Mark IIC+ Schematic (correct!)

[Jay Omega]

Dan_O (or anyone else)....I built a Mark IIC+ preamp based on the SloClone schematic and struggling with the EQ circuit. The power section is a basic Marshall 100W. The Amps works without the EQ by jumpering the preamp in the PI input. When I try the EQ board, I get what sounds like 60 cycle hum. I've re-done this multiple times....cleaned up my grounding and I keep getting the same result. At one point it did work with a different layout, but it was incredibly messy and I've not been able to get this back working.
Any help / input would be greatly appreciated.

It looks OK on paper. I’d double check the transistor pinout, if that’s good check your DC voltages, and tracing the signal thru on a scope

 

[Henz]

Dan_O (or anyone else)....I built a Mark IIC+ preamp based on the SloClone schematic and struggling with the EQ circuit. The power section is a basic Marshall 100W. The Amps works without the EQ by jumpering the preamp in the PI input. When I try the EQ board, I get what sounds like 60 cycle hum. I've re-done this multiple times....cleaned up my grounding and I keep getting the same result. At one point it did work with a different layout, but it was incredibly messy and I've not been able to get this back working.
Any help / input would be greatly appreciated.

I'll be following. By your post, you are well trained in C+'s. I'm sure that you've done this but trace everything back to the diode and 220k on power board. For my own interest, when you say you bypassed the EQ card, you took the lead from the output of V1A and attached it to the grid of first stage of PI? I love learning. I hope you get it sorted out. Lots of smart ppl on here. Jim.

 

[BigW72]

It looks OK on paper. I’d double check the transistor pinout, if that’s good check your DC voltages, and tracing the signal thru on a scope

Yes….turned out to be an issue with the transistor orientation. I built two boards....one using the stock transistors MPSA63, 2N2906, MPSA20. I also did a different one with BC557C, BC547C. The orientation of these are opposite.

For this amp, I’m using a custom PT from Heyboer that has it’s own secondary 60vac winding for the EQ power. Actual measurement is 63vac. Using the specs from the Dan_O schematic (1N4006, 220ohm, 220uF, 470ohm, 220uF) the output DC for the EQ board is a whopping -73.5vdc. I read in other threads that the amounts vary, but that seems like overkill and would guess it’s too much. I added a voltage divider 10K / 10K voltage divider after the DC conversion / filtering and before the board. Here are the voltages I’m getting:

Q2 E = -37.5vdc
Q2 B = -29.8 vdc
Q2 C = 0

Q1 C = -37.5 vdc
Q1 B = 0
Q1 E = 0

Q3 C = -29.8 vdc
Q3 B = 0
Q3 E = 0

Q4 C = -37.5 vdc
Q4 B = 0
Q4 E = 0

I've built quite a few amps, but never anything with transistors. I have no idea how these are supposed to read or if they fry out easily from soldering

Any help would be appreciated.

I'll be following. By your post, you are well trained in C+'s. I'm sure that you've done this but trace everything back to the diode and 220k on power board. For my own interest, when you say you bypassed the EQ card, you took the lead from the output of V1A and attached it to the grid of first stage of PI? I love learning. I hope you get it sorted out. Lots of smart ppl on here. Jim.

LOL, thanks Henz. I wouldn’t say I’m well read in this amp at all, but I’ve worked at a very slow pace on this one. The guys at the SLOCLONE forum did all the work breaking this thing down. To bypass the EQ, I simply put a jumper wire from the 22K resistor at the end of the preamp (R17) to the input of the PI. In this case, I’m using a standard Marshall 100W power section / PI, so it’s the .022 capacitor that then goes into grid of the PI tube.

 

[Jay Omega]

Yes….turned out to be an issue with the transistor orientation. I built two boards....one using the stock transistors MPSA63, 2N2906, MPSA20. I also did a different one with BC557C, BC547C. The orientation of these are opposite.

For this amp, I’m using a custom PT from Heyboer that has it’s own secondary 60vac winding for the EQ power. Actual measurement is 63vac. Using the specs from the Dan_O schematic (1N4006, 220ohm, 220uF, 470ohm, 220uF) the output DC for the EQ board is a whopping -73.5vdc. I read in other threads that the amounts vary, but that seems like overkill and would guess it’s too much. I added a voltage divider 10K / 10K voltage divider after the DC conversion / filtering and before the board. Here are the voltages I’m getting:

Q2 E = -37.5vdc
Q2 B = -29.8 vdc
Q2 C = 0

Q1 C = -37.5 vdc
Q1 B = 0
Q1 E = 0

Q3 C = -29.8 vdc
Q3 B = 0
Q3 E = 0

Q4 C = -37.5 vdc
Q4 B = 0
Q4 E = 0

I've built quite a few amps, but never anything with transistors. I have no idea how these are supposed to read or if they fry out easily from soldering

Any help would be appreciated.

