Hallicrafters 38B
5/24/2008 6:58:12 AMMike C(62514:0)
Found this old set at a yard sale in non working condition. Have not plugged it in yet as I noticed a couple of oddities. First it had a 12SG7 tube in place of the 12SK7???? Then I noticed a large brown stranded wire was not connected to anything but was near the 12SG7 tube with the other end connected to the connected to the CW AM switch. It appears to me that this is C41 wiring capacity. If so, what exactly does this mean and what does it really do. I guess you can see that I am just a novice in this area. Thanks for any info and insight on this old radio.
5/24/2008 5:06:49 PMEdd(62529:62514)
The BFO circuitry in your somewhat older receiver varies a bit from the newer Halli-Scratchers wherein there is a separate and dedicated 455 Khz BFO oscillator circuit and its CW signal then gets routed up towards the receivers I.F. amplifier first grid circuitry where that signal is then coupled into the set via what is known as a “gimmick capacitor”.
In that particular case they would merely take the end of the INSULATED wire and possibly twirl any where from a mere single turn of wire or on upwards of a few turns. That all being in design accordance as to the required degree of capacitance coupling being needed.
In your set, the situation is a bit different, where there is a run of insulated wire on one end at the plate wiring and then back to the first grid of the IF amplifier stage with a prescribed degree of gimmick capacitance being introduced at each end of the wire. Then in the center area of the wire it actually makes connection into the sets AM / CW(BFO) switch.
For normal AM reception the wire and its gimmick capacitance is brought to ground with minimal effect upon the passage of the IF signal thru the set. HOWEVER when you are wanting to receive code or SSB reception you need a CW carrier inserted in the signal, so now, you drop the switch to CW.
By doing that, the grounding of the gimmick capacitance is lifted and then some degree of plate to 1st grid feedback will then be initiated, along with there additionally being the insertion of a 470 ohm resistor into the IF amplifier 1st grids biasing such that then, a weak oscillation at 455 is also initiated in that stage, YET with the stage still concurrently amplifying 455 IF signal that is being fed to it by the converter stage.
”Fine tuning” of the received CW or SSB signal is then done by roughing in with the tuning knob and then the finer adjustment being made with the band spread tuning knob.
As per the 12SG7 versus 12SK7 syndrome, common to find either tube used in those sets.
Thassit….
73's de Edd
5/24/2008 6:23:45 PMMike C(62537:62529)
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:The BFO circuitry in your somewhat older receiver varies a bit from the newer Halli-Scratchers wherein there is a separate and dedicated 455 Khz BFO oscillator circuit and its CW signal then gets routed up towards the receivers I.F. amplifier first grid circuitry where that signal is then coupled into the set via what is known as a “gimmick capacitor”.
:
:In that particular case they would merely take the end of the INSULATED wire and possibly twirl any where from a mere single turn of wire or on upwards of a few turns. That all being in design accordance as to the required degree of capacitance coupling being needed.
:
:In your set, the situation is a bit different, where there is a run of insulated wire on one end at the plate wiring and then back to the first grid of the IF amplifier stage with a prescribed degree of gimmick capacitance being introduced at each end of the wire. Then in the center area of the wire it actually makes connection into the sets AM / CW(BFO) switch.
:For normal AM reception the wire and its gimmick capacitance is brought to ground with minimal effect upon the passage of the IF signal thru the set. HOWEVER when you are wanting to receive code or SSB reception you need a CW carrier inserted in the signal, so now, you drop the switch to CW.
:
:By doing that, the grounding of the gimmick capacitance is lifted and then some degree of plate to 1st grid feedback will then be initiated, along with there additionally being the insertion of a 470 ohm resistor into the IF amplifier 1st grids biasing such that then, a weak oscillation at 455 is also initiated in that stage, YET with the stage still concurrently amplifying 455 IF signal that is being fed to it by the converter stage.
:”Fine tuning” of the received CW or SSB signal is then done by roughing in with the tuning knob and then the finer adjustment being made with the band spread tuning knob.
:
:
:
:As per the 12SG7 versus 12SK7 syndrome, common to find either tube used in those sets.
:
:Thassit….
:
:
:
:73's de Edd
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:Soooo, from what I understsnd there is no difference that it had a 12SG7 tube versus the shown 12Sk7. Would it make a difference if I used a new 12SK7 or just keep the 12SG7 that checks OK but not great. NOwwww, the schematic shows the wiring capacitance wire connected to pin 4 of the 12SK7...should I go ahead and connect it to pin 4 of the 12SG7 tube??? I'm going real slow on this old fella!
:
:
:
:
:
:
:
:The BFO circuitry in your somewhat older receiver varies a bit from the newer Halli-Scratchers wherein there is a separate and dedicated 455 Khz BFO oscillator circuit and its CW signal then gets routed up towards the receivers I.F. amplifier first grid circuitry where that signal is then coupled into the set via what is known as a “gimmick capacitor”.
