Interesting valve multivibrator circuit.

[Murphyv310]

Don't be silly Terry. Not yours but the one David posted. I seem to remember this circuit on a home construction set. The way the output stage was biased caused the line transformer to overheat if no sync and no line drive from the oscillator stuck in one state and the set left unattended.


EDIT. I am sure I have the circuit on a disc and a modified oscillator to cure the problem 

[ppppenguin]

It's worth mentioning the general point about LOP stages running off speed. Usually pretty harmless if they're running fast, can be destructive if running slow. Simply on the basis of V = L * di/dt if the output device is on for too long then the current will rise too high.

I had a strange problem related to this on a Prowest 405/625 solid state monitor. This set has separate EHT, with what is effectively a 2nd LOPsystem to supply regulated EHT. The Thorn 2000 used a very similar system. The EHT output transistor (BU104) had blown so I put in a BU208 as it was in the spares box. Worked fine on 625 but drew excess current on 405. Turns out that the BU208 has a very low current gain compared to the BU104 and the base drive was marginal. The higher peak collector current on 405 took it out of saturation at the end of the forward stroke. Not healthy.

[Murphyv310]

Hi.
I have found the disc, it's written for Windows and doesn't run on Linux but I can see the various pictures and some of the text.
What I have gleaned is the oscillator is not a reliable starter without sync and it was modified to a regular Multi-vibrator in this set.
If I can extract the article I'll post it here but please don't hold your breath, computers and myself don't really get on!!!