The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
I haven't started experimenting yet, and would welcome any suggestions.
Doug
marv
:Prof. Hazeltine's patent kept a lot of Tuned Radio Freq sets of the 1920s from using his ckt to reduce oscillation in RF triode amplifier stages. I have quite a few TRFs, and am thinking about neutralizing all of them. (Prof. Hazeltine is long gone, and I trust that his patent expired many decades ago. But, I digress.)
:
:The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
:
:The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
:
:Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
:
:So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
:
:I haven't started experimenting yet, and would welcome any suggestions.
:Doug
Received your email. Neutralization cancels grid to plate capacity in a tube. Some signal couples through between grid and plate of a triode. For neutralization a small amount is fed back, out of phase, to cancel this unwanted signal.
You can experiment with small capacitors for neutralization but too much will also cause oscillation. As a neutralization cap is adjusted output signal will dip down. Too much capacity and signal increases again going into oscillation.
Your idea of gimmick cap would work. Early neutralization caps were made of glass tubing which could be adjusted.
Here is Majestic 70B:
http://www.nostalgiaair.org/PagesByModel/002/M0040002.pdf
Freed Eisemann NR5:
http://www.nostalgiaair.org/PagesByModel/885/M0039885.pdf
Due to differences between tubes best to use small trimmer caps. Caps need to be adjusted with tube to be used in the socket. Tape off a filament pin when adjusting a neutralization cap.
Manufactures also used wire wound resistors in series with grids of RF tubes which helped isolate internal tube capacity.
Some manufacturers mounted RF coils at different angles. Didn't have much effect on tube capacity but kept coupling between coils down, reduced oscillation.
Norm
:Prof. Hazeltine's patent kept a lot of Tuned Radio Freq sets of the 1920s from using his ckt to reduce oscillation in RF triode amplifier stages. I have quite a few TRFs, and am thinking about neutralizing all of them. (Prof. Hazeltine is long gone, and I trust that his patent expired many decades ago. But, I digress.)
:
:The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
:
:The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
:
:Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
:
:So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
:
:I haven't started experimenting yet, and would welcome any suggestions.
:Doug
I've measured the grid-to-plate capacitance of ten '01As that I pulled, at random, out of a box of tubes. (I measured them Pin 2 to Pin 3, not installed in a socket.) Here are the results:
Minimum: 7.4 pF
Maximum: 8.8 pF
Average: 8.2 pF
The data-sheet value is 8.1 pF - pretty darn close.
I've ordered some small caps to start experimenting with.
Doug
:Hi Doug
:
: Received your email. Neutralization cancels grid to plate capacity in a tube. Some signal couples through between grid and plate of a triode. For neutralization a small amount is fed back, out of phase, to cancel this unwanted signal.
:
: You can experiment with small capacitors for neutralization but too much will also cause oscillation. As a neutralization cap is adjusted output signal will dip down. Too much capacity and signal increases again going into oscillation.
:
: Your idea of gimmick cap would work. Early neutralization caps were made of glass tubing which could be adjusted.
:
: Here is Majestic 70B:
: http://www.nostalgiaair.org/PagesByModel/002/M0040002.pdf
:
: Freed Eisemann NR5:
:
:http://www.nostalgiaair.org/PagesByModel/885/M0039885.pdf
:
: Due to differences between tubes best to use small trimmer caps. Caps need to be adjusted with tube to be used in the socket. Tape off a filament pin when adjusting a neutralization cap.
:
: Manufactures also used wire wound resistors in series with grids of RF tubes which helped isolate internal tube capacity.
:
: Some manufacturers mounted RF coils at different angles. Didn't have much effect on tube capacity but kept coupling between coils down, reduced oscillation.
:
:Norm
:
:
:
::Prof. Hazeltine's patent kept a lot of Tuned Radio Freq sets of the 1920s from using his ckt to reduce oscillation in RF triode amplifier stages. I have quite a few TRFs, and am thinking about neutralizing all of them. (Prof. Hazeltine is long gone, and I trust that his patent expired many decades ago. But, I digress.)
