N(V)AD: Vox V100

Discussion in 'Amps and Cabs [BG]' started by wayshot, Aug 10, 2019.

  1. wayshot


    Sep 25, 2006
    Skage, Norway
    Hello, folks :)
    I recently bought a good-looking ~1970 Vox V100 tube head on the return from the summer holiday trip. Tried it with a guitar before shelling out the money, and it sounded pretty darn good. All original electrolytics, so I might replace those sometimes as preventive maintenance. I've done an inspection on the inside. One JJ and one Chinese 12AX7 tube in preamp and phase inverter. Four recent E-H EL34 output tubes. Components look all original, although the output transformer has been rewound - and there's only one secondary tap soldered to both "8" and "16" lugs on the impedance switch. Need to figure out that one to get it back to original condition.

    I haven't found a schematic for the V100 anywhere, so I intend to trace the circuit and make one. Some of the circuit is similar to the AC100, but there are differences.

    Check out this site for info and pictures of the V100: The VOX Showroom - Vox V100 Amplifier
  2. wayshot


    Sep 25, 2006
    Skage, Norway
    (Technical mumbo-jumbo alert)
    Had a little spare time at work today to do some measurements and inspections of the amp. First thing: Checking the fuses. Appeared that both the 3A mains fuse and the 1A B+ fuse were replaced with 5A. I don't have too many fuses around, but I replaced the 5A ones with a 2A for mains and 1A for B+. Turned the amp on, the fuses didn't blow.
    The voltage on the first B+ node seemed unusually low, around 305 V. The V100s and AC100s have a "brimistor" between the bridge rectifier and the first node. It's apparently been replaced with a 1Kohm/10W resistor on my amp, and it drops too much voltage. Tried bypassing that resistor. The B+ increased to 480 V after warmup, but one output tube started red-plating. I might have to mod the bias circuit and make it adjustable. I'll also have to order replacements for all electrolytic caps in the amp.
    I checked various resistors and coupling caps without finding any drifted values or DC leakage.
    Stay tuned for updates :)
    Last edited: Aug 16, 2019
  3. wayshot


    Sep 25, 2006
    Skage, Norway
    I just finished the overhaul on the amp. Here it is:


    I did quite a lot of work on this amp:
    Replaced the output transformer. It had been rewound, as mentioned earlier, but with a completely wrong turns ratio. It was 40:1, meaning that the four EL34 tubes would be happiest if the speaker load were 1.2 ohms. I bought a Marshall 100W-style output transformer from Tube-Town and installed it. Wired the 4 and 8 ohms taps to the switch instead of 8 and 16 like originally. I'll install a 3-way impedance switch later for 4/8/16.
    Replaced one of the EL34 tube sockets, as there had been an arc-over between plate and filament pins sometimes.
    Replaced all electrolytic caps, as they were all original. I also replaced the bias circuit coupling cap and the two phase inverter coupling caps, as the latter caps probably were leaking some DC.
    Replaced the 1K resistor in the power supply with a 33 ohms instead. The quiescent voltage on the EL34 plates is now 490 V instead of 310 V.
    Undid a mod that hard-wired the power transformer tap switch to 245V. This made the filament voltages too low - around 5.7 V~ . Using the 225V tap instead gave better voltages at around 6.1 V~.
    Removed the two Zener diodes in the bias circuit which originally should clamp the voltage to -35V, but it was only -31V on my amp. Replaced them with a 25K pot and a 24K resistor in series for bias adjustment. I set the bias for the four EH EL34s at 60-65% max plate dissipation.
    Put a new Tung-Sol 12AX7 in the preamp position and a JJ 12AX7 in the phase inverter.
    Haven't tested it with bass yet, I just used a sinewave generator and a dummy load for bench testing.

    Here's a schematic of the amp that I made after tracing the circuit. There are some interesting design choices here, like a 3300p coupling cap between the first gain stage and the volume control. This will work as a HPF at 100 Hz, so it might not be a bad thing after all :)
    AlexanderB, Raw N Low and biguglyman like this.
  4. wayshot


    Sep 25, 2006
    Skage, Norway
    UPDATE: Found an error in the schematic (phase inverter plate load resistor, to be exact). Corrected version added below.

    I just did one mod on the amp: Replaced the impedance switch with a third speaker jack and wired the three jacks to the 4, 8 and 16 ohms taps from the new output transformer. Nice to have all impedance options available :)
    Next task: Replacing the treble and volume pots. They're still scratchy even after two cleaning attempts. They are soldered directly to the PCB, so replacement might be a little tricky.

    Wasnex likes this.
  5. Balog


    Mar 19, 2009
    Everett, WA
    Thanks for the write up, this is really cool. Do you have any shots of the interior?
    wayshot likes this.
  6. wayshot


    Sep 25, 2006
    Skage, Norway
    See below :) By the way: Forgot to mention earlier that I moved the four screen-grid resistors from the PCB to the output tube sockets.
    Balog likes this.
  7. wayshot


    Sep 25, 2006
    Skage, Norway
    Tried the V100 at rehearsal last night, through a Behringer 4x10. It worked great, sounded nice, and had more than enough clean headroom for our not-too-loud blues band.
    pie_man_25 likes this.