Bush TR82 - general transistor bias question - UK Vintage Radio Repair and Restoration Discussion Forum

BenHoper · 26th Oct 2020, 9:21 pm

In the Bush TR82, in the pre-amp stage TR5, the voltage at the collector (output) is marked as 8.3V off a 9V battery.
Ref:
https://www.vintage-radio.net/forum/...3&d=1386851936

Why is that voltage so high? If you wanted a nice large clean voltage swing, surely you would have it closer to 4.5V?

Herald1360 · 26th Oct 2020, 10:40 pm

It's because it's driving a transformer primary rather than an actual resistor. This means that the collector can swing dynamically as far above the 9V rail as it can swing down before it hits the emitter voltage level.

The transformer coupled output of a typical valve set behaves in the same way with the standing anode voltage sitting as near HT+ as the primary resistance voltage drop allows.

BenHoper · 27th Oct 2020, 2:30 pm

HERALD1360 thank you for the explanation, very helpful. I'll do some more reading on that. Thanks you.

26th Oct 2020, 9:21 pm	#1
BenHoper Triode Join Date: Nov 2019 Location: Cambridge, Cambridgeshire, UK. Posts: 11	Bush TR82 - general transistor bias question In the Bush TR82, in the pre-amp stage TR5, the voltage at the collector (output) is marked as 8.3V off a 9V battery. Ref: https://www.vintage-radio.net/forum/...3&d=1386851936 Why is that voltage so high? If you wanted a nice large clean voltage swing, surely you would have it closer to 4.5V?

26th Oct 2020, 10:40 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 15,138	Re: Bush TR82 - general transistor bias question It's because it's driving a transformer primary rather than an actual resistor. This means that the collector can swing dynamically as far above the 9V rail as it can swing down before it hits the emitter voltage level. The transformer coupled output of a typical valve set behaves in the same way with the standing anode voltage sitting as near HT+ as the primary resistance voltage drop allows. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

27th Oct 2020, 2:30 pm	#3
BenHoper Triode Join Date: Nov 2019 Location: Cambridge, Cambridgeshire, UK. Posts: 11	Re: Bush TR82 - general transistor bias question HERALD1360 thank you for the explanation, very helpful. I'll do some more reading on that. Thanks you.