Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DummyLoad on November 14, 2019, 03:51:35 am
-
ran across this thing on the local CL. it is/was is very sad state of disrepair. missing the both original PT & OT - it had a 76 fender twin reverb OT hacked in. hacked in as in the mounting holes are not square and were elongated in hacked up manner. OT tests good so i'll probably flip it. read good things about the tone. seems that e. clapton and r. gallagher both used them.
the PCB is good condition, only 2 pads were lifted by the previous hackster. as for the wiring, most of it had been butchered. the wiring harness from the effects loop and power PCB to the control PCB was a salvaged and spliced together ugly POS - see pics in link. the original cover and bottom covers are missing, as are all of the original knobs. will need to fab those. looking at pics online provided by other owners/sellers, it seems that the original covers are made of perforated steel.
the PT replacement was a challenge - thought i'd use a 100W hiwatt style unit, but i have three (two now) twin reverb PT that were scratch and dent sale by mr. hoffman. perfect. made a mounting plate for the PT and use the mounting plate as the cut pattern for the chassis - BTW the chassis on these things is made entirely of soft .063" aluminum, with the front rack mount panel made of .125" aluminum plate. i have seen several of these amps online and most do not seem to have the original PT. if the mounting pattern of the original is any indication of it's size, then it seemed much too small to be a reliable power source for a 100W tube amp.
had to mod the bias supply to get down to -52V for the 450V B+ it's running on now - 15k resistor replaced with a 10k. the zener regulation for the op amps was running 3W resistors and they were burning the PCB. i installed 3W zeners (it had 1.2W) and 7W power resistors, both the zeners and power resistors were elevated off of the PCB. i used ceramic standoffs to help support and dissipate lead heat of the power resistors.
OT was replaced with a hammond 1760W - a twin reverb replacement model that has 4/8/16 ohm secondary taps - that allowed me to fill in most of the holes on the rear of the chassis and add 2 more load options as was.
replaced the AUX 120V outlet with IEC C13 power inlet. not too fond of strain relief power cords on a big amp such as this. plugged the rest of the unused rear panel holes.
kind of a chore to work on, but overall not too terrible for a PCB amp from the early 80's. spring reverb is op-amp based and uses FET switching for reverb foot control.1/2 TL072 op-amp drives a class B complementary bipolar pair to drive reverb tank, the second 1/2 of the TL072 op-amp is the reverb recovery. tone shaping for mids is also done with another pair of op-amps.
everything is working now. just needs some new covers fabricated and it's ready to flip.
pics. https://www.dropbox.com/sh/f6z3htujby476m9/AACscOb-3WXiWvjSTqdRet4ra?dl=0 (https://www.dropbox.com/sh/f6z3htujby476m9/AACscOb-3WXiWvjSTqdRet4ra?dl=0)
--pete