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The RCA "807 in Special Triode Connection" gets a new life. With Sweep Tubes!




 
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Author Topic: The RCA "807 in Special Triode Connection" gets a new life. With Sweep Tubes!  (Read 2934 times)
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Patrick J. / KD5OEI
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« Reply #50 on: December 23, 2018, 11:33:41 PM »


I have no idea what his "Beta Network" is supposed to do. Makes some sense right up to
the diode in the feedback loop!

Looks like the audio is fed to the output stage via pins 8 & 3 of the driver tube.
The driver is "standing" on 306 volts to keep the filament to cathode breakdown within
limits...

Looks like the lower 6550 also receives audio to its grid from the cathode of the
input tube, via that 0.33ufd cap...

It's a bit unclear how this screen modulator is coupled to the screen of an RF tube, since
the circuit presumably swings between the -300vdc and +700vdc, making the quiescent
point likely not where the screen of a big RF tube wants to sit? So maybe cap coupled and
then a DC screen bias on the screen side of the cap??

An unusual approach?

Pins 8 and 3 are the outputs of two identical cathode followers a 12AT7. There are two available inputs, the BNC jack and the jack from speech amp.

R1 is not shown except in the op-amp diagram. It would the the third input to set a DC level. In the tube diagram, the 20K 10 turn pot probably does this.

The control grid of the 6EJ7 is the (-) input of the op-amp diagram. It is the phase inverting voltage amplifier for all the voltage gain for the 6550.

The 12BH7 has a high plate voltage but also a large voltage drop and has to push those zener diodes around as well as the grid of the 6550.

The diode portion of the feedback network gives the modulator more gain when outputting positive voltage than negative voltage.
R4 in the network sets the overall gain.

C1-R6 set up the frequency response compensation via feedback and the overall gain at frequencies passed by C1. At some point the combined impedance of C1-R6 becomes more than R4 value and has less effect. at high frequencies it has much effect.

The lower 6550? the 6GK6? also has some gain and it is called a 'sink' there. So it is to shunt current to the -300V supply I guess, not having seen the whole article. As we might guess, a screen grid on a HV tube like the 4-1000 will, if left alone, gain charge and voltage rise toward plaste volts with bad results. This may be to prevent that, to eat negative screen current, or to just be the bottom half of ther totem pole output and do so without feedback, as it does not use the network.

That's the best I can answer for you without the article which I could not find.
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« Reply #51 on: December 24, 2018, 03:27:06 AM »

Whatever the concept of the W5BM modulator was, the 6550 (which has a triode amplification factor of 8 )
is not a good candidate for crazy drive. Using the crazy drive method is unlikely to be able to linearize the overall gm of such a tube by very much.

Don
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« Reply #52 on: December 24, 2018, 11:31:26 AM »

They are not the same application but the feedback network in the screen modulator is interesting. I believe the 6550 would do well in the 'special class B' circuit, which is not crazy drive. Anyway being a modulator those circuits are not perfect for minimizing distortion and the modulated stage is hardly a linear load, so some feedback is needed for improvement in all of them.
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« Reply #53 on: December 24, 2018, 05:14:36 PM »

Patrick,

I do expect that feedback will be beneficial in a crazy drive amplifier. I see feedback as being the last thing to apply to an amplifier after everything else has been done to get it right, using it to better the amplifier rather that cure its ills from the outset. So, as feedback goes, I haven't even been considering it at this point, but that time will come.

Along with modulated stages, I believe that loudspeakers are a non-linear load as well. While "perfect" is a good target to aim at, "practical" will inevitably win the day when the dust settles. But I don't mind getting my shoes dirty kicking up a little dust. Wink

Thanks,
Don
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« Reply #54 on: December 25, 2018, 03:28:25 PM »

