK to just not care much about the exact B+ there. We're talking about a 10% difference. That's mojo where some people come from.
In terms of the actual build, I'm planning to do point to point wiring, as there isn't really a pre-made turret board for this particular setup. If I go the PTP route (using terminal strips as needed of course), should I worry about getting shielded cable for any parts of the audio circuit?
That's cool, this is a great project can't wait to see it completed (hope you share progress). I've mounted that LND150 on terminal strips btw no problem.
I think it's good practice to do input jack to V1A grid (your instrument cable is shielded and V1A is sensitive, why not go the distance) but it's not necessary. Don't go buying a spool for that. Shielding should be unnecessary elsewhere in this amp, not a high gain design.