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811a vttc arcing issues

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Herr Zapp

Fri Jun 05 2009, 07:36PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Harry -

I suspect that you are correct. I think that simply monitoring the B+ with a HV probe & O'scope will confirm the bucking.

In all my years in electronics I have never heard the term "squegging". Based on the paucity of entries on any of the on-line dictionaries, this must be a uniquely "UK" term, with not very widespread usage.

Regardless, what a wonderfully onomatopoetic word!!

Regards,
Herr Zapp

Proud Mary

Fri Jun 05 2009, 08:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Herr Zapp wrote ...

In all my years in electronics I have never heard the term "squegging". Based on the paucity of entries on any of the on-line dictionaries, this must be a uniquely "UK" term, with not very widespread usage.

Simulation of a squegging oscillator
Richard F. Stollberg

Department of Electrical Engineering and Computer Science University of Wisconsin Milwaukee, Wisconsin 53201

George R. Steber

Department of Electrical Engineering and Computer Science University of Wisconsin Milwaukee, Wisconsin 53201

This paper presents an analog-computer simulation of a circuit familiar to many radio and electronics engineers â€” the squegging oscillator. A relatively simple model of a nonlinear transistor is shown to be sufficient to investigate stability conditions associated with the oscillator and to accurately duplicate the phenomenon of squegging. The success of the transistor model in this application also suggests its use in other simulations involving transistors.

SIMULATION, Vol. 21, No. 3, 77-80 (1973)
DOI: 10.1177/003754977302100304

Use Current-Mirror Biasing To Avoid Squegging In RF Oscillators

Electronics Design June 7 2007

Zappers, when you hear squegging in an audio amplifier, you'll understand how wonderfully onomatopoeic it can be!

But of course when it happens at RF, we can only know of it through our instruments.

I guess it is not that different to a kind of erratic relaxation oscillation - hiccupy* saw tooth patterns of wandering RF - like the output of a unijunction with a broken wing.

* Technical term related to human digestion

Steve Conner

Sat Jun 06 2009, 01:34PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Harry isn't crazy (well with respect to squegging anyway

) - it's a well known effect in analog electronics. Basically it refers to a parasitic relaxation oscillator, made from non-linearities and RC time constants that you failed to take into account.

That's as opposed to ordinary parasitic oscillations of the LC tank variety, in stray inductances and capacitances that you forgot to take into account.

plazmatron

Sat Jun 06 2009, 06:13PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Harry is a veritable mine of information as always! Unfortunately, I`m currently short of a `scope, so I`m working blind on this thing, but it is performing very well, giving 8 inches of arc.

Les

Proud Mary

Sat Jun 06 2009, 07:34PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

plazmatron wrote ...

I`m currently short of a `scope, so I`m working blind on this thing,

Leslie, I've only got a "twenty quid for 20MHz" twenty-year old relic - a two channel Gould 'scope that I bought on ebay for about Â£20 about five years ago.

But as I don't do many experiments at much more than 2MHz, there's no need for anything faster - and even a distorted picture is often much better than nothing at all if I wanted to look at 4Mhz for example.

The only thing I regret is not being able to take snapshots of individual pulses - as in a storage oscilloscope - but much more than twenty quid is needed for that!

If you had a "twenty quid for 20MHz" you'd be able to see exactly what is going on with your oscillator - or take a photo of the trace and put it up here for collective interpretation.

It's not a big investment.

Proud Mary

Sat Jun 06 2009, 07:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Harry isn't crazy (well with respect to squegging anyway

)

Is it Method in the Madness, or Madness in the Method?
I'm sure I'm not alone in that here!

Steve McConner wrote ...
Basically it refers to a parasitic relaxation oscillator, made from non-linearities and RC time constants that you failed to take into account.

On a more serious note re: Leslie's problem's, it will be seen that if feedback is increased beyond a certain point, then the operation of the valve will become increasingly non-linear, which, coupled with poor HT regulation would alone be able to account for Leslie's symptoms.

But without a scope, and knowledge of the operating parameters in the fault condition - especially Vg and Ig - anything I say can only be well-meaning conjecture.

If it works by reducing feedback, then it works!

Steve Conner

Mon Jun 08 2009, 09:36AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As long as there's more method in your madness than there is madness in your method, it's all good.

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