Well !
 This is a subject near and dear to my heart. The triode connection of tetrode tubes. rather than make an old buzzard transmission on the radio I will make a text presentation here to dispel untruths and old wives tales.
 My experince with triode connection goes back to 1966 when I was a boot. I had the basic build on the Crap Box 180 Originally a pair of 6146s modulated by a pair built on an old TV chassis. The plate voltage was about 700 volts. I had a copy of the  RCA transmitting tube manual. I saw the triode connected 807 modulator. I have a copy of RCA ham news or ham tips.One was GE the other RCA. This was dated from 1947. There was a whole write up on the glowing performance of the merits of triode connection of the 807. I fegured it would work well with the 6146 s . The audio driver was a pair of triode connected ( low Mu) 6BQ5s Plenty of audio power for the job. In triode connection mode the idle plate current was in the order of 50Ma. or so. I fed audio into the line level input from my microphonium preamp. It modulated but when I listened to it on my receiver it was loaded with grunge distortion. I was not happy with it. I decided to try a tube much like the 807, the 6BG6. Essentially an octal version of the 807. The idle current was about 15Ma. The audio driver did not quite have the peak audio voltage output to drive the 6BG6 s to full output. What I heard at lower modulation levels was the same type grunge distortion to a some what lesser extent. I gave up. I configured the modulator for the usual class AB 2 tetrode mode.It worked OK FINE !!!. I still was not to give up on the triode connection concept. I had started building my 4-1000 transmitter I chose a pair of 4-1000s as modulators in  RC coupled AB1. While all of the Ts were dotted and the Is crossed it would NOT make 100% modulation at low power(1500 volts) At medium power 3Kv it would barely make 100% mod. At full strap(6Kv) it made somewhat better than 100% positvive,if you could keep things from blowing up with that kind of voltage. Little did I realize that the problem was the ratio of the modulation transformer was the limiting factor. I had an 811 modulator deck from another transmitter. I pulled the plate caps off of the 4-1000s and placed them on the 811s  I made sure that the gear shift was in first gear I would image 6Kv on 811s would turn them into flash bulbs. To my surprise the 811s fully modulated the transmitter 100%. I disconnected the pair of 807s audio voltage amplifier and grabbed my HH Snot stereo amplifier and a good push pull audio transformer. I tried a few configurations. I tried tying the screen grid and the grid  in  in parallel at each tube. It appeared to work. I could make full modulation in first gear. I went into high gear to make some heavy duty changes to the modulator. As it is I had a 1500 volt power supply for the RC coupled 807 audio voltage amplifier. I also had a series screen regulator using a pair of 211 s as the pass tube regulator. I ran 850 volts on the screens of the 4-1000 s. I carefully diassembled the 807 audio voltage amplifier from it's sub chassis and built it on to the screen regulator chassis. The 211 s were replaced with a pair of 845 s . This was years before the audiophools discovered the 845.  I needed a driver transformer i I had an old UTC VM-4. I took the core apart and reassembled the core in the cross lamniated configuration. This it to obtain maximum inductance being that there is no appreciable unbalanced DC on the transformer. If you are familiar with that piece, Terminals7&12 to the 845 s 9&10 tied B+ CT. Secondary: 13 &14 to the screens of the 4-1000 s.2&5 are connected to the grids. driving the grids at much lower peak audio voltage.  3@4 tied. CT of secondary This is connected to chassis ground when transmitting. I used the bias voltage for the 845s about - 250 or so as cut off bias during standby.This configuration worked very well. No problem making full modulation. However I was getting comments about excessive band width. This was due to the poor performance of UTC vari- match modulation transformers. I went to an S 22-somewhat better. While trying to figure out the distortion issue I tried a few other configurations: Screen drive only Grids at audio ground , Grid driven,screens to ground. These schemes did not yield good results . I decided to try the RCA method. I came up with a pair of 20K 50 watt wire wound resistors. The modulator performed like it did with as I will call it the dual differential drive method. The RCA configuration suffered from the same grunge distortion the afflicts the 807 s. I went back to the differential drive method. Years later I came across a UTC 250 watt linear standard modulation transformer. This cleaned up the HF audio distortion.  In about 1977 after moving here to Skowhegan, Maine, a friend of mine had a Junkston Valiant that had a number of issues.  