Circuit Improvement (Tone Correction). - UK Vintage Radio Repair and Restoration Discussion Forum

JHGibson · 22nd Dec 2013, 4:45 am

In many valve radios there is a tone correction capacitor connected from the plate of the output pentode to chassis earth.

This is a highly stressed component. It not only has to withstand the plate voltage but added to that are the positive excursions of the audio output which can add one or two hundred volts at several watts of audio.

If this high voltage should cause the capacitor to fail, the output transformer could burn out and the rectifier become damaged.

This dire situation can be entirely avoided by rewiring the tone correcting cap directly across the output transformer primary where it will continue to act as a tone corrector.
John.

Peter.N. · 22nd Dec 2013, 9:54 am

True but I think it may go to earth for purposes of stability, to prevent parasitic oscillations in the output stage.

Peter

lesmw0sec · 22nd Dec 2013, 11:47 am

Hmm. Any output stage which requires a whacking great capacitor from anode to deck for stability purposes is a pretty poor design. As to tone "correction", a misnomer for excessive top cut, this could be achieved by placing a cap from the control grid to ground, although that would not disguise any effects of el-cheapo output transformers or over-stretched output stages!

Leon Crampin · 22nd Dec 2013, 6:26 pm

The tone correction capacitor as wired in the conventional position after the output valve provides some correction for the inductive load of the loudspeaker. It should (and sometimes did) also incorporate a series resistor when it becomes a Zobel network.

From a signal point of view, it's immaterial whether the capacitor is across the transformer primary or the valve - but I would always wire the capacitor across the transformer primary. This protects the transformer in the event of capacitor failure and gives a lower quiescent DC voltage across the capacitor. The peak voltage across the capacitor can be +/- 2 x HT voltage - so a suitably rated high voltage replacement should be fitted.

Leon.

G6Tanuki · 22nd Dec 2013, 6:39 pm

Quote:

Originally Posted by lesmw0sec

Hmm. Any output stage which requires a whacking great capacitor from anode to deck for stability purposes is a pretty poor design.

The issue I've experienced is with incompletely-decoupled IF signals leaking into the audio-chain. The traditional double-diode-triode detector/AGC/1st-audio valve is a favourite suspect here - it, and the following power-output stage, both have significant gain at IF and will amplify any IF-signals (and their harmonics) that come their way. The result can be quite a few volts of IF signal at the output-valve's anode. If this gets back on to the HT rail it can cause problems.

One of my standard tests of a rebuilt medium-wave radio is to disconnect the aerial, short the AGC line to earth (so the IF chain is running at full gain) and turn the volume up to max then tune around 2x the IF - 900KHz - 1MHz - If you get any hint of instability - whistles, screeches, swishes, changes in the pitch of the background 'mush' or motor-boating - you have decoupling issues which need to be fixed!

The anode-to-earth capacitor on the O/P stage is one device to control this feedback - often the problem can be further tamed by wiring a low-value resistor (10-50K) in series with the audio feed to the O/P valve's control-grid, with maybe a low-value (50pF)capacitor between said grid and cathode.

22nd Dec 2013, 4:45 am	#1
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Circuit Improvement (Tone Correction). In many valve radios there is a tone correction capacitor connected from the plate of the output pentode to chassis earth. This is a highly stressed component. It not only has to withstand the plate voltage but added to that are the positive excursions of the audio output which can add one or two hundred volts at several watts of audio. If this high voltage should cause the capacitor to fail, the output transformer could burn out and the rectifier become damaged. This dire situation can be entirely avoided by rewiring the tone correcting cap directly across the output transformer primary where it will continue to act as a tone corrector. John.

22nd Dec 2013, 9:54 am	#2
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Circuit Imrovement True but I think it may go to earth for purposes of stability, to prevent parasitic oscillations in the output stage. Peter

22nd Dec 2013, 11:47 am	#3
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,507	Re: Circuit Improvement (Tone Correction). Hmm. Any output stage which requires a whacking great capacitor from anode to deck for stability purposes is a pretty poor design. As to tone "correction", a misnomer for excessive top cut, this could be achieved by placing a cap from the control grid to ground, although that would not disguise any effects of el-cheapo output transformers or over-stretched output stages!

22nd Dec 2013, 6:26 pm	#4
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,872	Re: Circuit Improvement (Tone Correction). The tone correction capacitor as wired in the conventional position after the output valve provides some correction for the inductive load of the loudspeaker. It should (and sometimes did) also incorporate a series resistor when it becomes a Zobel network. From a signal point of view, it's immaterial whether the capacitor is across the transformer primary or the valve - but I would always wire the capacitor across the transformer primary. This protects the transformer in the event of capacitor failure and gives a lower quiescent DC voltage across the capacitor. The peak voltage across the capacitor can be +/- 2 x HT voltage - so a suitably rated high voltage replacement should be fitted. Leon.