This is an about -30 dBu sine wave being amplified to about 0dBu by the triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. The plate load is a single transistor gyrator/constant current source. Plate and heater currents are being supplied by a "China Special" boost converter module that's stepping up a 4 "C" cell battery pack. http://tinyurl.com/zeegduc Yick. Lots of switcher noise. The constant current source should be choking it out from the plate, so I'm guessing it's being injected from the heater to the cathode? Needs moar filtering...
Sloppy sine wave
Started by ●February 16, 2016
Reply by ●February 17, 20162016-02-17
On Tue, 16 Feb 2016 20:02:42 -0500, bitrex wrote:> This is an about -30 dBu sine wave being amplified to about 0dBu by the > triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. > The plate load is a single transistor gyrator/constant current source. > Plate and heater currents are being supplied by a "China Special" boost > converter module that's stepping up a 4 "C" cell battery pack. > > http://tinyurl.com/zeegduc > > Yick. Lots of switcher noise. The constant current source should be > choking it out from the plate, so I'm guessing it's being injected from > the heater to the cathode? > > Needs moar filtering...Frequency? Y'know, if you're going to build a circuit out of toobs, you really ought to build the WHOLE circuit out of toobs. How about using a triode as the plate load, with the grid held at about half of the plate potential with big-ass resistors (1M-ohm?) and a big- ass cap from grid to cathode. I'm pretty sure that the impedance looking into the cathode will be equal to 1/transconductance at DC, but be equal to the plate resistance at AC. Or, just resistively load the thing and be done with it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●February 17, 20162016-02-17
On 02/17/2016 12:35 PM, Tim Wescott wrote:> On Tue, 16 Feb 2016 20:02:42 -0500, bitrex wrote: > >> This is an about -30 dBu sine wave being amplified to about 0dBu by the >> triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. >> The plate load is a single transistor gyrator/constant current source. >> Plate and heater currents are being supplied by a "China Special" boost >> converter module that's stepping up a 4 "C" cell battery pack. >> >> http://tinyurl.com/zeegduc >> >> Yick. Lots of switcher noise. The constant current source should be >> choking it out from the plate, so I'm guessing it's being injected from >> the heater to the cathode? >> >> Needs moar filtering... > > Frequency? > > Y'know, if you're going to build a circuit out of toobs, you really ought > to build the WHOLE circuit out of toobs. > > How about using a triode as the plate load, with the grid held at about > half of the plate potential with big-ass resistors (1M-ohm?) and a big- > ass cap from grid to cathode. I'm pretty sure that the impedance looking > into the cathode will be equal to 1/transconductance at DC, but be equal > to the plate resistance at AC. > > Or, just resistively load the thing and be done with it. >The sine frequency is about 1 kHz. The noise from the switcher is around 250, 500 kHz, something (I'll have to check the specs again.) I'm not a tube purist - I'm content with a single transistor in the circuit. In this design the triode will hopefully be performing the dual function of RF amplifier (with the signal injected into the cathode) and demodulated audio amplifier (with the signal injected into the grid.) The cathode is bypassed for audio gain (but not RF as the signal is being injected there) with a small electrolytic capacitor in series with an RF choke. The single transistor gyrator load is nice because it presents a high impedance to RF, but just looks like a resistive load at low frequencies, and you can also take the audio output from the emitter of the PNP and get an emitter follower output for the demodulated audio from that. Not bad for a PNP, a 10nF capacitor and a couple of resistors. I'll try to post a schematic at some point... The main problem will be keeping the switcher noise out of the RF stage.
Reply by ●February 17, 20162016-02-17
bitrex <bitrex@de.lete.earthlink.net> Wrote in message:> On 02/17/2016 12:35 PM, Tim Wescott wrote: >> On Tue, 16 Feb 2016 20:02:42 -0500, bitrex wrote: >> >>> This is an about -30 dBu sine wave being amplified to about 0dBu by the >>> triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. >>> The plate load is a single transistor gyrator/constant current source. >>> Plate and heater currents are being supplied by a "China Special" boost >>> converter module that's stepping up a 4 "C" cell battery pack. >>> >>> http://tinyurl.com/zeegduc >>> >>> Yick. Lots of switcher noise. The constant current source should be >>> choking it out from the plate, so I'm guessing it's being injected from >>> the heater to the cathode? >>> >>> Needs moar filtering... >> >> Frequency? >> >> Y'know, if you're going to build a circuit out of toobs, you really ought >> to build the WHOLE circuit out of toobs. >> >> How about using a triode as the plate load, with the grid held at about >> half of the plate potential with big-ass resistors (1M-ohm?) and a big- >> ass cap from grid to cathode. I'm pretty sure that the impedance looking >> into the cathode will be equal to 1/transconductance at DC, but be equal >> to the plate resistance at AC. >> >> Or, just resistively load the thing and be done with it. >> > > The sine frequency is about 1 kHz. The noise from the switcher is around > 250, 500 kHz, something (I'll have to check the specs again.) > > I'm not a tube purist - I'm content with a single transistor in the > circuit. In this design the triode will hopefully be performing the dual > function of RF amplifier (with the signal injected into