BBC valve amp feedback query! - UK Vintage Radio Repair and Restoration Discussion Forum

John_BS · 23rd May 2015, 5:30 pm

I was browsing through some old BBC Technical Instructions and happened upon this amplifier (coded D/11). There were two preceding variants with what I would describe as "conventional" feedback around V2, using a tertiary winding on the output transformer. In the D/11 the text kind of implies that the feedback is negative, whereas I'm convinced it's actually positive??

With a 600 ohm load, the impedance referred back to the transformer primary is about 18k, which agrees with the text, which states that the anode load comprises equal contributions from R17 and the transformer.

So, if we perturbate the loop by, say, increasing the anode current by 1mA, 0.5mA will flow through R16, taking the cathode more negative, hence reinforcing the positive increase in anode current. Or am I missing something?

John

G8HQP Dave · 24th May 2015, 3:05 pm

My guess is that the heavy negative feedback which R16 would give on its own is reduced by positive feedback via the transformer primary. The net effect may still be negative feedback.

The circuit description describes how the circuit works (not very clearly!) but the part you showed does not say why they needed this peculiar arrangement. My guess is that they were trying to achieve a particular output impedance, by using a mixture of voltage and current sensing.

John_BS · 24th May 2015, 4:47 pm

This is the missing (first part) circuit description which implies that they were trying to adjust the output impedance of the valve. I find it misleading when it is implied that the "voltage" feedback is in the same sense as the "current".

kalee20 · 24th May 2015, 11:08 pm

You can call it what you like, it doesn't affect how it works - and trying to categorise it had my head going round last night!

Suppose the transformer is connected between the anode (as it currently is), and 0V instead than the cathode resistor tap (as shown).

The cathode resistors will give heavy negative current feedback, granted. But put a positive-going input on the grid. The anode voltage will fall slightly. This will cause a current to flow in the transformer primary, returning to 0V.

Now disconnect the transformer return connection from 0V, and reconnect to the cathode resistor tap. You'll see that the direction of this current is to drive the cathode voltage slightly negatively-going, which reinforces (not opposes) the positively-going grid signal. It won't be enough to overcome the heavy negative feedback already existing, but it will cancel it to a small extent.

So yes, you could say there's a measure of positive feedback at work.

G8HQP Dave is probably right in saying that the designers were trying to get a defined output impedance - and combinations of current feedback and voltage feedback can achieve this. But the fact that they still put padding resistors on the transformer secondary tells me they didn't quite achieve it!

24th May 2015, 3:05 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: BBC valve amp feedback query! My guess is that the heavy negative feedback which R16 would give on its own is reduced by positive feedback via the transformer primary. The net effect may still be negative feedback. The circuit description describes how the circuit works (not very clearly!) but the part you showed does not say why they needed this peculiar arrangement. My guess is that they were trying to achieve a particular output impedance, by using a mixture of voltage and current sensing.

24th May 2015, 11:08 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: BBC valve amp feedback query! You can call it what you like, it doesn't affect how it works - and trying to categorise it had my head going round last night! Suppose the transformer is connected between the anode (as it currently is), and 0V instead than the cathode resistor tap (as shown). The cathode resistors will give heavy negative current feedback, granted. But put a positive-going input on the grid. The anode voltage will fall slightly. This will cause a current to flow in the transformer primary, returning to 0V. Now disconnect the transformer return connection from 0V, and reconnect to the cathode resistor tap. You'll see that the direction of this current is to drive the cathode voltage slightly negatively-going, which reinforces (not opposes) the positively-going grid signal. It won't be enough to overcome the heavy negative feedback already existing, but it will cancel it to a small extent. So yes, you could say there's a measure of positive feedback at work. G8HQP Dave is probably right in saying that the designers were trying to get a defined output impedance - and combinations of current feedback and voltage feedback can achieve this. But the fact that they still put padding resistors on the transformer secondary tells me they didn't quite achieve it!