HMV car radio with type B amplifier - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

agardiner · 15th Aug 2022, 11:00 am

Fixed. Thank you to everyone who contributed.

Turned out to be man made. A snipped off component lead from the re-capping; in the radio itself, was shorting the tone corrector switch, killing the incoming audio. Clearly I had been careless and not noted that the lead had fallen into the set. I established it thanks to everyone's suggestions; ended up disconnecting C111 from the multiplug and connecting it straight to the grid of V102, proving plenty of gain. This meant the fault had to be around the volume control. Thanks again to all.

Now just got to correct quite heavy distortion and the set will be complete.

Synchrodyne · 18th Aug 2022, 11:27 pm

An observation in respect of the HMV B and D car radio amplifiers is that they appear to have had partial cathode loading, with the output valve cathodes connected to the output transformer secondary. This was the only feedback path, there being no overall feedback loop.

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Name: HMV B&D Amplifier Output Stage.jpg
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There was some discussion of the use of single-ended partial cathode loading in this thread: https://www.vintage-radio.net/forum/...d.php?t=182743.

In a general sense, the HMV circuit looks like the push-pull counterpart to the Ekco C273 single-ended circuit, albeit that it used fixed grid bias rather than cathode bias.

Click image for larger version

Name: Ekco A274, A277, C273 Output Stage.jpg
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The HMV output transformer secondary looks a little unbalanced, with the earthing point off-centre. That might simply be an artefact of the drawing. Or perhaps, given that the speaker load was taken from one side only (or one side and a bit for the higher impedance tapping), some asymmetry was need to equalize the signal voltages on the cathodes?

Cheers,

15th Aug 2022, 11:00 am	#21
agardiner Octode Join Date: Sep 2020 Location: Thetford, Norfolk, UK. Posts: 1,741	Re: HMV car radio with type B amplifier Fixed. Thank you to everyone who contributed. Turned out to be man made. A snipped off component lead from the re-capping; in the radio itself, was shorting the tone corrector switch, killing the incoming audio. Clearly I had been careless and not noted that the lead had fallen into the set. I established it thanks to everyone's suggestions; ended up disconnecting C111 from the multiplug and connecting it straight to the grid of V102, proving plenty of gain. This meant the fault had to be around the volume control. Thanks again to all. Now just got to correct quite heavy distortion and the set will be complete.

18th Aug 2022, 11:27 pm	#22
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: HMV car radio with type B amplifier An observation in respect of the HMV B and D car radio amplifiers is that they appear to have had partial cathode loading, with the output valve cathodes connected to the output transformer secondary. This was the only feedback path, there being no overall feedback loop. There was some discussion of the use of single-ended partial cathode loading in this thread: https://www.vintage-radio.net/forum/...d.php?t=182743. In a general sense, the HMV circuit looks like the push-pull counterpart to the Ekco C273 single-ended circuit, albeit that it used fixed grid bias rather than cathode bias. The HMV output transformer secondary looks a little unbalanced, with the earthing point off-centre. That might simply be an artefact of the drawing. Or perhaps, given that the speaker load was taken from one side only (or one side and a bit for the higher impedance tapping), some asymmetry was need to equalize the signal voltages on the cathodes? Cheers,