:
:In that particular case they would merely take the end of the INSULATED wire and possibly twirl any where from a mere single turn of wire or on upwards of a few turns. That all being in design accordance as to the required degree of capacitance coupling being needed.
:
:In your set, the situation is a bit different, where there is a run of insulated wire on one end at the plate wiring and then back to the first grid of the IF amplifier stage with a prescribed degree of gimmick capacitance being introduced at each end of the wire. Then in the center area of the wire it actually makes connection into the sets AM / CW(BFO) switch.
:For normal AM reception the wire and its gimmick capacitance is brought to ground with minimal effect upon the passage of the IF signal thru the set. HOWEVER when you are wanting to receive code or SSB reception you need a CW carrier inserted in the signal, so now, you drop the switch to CW.
:
:By doing that, the grounding of the gimmick capacitance is lifted and then some degree of plate to 1st grid feedback will then be initiated, along with there additionally being the insertion of a 470 ohm resistor into the IF amplifier 1st grids biasing such that then, a weak oscillation at 455 is also initiated in that stage, YET with the stage still concurrently amplifying 455 IF signal that is being fed to it by the converter stage.
:”Fine tuning” of the received CW or SSB signal is then done by roughing in with the tuning knob and then the finer adjustment being made with the band spread tuning knob.
:
:
:
:As per the 12SG7 versus 12SK7 syndrome, common to find either tube used in those sets.
:
:Thassit….
:
:
:
:73's de Edd
:
:
:Soooo, from what I understsnd there is no difference that it had a 12SG7 tube versus the shown 12Sk7. Would it make a difference if I used a new 12SK7 or just keep the 12SG7 that checks OK but not great. NOwwww, the schematic shows the wiring capacitance wire connected to pin 4 of the 12SK7...should I go ahead and connect it to pin 4 of the 12SG7 tube??? I'm going real slow on this old fella!
5/25/2008 6:36:42 AMWalt(62554:62537)
Mike- I'm a big fan of the S-38 series as it was basically an all-American 5 tube made to include the shortwave bands that gave a modern,inexpensive multi-band radio with a self-contained speaker to the masses. I believe that the cabinet was styled by Raymond Lowey,the man who gave us the bullet-nose Studebaker and Starlight Coupe. The dial seems to spell out 'C D' and meant 'civil defense' to me. Even though it required AC and not batteries. The RCA tube manual lists the 12SG7 as a Semi-remote cutoff pentode and the 12SK7 as a Remote cutoff pentode with slightly different electrical characteristics for each,but close enough to not make any difference in what you could hear. Either is an adequate substitute. If you do an IF coil peak adjustment,however,just do so with what tube you decide to run with. I would also replace any waxpaper capacitors with modern plastics and replace the electrolytic for safety. Note that these radios didn't have fuses. After that maybe just a general switch cleaning. Clean the tuning cap gently with compressed air as sprays can leave a residue that changes capacity of the plates. My first encounter with a gimmick capacitor was thinking that someone had cut the wires! Add a longwire antenna and enjoy it! - Walt.
5/25/2008 12:13:31 PMMike C(62563:62554)
: Mike- I'm a big fan of the S-38 series as it was basically an all-American 5 tube made to include the shortwave bands that gave a modern,inexpensive multi-band radio with a self-contained speaker to the masses. I believe that the cabinet was styled by Raymond Lowey,the man who gave us the bullet-nose Studebaker and Starlight Coupe. The dial seems to spell out 'C D' and meant 'civil defense' to me. Even though it required AC and not batteries. The RCA tube manual lists the 12SG7 as a Semi-remote cutoff pentode and the 12SK7 as a Remote cutoff pentode with slightly different electrical characteristics for each,but close enough to not make any difference in what you could hear. Either is an adequate substitute. If you do an IF coil peak adjustment,however,just do so with what tube you decide to run with. I would also replace any waxpaper capacitors with modern plastics and replace the electrolytic for safety. Note that these radios didn't have fuses. After that maybe just a general switch cleaning. Clean the tuning cap gently with compressed air as sprays can leave a residue that changes capacity of the plates. My first encounter with a gimmick capacitor was thinking that someone had cut the wires! Add a longwire antenna and enjoy it! - Walt.
Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike
5/26/2008 8:24:17 AMWalt(62588:62563)
:: Mike- I'm a big fan of the S-38 series as it was basically an all-American 5 tube made to include the shortwave bands that gave a modern,inexpensive multi-band radio with a self-contained speaker to the masses. I believe that the cabinet was styled by Raymond Lowey,the man who gave us the bullet-nose Studebaker and Starlight Coupe. The dial seems to spell out 'C D' and meant 'civil defense' to me. Even though it required AC and not batteries. The RCA tube manual lists the 12SG7 as a Semi-remote cutoff pentode and the 12SK7 as a Remote cutoff pentode with slightly different electrical characteristics for each,but close enough to not make any difference in what you could hear. Either is an adequate substitute. If you do an IF coil peak adjustment,however,just do so with what tube you decide to run with. I would also replace any waxpaper capacitors with modern plastics and replace the electrolytic for safety. Note that these radios didn't have fuses. After that maybe just a general switch cleaning. Clean the tuning cap gently with compressed air as sprays can leave a residue that changes capacity of the plates. My first encounter with a gimmick capacitor was thinking that someone had cut the wires! Add a longwire antenna and enjoy it! - Walt.
:
:Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
:
:Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
5/26/2008 9:03:01 AMMike C(62589:62588)
::: Mike- I'm a big fan of the S-38 series as it was basically an all-American 5 tube made to include the shortwave bands that gave a modern,inexpensive multi-band radio with a self-contained speaker to the masses. I believe that the cabinet was styled by Raymond Lowey,the man who gave us the bullet-nose Studebaker and Starlight Coupe. The dial seems to spell out 'C D' and meant 'civil defense' to me. Even though it required AC and not batteries. The RCA tube manual lists the 12SG7 as a Semi-remote cutoff pentode and the 12SK7 as a Remote cutoff pentode with slightly different electrical characteristics for each,but close enough to not make any difference in what you could hear. Either is an adequate substitute. If you do an IF coil peak adjustment,however,just do so with what tube you decide to run with. I would also replace any waxpaper capacitors with modern plastics and replace the electrolytic for safety. Note that these radios didn't have fuses. After that maybe just a general switch cleaning. Clean the tuning cap gently with compressed air as sprays can leave a residue that changes capacity of the plates. My first encounter with a gimmick capacitor was thinking that someone had cut the wires! Add a longwire antenna and enjoy it! - Walt.
::
::Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
::
::Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
Thanks for the clarification....I wasn't doing anything else until I found out about that wire. I don't want to cook the thing in the process of trying to restore. Mike
5/26/2008 4:00:04 PMMike C(62593:62589)
:::: Mike- I'm a big fan of the S-38 series as it was basically an all-American 5 tube made to include the shortwave bands that gave a modern,inexpensive multi-band radio with a self-contained speaker to the masses. I believe that the cabinet was styled by Raymond Lowey,the man who gave us the bullet-nose Studebaker and Starlight Coupe. The dial seems to spell out 'C D' and meant 'civil defense' to me. Even though it required AC and not batteries. The RCA tube manual lists the 12SG7 as a Semi-remote cutoff pentode and the 12SK7 as a Remote cutoff pentode with slightly different electrical characteristics for each,but close enough to not make any difference in what you could hear. Either is an adequate substitute. If you do an IF coil peak adjustment,however,just do so with what tube you decide to run with. I would also replace any waxpaper capacitors with modern plastics and replace the electrolytic for safety. Note that these radios didn't have fuses. After that maybe just a general switch cleaning. Clean the tuning cap gently with compressed air as sprays can leave a residue that changes capacity of the plates. My first encounter with a gimmick capacitor was thinking that someone had cut the wires! Add a longwire antenna and enjoy it! - Walt.
:::
:::Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
:
:Thanks for the clarification....I wasn't doing anything else until I found out about that wire. I don't want to cook the thing in the process of trying to restore. Mike
:::
:::Does the wire connect to pin 4 or just lay loose in there by the 12SK7 OR 12SG7??? It does not appear to have ever been connected. I also noted that a few of the resistors don't match the schematic....I replace the ones that did and the old paper caps....haven't plugged it in yet,though.....kinda reluctant until all issues are resolved in my mind.--Mike Mike- The brown gimmick wire doesn't connect to any of the tube socket terminals,it just kind of lays there in the field of the IF tube to give you your CW BFO function. As it approaches the 12SK7/12SG7 tube,it wraps loosely one turn around the black IF input coil wire,then lays under the tube socket entering between lugs 7 and 8 and exiting between lugs 4 and 5 with the end of the wire formed into a little hook shape. It is a cheap way of providing a loose coupling to the circuit without any components other than a piece of wire needed. The different values of resistors that you noted may be OK,as this model saw running changes and variants in production. See similar S-38,S-38A,C and D. I believe the S-38E (large rectangular style dial)used miniature tube types. Lafayette offered a S-38 looking radio with an S meter in a light gray cabinet and I wonder if they had purchased the tooling from Hallicrafters?
:
:Thanks for the clarification....I wasn't doing anything else until I found out about that wire. I don't want to cook the thing in the process of trying to restore. Mike
Old radio is up and growling and does now pick up a few stations. I really don't know how well it will do, but as I do a bit more here and there on all bands. First time it ever took me two hands to tune a radio!!!Thanks for all the help!