01-07-2017, 11:51 AM
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.
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.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
