Old Amps

[Wa Diddy]

Here’s a tip for anyone who’s about to turn an old amplifier on for the first time in years; don’t do it. Capacitors that haven’t been energized in a long time need to be ‘formed’ or they can act as fuses and blow (no conductivity), or short and take out a host of other components.

‘Forming’ is accomplished by ramping up the A/C voltage gradually. Take the amp to a repair shop and they can plug it into a ‘Variac’ [vari a/c], starting at 10 volts increase the voltage 10v every 5 seconds until it’s at 110v. Then you can turn the thing on and off at will.

I have three VERY old tube amps and learned about the above the hard way.

Hope this helps someone.

[dthumb]

i've heard similar stories from others. i have also been told by some tech buddies that an alternative is to make full use of the standby switch most older amps have and let them warm up slowly.have you heard this as well?

[Wa Diddy]

I’ve just looked at the schematic for a ‘65 Twin Reverb and the standby switch is on one leg of the rectifier. There is a 1000 volt capacitor upstream of the rectifier so I don’t think using the standby is a substitute for ‘forming’; certainly not for the big cap on the power supply. How it affects the rate the amp powers up I cannot say.

I do know someone who can answer your question and I will be seeing him next weekend (see thread about changing caps on a Les Paul). I have to use the standby mode when changing guitars or plugging in stomp boxes because I can’t help shorting the input as the jacks slide in and out, which usually sends a big crack to the speakers.

I noticed something interesting on the schematic; there is a screwdriver adjustable pot on the chassis for ‘hum adjustment’!!!!! Complete with instructions. Who’d of thought?

“Adjust for minimum hum from speakers with an open plug in J3, (#1 jack) vibrato channel. Volume, mid and bass controls full c/w. normal channel controls full cc/w reverb and vibrato off.”

I always wondered what that little pot was for. Thanks for asking your question or I wouldn’t have dug out the schematic. Now I have something to do this afternoon; adjust my hum.

Hmmmmmmm. (sounds better already)

Cheers,

[Taylorplayer]

i have also been told by some tech buddies that an alternative is to make full use of the standby switch most older amps have and let them warm up slowly.have you heard this as well?

Yes, both on the "warm up" side and the "cool down" mode. Steve Carr says that it's most important to follow that routine to extend tube life as well as giving the unit a proper chance to warm and come up to power evenly.

Hope this Helps!

ALL THE BEST!
Tp

[Wa Diddy]

I’ve just returned from consulting with my 87-year-old electronics resource, Don. He’s a retired electrical engineer who has lived through the tube age. He’s never heard of Leo Fender and refers to my Twin Reverb as ‘that upside down amplifier’. I showed him the schematic and I asked him about the standby switch.

Q: It is claimed that the standby switch affects the rates that tubes warm up and cool down. Is that true?
A: “That’s a lot of baloney. It has no effect on the rates.”

I tested this over several evenings last week. My amp took 14 seconds to produce a sound regardless of whether the standby switch was turned on with the power switch or in ever increasing 2-second intervals after the power switch was turned on.

Q: Does the type of full wave rectifier [tube vs. solid state] affect the quality of the sound?
A: “No. They both convert AC voltage to DC.”

Q: Does the type of rectifier [tube vs. solid state] affect the rate at which tubes warm up?
A: “Yes. A tube rectifier has to warm up which is more beneficial to the life of the capacitors and resistors than to the tubes.”

Q: What is the difference between amplifiers that use point-to-point wiring vs. ones that use a printed circuit board?
A: “In one, the manufacturer didn’t have to build a factory to make printed circuit boards.”

So to summarize; the affect of the standby switch on tube life [in Fender amps] is Nada. But the use of a tube rectifier, by virtue of it’s slow warm up period, prevents a voltage surge to the capacitors, resistors and tubes, thus increasing their lifespan. Steve Carr’s statement may apply to his circuit designs but it is invalid to apply it to Fender circuit designs.

Here is a primer on capacitors, http://www.antiqueradio.org/recap.htm
He also has a neat gallery of stylish tube radios.

And a link to a very cool site - forty kinds of retro grille clothes.
http://www.grillecloth.com/

Cheers,

[~j~]

your friend may be very talented with electronics, but he is very under-informed regarding guitar amplifiers.

The standby switch turns on the heater elements ONLY -- the filament is prepped to perform its duty but is not yet spewing electrons to be collected by the plate. This prevents wear on the tube as a whole. It is true that it does not affect the rates but this is not the importance of using the switch. Also, cathode's filaments react differently once they have reached a steady operating temperature so although the amp is operating the rate and saturation of the electrons being spit out differs after the filament is warmed up.

A solid-state rectifier does not differ from a tube rectifier in the job that is being performed, but for one every rectifier and power filtration circuit in general will behave differently, and for another the tube rectifier circuits have a "Sagging" voltage response that is considered desirable because due to a slower attack, it results in a warmer, smoother overdrive through the power tubes for a) less crossover distortion and as I said slower response, and b) they suffer from saturation artifacts as well which may or may not be desirable. The rectifier is part of the quality of power is supplied to the remainder of the circuit -- it is working in coordination with surrounding resistors/capacitors to filter the AC to DC for delivery to the power tubes. There are a variety of methodologies for integrating different rectifier configurations into amplifier circuits with different qualities that you might prefer depending on the style of music you play.

