squealing derailment amp question

[9 volts]

Hey there, just wondering if any one has any advice.
I've but a little two watt derailment amp. Works but squeals at some settings (eg vulome up) then squeal can be altered with guitar volume and treble control. Touching the chasis reduces the squeal so I figured it was a grounding problem (read Geo debug amp). Moved grounds around, tried star grounding, both input and output jacks are not connected to the chasis, tested with different tubes, no change. Grounding the input silences the squeal. It sounds like an oscillation as it grows when amp is turned on. I'm a little stumped. Thanks

[brett]

Hi
do you have a grid stop resistor (47k to 100k on the grid pin)?
These can stop RF and related audio oscillations.
cheers

[9 volts]

Thanks Brett,I found the solution, not sure why but the when the output speaker socket (common) is attached to the chassis the whine/oscillations disappear. I thought I was supposed to isolate this socket from the chassis? Nice sounding amp, in this situation I figure it was caused by internal oscillation rather than RF (sounded like it though) Thanks again

[R.G.]

Isolate from chassis, but connect it to ground with a wire. Also connect chassis to ground with a different wire. The point is to force the speaker currents not to flow in the chassis.

[9 volts]

Thanks RG, I looked at your star grounding pdf for guidance. One question - Is it a bad idea to connect the speaker ground the the point where the mains is grounded eg where base of the power transformer is bolted to chassis? Wouldn't this move the speaker currents directly out of the chasis. At present I've grounded the speaker via the base of the output transformer.
Also in relation to Bretts reply- I'm using a 1 meg grid stop- is there a difference between placing this before or after the resistor that leads from the input jack to the grid?
Much appreciate the replies. And once again thanks RG for GEO- always the first port of call.

[9 volts]

I found some info at 'aikens' website in relation to grid stoppers which answers my question.
'In order to take advantage of the parasitic suppression benefits of these grid resistors, they must be placed as close as possible to the socket pin of the tube, preferably soldered directly to the pin with a very short lead.  The resistor should be placed after the grid-to-ground resistor (usually 1 Meg or so), to avoid attenuation and to keep the signal path short. If the resistor is connected in series with the input jack and before the 1 Meg grid resistor, there is a small loss of the input signal, although, in most cases the attenuation is not enough to be concerned with (0.94 times for a 68K grid stopper and a 1 Meg grid resistor), and in amplifiers with a high and low level input, the grid stoppers also serve as attenuators.  When designing an amplifier, it is better to use separate resistors for input attenuation purposes in order to be able to locate the grid stoppers as close to the input grid pins as possible, rather than mounting them on the input jacks.'

Learning curve- This answers my question, same would apply to effects I suppose.