Don't forget to derate the transistor depending on power dissipated  (IcxVcc in watts) AND the ambient temperature - the chassis is rarely at room temperature for long (assume room temperature =Ta=25C = 77F). The transistor date sheet will usually give a thermal "resistance" based on case to ambient, junction to case, etc. Units are degrees/watt or something similar.The goal is to calculate the junction temperature of the transistor.

Once you calculate the junction temperature of the transistor using the Ta, Pd and appropriate thermal resistance(s) you can add a heatsink to get an even lower junction temperature. You are doing the same thing when you rebias a tube/valve based on plate voltage and cathode to plate current - note that tubes run at 250C while a transistor should not exceed a junction temperature of 125C - better to keep Tj at 80% of 125C.

You can guesstimate what's going on with your substituted part buy measuring the case temperature of the part with a thermocouple or (worse) IR scanner.

Strange problem here - I couldn't find an issue with the circuit and finally subed in another pan and now the reverb works. Trouble is the "bad" pan has continuity on the input and output coils - they measure 170 and 58 ohms. What else to check? I tried a third pan from an Orange Crush and it works fine too. The cables to the pan's in/out off the PCB were used on all three pan "tests".  :grr :grr :grr

Ask Blackstar if they have a old/previous revision of their "upgraded" firmware. 

Replaced R14 with a 300K resistor and this amp sounds amazing. Clean channel is what it should be, "lead" channel sounds fantastic and the amp' output level is as designed. SPICE simulation agrees with what's measured on the bench............

Phil thanks for doing the simulation - I believe you are right about the clean channel is a dirt box with a chip amp in the output. I going to mess around with the feedback until I get a nice clean signal through Q3/4. What SW tool are you using?

So if I haven't forgotten Fourier transforms  the non-symmetric signal, that this amp generates (distorts), will allow very rich third order harmonics - which gives a warmer sound  I'll try and do an FFT on some sinusoidal input waveforms, into the simulation, to see if the math works out.

PA section works fine.
Injecting a signal into reverb return gives nice clean un-distorted output signal at output of amp.
Didn't remove R14 from circuit yet
Q3/4 removed and tested - I even replaced these transistors with KSC2331 - no change in distortion.
All nodes of active devices are connected and DC voltages (Vcc measured at Q3/4 collectors and bases are one diode drop more positive than emitter voltage.

I've spent several hours trying to find out what's wrong with the components, traces, etc between R9-R15 and can not find any issues.  :grr Am I just hearing the "transtube" tone/design - I have no reference since this is the only Blazer I've every had to play with.

I need a sanity check here. This amp was driving me crazy   :loco Though the output level was "normal" (in spec with load) the clean channel was really distorted and really sounded terrible. I isolated the issue to be between R9 and R15. All components checked out good and DC voltages on the transistors and power supply rails were spot on.

I simulated the preamp from J1 to R15 using TI SPICE. Using the oscope utility, I thought I'd start experimenting with the feedback network around Q3/4 and found that the waveform (using Oscope utility in the program) at node at C9/R14 was distorted just like what I was seeing on the bench. The stock components were were used in the simulation - no values had been changed.  

I'll change out R14 when I get chance and see if simulated matches measured.

Any comments?

Texas Instruments and Analog Devices are two good sources for DSP audio processors. If you really want to explore a fascinating digital technology with both high powered DSP and flexible IO look at some of the products from Xilinx and Altera (FPGAs).

The Blazer III schematic matches the amp - thx.

I just noticed that if I switch S1 to "lead" channel the signal clears up nicely when scoping node C6/R8. So the issue is downstream between C7 and C23. I'll try your suggestion regarding the reverb send and try and isolate the issue.


If I inject a signal into C23 via J2 I get nice clean strong output.

I'm back on this little trouble maker and I've started at the input (J1) and worked back using a scope to trace a 1kHz signal through the clean channel.

VRMS voltages:

at J1 = 211mV
V@C4= 1.36V symmetric sinewave
V@C5= 1.36V      "                "
V@C6= 1.04V      "                "
V@C6 and R8= 41mV positive cycle of sinewave distorted

Something is loading down or distorting the signal after C6 - Checked VR1 with a DMM and I can't find an issue.
I've checked all the above capacitors/resistors and even replaced C6, resoldered all pads and verified that the switch (S1) is working. Can't find the issue. :grr  :grr

I'm unable to fix this little beast - symptom is that the clean channel distorts at almost any volume setting and the maximum output level is about 3-4VRMS into 4 ohms. If I put in a 1kHz sinewave at J1- I get a nice clean signal until the node between R9 and R10  (positive half of cycle is clipped). I assume these resistors are part of a feedback loop for the darlington pair of Q3/4 - is this right?

  DC voltages on Q3/4 look good and all the supply voltages are correct. If I inject a signal into the power IC U1 (lift C23) I get a nice clean signal that meets the rated power into a resistive 4 ohm load. There is a nice clean signal (undistorted) at R14/C9 - reverb send. Checked and repaired any broken solder joints too.

I'm baffled..

The class A amp on the same site uses even fewer components.

http://www.deeptronic.com/audio-amplifier-circuit-types-and-classes/

Odd, I've never seen a flyback diode used on a reverb transducer? This amp's reverb isn't working but the driver OPAMP has +-15V rails and pin 1 of U4 has a nice strong output along with a small signal at pin 7, the reverb tank has appropriate DC resistance at the input and output and all cables check out. I can hear a faint "ringing" when I tap the pan but the signal is very weak.

See the section of the schematic from a Fender Frontman 212. What is the purpose of D15? It's back biased by a -16V supply.

X5R for a small value ceramic capacitor - I don't think so - try C0G. X5R has a really nasty voltage coefficient and is microphonic due to the piezo effect. Do you have the BOM for this amp design?

Maybe it is easier to listen with one's eyes.