PTP are clearly easier to maintain and modify than PCB, as well as the possibility of customizing components, and also in PCB construction the machines that perform the algorithms for determining wire paths between circuit components do not consider other effects like the natural impedance of certain areas of the circuit. Impedance will create magnetic fields around components that can affect other areas of the circuit. If you look at a high-quality PTP you can see that they have taken this into account as much as they can in terms of winding the wires to prevent parasitic capacitance from becoming a problem with the particular circuit design.

Originally Posted by Wa Diddy

I’ve just returned from consulting with my 87-year-old electronics resource, Don. He’s a retired electrical engineer who has lived through the tube age. He’s never heard of Leo Fender and refers to my Twin Reverb as ‘that upside down amplifier’. I showed him the schematic and I asked him about the standby switch.

Q: It is claimed that the standby switch affects the rates that tubes warm up and cool down. Is that true?
A: “That’s a lot of baloney. It has no effect on the rates.”

I tested this over several evenings last week. My amp took 14 seconds to produce a sound regardless of whether the standby switch was turned on with the power switch or in ever increasing 2-second intervals after the power switch was turned on.

Q: Does the type of full wave rectifier [tube vs. solid state] affect the quality of the sound?
A: “No. They both convert AC voltage to DC.”

Q: Does the type of rectifier [tube vs. solid state] affect the rate at which tubes warm up?
A: “Yes. A tube rectifier has to warm up which is more beneficial to the life of the capacitors and resistors than to the tubes.”

Q: What is the difference between amplifiers that use point-to-point wiring vs. ones that use a printed circuit board?
A: “In one, the manufacturer didn’t have to build a factory to make printed circuit boards.”

So to summarize; the affect of the standby switch on tube life [in Fender amps] is Nada. But the use of a tube rectifier, by virtue of it’s slow warm up period, prevents a voltage surge to the capacitors, resistors and tubes, thus increasing their lifespan. Steve Carr’s statement may apply to his circuit designs but it is invalid to apply it to Fender circuit designs.

Here is a primer on capacitors, http://www.antiqueradio.org/recap.htm
He also has a neat gallery of stylish tube radios.

And a link to a very cool site - forty kinds of retro grille clothes.
http://www.grillecloth.com/

Cheers,

[Hotraman]

If stand-by switches are useless, then why are they on tube amps?
I always use it the right way.

[Wa Diddy]

J, I’ll print your argument out and bring it to him. I only ever see him on weekends so next Monday I’ll post his response.

Cheers,

[Herb Hunter]

Originally Posted by Hotraman

If stand-by switches are useless, then why are they on tube amps?
I always use it the right way.

They aren't useless they prolong the life of the tubes. Subjecting tubes to frequent temperature changes, as when the band takes a break, shortens tube life. The standby switch allows one to shutdown the amp while keeping the tubes warm.

[Herb Hunter]

Originally Posted by Wa Diddy

I’ve just returned from consulting with my 87-year-old electronics resource, Don. He’s a retired electrical engineer who has lived through the tube age. He’s never heard of Leo Fender and refers to my Twin Reverb as ‘that upside down amplifier’. I showed him the schematic and I asked him about the standby switch.

Q: It is claimed that the standby switch affects the rates that tubes warm up and cool down. Is that true?
A: “That’s a lot of baloney. It has no effect on the rates.”

Your friend was absolutely correct in his response to what you asked. He didn't say the stand by switch had no useful function.

[capnjuan]

The safest way to re-form caps on amplifiers that have been sitting for extended periods is with a Variac; a variable AC transformer; $75 - $150 depending age and condition. It has wall voltage 110vac on one side and a dial on the secondary ranging from 0 to 120 volts. The amp is plugged in to the variac, the variac is plugged into the wall, set to 0 voltage, and both amp and variac are turned on. If the amp has a tube rectifier, the initial setting is 60 volts. The voltage is slowly brought up over several hours to about 110 volts after which, if the caps are still good otherwise, the amp is ready for use.

Tube rectifiers will not conduct below 60 volts and since the capacitors are downstream of the rectifier, starting at 0 accomplishes nothing since the capacitors won't see any DC until the recitifier is conducting. If the amp has tubes but a solid state rectifier, then this process starts at 0 volts and continues over several hours because a solid state rectifier will supply DC voltage as soon as there is any AC voltage on it.

On tube amps, standby switches weren't necessary until the widespread use of solid state / diode rectifiers. With a tube rectifier, it takes nearly 8 - 10 seconds for the tubes' heaters or filaments to get the cathodes hot enough to conduct, including the rectifier. As a result, there is a built-in lag between turn-on and the presence of DC in amp. Without a standby switch, as soon as AC appears on the diode rectifier, DC voltage appears on the plates of the tubes stripping electrons from the cathode, weakening the functionality of the tube, and shortening its useful life.

Standby switches allow the tubes to heat up and delay the appearance of DC in the tubes ... not to mention allowing instrument cables be plugged/unplugged without unpleasant noise. Standby switches are of no value in forming capacitors because, as soon as the switch is set to the 'on' position, the capacitors are exposed to the full DC output of the rectifier, not a gradual ramp up; it is the tube heaters / filaments that are slowly warming up, not the capacitors. John