LOL, thanks Henz. I wouldn’t say I’m well read in this amp at all, but I’ve worked at a very slow pace on this one. The guys at the SLOCLONE forum did all the work breaking this thing down. To bypass the EQ, I simply put a jumper wire from the 22K resistor at the end of the preamp (R17) to the input of the PI. In this case, I’m using a standard Marshall 100W power section / PI, so it’s the .022 capacitor that then goes into grid of the PI tube.

With your circuit, the darlington emitter should be half the supply and the base should be a couple diode drops away from the emitter. The differential amplifier emitters should also be approximately half the supply. Transistors are sensitive to both heat and incorrect voltage, they’re easy to fry - I’ve done it once or twice with this circuit.

My best advice is to cut the suspect transistors out, test new ones for conductivity with a meter (put your positive lead on the “p” and negative lead on the “n” if you have a diode setting) and put them in one at a time checking your DC voltages along the way

 

[electric mayhem]

Jrb,
Thank you- mainly for making it much easier to trace all the boards around the 10A. A while back there was a place in N Colorado students would send simulated circuits to have printed. It was through hole even though more modern. There may be some smaller fabs that could do it.
If you do the IIB and need reference pics I have the SP-8B, RP-8C and SP-9A boards.

 

[BigW72]

I did find some more vintage transistors for MPSA20, MPSA70, and MPSA63 on eBay and re-did the board...now it's working. Just some final spots to clean up and I think this thing will be done.

Thank you, Jay Omega.....your help is greatly appreciated.

 

[Amimaster]

It's been quite some time I've tried simulating this circuit in LtSpice, just for the sake of understanding how it works since I'd like to start building tube amps for hobby. I know that simulations are not always accurate, but this one makes sense overall. However, every now and then I spot some issues that I don't understand. Here's one:

- If I set the input voltage to 1Vac, 1KHz, all knobs at 50% except the master at 100% (clean channel), I get a huge preamp output signal (pre eq), swinging from -120V to +120V. Other simulations I did months ago provided similar results.

Now, I've seen EQ being powered by anything between -22V and -66/70V. However, it's anywhere close to my estimated output voltage. I also don't see any voltage divider before the EQ (not in this, nor other mesa amp schematics). Wouldn't the signal be hard clipped by the EQ circuit, given it's powered by just 66V? Am I missing something or I just don't understand anymore how transistors work?
In the MkV the situation is similar, with just 22V eq, supply and a jfet with some diodes that I theorize are used to clip the input signal. I don't remember my quad clipping abruptly even with master cranked up at max...

 

[Jay Omega]

It's been quite some time I've tried simulating this circuit in LtSpice, just for the sake of understanding how it works since I'd like to start building tube amps for hobby. I know that simulations are not always accurate, but this one makes sense overall. However, every now and then I spot some issues that I don't understand. Here's one:

- If I set the input voltage to 1Vac, 1KHz, all knobs at 50% except the master at 100% (clean channel), I get a huge preamp output signal (pre eq), swinging from -120V to +120V. Other simulations I did months ago provided similar results.

Now, I've seen EQ being powered by anything between -22V and -66/70V. However, it's anywhere close to my estimated output voltage. I also don't see any voltage divider before the EQ (not in this, nor other mesa amp schematics). Wouldn't the signal be hard clipped by the EQ circuit, given it's powered by just 66V? Am I missing something or I just don't understand anymore how transistors work?
In the MkV the situation is similar, with just 22V eq, supply and a jfet with some diodes that I theorize are used to clip the input signal. I don't remember my quad clipping abruptly even with master cranked up at max...

Try transient analysis as opposed to AC analysis

 

[Amimaster]

Try transient analysis as opposed to AC analysis

I tried both, what I'm referring to is actually the transient analysis indeed.
My question was more on the theoretical standpoint: if I power the eq with 22V than it could handle input signals no larger than 22Vac (more or less), am I correct?
But I don't see anything preventing such large signals to be generated by the last stage (except for the master pot that can be still set at maximum).