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Humble OD twin preamp goes to gig and wins!

10/2/2000 5:51 PM
Humble OD twin preamp goes to gig and wins!
Hi there,  
So I took out my Humble twin preamp clone (97s layout and 70sOD aka sans tone ciruit after OD2) to a gig. This gig was fun since it was on top of a garage. I ended up using only the 97s channel since the 70sOD was still too harsh and a bit softer in volume compared to the 97s channel (I am talking about clean operation here). The second guitar player played through a 1965 Twin Reverb. With the preamp I drove a 2 EL84 power amp (12AX7 PI-tube) salvaged from a Hammond organ A100 into two 12" Rolas from 1962 in conjunction with another 2 EL84 amp that acted as a reverb amp. The main amp gives some signal directly into the reverb springs and the signal is then recovered in the reverb amp and goes out to another 12" Rola. All this in a 3 x 12 " cabinet the size and shape of a Marshall straight box. (Not tolexed yet btw!).  
During the first few Blues numbers it was clear my amp could blow the Twin away. Oddly enough my setup was loud and in the face compared to the 4X 6L6GC Twin amp.  
The band members declared the twin preamp clone a winner!  
But I still ended up tweaking the thing a bit:  
I put 220K plate resistors on the OD1 and OD2 of the 70s OD design, got rid of the feedback. I used 150 K plate resistors on the clean 1 and clean 2 in the OD70s as well. Boy this improved a lot in sound. I changed the coupling cap of the 97s OD1 to 0.1 uF for a thicker sound. Rolled of all the bass with the pot, but now it has mor lower mids I think. Replaced the 5.0 uF cathode bypass caps on the 70sOD with 1.0 uF non electrolytics. So now the amps is really what I wanted. Smooth and silky: the clean is really all I need with the EL84s breaking up nicely when playing loud. Would I do it again??  
Yes but different: Just one preamp ala 97s design and a 4 El84 output section all in one amp (head) driving a 4X12" box.  
BTW the twin preamp clone has the dumbleator installed. I have not yet used it on a gig with an effect since I have the above described reverb system. But I would include this in my future projects.  
So this is not a Humble anymore.  
More like a mongrel. (might call it Mumbrle)  
But exactly what I was looking for all those years!  
My thanks go out to the people that inspired me:  
Gil Ayen, Bill Morgan, Steve Aloha and all the others that contribiute so much wisdom to this forum. Definitely not my last homebrew amp.  
Thanks for reading,  
Happy camper,