Standby Switch squeal/bark sound on Marshall or Fender

[soundmasterg]

I had this same problem on a build I did that was a clone of a Vox AC100 with a higher gain channel added. It would squeal when put into standby but otherwise didn't squeal if power switch was flipped with standby in play mode, or if power was unplugged from wall with the same. I added a cap to ground after the switch and before anything else...I think it was a 0.1uF but I forget at the moment. It could be a bad standby switch I suppose, tho if I remember right I tried another switch. its been several years and I haven't been able to get back to that amp yet.

Greg

[pdf64]

I suggest tacking a small eg 0.47 - 10uF cap across each B+ node in turn, and noting how it affects the issue; a key node being immediately after the standby switch, before the choke.

[davman]

Sorry, I missed the model ref in post #7; does the standby interrupt the dc side, between reservoir and choke, as per the 60s arrangement, or the ac side, between winding and FWB, as per the 70s onward method?
http://bmamps.com/Schematics/marshal..._100w_1959.pdf
http://bmamps.com/Schematics/marshal..._year_1988.gif
Or maybe some other arrangement, eg between rectifier and reservoir?
Whichever, the HT seems no place for a switch, use of such a standby is pointless at best and often likely disadvantageous to reliable operation.
But at least it's not the worst standby implantation Marshall came up with http://bmamps.com/Schematics/marshal...d_50w_1987.pdf

There seems to be several variations of the Marshall Standby circuits.

The best-designed one I saw was on the 1959 schematic, where there were two 0.22uF caps going from the HT secondary ends to 0v.
If you don't have these caps on the HT winding (before the s/by switch), then as the switch is opened, the secondary HT winding acts as an ignition coil - and tries to keep the current flowing by creating a spark across the switch contacts.
This current depends on where in the mains cycle the s/by switch is turned off, which may account for the different squeals you are getting. The fact that the preamp volume setting affects the squeal may be that the spark is producing RF which is picked up by the preamp wiring.

On the Randy Rhoads version, there are no such capacitors before or immmediately after the switch, hence a loud bang can occur as s/by is turned off!

It seems like your fault is related to the circuitry around the s/by switch. Would need to see your particular model's schematic, but if there are suppressor caps fitted, it may be that one/both may be faulty.

A quick remedy would be to wire in a capacitor from each HT winding end to 0v - (use 0.22uF, 250v ac, X2 grade) for reliability. It may well solve the problem & increase the life of the switch!

PS. I always thought Standby was there to increase output valve life, esp at switchon-from-cold

[soundmasterg]

PS. I always thought Standby was there to increase output valve life, esp at switchon-from-cold

That would only be the case if receiving tubes needed to be warmed up fully before the high voltage B+ was applied, but that isn't the case. It says right in the RCA receiving tubes manual that cathode stripping is not a concern with receiving tubes, which are the kinds used in consumer electronics like guitar amps. When you get into transmitting tubes with their upwards of 1500 volts on the plates, then cathode stripping starts to become an issue, but with 450 volts or even 700 volts as in an Ampeg SVt, it isn't an issue. Standby is a useful thing to have on stage but leaving your amp powered on while in standby can result in cathode poisoning if you leave it in that condition for a long time. Standby is really a muting feature and there are other ways to accomplish it besides cutting off the B+ as is usually done. A standby switch is useful for the person working on the amp also. Standby isn't necessary at all.

[davman]

Thanks, I am enlightened!
This link gives a bit of history The Valve Wizard

[Justin Thomas]

I would think an alternative standby would be useful during testing. There have been occasions when I need to check high voltage, but the resulting noise coming out of the speaker scares the $#!+ out of me!

Justin

[olddawg]

I've had this problem in old Marshalls. It was the switches. Over time time they carbonize and wear. As the contacts separate there is some resistance and conduction for a longer period of time than there would be with a new switch. Then weird things happen in the amp during that time with the power supply. I never saw it as a concern.

[davman]

I should have stated that the 'quick remedy' proposed in my page1, #33 msg completely solved the problem on a Marshall 1959RR (Rhoads) version, which recently arrived for repair (at The Sound Garden, UK).

It eliminated the loud bang when the s/by switch was turned off....... all that could be heard was a barely perceptible 'click'.

On another forum, someone noted that the loud bang disappeared when an external Mains Power Conditioner/filter was used.
I suspect this is because the filter capacitor, introduced across the mains transformer primary is 'referred' to the secondary by transformer action - producing an effective capacitor load across all secondary windings.
This provides a simple 'non-techie' solution, but assumes the filter's output capacitor has a large enough capacitance to do the job, approx 0.1uF is needed. Maybe this could be measured before purchase!