By Eddie Pletka 09/02/11
U1561: This amp recently came in for some tubes and repair work. I had never
seen one before this encounter. The old ceramic sockets where cracked in several
places, it needed a new power cord and a fresh set of tubes. Once I dove into
the amp I noticed the power section was wired up in a somewhat peculiar fashion.
The primary taps on the OT are wired directly to the plates of the first pair of
power tubes. Then they jumper over to the plate connections for the second pair
of power tubes via 100ohm 2 watt resistors. You can see this in the schematic we
posted for the U1061. I have seen “plate stop” resistors before but these
were a bit higher in value then what I’m used to seeing. At any rate, when the
amp came in both of these 2 watt resistors were burned to bits. The cracked
sockets lead to some pretty serious arching which I believe was the cause for
these burned up resistors.
An hour or so later we had a new set of Belton sockets, fresh 100 ohm resistors and a matched quad of the JJ 6550’s ready to rock. After firing the amp up we checked the plate voltage and it came in just shy of 700VDC. We dialed the 6550’s in at about 32-35mA per tube and let her rip. It actually sounded great! Although the wire dress left a lot to be desired (customer was on a serious budget and specifically asked me not to address this unless it was functionally necessary), the amp was almost dead quiet. I was stoked to say the least.
In the end this Univox was a pleasant surprise! It sounded great with a guitar and stellar with a bass. This head combined with a Sunn 4X15 we keep in our studio was fat, and punchy as all get out. Considering the amp was acquired for about $40 at garage sale, the customer was very happy with the outcome.