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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.
I haven’t forgotten about this project! I ordered all the parts and most of them are here, but I’m not sure when I’ll get to putting it together. Have to finish up my outdoor projects first...
In the meantime, I’d appreciate any tips about point to point wiring layouts. I have a general idea of how to lay this out and I bought plenty of terminal strips, but I would like to be as neat as possible. I do know about twisting the heater leads and keeping them in phase, but any other thoughts would be appreciated!
For a Deluxe Micro I'd recommend the Weber Signature Alnico. Great tweed speaker at a great price.
Looks like a great project and PTP to boot!
So I'm getting down to my week off (well, home alone with my 18 month old) when I'm hoping to get this amp knocked out (when she's asleep!). Look at the number of components, I'm having second thoughts about doing 100% point to point wiring...I'm considering employing the use of a turret board to simplify things a bit, something like https://www.mojotone.com/amp-parts/...oards_x/Mojotone-Blackface-Generic-Fiberboard. Any thoughts for or against? I'm just afraid of making a rats nest of things by trying to go PTP, and either way I'm not sure when I'm going to get some time ahead of time to try to do a paper layout of the components.
Cleared out the chassis tonight and traced the profile on some paper so I can do a pencil component layout before the actual soldering begins.
Made some progress on the chassis at least, not sure I'll get to do much wiring yet this week...
The patient on the operating table:
During drilling, some mistakes were made. Namely, I drilled all the holes for 8-32 screws, but I ordered 6-32 blunt tipped self tapping machine screws. D'oh! I also bought nuts with captive lock washers, which doesn't help anything in terms of actually improving the ground connection between a terminal and the chassis. I have some chassis mount ground terminals that have teeth which I plan to stick under the couple items that will double as ground points in the circuit instead of making yet ANOTHER hardware purchase. I also had some real issues with the tube sockets, more on that below.
Post-op, I started some assembly today!
Short bat on-off-on switches between the "gain" and "equalization" will be for negative feedback and cathode bypass controls:
I'm going to redo the mounting screws that will also double as ground connections to use a star washer between the chassis and the actual terminal; as mentioned earlier I bought nuts with captive washers which doesn't actually help much...
I'm not really happy with how the tube socket mounts came out. I had planned to mount them with the screw heads on the outside and the nuts on the inside of the chassis, but there was very little clearance between the holes in the tube socket and the actual socket, so this is the only way they would fit. It was also a real pain to drill the holes for the screws as they were super close to the 1-1/8" holes I had to punch out (with a step drill bit) for the socket itself. In that case, self tapping screws really didn't matter either way! Luckily the screws are short enough that they don't interfere with the tubes anyway so it works, even if it's ugly!
I have to re-drill one hole to make it larger for my speaker output jack, other than that I'll mount the remaining components in the coming days and finally get to soldering!
So first, I forgot to mention, to figure out my chassis layout, I traced the chassis out and did a hypothetical "wiring" to figure out if my intended layout of tag strips and tube sockets would work. Just thought I'd share my method in case someone else in the future is wondering how to figure out a PTP layout.
I finally made some progress on the wiring tonight as well! I will likely revisit that resistor that's kind of "flying over" the input jack on the far left later on. I got so far as completing the input wiring to V1, from V1 to the LND150, the treble control (which is kind of complicated as I got a switched pot so it has a lot of connections), and the input volume jack.
Next will be completing the tone stack and moving on to the output tube. From there, I'll tackle the power supply, B+ wiring, then finally the filaments and other details.
Tonights progress...finished the tone stack! I also redid the wire from the input jack through the 33k resistor to the grid of the first half of the 12AY7 (blue wire). Hopefully I didn't make it *too* long...
I've been tracking my progress on the schematic by highlighting the components I've placed so far.
Today's progress...there's a small chance I'll get back to it later tonight, but I'm assuming this is all I'll do today.
It's definitely a little crowded, but then again there's a reason Fender was already using turret boards early on! I'm really hoping I don't have any trouble with the LND150 buffer, that will be interesting to get to if I have to work with it. My grounding terminals are also full. I'm not 100% proud of the soldering I did in some places; especially on the ground terminals, things got a little crazy, and in some places I wasn't able to wrap the lead/wire around the terminal; I just had to push it through, bend it as best I could, and solder it in. I may actually try a flatter soldering tip later on to touch up the grounding points in particular.
Also, I definitely think I'll need to get some magnifying goggles if I keep doing this - and I'm only 33 years old! I also need to remember to get a small fan, I keep giving myself soldering headaches...
Things left to do: install transformers, output transformer wiring, speaker jack/headphone jack wiring, power supply wiring, B+ wiring, and filament leads. Oh, and I still need to come up with some sort of power indicator lamp. I have a bunch of LEDs, I'm thinking I can just tie one in to the filament supply with a dropping resistor and series diode. Any yea or nay to that idea? Wasn't sure if that might introduce unwanted noise.