Chernobyl resistor melted down in a bad way. After cleaning up that mess I decided to work on the audio. I went overboard. I pulled the modulation transformer out, took the core apart and cross laminated it. In place of the 866 s after solid stating the HV supply I made use of the space for a 10Hy 300Ma filter reactor as a modulation reactor. I decide to triode connect the 6146 modulator tubes. I had a 15 watt modulation transformer that had two sets of primary taps as well as the full primary connection. I connected the full primary to the screens and the set of taps colsest to the CT to the grids. I used a single 6550 as the audio driver tube utilizing the secondary of the 15 watt modulation transformer as the primary. I fired up on 75 expecting to get glowing praises as to how good it sounded. I was crest fallen when I was told that the grunge distortion levels were prominent. I listened to it on a good broadband receiver. It sounded like crap. Surely some mistake was made in the speech amp. I pulled the 6146 mod tubes out and took an audio output transformer,hung it across the screen pins( full driver transformer secondary)and connected it to my large speaker system I had hanging on the wall. I patch an FM radio tuner into the line level input. The audio was clean and robust. So where is this distortion coming from ? I grabbed a 6.3 volt 20 A filament transformer, A pair of 4 pin sockets and a pair of 811 s. I JS ed it together at the side of the transmitter and tied the grids of the 811s to the taps that went to the grids of the 6146 modulators for the right step down impedance. I key up the transmitter. The audio was very clean and green. Hmmm I said. I had a pair of used TV sweepies ,a pair of 6KD6s. I happened to have a pair of 12 pin sockets kicking around as well. I hooked up the 6KD6s in differential drive method like I did with the 6146s . The grunge distortion was WORSE. The idle current was about 150Ma so I did not want to have things key down for any length of time. I tied the screens to ground and just drove the grids. The audio was very clean but low modulation because the tube would not pull much more current with audio. Finally I grounded the grids and drove the screens . That was the magic bullet. The 6KD6s worked every bit as well as the 811s. I then addressed the high idle current. I found a  Radio Shack multi-voltage wall wart. I lopped off the DC connector . I tied the + to ground and the - to the grids. About -6 volts brought the idle down to 60 Ma. I operated with the transmitter for a while. I returned to the 6146s with screen drive only. It worked it was clean. What I noticed is that it had trouble making full modulation. I figured I had spent enough time on it. Overall it sounded good so it went back to it's owner Jack, now W4PPT, the founder of the Boat Anchors list. Shortly thereafter P.W. Fallon,WA1IWQ was doing up a Valiant his way .He liked the idea of the triode connected 6146s. He was probably the first person to build up a solid state audio driver . A pair of LM 383s in bridge mode. I gave P.W. a push pull audio transformer to couple to the screens of the 6146s He used a UTC S 21 120 watt mod transformer. Much better suited than the stock transformer. It was set up in the 3:1 impedance step down ratio to deal with the low impedance of the modulated stage in the Valiant. While it sounded great he could NOT make full modulation.The solid state audio driver bucked and farted to try to make full modulation. P.W. had a Dynaco MK II 60 watt audio amplifier. He used that to drive the 6146 s The bucking and farting was gone but it STILL would not make full modulation. A new pair of 6146 s was placed in the modulator. No improvement. We were both at an impasse. While PW can up for a visit I told him how well the 6KD6s worked. I had a pair of 6CD6 s . Basically 2.5 amp filament tube. About twice the peak emission of the 6146. The 6CD6 is like a lighter version of the 6DQ5. P.W. wired them in and biased them correctly.Now the transmitter easily made full modulation. The solid state audio driver behave very well after that. I puzzled over the reasoning with the difference between the two sets of tubes. Certainly the 6146s in tetrode mode make 120+ watts of audio. Why not when in triode connected mode ? I gave it a lot of thought. I may be right or wrong here. Beam power tubes which we are using are beam power tubes due to the fact that the screen grid is wound at the same pitch as the control grid. This is to get maximum gain and minimum screen current draw. In the triode connection ,the screen being the only driven element, the control grid is a useless appendage. It is analogous a lunar eclipse with the shadow of the grid limiting the potential emission of the cathode. As it is many others have used the 6DQ5 in the Valiant and other transmitters in triode connection mode. I have a viking Valiant that I did up for a friend in 1984. I used a pair of 6KD6 s in the modulator and a Chicago CMS 1 250 mod transformer stuffed into it. I eventually wound up with the transmitter. I used it as a 100 watt class daily driver until something crapped out in the power supply. I have used it for
 better than 10 years before it died. The game plan over the Winter is to be able to extricate it and do some research on differential drive to see if there is anything to it-see last part of this posting.
