Pentadriver MKII + Red Star Drive MKII = COLD WAR DRIVE

[sweetwilly]

I've spent the last few days breadboarding the newer versions the Pentadriver and the Red Star Drive both designed by Rick Holt (Frequency Central) with the modifications suggested by Jasper Oosthoek at the AX84 forum.  I ended up combining elements of both to create the Cold War Drive which I submit here.  Please excuse my drawing skills!  The components marked with an asterix are either caps or resistors in parallel or series to obtain the value shown.



This soundclip is Gibson Firebird with minihumbuckers - Ross Comp clone (Sus and Level set at 12 o'clock) - Cold War Drive (Drive Vol at 50% and Drive fully down for 1st half, both maxed for 2nd half) - USB mixer - Cubase.  Gain and Volume both maxed throughout.

http://www.aronnelson.com/DIYFiles/up/Cold_War_Drive-02.mp3

Crunch anyone?

[frequencycentral]

That's sweet Willy!

What about an interstage tone control too, maybe a BM tonestack or something similar to the Prof Tweed? I think there's enough gain with the turbo circuit to allow one, but it would reduce the gain into the second stage a little.

Or

How about another turbo circuit on the second stage to really whack up the output, then put the tonestack after it?

[sweetwilly]

I have been considering a 2nd turbo stage as the 2nd stage could do with some boosting.  I was thinking it would be neat if the 1st stage's turbo mod could run the 2nd as well - save on parts, etc - but I'm not sure of the maths required to fiddle the values to get the same voltage to Pin 1 of both tubes.  Is this a possibility, or not within the operating range of the transistor?  To the datasheet!

[frequencycentral]

I was thinking it would be neat if the 1st stage's turbo mod could run the 2nd as well - save on parts, etc

You could try using two 1K resistors between the collector and both the plates, if you know what I mean. Then tweak in the trimpot. It might work..........? It might not...........?

[sweetwilly]

I have 2 datasheets for the 2N3906 - one from Vishay the other Fairchild which show opposite pinouts.  Confusion.  The emitter has the arrow though, doesn't it?

No joy with a 1k resistor between the collector and both Pin 1's.  No trim tweaking helped either. 

Am I just trying to get the same voltage from the collector to both pins?  I found just adding a jumper to the 2nd Pin 1 exactly halved the voltage (surprise surprise) so is it simple a case of (somehow) doubling the voltage at the collector - if that is even possible?

[frequencycentral]

Yes the emitter has the arrow. I guess it needs two turbo circuits then. That would also mean a change to the clipping diodes.

[sweetwilly]

I think that a 2 turbo version would HAVE to be called Nuclear Winter.   

[sweetwilly]

Diodes doubled?  The polarity would help prevent the extra transistor clipping wouldn't it? 

[frequencycentral]

"Nuclear Winter"  -I like that!

Ok, the first 5672 is working at higher voltage than the second. That means that the second has less headroom. So, if the voltage of the second in raised, does that mean that the effect of the first stage smashing into it will be less, as the second stage has more headroom to handle it? Cumulative effect - higher voltages but less drive?

The diodes would certainly have to be doubled to handle the increased voltage of the second stage.

The result of the whole thing might be less pentode clipping but the same diode clipping - just at higher voltages. I don't know. Experiment time.

I'm going through this same process with my "V2" design, trying to get the most 'blat' possible out of a single 6111, thinking of adding turbo to the second stage and more diode clipping.

These two ideas are kind of running in parallel, I'll let you know how the "V2" goes, which should help with "Nuclear Winter".

[sweetwilly]

Yup.  Is going to be VERY tight on the meagre breadboard available but doable.  I am really liking this version though.  I've swapped the 20uf and 5 uf caps for 22uf and 4.7uf and seem to get more range from the Drive Volume pot.  Will have to go back to the original 20 and 5 as it seemed to have more grunt from the somewhat limited range.  Don't know though, could be my ears. 

What did you say about never having too many drives Rick?  Will keep an eye on V2.

Thanks for the pointers.  As always.

[sweetwilly]

I bought a bigger breadboard and transferred the circuit over so I could spread out a bit and try out the 2 turbo stages.  Unfortunately something didn't tranfer correctly as the output is sounding like a blanket's been thrown over it.  The 2nd turbo does have an impact, but until I get the original working properly again I can't be sure how much.  I'm just hoping there wasn't a happy little mistake on the first one that made it work so well and that if so, I can make that mistake again!  I did substitute the 1M pot I was using for the trim for an actual trim pot and I've also tried out a dual gang pot for Drive and Drive Vol so either of these could be the culprit.  Or the junction box wire I salvaged for jumpers.  Or me...

[sweetwilly]

So, turns out if you run the 0.1uf cap from Pin 2 of stage 2 the output is rather muffled.   

All good and the output is now phenomenal!  Is the same crunch, just louder!  AND this is only phase 1 of experimentation.   

[frequencycentral]

That sounds cool. Yesterday I did similar with a 6111 - seperate turbo to both plates = more volume/gain/clipping. It sounds great but I'm having trouble figuring out how to clean the thing up, it seems to be a case of great full on sound but no option to mellow it down. It should work well with your 7327's too. I'll post a schematic over on the V2 thread when I'm settled with it.

[sweetwilly]

Started out by adding a 2nd 2n3906 with the wiper of a 1M trim attached to the base with 12V and ground to the outer lugs, and a 1k resistor between the emitter and 12V.  I was still using the dual gang pot for Drive and Drive volume, but still lost a little volume with clipping maxed so I duplicated the circuit from the 1st turbo stage and used the dual gang pot to control both of them, with a seperate pot for the Drive/clipping.  The clipping from the diodes isn't as pronounced so figure the 2nd pentode is being hit pretty hard by the both the first stage and the extra turbo stage added.

Will redraw the schematic, but basically - recreate the turbo section of the 1st stage for the 2nd, double the diodes and try a dual gang pot for both Drive Volumes.  Also changed R3 to 82k but didn't seem to make too much difference.

EDIT - and no, there is no clean that I can find in this either really. But then, I still chuckle when soundchecking with a different engineer and they ask me if I have a "clean" sound, so not overly bothered!