Brown Box / Variac with old tube amps

[agedhorse]

This concern over 120V operation extending to 130V is why I keep mentioning safety regulatory approvals. For an amp to be approved for use in the US under UL 60065 or 62368 must be able to operate at 10% over nominal 120V which is 132V. For amps that are not safety certified, this requirement doesn’t exist.

 

[Marc Speed]

I was in a Beatles tribute band for several years. One of the vintage Vox AC100s was very sensitive to voltage. If it saw 115 from the wall with the amp’s voltage knob set on 110, it would blow a fuse. Using a Variac to limit the voltage to around 107 solved the problem. With old amps, using a Variac is a reliability matter, not a tone matter, at least from my perspective. YMMV.

 

[Kevin Wolfe]

I built a bucking transformer for my vintage Bassman but not for the sound. My concern was keeping the heater voltage under 6.3 VAC.

 

[MultiScaleMale]

When I hear Variac, my mind jumps to Eddie Van Halen.

The TRUE Story of Eddie VanHalen Using A Variac With A Tube Guitar Amp

I knew a guy in '86-'87 that had the full EVH setup, including a Variac. No idea how he used it, but it sounded great.

 

[Boneless]

I use an Amp Maniac at most gigs. I primarily gig vintage Sunn 200s / Sorados. They were designed for 117v, but as I understand it, their capacitors are just barely adequate for that voltage and I don’t want them hit with the 123-125v from some of todays outlets.

Then just replace the capacitors.

 

[markjazzbassist]

Wanted to check in and see if anyone is using the AmpRX Brown Box or similar? I've got a 1972 Orange OR80 and it's spec'd at 115V (see below). i'm playing gigs at places where sometimes you're seeing 120-128v (welcome to middle america, freedom and building codes are free). if the amp was made for 120 i'd say sure that's within spec (as mentioned by AgedHorse 10% swing calculation above at 132v), but at 115v that 10% puts me at 126.5v.

Would something like the AmpRx Brown box make sense for me? it's an old and expensive amp (and i love it dearly) and i just want to prolong it's health.

 

[Wasnex]

Wanted to check in and see if anyone is using the AmpRX Brown Box or similar? I've got a 1972 Orange OR80 and it's spec'd at 115V (see below). i'm playing gigs at places where sometimes you're seeing 120-128v (welcome to middle america, freedom and building codes are free). if the amp was made for 120 i'd say sure that's within spec (as mentioned by AgedHorse 10% swing calculation above at 132v), but at 115v that 10% puts me at 126.5v.

Would something like the AmpRx Brown box make sense for me? it's an old and expensive amp (and i love it dearly) and i just want to prolong it's health.

The power supply in my vintage Matamp GT120 runs high on US voltage. AFAIK the OR80 uses a lot of the some circuits and transformers, so yes it could potentially benefit from using a brown box or variac. I haven't used the Amp RX, but I believe it has a good reputation.

I believe my GT120 was made in 76. It had the original capacitors and was running fine on German power. Five or so years after returning to the US, it started having some problems. I opened it up and noticed some of the capacitors were leaking. I checked the power supply voltages and they were running over the rating of several 450V capacitors.

At 120V line, the first two preamp decoupling caps were overvoltage; with 476V on the first cap, 455V on the second, and 326V on the plate of V1b. Dropping the line voltage to 110 brought both caps under 450V.

I replaced all of the electrolytics with higher voltage components. I also increased the value of the 10K resistor feeding the preamp voltage ladder to 16K. The higher resistance lowered the voltages in the preamp. My notes say Orange used 33K for the resistor, so your preamp voltages may be fine.

With the amp biased to about 60%, the plate voltage runs about 516V on 120V 60hz. I believe the schematic says it should be about 467V. I did not do anything to lower the plate voltage. Since the OR80 only runs two tubes so the plate voltage may run a bit higher.

 

[Wasnex]

I'll also mention that I have an early 60s Vox AC30 that I run at 110V on a variac. I don't play guitar, but I used to fake it in my basement, so the variac is fine for my needs. If I were going to gig the amp I would probably go old school and install a bucking transformer.

 

[agedhorse]

Wanted to check in and see if anyone is using the AmpRX Brown Box or similar? I've got a 1972 Orange OR80 and it's spec'd at 115V (see below). i'm playing gigs at places where sometimes you're seeing 120-128v (welcome to middle america, freedom and building codes are free). if the amp was made for 120 i'd say sure that's within spec (as mentioned by AgedHorse 10% swing calculation above at 132v), but at 115v that 10% puts me at 126.5v.

Would something like the AmpRx Brown box make sense for me? it's an old and expensive amp (and i love it dearly) and i just want to prolong it's health.

120V is the nominal standard, it's not at all uncommon to see 128-130V under light load conditions because the distribution voltages sag under heavy loading and the utility has to establish a range of voltages at the end points of their grid that are still high enough to be within range, which means that the voltages closer to the transmission portion of the grid would be a little higher than nominal.

At the venue I managed for about 30 years, I recorded the line voltage over many months and times of day, we ran between 118V and 130V depending on tome of day and day of week. My switchgear was fed directly off of a 3 separate feeder network transformer bank (3 sources fed this transformer bank, with network interrupters for fault protection). This results in a very stiff supply without much variation other than our own feeder and distribution losses. My in-house gear ran for many years without a single issue.