20-05-2024, 03:03 PM
(This post was last modified: 20-05-2024, 03:04 PM by Mike Watterson.)
Testing the output valve using the equipment instead of a valve tester
For this we need to decide what kind of power supply is used and what kind of valve and output stage and what kind of bias.
1) Supply.
Series or parallel heaters / filaments, or even a pair of paralleled series chains.
Isolated transformer for heaters and HT or partial transformer/autotransformer/ballasts only (Live chassis). WARNING some USA sets have metalwork connected via 0.1 uF paper cap to one side of mains. MUST be replaced by safety 10nF rated 500V to 1kV. Some sets have the ballast resistor as part of line cord to convert from 120V to 240V. Needs replaced.
Valve or Metal/Selenium or Germanium or Silicon rectifier(s). A rectifier valve can have its own heater supply, or be part of the series chain, or run direct off USA 117V nominal mains, or off main heater supply.
Is there a Barretter? A big glass tube that looks a little like a festoon light bulb. It's a kind of Thermistor to regulate the mains voltage. Is there a regular series thermistor? It limits inrush current as it may damage panel lamps or some valves. The inrush otherwise can be x2.5!
Is it battery only or mains and battery using 1.4V or 2V battery tubes?
Is it battery using selected mains 6.3V or 12.6V valves direct off car battery? 10.8V to 14V. Then the audio will be transistors.
Is it battery using a normal HT on valves by a vibrator pack from 6V or 12V. Some of these use an extra pair of contacts to "rectify" the transformer secondary. May be 80Hz to 200Hz. Some can also use mains.
This can all be deduced without a schematic. A taperecorder, transmitter or TV needs a schematic. Most radios and almost all record players can be figured out without one.
2) Audio output Valve.
a) Direct Triode with 2, 4 or 6V filament. Mostly before 1931, but a few sets used them up till 1941. There is often a tapped grid bias pack.
b) Direct Pentode with 1.4V or 2V filament. The 4V was rare. Very early sets used a tapped grid bias pack. Some used a Mallory mercury cell. This is what the button format and mercury battery was invented for. Company named after Mallory and became Duracell. Later sets used a resistor in the -HT connection to 0V. A very few sets had the grid bias in the HT pack. Some 1950s sets use filtered connection to the local oscillator grid for a -9V grid bias. A 7.5V to 12V battery (or battery mains) set using 1.4V valves will put the audio output valve(s) at the +7.V, 9V or 12V battery connection so that the grid leak resistor to 0V is neagtive. The first audio preamp is usually at the 0V end of battery (and mains supply for least hum) and then the Mixer/oscillator is next.
c) Pentode or Beam Tetrode with indirect heater. Grid Bias is usually derived by the voltage on the cathode series resistor. On push pull there may be a negative supply and always on transmitters. Occasionally this cathode resistor has a 1.35V tap for a DM70/DM71 magic eye filament.
3) Kind of output
a) Very old sets, with a triode or pentode, used a direct 2,000 or 4,000 load such as a horn, moving iron cone speaker or 2K + 2K headphones. This would burn out when the grid bias battery failed after about a a year, or if the tap came off.
b) Later a transformer with a 2 Ohm to 16 Ohm moving coil and permanent magnet was used. The output transformer would burn out when the grid bias battery failed after about a a year, or if the tap came off.
c) Later still a capacitor rather than a transformer was used to drive the output from the prior anode. This would go leaky in 5 to 10 years and the output transformer would burn out.
d) Mains radios are common from 1934. Early models ran the HT through a series winding on the speaker instead of a magnet. This might include a humbucking winding. Now the leaky cap can result in the output transformer or the field winding burning out.
e) Due to design of transformer a capacitor called a tone corrector (a snubber) is added to the output anode. it can go to HT or OV. When it fails the HT rectifier or main HT transformer burns out. The failing grid to proir anode cap may have already damaged the output valve and output transformer.
f) The output may use a centre tap transformer and two identical valves. Sometimes these are in the same envelope such as KLL32 and ELL8x
g) A few mains / battery radios use all battery valves and switch from a battery Pentode to regular mains radio Pentode on mains. These use a custom audio transformer, inevitably burnt out.
h) There may be negative feedback from anode, or loudspeaker or an extra winding to grid or volume control. Check the leakage of capacitors.
