Hello, everyone.
First of all, I'm sorry if this question has been asked and dealt with before. I have done some searching, but haven't been able to figure out a straightforward answer. I'm new to solid state amplification. Now, to the point.
I've been looking at Rod Elliott's 40W power amp design and I don't know which components should be matched. I started reading teemuk's book, but for now it's quite a bit to digest, so I'm taking my time.
So, should I look for matched pairs of any devices found here?
The whole article:
https://sound-au.com/project215-p27-revisit.htm

Circuit diagram:
https://sound-au.com/p215-p27-revisit-f6.gif

Thanks for any input.
M.

Quote from: edvard on June 01, 2024, 10:45:18 PMI've asked before on other forums that if there were such thing as a "clean channel" pedal, what would it consist of, and most folks suggested either completely dry, like plugging into the power section with a tone control, or light-to-medium compression with no clipping.

Thanks for the tip. There are a couple of ideas I've been looking at, like the Fetzer Valve with a tone stack, Rod Elliott's preamp, or Umlaut's Keelhauler. I'll start a new thread once I've found what I'm looking for.

QuoteHe hangs mostly at diystompboxes.com, maybe pop in there and send him a personal message.  At the very least, I'd say draw your own schematic and highlight the differences, so you won't be straight-up copying.  Maybe include "modified from Joe Davisson's Vulcan" and a link to his page.
IANAL, so I'd ask him about it first.

I sent him a PM this morning. We'll see what he says.

Quote from: Miyagi_83 on May 29, 2024, 09:13:26 AMUpdate

So, I have been tweaking the circuit for the past couple of days and I think I've got it where I like it. Actually, there are a few versions that I'm fond of, so I'm going to make it a switchable two or three-channel preamp. Honestly, this design has enormous potential, afaic. I haven't tried it on 18V yet. That's still on my to-do list.
Now I'm going to need a nice clean channel to pair with it. Any suggestions are welcome.

I've asked before on other forums that if there were such thing as a "clean channel" pedal, what would it consist of, and most folks suggested either completely dry, like plugging into the power section with a tone control, or light-to-medium compression with no clipping.

QuoteIs it OK to post a schematic with my changes? On his site, Joe Davisson put the 'do not copy / all rights reserved' clause, so I wouldn't like to infringe his copyrights, annoy him or anything like that, hence my question.

He hangs mostly at diystompboxes.com, maybe pop in there and send him a personal message.  At the very least, I'd say draw your own schematic and highlight the differences, so you won't be straight-up copying.  Maybe include "modified from Joe Davisson's Vulcan" and a link to his page.
IANAL, so I'd ask him about it first.

Generally, to prevent hard saturation of transistors (the kind that takes significant time to undo) you install a Schottky or Germanium diode between Base and Collector. I question whether it makes an audible difference in an audio circuit. The way this guy has installed the diodes, it makes sure any big negative input is able to degrade the Base-Emitter junction. 

Quote from: joecool85 on May 27, 2024, 08:56:13 PMFor those looking for the link to the schematic: https://www.diystompboxes.com/analogalchemy/sch/vulcan.html

I'm curious what the diodes are for between the stages though.  Not sure I've seen that before.

Per the description below the schematic; "The diodes in the bias network prevent hard-saturation of the transistors."

I won't pretend I'm smart enough to know how that works, but it sounds to be in the same vein of putting a diode across base & emitter to prevent zener breakdown events that degrade the junction and cause noise to build up over time.
See here: https://www.diystompboxes.com/smfforum/index.php?topic=132039.0

I wonder if those two arrangements might be considered general good practices for BJT-based preamp circuits?
 

Those seem good.  Can you do the same checks (DC volts, no signal) for e,b,c of Q13 and Q14.

Thanks g1, that was helpful.  The pinout shape on my schematic and what the redistor looks like are a bit different (still the correct value though), and you're indeed right that the pinout was backwards.

So with no signal, this is what I got:

Q8
C 39.26
B -0.128
E -0.713

Q9
C 39.79
B -0.149
E -0.709

The 30 vac refers to the AC voltage but after rectification to DC voltage it will be higher. 30vac * 1.414 = 42.42vdc.

Quote from: g1 on May 18, 2024, 02:25:27 PMYou need a power transformer that puts out 60V center-tapped (30-0-30) good for at least 120VA.  If it's not the exact peavey unit, you will have to drill and cut to make it physically fit the chassis.

Im looking at the schematic... looks like it says 42.8V+, 42.8- coming out of the transformer after the diodes. Then goes through a pair of resistors down to 15V+, 15V-. Dont I need a transformer that is 42.8-0-42.8? Im not good with transformers so bear with me..Thanks.
 Rick.

Those Voltages are correct, you just measured them in the wrong place.
68.2 divided by 2 = 34.1 which is close enough to spec.
You just measured across both + and - Voltage rails, forgot the common center tap.
Same with low supply, 29.4 is the potential across + and- rails.
Both Preamp and poweramp supplies use a split supply where you have 3 wires from transformer the center tap is the common or Zero point.
Sadly none of this helps much as the amp is faulty.
There looks to be a mute circuit hung off pin 6 of U1b consisting of Q1 & Q2 which may have failed causing very low output.
Other here will know more.
Phil.