We revisit the 807 triode connection method. I had acquired a home brew modulator with a pair of triode connected 807s and an ART 13 mod transformer a 6BL7 audio driver using a UTC S 9 push pull plate to push pull grid driver transformer . This transformer has several sets of secondary taps for a number of turns ratios. I fixed up the lower level stages. Added bass and treble tone controls and decided to try it out. I did the audio output transformer hookup to the big monitor speaker on the wall. The FM tuner was patched in. The grunge distortion at low signal levels was apparent. I noticed a strange phenomenon . As the audio level is slow increased the 807 plate current went DOWN instead of up. This was when the distortion was most evident. As the levels were increased the distortion went down along with a greater plate current draw. Even at a good loud level from the speaker there was always an underlying distortion. After examining the  S 9 transformer. I took the secondary taps closest to the center tap and connected them to the grids of the 807s therefore eliminating the 20 K resistors. That did it ! Differential drive works well with the 807 in triode connection mode. I did the same thing many years later when I built the 50 watt SBE rig. I am using a pair of triode connected 807 s with 1000 volts on the plate and an ART 13 mod transformer. I used a UTC S 8 and a single 6V6 as the audio driver. The S 8 has a number of secondary taps like that of the S 9 . This arrangement also work very well.  Even though it has become standard procedure to just use screen drive I am ov the opinion that differential drive of big TV sweepies will yield more gain and perhaps better liearity. However recently Steve WA1QIX stuffed a pair of triode connected 6DQ5s in a Ranger owned by Mike N1HXA. A Stancor A 3893 60 watt mod transformer was used. A direct coupled FET audio driver was used. According to Steve he has researched the 6DQ5 s characteristics. In screen drive mode it is VERY linear. Being that there is no driver transformer used it is possible to wrap large amounts of negative feed back around the whole modulator to obtain superlative audio performance. While the use of a conventional type driver transformer circuit is very reliable and fool proof it can't quite measure up to a direct coupled driver scheme. The driver can also be hollow state based perhaps using a modification of the cathode follower screen modulator circuit used in the DX 60 and other transmitters of that type. Using two 6DE7 tubes with a phase inverter ahead of them to work them in push pull would probably make a very good driver.  A 6BL7 would also make a good push pull cathode follower with a push pull audio voltage amplifier ahead of it. I have not worked out the circuit details yet. A last observation: sometime back Terry W2PFY brought by a Sonar SRT 120 transmitter. A single 5894 with both sections in parallel modulated by a pair of 6L6s in triode connection like that of the 807. The audio driver consisted of triode section of one of the lower level stages into a small 1:3 audio transformer to a 12AU7 as a cathode follower to drive the screens of the 6L6s. The mod transformer is a big Triad unit. I did the following Used a much better 1:3 audio transformer and upgraded to 6CG7 dual triode as the cathode follower. A 12BH7 is a drop in replacement. I did not have one available at the time. The 6L6s are numb with just the screens driven. I went to a pair of 6Y6 s .Same pin out as the 6L6 but a 1.2 amp filament. This tube has similar peak emission to a 6146. The configuration worked quite well. Problem being that cathode follower was positive biasing the 6Y6s which were pulling too much current. I needed a lower resistance return to ground. I decided to use a Hammond push pull plate to 150/600 line output transformer. I used a resistor from the primary center tap to ground. There is bias voltage in the SRT 120. This worked very well. I later tied the secondaries in series I tied the center tap point to ground and each side of the total 600 ohm winding to each grid. The audio did not appear to have any grunge distortion. The power sensitivity improved. For the hell of it I reversed the lead to the grids to observe how it would work. The audio went to zip. To me this proves that I hit upon the correct level of differential drive to high gain tetrodes. As it is Terry hardly ever used the transmitter. He left it on at his basement work bench. He was greeted by the stench of burning power transformer. It stank up the house. His Wife insisted that he get rid of the transmitter. I wound up with it. It is in my collection. I replaced the power transformer with an old black and white TV power transformer. The transmitter makes about 25 watts .
In closing I hope this long old buzzard posting will shed light on the best ways of triode connected big TV sweepies and other tetrodes . All are a bit different. The 813 for example is utilized by tying the screen and grid together and driving it with audio. There are many possibilities with other tubes.
Christmas day-Tim WA1HnyLR
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« Reply #55 on: December 25, 2018, 06:03:44 PM »