::
::The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
::
::The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
::
::Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
::
::So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
::
::I haven't started experimenting yet, and would welcome any suggestions.
::Doug
marv
:Prof. Hazeltine's patent kept a lot of Tuned Radio Freq sets of the 1920s from using his ckt to reduce oscillation in RF triode amplifier stages. I have quite a few TRFs, and am thinking about neutralizing all of them. (Prof. Hazeltine is long gone, and I trust that his patent expired many decades ago. But, I digress.)
:
:The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
:
:The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
:
:Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
:
:So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
:
:I haven't started experimenting yet, and would welcome any suggestions.
:Doug
:Doug,
:An 8.1 pf gimmick cap would probably require solid #12 wire and several inches at that, creating more unwanted feedback problems. Best to use a small variable if possible. My Edison 7R had phenolic/brass variable neutralizing caps rigidly attached to the sockets at the plate-grid pins of the 27's.
:
:marv
:
::Prof. Hazeltine's patent kept a lot of Tuned Radio Freq sets of the 1920s from using his ckt to reduce oscillation in RF triode amplifier stages. I have quite a few TRFs, and am thinking about neutralizing all of them. (Prof. Hazeltine is long gone, and I trust that his patent expired many decades ago. But, I digress.)
::
::The howling of un-neutralized TRFs can usually be controlled by fiddling with the filament rheostats, but I'd like to avoid that by adding a permanent neutralizing fix.
::
::The oscillation was caused by the grid-to-plate capacitance in the old triodes, such as '01As, which really weren't suitable for RF frequencies, even broadcast band. The neutralization idea was to put a cap between the plate and the bottom end of the RF coil. The signal at the negative end of the RF coil is 180 deg out of phase with the top end of the coil, which was connected to the grid - so, the two capacitances cancelled.
::
::Typically, the neutralizing cap was adustable to accommodate variations in each tube's inter-electode capacitance. (The rated capacitance for an 01A is 8.1 pF.) I'm thinking the adjustment feature might be gilding the lilly, and anyway, it theoretically keeps me from arbitrarily swapping tubes around. (I'm not sure I have enough tubes for all my old TRFs.)
::
::So, my idea, is to just strap a 10-pF cap (the closest standard size to 8-pF) from each RF tube's plate to the negative side of its RF coil. I might, alternatively, fashion gimmick caps.
::
::I haven't started experimenting yet, and would welcome any suggestions.
::Doug
I have a Philco 531 (511 chassis) that has neutralization padders - schematic here:
http://www.nostalgiaair.org/Resources/735/M0013735.htm
They are noted as numbers 5,10 and 15 on the print.
TonyJ
:Marv, can you link me to a schematic for your Edison? Or, can you tell me exactly how the neutralizing trimmers were wired into the RF ckts? Thanks.
:Doug
:Doug:
:
:I have a Philco 531 (511 chassis) that has neutralization padders - schematic here:
:
:http://www.nostalgiaair.org/Resources/735/M0013735.htm
:
:They are noted as numbers 5,10 and 15 on the print.
:
:TonyJ
:
::Marv, can you link me to a schematic for your Edison? Or, can you tell me exactly how the neutralizing trimmers were wired into the RF ckts? Thanks.
::Doug
This link containing an article by Lane Upton may be of some help to you:
http://www.antiquewireless.org/otb/aligntrf0106.htm
Radiodoc
***************
:Thanks, Tony. I didn't realize there were some many wiring variations for the neut caps.
:Doug
:
::Doug:
::
::I have a Philco 531 (511 chassis) that has neutralization padders - schematic here:
::
::http://www.nostalgiaair.org/Resources/735/M0013735.htm
::
::They are noted as numbers 5,10 and 15 on the print.
::
::TonyJ
::
:::Marv, can you link me to a schematic for your Edison? Or, can you tell me exactly how the neutralizing trimmers were wired into the RF ckts? Thanks.
:::Doug
marv
:Marv, can you link me to a schematic for your Edison? Or, can you tell me exactly how the neutralizing trimmers were wired into the RF ckts? Thanks.
:Doug