This Week on the Breadboard: the Subdecay Variac (Brownout OD)

Chuck D. Bones

Well-known member
Another FET-based tube amp emulator pedal! I like this one. Here's a preview schematic, courtesy of PedalPCB. The first two stages (Q1-Q3) emulate a Marshall preamp. The tone contols are a Fender stack with the MID control hardwired at zero. After playing with it a bit, I bumped C7 up to 1nF, C11 up to 15nF and changed the TREBLE pot to B250K. I had to disconnect C4 on my breadboard because at higher gain settings it oscillated between 10KHz and 20KHz. The schematic calls for 2N5457, but you have to select ones that put Q1 and Q2's drain voltages between 5.0V and 5.5V. I used 2SK193s and lowered R3 & R7 to 22K to dial-in the drain voltage. I tried PF5102 and PN4393, but they have too much gain and too much capacitance which choked off the tone. The switch between Q4 and Q5 tightens up the bass a little when it's ON. Its effect is noticeable, but subtle. The PENTODES control is a second gain control that affects the distortion in the last stage (Q5 & Q6). The last stage is intended to simulate the phase inverter in a push-pull amp. At moderate settings of PREAMP and PENTODES, you get smooth, symmetric clipping. Even with PREAMP and PENTODES at zero, you have to back off the guitar volume a bit to play clean. It's pretty easy to dial-in the desired tone. Good range of tones and the pedal responds well to touch and guitar controls. Subdecay has a good description of the operation and control settings on their website. I tried two different transistor sets for Q5 and Q6. First, I installed a silicon dual transistor. The two transistors in one 6-lead package are matched and track each other with temperature. Their HFEs were over 300. The second transistor set I tried was a pair of MP38As from the batch I just bought. Their HFEs were between 50 and 60. I tried a few 'til I found a pair that were pretty well balanced. I checked the balance by measuring the voltage from Q5-C to Q6-C. With the dual transistor, the difference in collector voltages was about 220mV. With the MP38A's, the difference was closer to 800mV and wandered around. In this circuit, HFE does not matter much. The two transistor sets did not sound all that different. I would recommend that anyone building this should match the transistors for Vbe. It's too bad Subdecay didn't put the spare opamp to use as a DC servo to keep Q5 and Q6 in balance. I'll give that a try to see if it does any good.

I will definitely be building one of these on a pedalPCB board.

Subdecay Variac [Brownout OD] sch.jpg

Brownout OD breadboard.jpg
 
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jubal81

Well-known member
Very cool. I've been waiting to see the schematic for this one. My guess would have been Muamp, like Catalinbread.
Source and drain resistors on those input gain stages look kinda wonky. I'd expect them to be about 20% of those values.
 

Chuck D. Bones

Well-known member
Another opportunity for improvement. I usually don't go over 47K for JFET drain loads. Sure, tube amps have 100K and higher for plate loads in the preamp section, but 12AX7A's have a 30K output impedance. That means the effective plate load is more like 23K.

The Blackout bears a strong resemblance to the Mofeta. That's next on the breadboard list.
 
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Chuck D. Bones

Well-known member
With the addition of four resistors and two capacitors, the spare opamp is now an auto-balance for Q5-Q6. No more Germanium drift. The collector currents are matched to ±1%.

I also edited my original post to include changing C11 to 15nF.
 
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jubal81

Well-known member
Love where this going. I've been meaning for a long time to get around to see what I can do as far as making the Wampler and other designs work with servos and gate bias voltage.

Definitely adds a lot more parts than a trim pot, but I think it'd be fun to play with. Cool thing here is you've already got that spare opamp doing nothing.

This is from the old Stompboxology articles:

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Chuck D. Bones

Well-known member
What I did was similar. Most, but not all, of the ref des in this LTSpice schematic match the schematic at the top of this thread. C5 & C21 are film. Note that I deliberately unbalanced Q5 & Q6 in this simulation so I could see the auto-balance working. This circuit is implemented on the breadboard using MP38As for Q5 & Q6. Their collector voltages now match to within ±0.5mV.

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Chuck D. Bones

Well-known member
Jubal81,
That JFET servo you posted works, but it's a bit much for something that only needs to be set once during manufacturing. These DC servo circuits can motorboat; the R-C rime constants must be chosen to ensure stability.

The auto-balance needs to run all of the time because a differential amplifier made from BJTs is inherently thermally unstable unless some DC feedback is used to stabilize it. With Si transistors and a carefully selected operating point, the thermal instability is very minor.
 

jubal81

Well-known member
Wow, that seems like a huge design improvement.
Yuge - and it's just adding a few passives.
I'm sure there's a reason more tube amp manufacturers don't do more of this, like Bugera, but I don't know what it is.

So would a simpler approach like this one from Elliot Sound be better suited for a typical common-source JFET amplifier stage?

Screen Shot 2021-07-22 at 1.56.42 PM.png
 
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