Tim,

Thanks for sharing some of your 50+ years of experience experimenting with modulator design. You are a font of knowledge on the subject and many have benefited by it in your on-air discussions.

When I first noticed your posting here, I worried that it might be a very, very, long one. Thankfully though, you kept it down to only one paragraph. Grin

From what I've read in the DIYaudio forum, where many hours of research have been done by top experts in the field of audio amplification using vacuum tubes, proportional drive of the grids of sweep tubes will yield linear amplification heretofore unknown in the realm of tube-type audio amplifiers. I intend to implement what I've learned there, in a modulator which I will briefly describe here.

The difference between screen drive and proportional 'crazy drive' is a resistor from G2 to G1 and a resistor from G1 to ground. For a push-pull amplifier that's only four additional resistors to accomplish the task.

However, I'm going to be fancy about it and use a dual potentiometer. One end of each pot will go to G2, the other ends going to ground. The wipers will go to each tubes G1. G2 will be directly driven by a source follower.

With this, I can tune the ratio of drive from zero (G1 at ground, just like conventional screen drive), to a point that causes the overall gm of the tube to be even more linear than with screen drive alone. At the same time, this should reduce the drive voltage requirement. The increased gm linearity will permit the idling current to be lowered as well, increasing power efficiency. This is the hope.

Rather than scratch-build a transmitter and modulator, I've opted to modify a Heathkit DX-100. I've replaced the 1625 modulator tubes with a pair if 12GC6 sweep tubes which will be driven by a pair of STP7N95K3 MOSFET source followers. This is similar to a portion of the schematic that Steve, WA1QIX has posted, elsewhere in this forum, of a modulator that he designed and built for a Johnson Ranger transmitter - sweep tube modulators driven by 'FET source followers. The similarity ends there, as I will explore the proportional drive of the sweep tubes and use different line amp stages.

I have been moving forward on this project as time and funding become available to me. I'm following the learn-as-I-go building method on this project rather than amass a pile of paper notes that, in the end, never get built.

I greatly appreciate the members of this forum who have taken the time to follow and post here. It's my hope that what we find here together will be of benefit to those, like yourself, who enjoy building their own AM equipment.

Thanks again for your contribution!


Don
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« Reply #56 on: December 28, 2018, 11:24:03 AM »

Here's the latest drawing... for those of you who are following along. Wink

Don

Edit: I think I finally figured out how to get the image correct.  Roll Eyes


* 12GC6 crazy drive modulator for DX-100 rev7_A.png (130.88 KB, 2236x1192 - viewed 30 times.)
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« Reply #57 on: December 30, 2018, 02:39:47 AM »

That looks great including the dual pot.
You could always label the knob for the dual pot on the grids "Linearity" or "English" or something.
It needs a name - you ought be the one to name it if you make it work.
Or use an audiophool mystery name like "Differential Mu Compensator"..  Grin
Illudium Q-36 Modulator?
20K to GND -may dissipate some peak power 660mW? in the pots. Might pay to try to use 2W parts if they can be had.
I Like what you did.
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« Reply #58 on: December 30, 2018, 09:22:20 AM »

Phil,

I had ordered a dual 2 watt pot from Hong Kong - still waiting for delivery. It didn't even occur to me to "name" it. You proffer some interesting suggestions. Smiley
It's just a standard "Manually Operated Dual Carbon Rotating Armature Potentiometer." or "MODCRAP". With that in mind, I'll update the drawing with the label "Set Linearity Under Test". Grin Grin Grin Grin

Don
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« Reply #59 on: January 02, 2019, 01:55:44 PM »


Don,

   A few talking points:

In your schematic, the 12AY7 phase inverter needs to have the right side tube's grid at an AC ground, NO?

