DESIGN ThunderZone

[Chuck D. Bones]

A couple of weeks ago I breadboarded the Apocalypse Audio Thundertone. It sort of models the preamp section of the Supro Thunderbolt from the mid 60's. Mr. Linders chose to use Germanium transistors. It sounded ok, but didn't get me too excited. The TONE control had a big volume drop in the middle and I did not like the biasing. So I thought I'd take a bash at it myself. After several incarnations, one of which bore more than a passing resemblance to the Quarantine SE, I ended up with this. I managed to get one Ge transistor in there! It started out as a 3-knob pedal, but the last circuit change was adding the PRESENCE control at the end to be able to reduce the harshness with high GAIN settings. The first two stages are pretty close to the Supro and the 3rd stage is supposed to represent the phase splitter and output stage. It's a medium gain pedal that will play clean with lower GAIN settings. The breakup is gentle at first and gets more aggressive as you advance the GAIN. I also tried it with MPF4393s and it sounded the same after a little bias tweaking. Other JFETs should work too, just tweak the source resistors. I tried 2N1308 and MP38A in the 3rd stage; both worked and no bias adjustment was req'd. It's nice being able to tailor the tone before the distortion stages.



VOLUME & GAIN on the top, PRESENCE & TONE on the bottom.

 

[cooder]

Pretty in pink! Nice one, another schematic to wrap my head around...
So what source resitor tweaking would I need to do if I have mmbf4393 at hand... which I do..
What am I shooting for? Give me bearings, Sir Chuck...

 

[music6000]

Mmmm, Thunder hey!

 

[Chuck D. Bones]

Shoot for around 5V on the JFET drains. Increasing the source resistors (R5, R11) makes the drain voltage go up.

 

[HeliosPAF]

Hello! have the layout for this build?

 

[Chuck D. Bones]

Are you asking for the Vero layout?

 

[temol]

I have it on a breadboard now. PF5102 (Vp around -1V) for Q1 and Q2, different Ge transistors for Q3. It's very good sounding circuit. I like it a lot!

Here's very short comparison between different Ge transistors. Signal chain: strat, single coil in bridge position -> breadboard -> audio interface. Mesa Boogie 4x12 V30 impulse response. Gain @ max, tone @ 14 o'clock, presence @ 15 o'clock.

In order of appearance: MP38A, MP10A, KT601A, AC187K, GT404G, MP38A, KT603D, KT611G, noname transistor.

Dropbox

www.dropbox.com

 

[HeliosPAF]

Are you asking for the Vero layout?

Yep!

 

[Chuck D. Bones]

You can find one here.

Or use mine...

 

[zgrav]

I have it on a breadboard now. PF5102 (Vp around -1V) for Q1 and Q2, different Ge transistors for Q3. It's very good sounding circuit. I like it a lot!

Here's very short comparison between different Ge transistors. Signal chain: strat, single coil in bridge position -> breadboard -> audio interface. Mesa Boogie 4x12 V30 impulse response. Gain @ max, tone @ 14 o'clock, presence @ 15 o'clock.

In order of appearance: MP38A, MP10A, KT601A, AC187K, GT404G, MP38A, KT603D, KT611G, noname transistor.

Dropbox

www.dropbox.com

thanks for sharing the audio for the different transistors. I liked both the third and fourth ones which oddly enough looked like the lowest level and highest level outputs. what did you decide to keep in the board?

 

[temol]

what did you decide to keep in the board?

I installed the MP38A. The board is waiting for a box so I can still change the transistor

 

[lrgaraujo]

@Chuck D. Bones, this may be a little off-topic, so feel free to delete this post: have you tried piggybacking transistors (like Q2 and Q3 in here, for example) in place of the germanium transistors in your hybrid germanium-fet designs? I'm having trouble sourcing germanium transistors and those seem like a really simple (and cheap) option.

 

[Chuck D. Bones]

It is off-topic, but since you are a Well-known member, we can have a brief discussion here.
I have never felt the desire to try that little trick. It doesn't make a silicon transistor behave like germanium. By shunting some of the base current to the emitter, it makes the transistor behave like it has a lower HFE. Sort of. Why not just use a low-HFE Si tranny?

Ge transistors are not that hard to find. I have sourced Ge trannys from the US, the UK and Eastern Europe, mostly via eBay. It really comes down to shipping costs and how much you are willing to gamble. When I buy transistors on eBay or Amazon, I test them and if they are fugazi, I obtain a refund. So far, most of the problem suppliers I've dealt with were in China.

BTW, J201 is not a good sub for 2N5103. 2N5103 has Vp between -0.5 and -4.0V. J201 has Vp between -0.3 and -1.5V. 2SK30A-Y or 2N5485 would be better subs in this circuit. I'd look for something with Vp between -2V & -6V.

 

[lrgaraujo]

Cool, thanks! I'll try to find some good deals on germanium transistors and stock on some low-hfe Si