R209 audio amplifier - how is it supposed to work? - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 10th Dec 2018, 5:32 pm

I'm doing a bit of repair to my old R209 (a few resistors gone high, leaky decoupling capacitors).

One thing I've always been confused by is the audio amplifier.

it uses two different valves - a 1T4 and a 1S5 - in push-pull. See attachment.

Audio input is via C81 to the grid of V9 (1T4) - the grid of the 1S5 (V10) gets fed seemingly from the screen-grid of V9 by way of C80

Apart from the oddity of using two dissimilar valves (one has a Gm of 0.72 the other 0.9!) how is this supposed to work? Is the screen of V9 somehow generating an antiphase voltage of the 'right' value to allow V10 to make a contribution?

It's not like any other P-P amplifier design I can remember!

ms660 · 10th Dec 2018, 5:44 pm

RDH4, book page 585:

http://www.tubebooks.org/books/rdh4.pdf

Lawrence.

ukcol · 10th Dec 2018, 5:48 pm

I think you have it right. The "triode" formed by V9 with the screen grid playing the anode roll provides the phase inversion for V10. Hardly Hi-Fi but that is not required.

Maybe the lower gm valve is required to prevent the circuit going into oscillation? That I admit though is a totally wild guess.

G6Tanuki · 10th Dec 2018, 5:50 pm

Thanks for the info: it may "work" but it seems an utterly crazy bit of design! I wonder what the guys at Murphy had been adding to thir tea the day they came up with it?

turretslug · 10th Dec 2018, 5:56 pm

That's worthy of a "freaky Friday" at Philips R + D.....

To be fair, it probably represents an ingenious way of making an efficient, minimum-power and minimum bottle-count AF amp, with one also acting as phase-inverter via its screen-grid and the lower gm of the "slave" valve of the push-pull pair compensating for the slight gain of the first valve's control-grid-to-screen-grid circuit.

I'm quite liking it now....

ms660 · 10th Dec 2018, 6:12 pm

Here's one from another thread that takes the feed from the other valve:

https://vintage-radio.net/forum/showthread.php?t=139740

Lawrence.

G6Tanuki · 10th Dec 2018, 6:36 pm

The ingenuity of designers striving for adequate-performance-with-minimal-component-count never fails to astound me.

[I recall a similar circuit using a P-P pair of 6V6, grid drive to one of them, the other grid earthed, and a shared unbypassed cathode-resistor to provide the coupling, so one half is grid-driven common-cathode in the traditional way and the other half is cathode-driven grounded-grid!]

TonyDuell · 10th Dec 2018, 6:42 pm

Quote:

Originally Posted by G6Tanuki

[I recall a similar circuit using a P-P pair of 6V6, grid drive to one of them, the other grid earthed, and a shared unbypassed cathode-resistor to provide the coupling, so one half is grid-driven common-cathode in the traditional way and the other half is cathode-driven grounded-grid!]

That sounds a bit like the well-known 'long tailed pair' differential amplifier in concept.

Ambientnoise · 10th Dec 2018, 6:55 pm

Isn’t it a cascode circuit ?

G6Tanuki · 10th Dec 2018, 6:55 pm

Yes I guess you could call it a 'power output' long-tailed-pair load-driver!

Jon_G4MDC · 10th Dec 2018, 10:11 pm

Haha, as my brother in law is fond of saying "do we like this?"
It gathers opinions for sure..

10th Dec 2018, 5:32 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	R209 audio amplifier - how is it supposed to work? I'm doing a bit of repair to my old R209 (a few resistors gone high, leaky decoupling capacitors). One thing I've always been confused by is the audio amplifier. it uses two different valves - a 1T4 and a 1S5 - in push-pull. See attachment. Audio input is via C81 to the grid of V9 (1T4) - the grid of the 1S5 (V10) gets fed seemingly from the screen-grid of V9 by way of C80 Apart from the oddity of using two dissimilar valves (one has a Gm of 0.72 the other 0.9!) how is this supposed to work? Is the screen of V9 somehow generating an antiphase voltage of the 'right' value to allow V10 to make a contribution? It's not like any other P-P amplifier design I can remember! Attached Thumbnails

10th Dec 2018, 5:44 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: R209 audio amplifier - how is it supposed to work? RDH4, book page 585: http://www.tubebooks.org/books/rdh4.pdf Lawrence.

10th Dec 2018, 5:48 pm	#3
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: R209 audio amplifier - how is it supposed to work? I think you have it right. The "triode" formed by V9 with the screen grid playing the anode roll provides the phase inversion for V10. Hardly Hi-Fi but that is not required. Maybe the lower gm valve is required to prevent the circuit going into oscillation? That I admit though is a totally wild guess.

10th Dec 2018, 5:50 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: R209 audio amplifier - how is it supposed to work? Thanks for the info: it may "work" but it seems an utterly crazy bit of design! I wonder what the guys at Murphy had been adding to thir tea the day they came up with it?

10th Dec 2018, 5:56 pm	#5
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,396	Re: R209 audio amplifier - how is it supposed to work? That's worthy of a "freaky Friday" at Philips R + D..... To be fair, it probably represents an ingenious way of making an efficient, minimum-power and minimum bottle-count AF amp, with one also acting as phase-inverter via its screen-grid and the lower gm of the "slave" valve of the push-pull pair compensating for the slight gain of the first valve's control-grid-to-screen-grid circuit. I'm quite liking it now....

10th Dec 2018, 6:12 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: R209 audio amplifier - how is it supposed to work? Here's one from another thread that takes the feed from the other valve: https://vintage-radio.net/forum/showthread.php?t=139740 Lawrence.

10th Dec 2018, 6:36 pm	#7
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: R209 audio amplifier - how is it supposed to work? The ingenuity of designers striving for adequate-performance-with-minimal-component-count never fails to astound me. [I recall a similar circuit using a P-P pair of 6V6, grid drive to one of them, the other grid earthed, and a shared unbypassed cathode-resistor to provide the coupling, so one half is grid-driven common-cathode in the traditional way and the other half is cathode-driven grounded-grid!]

10th Dec 2018, 6:55 pm	#9
Ambientnoise Heptode Join Date: Feb 2006 Location: Marlborough, Wiltshire, UK. Posts: 915	Re: R209 audio amplifier - how is it supposed to work? Isn’t it a cascode circuit ?

10th Dec 2018, 10:11 pm	#11
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: R209 audio amplifier - how is it supposed to work? Haha, as my brother in law is fond of saying "do we like this?" It gathers opinions for sure..