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Messages - willpirkle

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31
Schematics and Layouts / IN SEARCH OF: Studio Supply Company
« on: March 16, 2020, 02:44:27 PM »
Well technically not a solid state guitar amp schematic, I figured I would try here to track down schematics by the Studio Supply Company. These two cards came from a distribution amplifier out of a concert hall built in 1974. I believe the distribution amplifier fed individual offices in the bottom of the concert hall. I am trying to track down the schematic for what appears to be a multi band graphic EQ, all discrete which is in the second image. The first image is the backside of the only card that actually shows the logo of the company. Any help would be appreciated as always. Thanks.

32
Amplifier Discussion / Re: Peavey Stereo Chorus NEW noise problem
« on: February 10, 2020, 06:37:23 PM »
Yep. That’s it. But you can use 8 ohms or even 16. You won’t need to crank up the amp to see crossover distortion (and you get the noise at relatively low volume I believe).

33
Amplifier Discussion / Re: Peavey Stereo Chorus NEW noise problem
« on: February 09, 2020, 09:00:22 PM »
Honestly, if you haven’t used the scope in a while then I would be weary of poking around the output section of a power amp without some guidance. The scope probe has a ground ring and can easily short stuff and damage your amp if you don’t have the protective sleeve (condom) for it. Or if the alligator clip snaps off and drags across the PCB. I’ve done it - not fun.  :(

To test the power amp alone, you’ll need to run a sinusoid through it via the fx return. Do you have a dummy load to connect to the output of the amp? Then you can run a signal into that rather than the speaker and look with the scope hooked across the dummy load. That will keep you out of the amp chassis and is safer. You can also probe the output jack (speaker leads - be careful) with no load connected, but my old boss (guru) always made us test for that with a dummy load. With the speaker itself you are going to get a noisy signal and the crossover distortion “glitches” do not necessarily occur in the center of the waveform, so might be hard to see, or obscured. Be really careful and get a dummy load if you can.

34
Amplifier Discussion / Re: Peavey Stereo Chorus NEW noise problem
« on: February 09, 2020, 06:20:53 PM »
Get an oscilloscope to look at the output waveform - crossover distortion will be easy to see. If the output transistors are biased just into conducting at idle, then there should be little/no crossover distortion. While you can measure the voltages across the emitter resistors, the scope will tell all.

35
The Newcomer's Forum / Re: Hello and a question about a Line 6 Spider 3
« on: February 06, 2020, 09:15:20 AM »
Cool! :) Bonus - there is no top plate on the chassis. When you pull it out, the top will be open and you can immediately look inside.

36
The Newcomer's Forum / Re: Hello and a question about a Line 6 Spider 3
« on: February 05, 2020, 09:19:18 PM »
The cabinet does not come apart. The chassis of the amp slides out through the slot/back. Take out the two screws, one on each side, near the top. Lay the amp on its side (speaker perpendicular to table) and remove the top 4 chassis screws. Being on its side, the chassis will not fall down. Then you can slide the metal chassis out, like pulling a book out of a library shelf, from the back side. After that you will need a way to support it while working on it. Not sure if the last Spider I took apart was a v3, so the screw locations may be different, but it was pretty easy to work on. Hope that helps.

37
Schematics and Layouts / Re: Schematic sites for different brands of amps
« on: January 25, 2020, 11:07:41 AM »
Thanks!!!

38
Tubes and Hybrids / Re: 12 v tube preamp
« on: January 20, 2020, 01:27:15 PM »
That’s pretty cool, and only $13. Looks to be another switching charge multiplier. That site has a bunch of kits that look to range from beginner-type too so you might start there? I’m in solid-state mode these days, so only looking at that stuff right now.

39
Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 05:05:45 PM »
Here’s the screamertube I built many years ago that might split the difference and give you something to experiment with that isn’t overly complicated. Note that there is a small error concerning the heaters - they should be wired in series, not parallel (causes excess heat). The circuit was changed, but not the blog.

http://screamertube.blogspot.com

Disclaimer: tube purists will hate it, but it was a fun project to put together!

40
Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 03:32:42 PM »
PAIA used to sell just the PCB for their project kits. If you can find one of those, could be do-able! And I’d guess someone is probably cloning the PCB by now.

41
Tubes and Hybrids / Re: 12 v tube preamp
« on: January 19, 2020, 10:23:38 AM »
This would be considered starved plate and the B+ supply is +15V and not internally boosted, but the design does not include biasing so the first half 12ax7 only amplifies the bottom (-) half of the waveform, which is supplied via the boost amp. The second half 12ax7 reamplifies the flipped, attenuated signal (100K/4.7K resistor divider). I ran a SPICE simulation this morning and attached the results. The lowboost.png file is the output of the two tube halves (top is 1st half, bottom is the 2nd tube output) with a small input signal (low boost knob setting), hiboost.png is the same thing with a massively boosted input, and combogain.png is a combination plot. The harmonic distortion was 38% for the most distorted output.

I am not a fan of this design and I agree with Enzo - it does *something* --- but there is no duty cycle modulation you expect out of a true high-gain cascaded tube preamp when you overdrive the input, and it's going to add noise.

Have you looked at the classic PAIA Stack in a Box (SIAB)? it used a switching charge multiplier circuit to get the plate voltage up from 15V to almost 50V or so, getting it out of starved-plate region. I never built it, but it does seem to have a following of sorts.

42
Liquids + electronics + pedalboards + bar gigs = good idea?

Definitely fun idea.

43
Amplifier Discussion / Joël Huser amp module
« on: January 13, 2020, 07:42:01 PM »
This popped up on my LinkedIn feed, thought you might like it. MOSFET power amp section, claims 242W into 4 ohms. No connectors, leads solder directly to traces. I have not heard it, nor tried it, nor bought it. At the bottom of the linked page is a google drive link with the schematic and PCB layout info. Purely FYI.

https://www.tindie.com/products/jojo83/242-wrms-single-channel-audio-amplifier-core/


44
Two years ago, one of my students did his senior project with a NuTube (which was practically brand new at the time, and he was able to get free parts from Korg somehow). The project was a guitar preamp, and he had nothing but problems with it; noise levels were awful, and it took a lot of shielding and HF compensation to keep the thing from squealing/hissing. Now, the student did not lay out a proper PCB (against my protests, but he ultimately ran out of time) and tried using prototype-boards (solder-type) but man, he had a ton of noise problems with that thing. Perhaps Korg has some (actual) reference designs now that address the noise issues. I got the impression that it was a sort of gimmick device.

45
Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 30, 2019, 11:03:25 AM »
Agree with g1 — the subbed parts are not identical with respect to the built-in base-emitter shunt resistors (which are slightly larger in the new part) and the abs max Ib, which is more than double the original (0.5A vs. 0.2A). You may need to push the bases a bit harder on the new parts. Following up on g1’s question, are your output Vbe’s healthy at idle?

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