1969 Fender Vibrolux Reverb Amp
The customer brought me this lovely 1969 Vibrolux Reverb, complaining of volume drop-out, and popping noise. He asked if I would convert the amp to blackface specifications – a common modification, since the pre-CBS (blackface) Fender circuits are arguably more desirable than the CBS (Silverface) circuits.
The first thing to do is assay the amplifier – determine what condition it is in, how it is operating, and the status of the circuit. I look for burn marks, dirt, dust, and contamination, including corrosion, which can be common in vintage amplifiers. I check the tubes for function and health, and measure the B+ voltages of the circuit. I test the critical capacitors and resistors for value and inspect all of the solder joints in the amp.
This amp made some crackling / popping noise at start up, and then was relatively quiet.
The amp is in good exterior condition. The amp was manufactured in 1969, in the Silverface CBS era. Transformers are original. Speakers are original Oxford 10L5-3. There is a non-factory ½” hole by the rear courtesy outlet. I don’t know why – maybe a failed attempt to add a master volume or a midrange control. Regardless, you do not want to put your finger in there.
The tube chart identifies the circuit as AA864; this circuit does not exist for the Vibrolux Reverb. It may have been a “transition” circuit, or a mistake at the factory. This unknown circuit, AA864, is well documented for the Vibrolux Reverb on online forums; evidently a number of amps have this tube chart.
The actual circuit in the amp is AB568, a CBS – Silverface circuit.
The circuit had been extensively modified in the past. There were numerous changes to capacitor values and key resistor values. A 3-conductor power cord had been added.
TUBES:
The results of TV-7D/U tube testing were as follows:
5AR4/GZ34 Rectifier RCA (Min.) Value
(40) 55/56
6L6GC Output JJ (Min.) Value
#1 (25) 37
#2 (25) 36
6L6GC RCA Black Plate (Min.) Value
#1 (25) 50
#2 (25) 50
12AX7 Preamp Type (Min.) Value Circuit
#1 (generic) (32) 53/40 Normal Channel
#2 (Tung-Sol) (32) 55/53 Vibrato Channel
#3 12AT7 (Penta) (50) 50/40 Reverb Send
#4 12AX7 (Tung-Sol) (32) 50/60 Rev. and Gain
#5 (generic) (32) 51/53 Vibrato
#6 12AT7 (Zenith) (50) 88/78 Phase Inverter
V3, the Penta 12AT7, was weak and was replaced with a Tung-Sol reissue 12AT7. The other preamp tubes were in good condition. The output tubes were JJ, and were tired, and starting to sound ratty. These were replaced with strong vintage RCA Blackplate 6L6GCs. The RCA Blackplates were of different bottle shapes, and one had a missing guide pin which was repaired with an adapter. RCA Blackplates are considered the “Holy Grail” of output tubes, and were probably original equipment for this amp in 1969.
Assay and Repair:
Before we even think of starting the conversion to blackface specs, the existing problems must be identified and repaired – otherwise we will be changing too many things at once, and risk making things even worse.
The circuit board had been extensively re-wired, using small gauge, plastic-insulated wiring. The input signals had been re-done using shielded wire. All of the tube filaments had been re-wired. It is a very curious amp – I’m not sure why anyone would do some of these things.
At some point, many of the electrolytic capacitors had been replaced. The filter caps are very high in value – the first filter cap is an astonishing 147 uF, where the design calls for 32 uF. The GZ34 rectifier has a specified maximum first capacitance of 40 uF. Similarly, the #2 and #3 caps are each 80 uF, where the schematic calls for 16 uF. The #4 filter capacitor is 47 uF, and it should be 16 uF. All these filter caps were replaced with new F&T 500V 22 uF capacitors of the correct values.
There were numerous poor solder joints on the eyelet board which were repaired. Most of the cathode bypass capacitors had been replaced and also bypassed with 1.0 uF film capacitors, presumably (perhaps) to enhance bass response. All the bypass capacitors were replaced with new Sprague electrolytic 25/50 axial capacitors. The 0.1 uF film capacitors were removed from the circuit.
The 0.1 uF coupling capacitors were suspect and were replaced.
Several of the 100K preamp plate resistors were ¼ Watt, and were replaced with the correct ½ Watt resistors. The tremolo LDR unit and the phase shift capacitors were re-dressed and re-soldered.
Several of the tube socket pins were re-soldered. All of the tube sockets were cleaned and re-tightened.
The NFB (negative feedback) resistor was of the wrong value and was replaced. The bias control resistor was of the wrong value and was replaced.
BLACKFACE CONVERSION:
The amp was now sounding pretty good, an excellent example of the CBS-era Vibrolux Reverb. Now we can start the conversion to the blackface circuit.
The following changes were made to convert the Silverface AB568 circuit to the Blackface AA964 circuit:
1. The 47K phase invertor plate resistors were changed to 100K and 82K.
2. The 100K grid bias resistors were removed, and the circuit changed to a bias adjust from a Hum Balance circuit.
3. The 2000 pF ceramic disc capacitors on the output tube grids were removed.
4. The 25uF capacitor was removed from the output tube cathodes.
5. The first B+ voltage resistor was changed from 4.7K / 1Watt to 10K / 3Watt.
6. The 15K bias set resistor was changed to 27K.
7. The bias voltage capacitor was changed to 100 uF / 100 Volts.
COMPLETION:
All other resistors and capacitors were within 20% of their rated values, as original.
The output tubes bias was set to 47 mA with a plate voltage of 408 VDC (64%) for a total static dissipation of about 19.2 Watts X 2 = 38.4 Watts.
The GZ34 rectifier can be replaced with a 5U4GB, which will lower the amp’s headroom and give more sag and compression. (The bias is reduced from 47 mA to about 39 mA).
The right hand speaker had a small hole in the cone, which was repaired with corona dope. This speaker may be making some occasional cone rub noise, I’m not sure. The speakers are 52 years old. The amp may benefit from replacing the original speakers with new, efficient speakers like the Weber 10A150 or Jensen C10Q, or better yet, one of each.
