I picked up this Hohner 12R on ebay.
Thought it looked cool and it was cheap. :)
When it got here it was damaged in shipping, :(
and I ended up getting a full refund including the shipping. :)
Pulled the chassis out, with the idea that I might repair the cabinet or make a new one.
The amp still worked and didn't sound half bad.
With the chassis out of the cab I found "Marlboro" etched on the circuit board and Marlboro branded TO-3 output transistors.
A little research an it seems Marlboro Sound Works made many amps that were re-branded,
Hohner
Paramount
and probably others.
Looking at the chassis on my bench... ideas began to form... and I abandoned the idea of fixing the cabinet....
More to come later.... ;)
Here's the guts of the Hohner and a schematic for a Marlboro G40R.
The Hohner has some minor differences in part values, plus a second 100R bias trim and 220R on Q10.
Sorry about the poor schematic, it was all I could find.
With the Hohner chassis on my bench I replaced all the electro caps and added a 3 wire grounded power cord.
Also clipped out the death cap.
Playing it thru a 1X12 sealed cab loaded with a Celestion G12H it sounds pretty good.
Some minor hum that got better (less) when I adjusted the bias.
The bias trimmers are glitchy and I need to replace them.
So here's why I call this a Frankenampmonster.
I'm gonna stuff the guts of the Hohner into this tiny Teisco Checkmate 21. :duh
The original Teisco has maybe 2-3 watts at best.
Just eyeballing this I think I can make it work....
Stuffing a 500 lb amp into a 10 lb space... well... that's a slight exaggeration...
The heat sinks for the TO-3's are 1/4" thick aluminum angle stock.
At full volume playing for 20 minutes the transistors are barely warm.
This thing is loud and punchy. 8) <3)
Has a thick saturated distortion at full volume. Very usable.
Looks innocent... Packs a Punch! :)
I found another Hohner schematic, this for a 1040 model.
Not the same as the 12R but the output section is the same with the two 100R bias trimmers.
I adjusted the bias as per the instructions on this 1040 schematic.
The adjustment trim is very touchy so I may install multi turn trimmers at some point.
Cool.
You may "spread" the adjustment range.
Say now you have to set the 100 ohm trimmer to somewhere around 62 ohms?
Replace it with a series combination of fixed 47 ohm resistor and 25 ohm trimmer, you physically widen the sweet spot 4X .
If you dare: use a fixed resistor 5 ohm below needed value , in series with a 10 ohm trimmer.
Now THAT will become a "fine adjustment".
Although the main problem remains: lack of thermal tracking.
Quote from: J M Fahey on October 18, 2017, 04:14:46 PM
Cool.
You may "spread" the adjustment range.
Say now you have to set the 100 ohm trimmer to somewhere around 62 ohms?
Replace it with a series combination of fixed 47 ohm resistor and 25 ohm trimmer, you physically widen the sweet spot 4X .
If you dare: use a fixed resistor 5 ohm below needed value , in series with a 10 ohm trimmer.
Now THAT will become a "fine adjustment".
Although the main problem remains: lack of thermal tracking.
Thanks much J M!
Ya, thermal tracking would be great.
I just have to make sure I set the bias when it's good and warmed up.
Update, I installed multi turn trimmers for the bias.
This made setting bias very easy.
Set it up slightly hot and got the amp good and warm (hot!)
then while still hot, backed it down to 25mV across each resistor like it says on the schematic.
This seems to work well.
Starting the amp from cold and monitoring the voltage across each resistor,
it starts at ~ 18mV and gradually drifts up to ~ 22-24mV as the amp warms up.
Never gets hot, just slightly warm.