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allan · 19th Oct 2019, 4:34 pm

I think V12 might have been used once but no longer om. I thought V12 and V13 were originally push pull drivers for the external power amp.
VR1 has an on/off switch so may be a basic volume control.
I think you are right about VR3 and VR4 being a loudness control.
VR4 appears to be able to turn off the input to V8 which presumably feeds in via C26 as that circuit can be the Gram input. I might have misdrawn the RHS of the source switch inputs as the ground shouldn't short the QAVC/Radio input except when Gram is selected.
Loudness is only selected at the source switch for Radio and QAVC.
It looks like VR3/VR4 is a combined Loudness + volume because of the grounding of V8 grid at the min setting of VR4.
VR1 also grounds its valve grid so must be a second volume control.
Presumably you juggle VR3/VR4 and VR1 to get the best fidelity?
VR2 seems to sit in the centre element of a high pass filter which would make sense if it has a value in megohms? A few calculations may show its effect (if V13 were to be re-connected)??
Allan G3PIY

19th Oct 2019, 4:34 pm	#81
allan Pentode Join Date: Oct 2005 Location: South coast near Ringwood/Christchurch, Dorset, UK. Posts: 230	Re: Moreton Cheyney I think V12 might have been used once but no longer om. I thought V12 and V13 were originally push pull drivers for the external power amp. VR1 has an on/off switch so may be a basic volume control. I think you are right about VR3 and VR4 being a loudness control. VR4 appears to be able to turn off the input to V8 which presumably feeds in via C26 as that circuit can be the Gram input. I might have misdrawn the RHS of the source switch inputs as the ground shouldn't short the QAVC/Radio input except when Gram is selected. Loudness is only selected at the source switch for Radio and QAVC. It looks like VR3/VR4 is a combined Loudness + volume because of the grounding of V8 grid at the min setting of VR4. VR1 also grounds its valve grid so must be a second volume control. Presumably you juggle VR3/VR4 and VR1 to get the best fidelity? VR2 seems to sit in the centre element of a high pass filter which would make sense if it has a value in megohms? A few calculations may show its effect (if V13 were to be re-connected)?? Allan G3PIY