Low line voltage and tube amps

[brndn123]

I had issues with my Fender Bassman 300 (all tube) a few nights ago at a gig (low volume, no definition in lower notes etc). Presumed it was tube related but after trying the rig at my house discovered my amp sounded fine. TBer B-String suggested it was low line voltage at the club causing the amp to run with lower power. I've been researching this a bit because my amp had been running that way for a few hours that night and I want to ensure nothing internal was damaged. I seem to be getting different views on whether running an amp at lower voltage is damaging to the amp itself. Here are the theories I've come across:

1. Causes cathode stripping and ages tubes
2. Low voltage causes higher current in the amp which fries internal components.
3. Common practice for guitards thanks to EVH. Modern tube amps have better design and can handle low voltage well, older amps not so much.

So, whats the deal?

[father of fires]

I think any long term ill effects from low voltage would be after lots of use in that condition. One gig shouldn't cause any lasting damage. Not an expert just a DIY enthusiast.

[beans-on-toast]

Tube amps, with traditional power supply designs, are fairly tolerant of variations in power line voltage. All amps are designed to safely operate over a range. Look at the back of your amp where the power cord connects and see if a range is specified. If might just say 120VAC 50-60Hz, or it might provide a range which is often +/- 10% of the normal line voltage. In your case this would be to 108 to 132VAC. If it doesn't give a range, call Fender's customer service department and ask what the acceptable line voltage range is for your amp. These amps have to be capable of operating over a range because line voltage vary in North America, for example, typically between 117 and 130 VAC.

Once you know the range, if it concerns you, you can bring an inexpensive voltmeter to your gigs and measure the line voltage. It wouldn't hurt to have an outlet tester to ensure that the power line circuit is safe to plug into. Nether of these devices will tell you how clean the power is, voltage spikes can be dangerous for your amp as well. But that's another issue.

One concern with plugging in to a lower line voltage is that, with your amp, the bias voltage applied to the power tubes will be lower. This will increase the current running through the tubes. A bit and the tubes distort earlier and it wears them out faster, too much current and the tubes can blow.

Since your amp sounds fine at home, I wouldn't worry about it.

[brndn123]

So in theory, the only thing adversely affected by the higher current are the tubes? The caps, internal electronics are safe?

[brndn123]

On a side note, tho the sound was not as defined it was way cool nonetheless, like a ric through an ols B15

[beans-on-toast]

Originally Posted by brndn123

So in theory, the only thing adversely affected by the higher current are the tubes? The caps, internal electronics are safe?

When you use the amp at a lower line voltage, the high voltage power supply (B+) and the (12.6 or) 6.3 VAC heater supply will be lowered proportionally. The heater supply specification is typically +/- 10%. Below a certain level, the tubes are going to shut down.

When the B+ supply is lower, the output power is going to fall, the power supply caps are not going to fully charge, they will not be able to supply the current that the amp was designed for. The voltages at the tube plates will be lower. The operating point of the tubes is going to shift resulting in compression and then distortion. If the operating point shifts too much the tubes will shut down or partially shut down depending on the input swing of the signal that is feeding them.

A variac is a device that allows you to reduce or boost the line voltage by turning a knob. The variac plugs into the wall, the amp plugs into the variac. Sometime people use these on guitar amps to lower the line voltage. Van Halen for instance used to lower the line voltage to 90 volts on his Marshall 100W amp. At 90VAC, the amp distorted in a good way. Below 90 VAC and the amp lost its sweet sound and farted out. I mention this because dropping from 120 to 90 is 30 volts. Quite a bit. He used to re-bias the power tubes to operate them properly at this voltage. You can't do this if the amp has a tube rectifier. Your amp doesn't. I should add that some amps have low voltage protection circuits that shut the amp down if the line voltage falls below some predefined level.

At a lower line voltage, the character of your amp is going to change. How much so depends on the line voltage drop, the design of the amp, and the specs of the components. In general, the power supply caps and resistors will do fine at a reasonably lower B+ voltage. The amp will operate at a lower line voltage within the specified operating voltage range of the amp. Beyond that, the power tubes are pushed too hard. The higher current draw through the power tubes also stresses the power transformer as B-string pointed out. This could put a strain on some of the components in the power amp. If the tubes blow, they could take other components and even the output transformer with them.

As I mentioned above, based on your description of what happened, it sounds like you have not harmed your amp.

[rickdog]

As I remember the story back in the day, Eddie Van Halen used a variac to turn up the voltage on his Marshall. He'd burn them up, but what did he care? But my memories of those days, well, you'll understand if they're a little faded

I used to play bass through a 100W Marshall Super Lead (which sounded pretty good if you liked some distortion, which I did). One gig it was sounding not so good, farty and weak... something inspired me to pull out my voltmeter, and I found I was getting about 105V out of the socket.