the cathode) and > demodulated audio amplifier (with the signal injected into the grid.) > > The cathode is bypassed for audio gain (but not RF as the signal is > being injected there) with a small electrolytic capacitor in series with > an RF choke. > > The single transistor gyrator load is nice because it presents a high > impedance to RF, but just looks like a resistive load at low > frequencies, and you can also take the audio output from the emitter of > the PNP and get an emitter follower output for the demodulated audio > from that. Not bad for a PNP, a 10nF capacitor and a couple of > resistors. I'll try to post a schematic at some point... > > The main problem will be keeping the switcher noise out of the RF stage. > > >If the switcher is at hundreds of kHz and the sine is at 1 kHz, I'm thinking oscillation. -- www.wescottdesign.com ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●February 17, 20162016-02-17
bitrex wrote:> On 02/17/2016 12:35 PM, Tim Wescott wrote: >> On Tue, 16 Feb 2016 20:02:42 -0500, bitrex wrote: >> >>> This is an about -30 dBu sine wave being amplified to about 0dBu by the >>> triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. >>> The plate load is a single transistor gyrator/constant current source. >>> Plate and heater currents are being supplied by a "China Special" boost >>> converter module that's stepping up a 4 "C" cell battery pack. >>> >>> http://tinyurl.com/zeegduc >>> >>> Yick. Lots of switcher noise. The constant current source should be >>> choking it out from the plate, so I'm guessing it's being injected from >>> the heater to the cathode? >>> >>> Needs moar filtering... >> >> Frequency? >> >> Y'know, if you're going to build a circuit out of toobs, you really ought >> to build the WHOLE circuit out of toobs. >> >> How about using a triode as the plate load, with the grid held at about >> half of the plate potential with big-ass resistors (1M-ohm?) and a big- >> ass cap from grid to cathode. I'm pretty sure that the impedance looking >> into the cathode will be equal to 1/transconductance at DC, but be equal >> to the plate resistance at AC. >> >> Or, just resistively load the thing and be done with it. >> > > The sine frequency is about 1 kHz. The noise from the switcher is around > 250, 500 kHz, something (I'll have to check the specs again.) >Those smell like (sub)harmonics. IM?> I'm not a tube purist - I'm content with a single transistor in the > circuit. In this design the triode will hopefully be performing the dual > function of RF amplifier (with the signal injected into the cathode) and > demodulated audio amplifier (with the signal injected into the grid.) > > The cathode is bypassed for audio gain (but not RF as the signal is > being injected there) with a small electrolytic capacitor in series with > an RF choke. > > The single transistor gyrator load is nice because it presents a high > impedance to RF, but just looks like a resistive load at low > frequencies, and you can also take the audio output from the emitter of > the PNP and get an emitter follower output for the demodulated audio > from that. Not bad for a PNP, a 10nF capacitor and a couple of > resistors. I'll try to post a schematic at some point... > > The main problem will be keeping the switcher noise out of the RF stage. > >Are you getting IM from the switcher or what? I would think a nice RC or pi filter at the switcher would work as an LPF. I have a cheap condenser mic than can whistle when loaded by certain preamps. It uses an RF stepup switcher for the FET Vcc in the mic. That couples with the phantom power somehow. -- Les Cargill
Reply by ●February 17, 20162016-02-17
On 02/17/2016 09:45 PM, Les Cargill wrote:> bitrex wrote: >> On 02/17/2016 12:35 PM, Tim Wescott wrote: >>> On Tue, 16 Feb 2016 20:02:42 -0500, bitrex wrote: >>> >>>> This is an about -30 dBu sine wave being amplified to about 0dBu by the >>>> triode section of a Sylvania 12EC8 triode/pentode "space charge" tube. >>>> The plate load is a single transistor gyrator/constant current source. >>>> Plate and heater currents are being supplied by a "China Special" boost >>>> converter module that's stepping up a 4 "C" cell battery pack. >>>> >>>> http://tinyurl.com/zeegduc >>>> >>>> Yick. Lots of switcher noise. The constant current source should be >>>> choking it out from the plate, so I'm guessing it's being injected from >>>> the heater to the cathode? >>>> >>>> Needs moar filtering... >>> >>> Frequency? >>> >>> Y'know, if you're going to build a circuit out of toobs, you really >>> ought >>> to build the WHOLE circuit out of toobs. >>> >>> How about using a triode as the plate load, with the grid held at about >>> half of the plate potential with big-ass resistors (1M-ohm?) and a big- >>> ass cap from grid to cathode. I'm pretty sure that the impedance >>> looking >>> into the cathode will be equal to 1/transconductance at DC, but be equal >>> to the plate resistance at AC. >>> >>> Or, just resistively load the thing and be done with it. >>> >> >> The sine frequency is about 1 kHz. The noise from the switcher is around >> 250, 500 kHz, something (I'll have to check the specs again.) >> > > Those smell like (sub)harmonics. IM? > >> > > Are you getting IM from the switcher or what? I would think a nice RC > or pi filter at the switcher would work as an LPF. > > I have a cheap condenser mic than can whistle when loaded by certain > preamps. It uses an RF stepup switcher for the FET Vcc > in the mic. That couples with the phantom power somehow. >It looks just like the standard switcher ripple that I can see on the switcher's output, but superimposed on the audio. An LC between the switcher's output and the terminal block that feeds the B+ and filament voltage is what I'm going to try.... ; )
