Tube Amp Care

Hi fellow Gearheads,
As Doug knows I run a small business in the Claire and Gilbert Valley of South Australia, doing a variety of things involved with music, most of which are legal, and one of which is repairing valve amps (are there any other kind!!), and guitars.

During the last week I have had a number of Marshall's in for repair and they are all suffering from faults related to the same problem.

When a valve amp is switched on it should be run in standby for at least 30 seconds, preferably a minute before applying the DC rail voltage to the power amp valves.

On Marshall's, Fenders and most other valve amps, this is achieved by switching on the power at the main switch, leaving it for the necessary time, then switching from standby to normal operation.

The results of not carrying out this procedure can vary from annoying, to downright expensive.

Annoying:
(1) Power Supply fuses may blow, particularly the DC fuse.
(2) Stripped cathodes in the power amp valves will degrade the sound of your amp

Expensive:
(3) Power Supply capacitors may fail.
(4) Bypass capacitors may fail.
(5) Your power amp valves may only last half the time.
(6) When items 1 - 5 cause your amp to let you down in the middle of a gig, frustration may lead you to doing serious harm to your amp, as per Pete Townsend.
(Note PT can probably afford it, most of us can't).

All this can be avoided by following the above procedure.

One common problem that leads to not following the above is switching off the amp at the end of a show/rehearsal by just switching off the main power switch. Next time the amp is turned on the standby switch is already on and DC is applied to the power amp valves before the heaters have had time to warm up.

I am probably doing myself out of easy money as I am quite happy to repair all the amps which fail due to the above problem but that's not the way I like to do business. Follow the above and save yourself money and frustration.

By the way, the above does not apply to solid state (WHAT!!!) amps, just turn them on and play.

Regards, Bob Charman
Stockport Sound Studio
PO Box 1, STOCKPORT SA 5410, Australia
Telephone 08 8528 2098

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This first appeared in GearheadEzine.com which was Australia's On-Line Guitar Gear Ezine at the time
and later in Guitar News Weekly Edition #95.

See also: AMPS @ GuitarSite.com

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but....

Great advice, but what if my tube amp has no standby switch? It's a Peavey Classic 30. Do these amps have a built-in safety system, or are they just designed to blow parts early? Might explain the low price tag...

No standby switch.

If the amp has a tube rectifier, you don't need a standby switch as the rectifier won't rectify until it heats up and by the time it does, the other tubes are hot enough as well.

If an amp has no standby switch and has solid state diodes, the designer was an idiot. I had a Reverend amp built this way and had to educate Joe Naylor, of all people!!!

No switch

Really,
I got Peavey Delta Blues and it neither has a standby switch.
How it deals with the matter above?

Cathode Stripping is a Myth

Number 2 is a very common urban myth. In reality, you'd need voltages closer to 1000V to achieve cathode stripping in receiving tubes (amplifier tubes, etc.). Cathode stripping is a problem on transmitting tubes, but we don't use those in guitar amps. So the power tubes do not need to warm up.

Standby switches were started when Fender used them on its early amps. This was because Fender used cheaper power capacitors that were not rated for the voltage that the power supply produced. By allowing the tubes to warm up, the resistance of the tubes went down. So when the standby switch went on, the tubes would immediately draw current, pulling the power voltage down and protecting the filter capacitors. Marshall copied the switch when they copied Fender amps, and soon all other guitar amp manufacturers were using them. Today, any amp built with a properly-designed power supply shouldn't need a standby switch. Note that tube hi-fi amps rarely have standby switches and they do just fine.

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