Proceeding
Some people talk about a "THAT" capacitor. There are many problematic capacitors. Leaky paper caps on the screen grids (g2) will kill gain. Leaky paper caps from mains to metal work may kill users. The "Tone Correction" cap is a problem.
The power supply needs to be got working first, but after replacing / reforming caps.
Unplug all the valves. Do not connect to supply. Connect LV PSU to any Electrolytic on PSU HT and adjust current limit to a few mA and turn up voltage to 30V. Leave at least all day. If the current doesn't drop a cap is leaky and not reforming. If the voltage vanishes the instant the PSU disconnected, then the cap has dried out.
Replace any metal/selenium rectifiers with 1N4007 and 47 Ohm to 100 Ohm 5W resistor. Valves in a series chain need replaced if filament/heater open circuit.
Check resistance of ballasts/transformer winding and leakage of transformer. Check primary of audio transformer.
Disconnect + end of cathode capacitor and reform it at 25V and current limit. Replace if needed.
If parallel or no valve rectifier, you can power up with all the valve out except rectifier once caps, leakage etc all checked. Check HT. Measure g1, and cathode of o/p valve socket that they are 0V. Measure g2 and anode that they are close to HT volts.
Add output valve and measure cathode voltage. Switch off
Calculate current from V/R where R is cathode resistor. if it's very low and all voltages are OK, then the valve is worn out. If it's very high, recheck the grid circuit.
Basically any paper capacitor (most old ones from 1000pF to 1.5uF) is going to need replaced unless it's a hermetic sealed type in a metal can (not the TCC as the end bungs leak). Cathode and Discriminator electrolytics tend to dry out (low volume and no FM) and HT ones often "reform". Only USA Silver Mica are prone to failure. Polystyrene are OK unless you melt them soldering. Mustard and rainbow sweet caps are usually plastic & foil or metalised foil so usually OK, though I've had faulty poly caps in a cassette deck. The two colour plastic body low voltage Electrolytics are usually scrap.
If the set is an isolated chassis (decent transformer) you can now get hum touching the o/p valve grid via a 10nF cap and 100K resistor for safety.
If in doubt, don't turn it on and ask!
For this we need to decide what kind of power supply is used and what kind of valve and output stage and what kind of bias.
1) Supply.
Series or parallel heaters / filaments, or even a pair of paralleled series chains.
Isolated transformer for heaters and HT or partial transformer/autotransformer/ballasts only (Live chassis). WARNING some USA sets have metalwork connected via 0.1 uF paper cap to one side of mains. MUST be replaced by safety 10nF rated 500V to 1kV. Some sets have the ballast resistor as part of line cord to convert from 120V to 240V. Needs replaced.
Valve or Metal/Selenium or Germanium or Silicon rectifier(s). A rectifier valve can have its own heater supply, or be part of the series chain, or run direct off USA 117V nominal mains, or off main heater supply.
Is there a Barretter? A big glass tube that looks a little like a festoon light bulb. It's a kind of Thermistor to regulate the mains voltage. Is there a regular series thermistor? It limits inrush current as it may damage panel lamps or some valves. The inrush otherwise can be x2.5!
Is it battery only or mains and battery using 1.4V or 2V battery tubes?
Is it battery using selected mains 6.3V or 12.6V valves direct off car battery? 10.8V to 14V. Then the audio will be transistors.
Is it battery using a normal HT on valves by a vibrator pack from 6V or 12V. Some of these use an extra pair of contacts to "rectify" the transformer secondary. May be 80Hz to 200Hz. Some can also use mains.
This can all be deduced without a schematic. A taperecorder, transmitter or TV needs a schematic. Most radios and almost all record players can be figured out without one.
