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hmv1944 · 6th Dec 2008, 6:25 pm

I have been working on a power amp from a large radiogram made by Sound Sales of Farnham (see pictures) which dates from around 1947 but have failed to find any service information or schematic diagram despite many months of searching on the web. The valve line-up is: 6J7, 6SN7GT, 6SN7GT, PX4, PX4, 5Z4G. I have created my own schematic from inspection of the amp which I hope is correct and have uploaded it for the information of others. (Do ask me if you would like a higher res image.) The component values shown have been taken from the components in the existing unit. There is a large carbon resistor running from a centre-tap of the PX4 heater winding to ground which has lost almost all of its paint due to sixty years of heat dissipation. It does, however, retain a little mauve paint on one end and measures 52.5 ohms (with DMM) so I have assumed it was originally of nominal value 47R.

After electrolytic capacitor reforming and replacement of wax paper caps the amp sounds wonderful using an FM radio as signal source and using the existing speaker in its large padded cabinet. There is just the barest suggestion of a 50Hz hum. (Happily SWMBO loves the sound as well!) The two PX4s are not identical, one having extra fins on its anode plates, the other not. The DC voltage drop across the large carbon 47R resistor that has been hot in the past is around 28 volts and after about an hour of running is only warm. Is this resistor really dissipating 15 watts? If it needs replacing what could be used instead? There is an adjustment pot from the grid of one PX4 to ground which I assume is there to balance the two output valves. My question is what measurement should be used to make the adjustment? Using the DMM on DC there is 0.04 V on the grid of V4 and 0.13 V on the grid of V5. By measuring the voltage drop across the 100R resistors in the anode circuit of the output valves it shows there is a difference in the current flowing in each valve; around 20mA for V5 and 36mA for V4. Should these values be the same or does this just reflect a difference in the two valves? I saw with alarm that used PX4s retail for £80-£90 each! In order to be gentle to the old lady I have reduced the mains voltage to give an HT of around 225 volts.

Alan.

6th Dec 2008, 6:25 pm	#1
hmv1944 Tetrode Join Date: May 2006 Location: Sheffield, South Yorkshire, UK. Posts: 76	Sound Sales PX4 Amplifier I have been working on a power amp from a large radiogram made by Sound Sales of Farnham (see pictures) which dates from around 1947 but have failed to find any service information or schematic diagram despite many months of searching on the web. The valve line-up is: 6J7, 6SN7GT, 6SN7GT, PX4, PX4, 5Z4G. I have created my own schematic from inspection of the amp which I hope is correct and have uploaded it for the information of others. (Do ask me if you would like a higher res image.) The component values shown have been taken from the components in the existing unit. There is a large carbon resistor running from a centre-tap of the PX4 heater winding to ground which has lost almost all of its paint due to sixty years of heat dissipation. It does, however, retain a little mauve paint on one end and measures 52.5 ohms (with DMM) so I have assumed it was originally of nominal value 47R. After electrolytic capacitor reforming and replacement of wax paper caps the amp sounds wonderful using an FM radio as signal source and using the existing speaker in its large padded cabinet. There is just the barest suggestion of a 50Hz hum. (Happily SWMBO loves the sound as well!) The two PX4s are not identical, one having extra fins on its anode plates, the other not. The DC voltage drop across the large carbon 47R resistor that has been hot in the past is around 28 volts and after about an hour of running is only warm. Is this resistor really dissipating 15 watts? If it needs replacing what could be used instead? There is an adjustment pot from the grid of one PX4 to ground which I assume is there to balance the two output valves. My question is what measurement should be used to make the adjustment? Using the DMM on DC there is 0.04 V on the grid of V4 and 0.13 V on the grid of V5. By measuring the voltage drop across the 100R resistors in the anode circuit of the output valves it shows there is a difference in the current flowing in each valve; around 20mA for V5 and 36mA for V4. Should these values be the same or does this just reflect a difference in the two valves? I saw with alarm that used PX4s retail for £80-£90 each! In order to be gentle to the old lady I have reduced the mains voltage to give an HT of around 225 volts. Alan. Attached Thumbnails