HRO AVC operation - UK Vintage Radio Repair and Restoration Discussion Forum

G4_Pete · 11th Feb 2014, 10:40 pm

Hi First post on the board. I could not find any info on the following so apologies for starting a new thread.

I have an HRO 5 to restore and another MX that works fine, using the MX that works I am trying to understand the reason the designer used the 2nd diode in V7 6B7 for the OFF AVC. I understand Rectifying the IF and generating the negative bias in the AVC ON position but I have monitored all the waveforms and voltages round this valve and unless someting is wrong I really cant see what the 2nd diode on Pin 5 does for the AVC OFF selection.

Can someone enlighten me before I tear my hair out!

Pete

Stockden · 12th Feb 2014, 8:43 am

Hi Pete,

According to the EMER (obtainable from the VMARS web site) the second diode is maintaining the mean grid bias voltage of the controlled valves "at about the same potential as that existing under no signal conditions with AVC on".

Hugh

G4_Pete · 12th Feb 2014, 12:21 pm

Thanks.

the ref manual I have is TM11-885 which redraws the AVC in detail and explains the operation but whilst my HRO MX AVC appears to work correctly as described I NEVER see any potential across the diode connected to the OFF side of the AVC, regardless of the gain, switch combinations I try. All the components appear to check out ok but the diode stays at cathode potential strapped by R14.

I have attached the diagram but I clearly must have missed something here as in my view the diode on the "OFF" circuit can only do anything if the end of R-14 marked " - " rises positive to the cathode, that would put the AVC controlled grids positive to V7 cathode.

However I measure this or draw it out it does not add up so I am clearly missing something here but I just cant figure it out. Can someone enlighten me please.

Herald1360 · 12th Feb 2014, 2:27 pm

I think the words "contact potential" are the significant ones here. The anode and cathode of the diode are of dissimilar metals so a contact potential (as in a thermocouple) exists between them. They are "connected" together by the fewish high energy electrons that escape from the cathode regardless of the lack of any positive voltage on the anode. The effect is that if the "load" between the anode and cathode is sufficiently high impedance, the anode will sit slightly negative wrt the cathode.

In the HRO, if both diodes are similar, with similar loads, they will both exhibit this effect to the same degree, but the AVC off diode will not be affected by signals. It's an elegant way to make the agc-off bias the same as the no signal agc-on bias which is itself not the same as if the diode weren't there.

(I think).

G4_Pete · 12th Feb 2014, 7:50 pm

Chris,
Thank you, following your comments I set up a 6SQ7 heater only and measured the signal diodes wrt to the cathode. Sure enough they drop negative to 0.6 Volts. Interestingly if you physically disconect the heater, that is hot valve with a 10 Meg DVM between the cathode and the signal diode anode only, you see the 0.6V drop off slowly. The hot valve is generating a voltage on its own, stand alone. The HRO manual AVC explanation now makes sense as I was certain it would eventually, as it was compiled by better valve brains than mine.

Thanks again, Pete.

11th Feb 2014, 10:40 pm	#1
G4_Pete Hexode Join Date: Feb 2014 Location: Southampton, Hampshire, UK. Posts: 419	HRO AVC operation Hi First post on the board. I could not find any info on the following so apologies for starting a new thread. I have an HRO 5 to restore and another MX that works fine, using the MX that works I am trying to understand the reason the designer used the 2nd diode in V7 6B7 for the OFF AVC. I understand Rectifying the IF and generating the negative bias in the AVC ON position but I have monitored all the waveforms and voltages round this valve and unless someting is wrong I really cant see what the 2nd diode on Pin 5 does for the AVC OFF selection. Can someone enlighten me before I tear my hair out! Pete

12th Feb 2014, 8:43 am	#2
Stockden Heptode Join Date: Mar 2008 Location: Forest of Dean, Gloucestershire, UK. Posts: 648	Re: HRO AVC operation Hi Pete, According to the EMER (obtainable from the VMARS web site) the second diode is maintaining the mean grid bias voltage of the controlled valves "at about the same potential as that existing under no signal conditions with AVC on". Hugh

12th Feb 2014, 12:21 pm	#3
G4_Pete Hexode Join Date: Feb 2014 Location: Southampton, Hampshire, UK. Posts: 419	Re: HRO AVC operation Thanks. the ref manual I have is TM11-885 which redraws the AVC in detail and explains the operation but whilst my HRO MX AVC appears to work correctly as described I NEVER see any potential across the diode connected to the OFF side of the AVC, regardless of the gain, switch combinations I try. All the components appear to check out ok but the diode stays at cathode potential strapped by R14. I have attached the diagram but I clearly must have missed something here as in my view the diode on the "OFF" circuit can only do anything if the end of R-14 marked " - " rises positive to the cathode, that would put the AVC controlled grids positive to V7 cathode. However I measure this or draw it out it does not add up so I am clearly missing something here but I just cant figure it out. Can someone enlighten me please. Attached Thumbnails

12th Feb 2014, 2:27 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: HRO AVC operation I think the words "contact potential" are the significant ones here. The anode and cathode of the diode are of dissimilar metals so a contact potential (as in a thermocouple) exists between them. They are "connected" together by the fewish high energy electrons that escape from the cathode regardless of the lack of any positive voltage on the anode. The effect is that if the "load" between the anode and cathode is sufficiently high impedance, the anode will sit slightly negative wrt the cathode. In the HRO, if both diodes are similar, with similar loads, they will both exhibit this effect to the same degree, but the AVC off diode will not be affected by signals. It's an elegant way to make the agc-off bias the same as the no signal agc-on bias which is itself not the same as if the diode weren't there. (I think). __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

12th Feb 2014, 7:50 pm	#5
G4_Pete Hexode Join Date: Feb 2014 Location: Southampton, Hampshire, UK. Posts: 419	Re: HRO AVC operation Chris, Thank you, following your comments I set up a 6SQ7 heater only and measured the signal diodes wrt to the cathode. Sure enough they drop negative to 0.6 Volts. Interestingly if you physically disconect the heater, that is hot valve with a 10 Meg DVM between the cathode and the signal diode anode only, you see the 0.6V drop off slowly. The hot valve is generating a voltage on its own, stand alone. The HRO manual AVC explanation now makes sense as I was certain it would eventually, as it was compiled by better valve brains than mine. Thanks again, Pete.