Deluxe reverb woes

[Chippo]

Hey guys. I've been reading posts on this site for a long time but have only just joined. Thanks so much for all the help you guys give. I really hope someone can help me with an issue before it drives me crazy

I have a silverface deluxe reverb that I've loved and worked on for a couple of years. I've made a few changes to blackface it over time but nothing too drastic. For a while now i have had some trouble when I first turn the amp on. When I flick the standby switch it oscillates as if the tremolo is active. I originally thought that it was yhe tremolo cutting in and out when I didn't want it to but later discovered that it was the B+ line voltage fluctuating rapidly and creating a random tremolo like effect. From memory this started happening soon after I changed one of the dropping resistors in the filter section to bring the preamp voltages down to match those on the schematic.

The amp only does this for about 5 seconds usually before it kicks in and it usually doesn't cause me any trouble once it's going. My biggest fear is that when I really need it it won't kick in and continue to freak out. Does anyone have any ideas? I've checked so much stuff but every time I think I've fixed it it comes back a few weeks later. It's driving me nuts. I really just want to not worry about my amp.

Any help would be extremely appreciated. Thanks so much guys

[Stevem]

If you have the preamp voltages more in line with what they should be then that's not the issue, that being said if you have not replaced all the power supply filters by now including the one in the bias circuit then the amps preamp stages and circuit may not not be able to uncouple from each other as they should, and this would include all the cathode bypass filters in all the preamp stages!
This is the first thing I would do, and while you are at it step up that bias circuit filter to a 80 to 100 uf rating and kick its voltage rating up by double that factory number.
Also while you are at it the tremolo opto coupler may be going south on you, so spending money on replacing that will be money well spent !

[pdf64]

Don't use standby!
It's bad.
Like the human appendix, it lacks useful function and causes problems.

Have you replaced all electrolytics (B+, C-, cathode bypass)?

[vibratoking]

Don't use standby!
It's bad.
Like the human appendix, it lacks useful function and causes problems.

Have you replaced all electrolytics (B+, C-, cathode bypass)?

Yea, well there are plenty of properly working amps with standby. Whether it serves it's intended purpose is questionable. It depends on what you think the purpose is.

[gui_tarzan]

What kind of problems could it possibly cause?

[Stevem]

It does not make for ANY problems but they came to be to stop the potential problem of cathode stripping and they are very handy to drain the power supply before working on a amp!
The only issue they have is when they go bad and start to make noise, and if it happens at a gig and you can not jump the switch your done! Don't ask me how I know that! Lol.

[JazzGuitarGimp]

Cathode stripping is a transmitting tube affliction. We use receiving tubes in guitar amps. Cathode stripping does not occur in receiving tubes. The only reason to use a standby switch on a guitar amp is if you want to mute the amp for a time without having to wait all of 11-seconds for it to warm up again when you're ready to play.

[pdf64]

What kind of problems could it possibly cause?

Most common standby implementations result in heavy surge currents when it's flipped to operate mode.
The stress of that is wearing for B+ fusing, rectifiers and reservoir caps.
If a tube rectifier, then hot switching of a capacitive load is probably especially taxing, as it may push / exceed the peak plate current limiting value.
Switches used for standby are generally not rated for such high voltage use.
The lack of load on the PT may result in the heater voltage rising, possibly above the limiting value of the tubes (PT regulation being ~5 to 10%, tube heater spec may only be 6.3 +5% or 10%.

A very problematic way to mute the amp, which is probably it's most common use.
That, and satisfying customer requirements for an amp switch on ritual.

[Stevem]

All true, my two preferred switch set up's is to just short across the output tube grids in a PP amp, or switch in a large 25K resistance on each output tube cathode!

[gui_tarzan]

I don't get why the heater voltage would rise, that has its own winding and is hot when the power switch is on. And if you look at the typical switch rating, it says something like "125v, 3A" and "250v, 1.5A" so putting 450v+ at a couple hundred mA shouldn't put any real stress on it.

[pdf64]

if you look at the typical switch rating, it says something like "125v, 3A" and "250v, 1.5A" so putting 450v+ at a couple hundred mA shouldn't put any real stress on it

I don't understand why you think that?
As voltage increases, so does the propensity for an arc to form.
Edit - Also I think that an arc is more likely to form with dc than ac, as for ac the current reduces to zero twice per cycle.
So generally switch ratings for dc are lower than for ac, perhaps significantly so, eg in the case of a slow switching action type.

[Mark]

Wiki has this to say about arcs forming:

The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm.[3] This is only as a rough guide, since the actual breakdown voltage is highly dependent upon the electrode shape and size. Strong electric fields (from high voltages applied to small or pointed conductors) often produce violet-colored corona discharges in air, as well as visible sparks. Voltages below about 500–700 volts cannot produce easily visible sparks or glows in air at atmospheric pressure, so by this rule these voltages are "low". However, under conditions of low atmospheric pressure (such as in high-altitude aircraft), or in an environment of noble gas such as argon or neon, sparks appear at much lower voltages. 500 to 700 volts is not a fixed minimum for producing spark breakdown, but it is a rule-of-thumb. For air at STP, the minimum sparkover voltage is around 327 volts, as noted by Friedrich Paschen.[4]

While lower voltages do not, in general, jump a gap that is present before the voltage is applied, interrupting an existing current flow often produces a low-voltage spark or arc. As the contacts are separated, a few small points of contact become the last to separate. The current becomes constricted to these small hot spots, causing them to become incandescent, so that they emit electrons (through thermionic emission). Even a small 9 V battery can spark noticeably by this mechanism in a darkened room. The ionized air and metal vapour (from the contacts) form plasma, which temporarily bridges the widening gap. If the power supply and load allow sufficient current to flow, a self-sustaining arc may form. Once formed, an arc may be extended to a significant length before breaking the circuit. Attempting to open an inductive circuit often forms an arc, since the inductance provides a high-voltage pulse whenever the current is interrupted. AC systems make sustained arcing somewhat less likely, since the current returns to zero twice per cycle. The arc is extinguished every time the current goes through a zero crossing, and must reignite during the next half-cycle to maintain the arc.

I'm inclined to think arcing will depend on where the standby switch is in the power supply circuit. I'd be keeping it away from the choke, but Ken Fischer didn't. If it preceded the filter caps with silicon rectifiers, it would have to face the same issues as the on-off switch.

How did you go with that oscillating fault, is does sound like motor boating and thus replacing the filter caps would rectify that fault.

[Stevem]

The appendix is not us less, as its been found to be the storage location for the healthy bacterium in our immune system that we all need, of which 70 % of the Human immune system is in our gut!!!

[pdf64]

I don't get why the heater voltage would rise, that has its own winding and is hot when the power switch is on

Agreed, the copper loss aspect for the heater winding should be the same whether the B+ winding is loaded or not.
But the transformer's iron loss will probably increase as the total secondary loading increases, and the effect of this will probably be the same on each of the secondary windings.

[Structo]

Most switches are not rated for DC current.

As mentioned, DC tends to arc more easily than AC.

But, there are plenty of vintage amps with standby that still work properly.
So maybe it isn't that great of a concern.

It also depends where the standby switch is located in the circuit.
I like to put it after the first filter cap since those are usually the biggest caps.
So when you finally flip the standby off, the surge to charge the remaining caps is reduced.

What I do is turn on the power switch, let it warm up a few minutes and take it off standby.

When I power down, I just turn the power switch off thereby eliminating one power cycle on the standby switch.