Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Another 807 Modulator




 
The AM Forum
August 11, 2020, 12:18:07 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Another 807 Modulator  (Read 442 times)
0 Members and 1 Guest are viewing this topic.
KI4YAN
Member

Offline Offline

Posts: 161


« on: July 16, 2020, 10:18:57 PM »

Spurred on by N4LTA's progress and assistance with getting some filament transformers, here's my first draft at a 807 modulator for a single 4-65A final.

I've used this input circuit before several times at the 25V output swing level, and given enough B+ it will generate under 1% distortion with no additional negative feedback, with pretty much any bogey 6BL8/ECF80 tube. Here, it's being asked to swing 32 volts, which I'll need to verify still.

Not sure where/how I'll pull my negative feedback source in, I've done it off an 8 ohm secondary before but here I don't have one. Could pull it from the plate of one of the 807's or from the screen modulation winding easy enough.

Might need more voltage-the circuit in the input/phase splitter section will work from 300 to 400V and gets lower distortion numbers at the upper end of the range.



Once I run a few more numbers and make sure I'm going to have enough headroom to get the 32v output swing and keep the direct-coupled triode happy, I'll start on the power supply section.
Logged
KI4YAN
Member

Offline Offline

Posts: 161


« Reply #1 on: July 17, 2020, 12:50:23 AM »

So, looking at my own drawing I see some things I'll need to add.

If the grid-bias pots open up, it'll definitely break something. Need to add a 1 ohm current sense resistor to the 600V line and set up a relay to open the 600V line if over 350mA is drawn (that'll definitely be a fault condition!)

Can probably adjust the 0.1uF coupling caps to work with the grid bias resistors to adjust the audio bandwidth down some, right now the input circuit is pretty flat to 40kHz and the transformer is flat to 15Khz. I'd like to limit the frequency response of the amplifier to around 100Hz-15Khz to curtail some of the 807's aspirations of being transmitter finals. (plus the grid and screen stoppers already present)
Logged
KI4YAN
Member

Offline Offline

Posts: 161


« Reply #2 on: July 18, 2020, 01:15:09 AM »

happy to report that rigged up on the workbench in the most dangerous manner possible, I can get this thing to put out 45W of noise into a resistor. I don't have a big enough 10K resistor to go more than that. That tells me it's workable enough to start metalwork and making it less dangerous and prone to oscillation.

Still don't have the negative feedback sorted out, I'm thinking perhaps an AC-coupled loop from the unused end of the screen modulation winding? Not sure if having a capacitor there with its phase shift is going to be workable or not.

Found a power transformer from an old Wurlitzer organ today too. According to the schematic faded and taped up inside, it's designed for a 350V supply, and measuring it with no load, it outputs 398-0-398V. It did power a push-pull 6L6 amp, plus 26 more tubes, so should have some current available. It should deliver plenty of current at 350V to power a couple voltage regulators for different parts of the rack.
Logged
W4RFM
Member

Offline Offline

Posts: 349



WWW
« Reply #3 on: July 18, 2020, 01:50:35 PM »

I may be thinking very wrong, but I believe the cathode would be a good place to tape off to get a lower impedance source for your feedback.  Again, I am probably forgetting some electronics law I knew 40 years ago, but...
Logged

BOB / W4RFM  "I have looked far and wide, (I also checked near and narrow)"
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1511


« Reply #4 on: July 18, 2020, 02:24:53 PM »

I would up R5 and R6 to 100k's.

R3 and R4 really do not need added inductance at the grid so I would delete any coils here.

I would place 22 ohm Ohmite OY resistors in series with the plate leads to T1 for added parasitic suppression.  

807's really prefer a well regulated 275 to 300V SG supply so I would place 0.1 caps after each 100 ohm SG resistor at the SG grid pin.


Phil  
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
KI4YAN
Member

Offline Offline

Posts: 161


« Reply #5 on: July 18, 2020, 05:42:56 PM »

I originally had R6 and R6 at 100K, but knocked them down after adding in the bias potentiometer. I figured half and half should do, but I guess it would depend on what my bias voltage actually comes out to be, as to where the potentiometers get twisted to. They can easily go to 82K or 100K, the datasheet calls out not more than 100K.

R3 and R4 were an attempt at an Ohmite Z50 type stopper, I actually had some of those at one point. I'll just change these to 100R resistors and if they need any help later they're easy enough to get at.

As for the 22R OY's in the plate leads, do I fit them up inside the plate cap or is down under the chassis OK? I suspect up under the plate cap is where they should be. I can heat-shrink them for added insulation if they stick out of the ceramic cap.

I'll move the bypass caps, and the 807 screens get their own +300V regulator. The plan is currently 3 regulated supplies: Modulator Screens, RF Final Screen,and then Modulator input/phase splitter/RF exciter low voltage. Then there's the 600V modulator high voltage and 1300V RF final high voltage. And then the heater supplies, and low voltage for the local oscillator and audio processing chain.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1511


« Reply #6 on: July 18, 2020, 11:45:57 PM »

I originally had R6 and R6 at 100K, but knocked them down after adding in the bias potentiometer. I figured half and half should do, but I guess it would depend on what my bias voltage actually comes out to be, as to where the potentiometers get twisted to. They can easily go to 82K or 100K, the datasheet calls out not more than 100K.

R3 and R4 were an attempt at an Ohmite Z50 type stopper, I actually had some of those at one point. I'll just change these to 100R resistors and if they need any help later they're easy enough to get at.

As for the 22R OY's in the plate leads, do I fit them up inside the plate cap or is down under the chassis OK? I suspect up under the plate cap is where they should be. I can heat-shrink them for added insulation if they stick out of the ceramic cap.

I'll move the bypass caps, and the 807 screens get their own +300V regulator. The plan is currently 3 regulated supplies: Modulator Screens, RF Final Screen,and then Modulator input/phase splitter/RF exciter low voltage. Then there's the 600V modulator high voltage and 1300V RF final high voltage. And then the heater supplies, and low voltage for the local oscillator and audio processing chain.

As for the plate suppressor it can go anywhere between the transformer lead and the plate cap. I prefer to avoid the plate cap location as there is usually a lot of heat there.

The concern I had with the grid resistor value was the output impedance of the triodes seemed to be working into a very low grid impedance so the low frequency response might suffer as well as the gain of the first stage would be compromised.

Good work and when finished it should sound really good.

Phil
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.057 seconds with 18 queries.