Thanks guys.
My friend has already listened to the amplifier prior to installing the binding posts. He likes it.
This Motorola was inside of a portable record player. (Portable by 1960 standards, it's not very small.)
It used two 12AX7 drivers for each side. One section was for tone compensation, and the other was the gainer stage. (Or was it the other way around?) I didn't want to deal with the daisy-chained tone pots, balance, and volume control, so I just made it into a stand alone amp.
No need for the extra halves of 12AX7 without the tone comp crap, so I just used one 12AX7 for a gainer stage. My friend doesn't have a pre-amp or a CDP with a VC, so I put 100K a volume control on the amp.
Those cheap radio shack 100K pots are pretty good little units, it work rather good.
I went ahead last night and installed 1 meg grid leak resistors, I had the grids just connected straight to the VC.
Somebody brought up around here the possiblity that if one of the wipers on the pot looses contact, then the grid bias can go positive?
I dunno, the 1 meg grid leak R's didn't effect the sound at all, I geuss it can't be a bad thing....
Other than that, the circuit is pretty much stock.
This amp has plate to plate feedback. A 2.2 megaohm resistor with 1500 picofarad capacitor. I left the values stock. Except I think the original ceramic cap was 1200 picofarad, and I swapped to a 1500 pf silver-mica.
The coupling caps are vintage Pryramid generic vitamin Q looking oils.
This amplifier seems to be picky with recordings, if the CD is thin sounding, the amp sounds thin too. You put in a good recording with some strong bass, the amp sounds good has strong deep bass. With my Cornwalls.......
Plenty of gain, almost too much as the amp get's kinda squirley when cranked.
Using a GE 7025 does help the issue with gain.
The amp does have a little sharpness about it, which Mullard EL-84's helped, but he's not gettin' Mullards. Besides, the stock Motorola Austrian made Philips EL-84's still test like new.
This is one amp I need to connect to a scope and noodle around with that plate to plate feedback. I probably will still do it, once I quick cocking around and get some scope probes.....
He's going to use the amp with a older Pioneer 6 disc changer, and a pair of Marantz Imperial 6's. (I think they are 6's, they are just a two-way with the blue coned 10" woofer.)
I just hope the little amp will mate up with the Marantz speakers, If not, he's got some vintage Grundigs. Though a woofer is fried on a Grundig....
I scored him a cherry Harmon Kardon 330B receiver with walnut case from the original owner at a yard sale. I love that little receiver.......
It was a nice break from my billion year long Eico ST-70 project.
