Nic brought me a Fender Twin Reverb that he had owned for 40 years or more. He had grown quite sentimentally attached to it. The pilot lamp went out, and then shortly thereafter the fuse blew, and he noticed a horrible burning smell.
The Twin is probably from 1968 or 1969. The serial number is on a sticker and is not a standard Fender number, it may have been added when the amp was modified – probably in 1970, as there is a date marked inside the chassis. The original Fender serial number may be underneath the current sticker, but I did not remove it.
The amp was in fair to poor condition. The original baffle board and speakers had been replaced with a plywood baffle and 2- 12” Guya speakers. The speakers are of Japanese manufacture and are similar to Celestion speakers of the period. The grill cloth had been replaced with metal bird screen. The back panels are missing. One speaker was not installed and had broken connector terminals.
The first thing I noticed was that the chassis was badly damaged on the right side, breaking off one of the power transformer bolts and leaving a gap between the chassis and the edge of the transformer. The bottom and side of the chassis had large dents. This probably happened some time ago, as someone had repaired the ground lug which had broken off due to the damage to the chassis.
The circuit boards were dirty and mildly corroded. The eyelet board is slightly warped, which is typical in these amps. The filter capacitors are all original, as are the electrolytic cathode bypass capacitors and coupling capacitors. The entire circuit appears to be as original, with no replacement components or modifications, other than the speakers and baffle board and grill. The power cord was damaged, and the wires exposed.
Tracing the Bad Electrons:
The vacuum tubes were tested on a TV-7D/U, and all tubes are fairly weak and needed replacement.
The #3 filter capacitor showed a dead short to ground. The choke was burned and had very low resistance and inductance; it had leaked melted varnish onto the chassis. This may have been the failure that blew the fuse and created the burning smell. ESR (equivalent series resistance) was high on all electrolytic capacitors. The power transformer seemed unaffected.
All of the filter capacitors and other electrolytic capacitors were replaced. The bias supply electrolytic capacitors were replaced. The B+ dropping resistors were replaced. The 4Hy choke was replaced.
The power transformer was removed in order to straighten out the chassis mounting area.
A new power cord with ground was installed, and the circuit modified to delete the ground switch and “death capacitor” for safety. The dents were repaired with a large hammer.
Both ½ Watt, 100 Ohm ground reference resistors on the 6.3VAC filament source were physically broken. These were replaced with ½ Watt, 100 Ohm resistors.
The four 470 Ohm, 1 Watt screen resistors on the output tubes had drifted more than 20% and one was burned and open. All were replaced with 470 Ohm, 2 Watt resistors.
The reverb cables were replaced with new vintage-style cables. I replaced the 2.5A fuse.
The speaker terminals were repaired, and the loose speaker mounted. A new spider connector for the speaker harness was installed. The speakers were wired in parallel @ 4 Ohms.
All pots and jacks were cleaned. The eyelet boards were cleaned. The chassis under the choke was cleaned, as it had a lot of melted varnish on it from when the choke failed.
The Route to Failure:
The B+ power had a dead short to ground through the filter capacitor. The power transformer immediately unleashed a vast quantity of electrons. The rogue electrons took out the choke, and probably the filament supply resistors and the screen resistors of the output tubes. It probably didn’t do the pilot lamp any good, either.
The yellow points on the Twin schematic are all components that were repaired/replaced.
The bias of the output tubes was as follows:
Tung-Sol 6L6GC power rating = 30 Watts
Plate voltage = 444 VDC
Plate – Cathode current = 44 mA
Dissipation = 19.5 Watts per tube = 65.1%
This bias setting is correct for the 1969 Fender Twin Reverb.
The repaired amp made Nic very happy. It sounds great, as a Twin should, and he said it also “sounded louder”. Which is good.
