Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Replaced Q301/2 with 2N4248

voltage around diff pair

Q301
VB=7.5

Q302
VB=7.8

VE=8.2

UPDATE - I found that Q303 was mismarked and has been replaced with NOS 40408

Voltage around Q303
VC=7.2
VE=.15
VB=.75

Q304
VC=9.5
VE=7
VB=7.7

Q301
VB=7.6

Q302
VB=7.8

VE=8.2

I have the amp on a light bulb limiter and at turn on you see a bright flash (charging caps) and then a very very slight glow. I haven't tried to drive a signal into the board yet and it still looks like Q303 collector is no where near VCC/2.

I see R317, connected to C ot Q304, but I don't see R339 anywhere, help me out.

The voltage at the bottom end of R317 is controlled by Q303 (including the 2v through Q304), which in turn is controlled by the diffy pair.  It seems that the diffy pair is turning on Q303 too hard.

The voltage at C of Q304 is so low because the voltage at C of Q303 is so low.  Q304 is working, as it is keeping the 2v difference across itself.  Q304 simply follows whatever C of Q303 does.  The reason C of Q303 is so low is what Loudthud is trying to help you with at Q301/Q302.   They control Q303.

One side of R316 is at 83V and between R316/7 I get 43V but on the other side of R317 it's sitting at 4V. I have lifted one side of CR308 (zener) which doesn't change voltage at node - what is dropping the voltage?  All measurements referenced to minus terminal on C403.

This is all consistent with Q303 being ON as well as the measurements at the top of the page.

Voltages around Q303 are:
VC=4.3
VE=1.9
VB=2.6

I'm still seeing LT 3V at C404.

This leads us back to the input pair, Q301, Q302.

What are the Voltages around those two transistors now ?

Q303 is what is called the "VAS" (Voltage Amplifier Stage) transistor or stage. It provides most of the Voltage gain needed to drive the output, because all those transistors, Q305 to Q313 have a Voltage gain slightly less than one, but have a huge current gain. Probably less than 1/10 of a milliamp can cause over 20 Amps to flow in the speaker. The Diff pair, Q301 and Q302 compare the input to a portion of the output (provided by the feedback network) and send a correction to the VAS stage.

Current for Q303 starts at ground, flows through R341, and Emitter to Collector in Q303. A small portion flows into the Base of Q310. The current continues through Q304 Emitter to Collector, and a small portion flows into the Base of Q305. The current continues through R317 and R316 at to the +83 supply. The diodes CR306, CR307 and CR308 only turn on when too much current flows through R322 and/or R323.

C307 is called the "Bootstrap" capacitor. It's job is to keep the Voltage across R317 relatively constant for AC signals. Don't worry about it, you have to get the amp running at DC first.