2) Audio output Valve.
a) Direct Triode with 2, 4 or 6V filament. Mostly before 1931, but a few sets used them up till 1941. There is often a tapped grid bias pack.
b) Direct Pentode with 1.4V or 2V filament. The 4V was rare. Very early sets used a tapped grid bias pack. Some used a Mallory mercury cell. This is what the button format and mercury battery was invented for. Company named after Mallory and became Duracell. Later sets used a resistor in the -HT connection to 0V. A very few sets had the grid bias in the HT pack. Some 1950s sets use filtered connection to the local oscillator grid for a -9V grid bias. A 7.5V to 12V battery (or battery mains) set using 1.4V valves will put the audio output valve(s) at the +7.V, 9V or 12V battery connection so that the grid leak resistor to 0V is neagtive. The first audio preamp is usually at the 0V end of battery (and mains supply for least hum) and then the Mixer/oscillator is next.
c) Pentode or Beam Tetrode with indirect heater. Grid Bias is usually derived by the voltage on the cathode series resistor. On push pull there may be a negative supply and always on transmitters. Occasionally this cathode resistor has a 1.35V tap for a DM70/DM71 magic eye filament.
3) Kind of output
a) Very old sets, with a triode or pentode, used a direct 2,000 or 4,000 load such as a horn, moving iron cone speaker or 2K + 2K headphones. This would burn out when the grid bias battery failed after about a a year, or if the tap came off.
b) Later a transformer with a 2 Ohm to 16 Ohm moving coil and permanent magnet was used. The output transformer would burn out when the grid bias battery failed after about a a year, or if the tap came off.
c) Later still a capacitor rather than a transformer was used to drive the output from the prior anode. This would go leaky in 5 to 10 years and the output transformer would burn out.
d) Mains radios are common from 1934. Early models ran the HT through a series winding on the speaker instead of a magnet. This might include a humbucking winding. Now the leaky cap can result in the output transformer or the field winding burning out.
e) Due to design of transformer a capacitor called a tone corrector (a snubber) is added to the output anode. it can go to HT or OV. When it fails the HT rectifier or main HT transformer burns out. The failing grid to proir anode cap may have already damaged the output valve and output transformer.
f) The output may use a centre tap transformer and two identical valves. Sometimes these are in the same envelope such as KLL32 and ELL8x
g) A few mains / battery radios use all battery valves and switch from a battery Pentode to regular mains radio Pentode on mains. These use a custom audio transformer, inevitably burnt out.
h) There may be negative feedback from anode, or loudspeaker or an extra winding to grid or volume control. Check the leakage of capacitors.
Proceeding
Some people talk about a "THAT" capacitor. There are many problematic capacitors. Leaky paper caps on the screen grids (g2) will kill gain. Leaky paper caps from mains to metal work may kill users. The "Tone Correction" cap is a problem.
The power supply needs to be got working first, but after replacing / reforming caps.
Unplug all the valves. Do not connect to supply. Connect LV PSU to any Electrolytic on PSU HT and adjust current limit to a few mA and turn up voltage to 30V. Leave at least all day. If the current doesn't drop a cap is leaky and not reforming. If the voltage vanishes the instant the PSU disconnected, then the cap has dried out.
Replace any metal/selenium rectifiers with 1N4007 and 47 Ohm to 100 Ohm 5W resistor. Valves in a series chain need replaced if filament/heater open circuit.
Check resistance of ballasts/transformer winding and leakage of transformer. Check primary of audio transformer.
Disconnect + end of cathode capacitor and reform it at 25V and current limit. Replace if needed.
If parallel or no valve rectifier, you can power up with all the valve out except rectifier once caps, leakage etc all checked. Check HT. Measure g1, and cathode of o/p valve socket that they are 0V. Measure g2 and anode that they are close to HT volts.
Add output valve and measure cathode voltage. Switch off
Calculate current from V/R where R is cathode resistor. if it's very low and all voltages are OK, then the valve is worn out. If it's very high, recheck the grid circuit.
Basically any paper capacitor (most old ones from 1000pF to 1.5uF) is going to need replaced unless it's a hermetic sealed type in a metal can (not the TCC as the end bungs leak). Cathode and Discriminator electrolytics tend to dry out (low volume and no FM) and HT ones often "reform". Only USA Silver Mica are prone to failure. Polystyrene are OK unless you melt them soldering. Mustard and rainbow sweet caps are usually plastic & foil or metalised foil so usually OK, though I've had faulty poly caps in a cassette deck. The two colour plastic body low voltage Electrolytics are usually scrap.
If the set is an isolated chassis (decent transformer) you can now get hum touching the o/p valve grid via a 10nF cap and 100K resistor for safety.
If in doubt, don't turn it on and ask!
