Jeff said:
... Any guesses on the cost to make this reliable??
Hi Jeff: Short answer is probably +/- $125 including a 3-wire cord .... but ... well ... there are several things.
Good chance of pcb construction dominated by old oil-filled capacitors like the ones shown below ....
... that go south; see the 150V cap (not very much voltage) spitting the bit:
Good chance it uses a selenium recitifer in the bias circuit; selenium in anything other than dietary trace amounts is poisonous (old left, fix on right):
Since there aren't any aluminum can caps showing on the transformer or tube decks, chances are there's one lurking under the hood; good news is that I found one for my U305R ... a voltage-for-voltage/value-for-value replacement .. bad news is that I had to pay for it (old left, new right):
I found a schematic for a sister amp; the 1236 - also 3 X 12A_7 and 2 X 6L6 ...interesting orientation of the 6L6s; like exhaust pipes with a heat shield. Unlike the U305R above - a reverb/trem amp - the 1235 / 1236 is a straight-ahead, single-channel Thunderbass-style amp; 3 inputs including bass and a bright switch. If the 1235 and 1236 are related, it has an odd-ball bias scheme including a un-adjustable negative voltage from the power supply
and a cathode resistor on the 6L6s ... usually one or the other ...
As a minor plus and unlike the U305 design which I have concluded was done by an in-house engineer at the Japanese company that manufactured the caps used in the amp as part of a design competition; who can design an amp that uses too many caps in the most obscure values? ...... and the winner was ... Ishura Turda :shock:
If like the 1236, this amp has fairly common cap values; if they needed to be replaced, finding them is no biggie. For all the risk on the downside, other than cheap 6L6 power in the door, do not know if the amp has an upside.
Highest and best use: buy, tear out pcb, buy a Bassman circuit board in a kit and make it work. Suggested offer: $50, a decent used pocketknife, and you'll pick it up ...