So based on the EVH stories I'd heard, and my experience with low voltage to a tube amp, I've always thought high voltage good (to a point), low voltage bad.

[B-string]

Originally Posted by brndn123

I had issues with my Fender Bassman 300 (all tube) a few nights ago at a gig (low volume, no definition in lower notes etc). Presumed it was tube related but after trying the rig at my house discovered my amp sounded fine. TBer B-String suggested it was low line voltage at the club causing the amp to run with lower power. I've been researching this a bit because my amp had been running that way for a few hours that night and I want to ensure nothing internal was damaged. I seem to be getting different views on whether running an amp at lower voltage is damaging to the amp itself. Here are the theories I've come across:

1. Causes cathode stripping and ages tubes
2. Low voltage causes higher current in the amp which fries internal components.
3. Common practice for guitards thanks to EVH. Modern tube amps have better design and can handle low voltage well, older amps not so much.

So, whats the deal?

I can further offer my views on one and two?
1) The cathode is still hot and stripping is not likely. Stripping happens when B+ is applied when the cathode is still cold and in the process of heating.

2) The power transformer is the most stressed.
Have you heard stories of cracking Fenders from 1/2 watt carbon comp plate resistors? Under low voltage the dissipation can exceed their rating, 1 watts will hold up under the same conditions.
As I said before, a little clearer this time though, your amp is still in fresh condition. At least without decades of use and I doubt it suffered any permanent damage

As far as building the amp to be tolerant of wide voltage variances? Didn't CBS try that with Fender amps, as in build them to be more stable? How did that do for them. Anyone suggesting all amps include a (large and heavy) autoformer so a wide voltage range can be utilized?

[beans-on-toast]

I recall a Van Halen interview where he described how he lowered the voltage with a variac. That's why people call it a brown sound. Brown as in brownout. There has always been a lot of controversy over this.

A higher voltage is bad. It stresses everything. That is a problem that a lot of vintage amps encounter. They were designed to run at 117, a lot of people have line voltages of 125 today. It can be a strain on an old amp that hasn't been in service or serviced.

Getting 105 VAC out the socket would make the amp sound farty and weak. The caps are not going to charge fully, there will be less charge reservoir available, you hit a note, the current gets drawn but it is insufficient, the power supply sags and the amp looses headroom and eventually farts out. You can get the same thing at a normal wall voltage if the amp is pushed hard enough. Even more so if the amp has a tube rectifier.

This distortion sounds great with a guitar. With a bass, some sag is good but too much is not so good.

[beans-on-toast]

Originally Posted by B-string

As far as building the amp to be tolerant of wide voltage variances? Didn't CBS try that with Fender amps, as in build them to be more stable? How did that do for them. Anyone suggesting all amps include a (large and heavy) autoformer so a wide voltage range can be utilized?

Above I mentioned amps being tolerant of a range of line voltages. I was referring to the plus or minus 10% line voltage spec that you see in some amp manuals. With the variation in line voltages across the US, often 117 to 125, amps have to be somewhat tolerant.

With the classic tube amp power supply design, other than using over-specified transformers and components that can tolerate a range of voltages and currents, I don't know if there is a cost effective way to make an amp tolerant of wide range of line voltages. You can DC regulate the heater supply but not the B+, the amp looses all its character.

I have used constant voltage transformers that regulated the output voltage. As I recall, they were large and heavy as you said. They also had quite a LOUD hum.

[B-string]

Originally Posted by beans-on-toast

Above I mentioned amps being tolerant of a range of line voltages. I was referring to the plus or minus 10% line voltage spec that you see in some amp manuals. With the variation in line voltages across the US, often 117 to 125, amps have to be somewhat tolerant.

With the classic tube amp power supply design, other than using over-specified transformers and components that can tolerate a range of voltages and currents, I don't know if there is a cost effective way to make an amp tolerant of wide range of line voltages. You can DC regulate the heater supply but not the B+, the amp looses all its character.

I have used constant voltage transformers that regulated the output voltage. As I recall, they were large and heavy as you said. They also had quite a LOUD hum.

Yes they are! I was speaking more to those that want amps to run to spec at 96Vac or less actually, not the +/- 10%.

[brndn123]

A lot of good info in here. Thanks to the two techies we got in B-String and beansontoast. I just feel so upset because though my gig was only about 2 and half hours of playing, i tend to fire my amp up, leave it in standby for a minute or two, then keep it on through the duration of the gig. There was 3 hours of setup time for the crew at the venue while I left my amp on which totals about 5 hours that the amp was on and out of the standby position. Normally, things are cool but considering it was on for that long on a low voltage outlet I'm kind of stressing. 3 or so hours of idle time and 2 and half of actually playing. Again, I'd be lying if I didnt think the tone was kind of cool. Really added character to my sound