In some if the DIY Audio forum notes on cray drive, one recommendation kind of stuck out. That was to bias G1 negative, and G2 positive. The idea was to set the resistive divider ratio such that G1 peak voltage hits zero volts when G2 is at the positive peak. This keeps grid current from flowing into G1, and therefore the resistive divider ratio stays constant with drive level. Might need something like -30v on G1 with +60v on G2 (or thereabouts) to establish the desired idle current (maybe 10ma cathode current per tube?).

In other DIY audio discussions about G2 drive, it is mentioned that some sweep tubes can get the plate voltage minimum as low as 40v when the G2 voltage is at the Maximum peak (maybe about +150v peak). The screen then becomes the plate and therefore the screen current soars. There are mentions of the need to limit the screen current to protect the sweep tubes from a catastrophic destruction of the screen grid. This could be as simple as a drain resistor added on the FET cathode follower. The caution exists if excess drive overload happens too often, or real quickly if the overload is sustained (i.e square waves).

Jim
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« Reply #60 on: January 02, 2019, 05:53:28 PM »

Jim,

In your schematic, the 12AY7 phase inverter needs to have the right side tube's grid at an AC ground, NO?
Yes it does. I totally missed that - added it to my next drawing revision. Thanks.

Re: Biasing G1 negative and G2 positive.
I hadn't seen any mention of that in my readings. Not sure how to impliment it, or if it's going to be needed. I just don't know. I'll look at this when experimenting.

Re: Limiting screen current.
I'll add the drain resistor to offer some protection. Fortunately, the tubes I'm using, 12GC6, are inexpensive and readily available so I can make a few mistakes. Roll Eyes


Obviously, you've been reading the audio forums on crazy drive. That's a good thing! I'd be lost going this alone. Thanks for your assistance.

And you've been helpful to me in ways you don't realize. While searching this forum last night as to why my DX-100 can't make more than 80% modulation, this posting you made, almost 6 years ago, proved to be very informative.
http://amfone.net/Amforum/index.php?topic=33195.msg258713#msg258713

Thanks again!

Don
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« Reply #61 on: January 03, 2019, 10:59:12 PM »

At this point...

I have the 12GC6 sweep tubes in the DX-100 modulator. With G1 grounded and driving G2 with a transformer, and -7.7V bias, the cathode resting current is about 20mA.

Driving it with an external amplifier, the audio sounds pretty good through this lash-up but positive modulation won't go above about 80%. Negative modulation can easily white-line the baseline. It's got plenty of power! Swapped input phase - same thing. Swapped plate cap leads - same thing. At 100% negative it's only 80% positive. Increasing the drive level, I can make the negative clip hard at the baseline but the positive just sits at 80%. It's weird looking on the old Eico 460 'scope.

It seems like the problem is at the RFPA screens. I have the original 20k screen dropping resistor in there. I'm going to increase that to 30k (as is often recommended to lower the screen voltage) and see what happens.

On the DX-100 there's a 500/2300 ohm tap on the 2800 ohm modulation transformer's secondary. As a lark, I may tap-off it with the 30k dropping resistor. That gives me the option of 18% or 82% modulation for the screens (relative to 100% on the plates) depending on how I wire it. This might prove interesting.

Either way, I need to get the rig modulating properly before I attempt any crazy drive setup. It should make for a fun weekend!

Don
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« Reply #62 on: January 06, 2019, 12:13:42 AM »

Well...

After spending a few hours going around in circles trying to figure out why the DX-100's modulation envelope displayed on my old Eico 460 'scope looked so weird, I schlepped the HP 'scope up from the basement to get a second opinion. Looks much better with the HP - making 100% positive modulation now. The Eico's fate is sealed.

Next step will be pulling out the amplifier tubes from the Eico and going straight to the deflection plates with RF and audio for a trapezoid pattern modulation monitor.  It's probably the best way to get an accurate measurement.

You know, I think the old girl would look good sitting next to the DX-100... and so will the Eico. Grin

Don
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