Eico 720

[WA4WAX]

I started working with a nice Eico 720.  I noticed that it does not have a variable neutralization cap.  What is the design approach here?  Most 6146 rigs I have seen have a variable neutralization cap.

[DMOD]

I started working with a nice Eico 720.  I noticed that it does not have a variable neutralization cap.  What is the design approach here?  Most 6146 rigs I have seen have a variable neutralization cap.

Are you experiencing high plate currents on 20-10 meters or seeing VHF spikes on a spectrum analyzer?

Look at the output impedance specs of the Eico 720 verses the DX-60, in which the latter has a narrow output impedance of 50-75 ohms and Neutralization. The Eico has a 50 to 1000 ohm impedance pi net without neutralization feedback.

My theory is that a pi-net with a narrow ranging output impedance implies a pi-net with a high Q. Conversely, a pi-net with a wide ranging output impedance implies a pi-net with a low Q.

A low Q pi-net is considered less efficient but it also means more dampening of potential parasitics'.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/serres.html#:~:text=The%20resonance%20of%20a%20series,is%20useful%20in%20tuning%20applications.parasitics'.

Phil - AC0OB

[WA4WAX]

I have yet to fire the beast up, but do not anticipate trouble. It is like new inside.

Eico did a nice job with that transmitter from both a design and build perspective.  I think your analysis is on target.  If you follow the wiring diagram, you will see a low value (40 pF?) fixed after the plate blocking cap that goes to ground.  A similar cap goes from 6146 signal grid to ground, putting them in series.  I suspect that the low Q Pi allows you to get away with fixed caps, obviating the need for variable caps and adjustment with final changes.

I hope to pair the 720 with my 2B and Johnson T/R switch.  Yes, I have a low pass filter.

[K9MB]

I started working with a nice Eico 720.  I noticed that it does not have a variable neutralization cap.  What is the design approach here?  Most 6146 rigs I have seen have a variable neutralization cap.

I believe that Eico used the almost identical circuit that was used in the DX100 by Heathkit.

What it looks like is a pinet coupling circuit between the driver and the grid of the final amp.
The idea was that the pinet output capacitor served as a capacitve divider between the grid plate capacitance and between the grid and ground so that any feedback voltage is reduced by the ratio of the grid-plate capacitance and grid to ground capacitance and this lowered impedance would serve to stabilize the amplifier without neutralization.

There is actually an article in the 1968 ARRL Handbook on this technique.
Page 160-161
One can download the 1968 ARRL HANDBOOK at this link.

https://archive.org/details/arrl_1968_handbook

[DMOD]

I have yet to fire the beast up, but do not anticipate trouble. It is like new inside.

Eico did a nice job with that transmitter from both a design and build perspective.  I think your analysis is on target.  If you follow the wiring diagram, you will see a low value (40 pF?) fixed after the plate blocking cap that goes to ground.  A similar cap goes from 6146 signal grid to ground, putting them in series.  I suspect that the low Q Pi allows you to get away with fixed caps, obviating the need for variable caps and adjustment with final changes.

I hope to pair the 720 with my 2B and Johnson T/R switch.  Yes, I have a low pass filter.

There is a 140pF variable cap (C16) after the DC blocking cap (C15) but that is for plate tuning. C50 is additional capacitance for loading up on 80 meters.

Mike (MB) is also correct as in fact, page 2 of the 720 manual states this very thing, as C13 (68pF) is the "de-harmonicizer."  

The Hallicrafters HT-40 and the Allied Knight T-60 both used a Pi-net grid driving circuit as well, but their grid shunt values were on the order of 9-10pF, because these sweep tubes have an input capacitance of about 23 pF. I have never seen any parasitic oscillations in either the HT-40 or the T-60.

I would recommend adding a 2.5mH choke (of at least 160mA) between the SO-239 center conductor and ground in case C15 shorts so that no HV DC gets into the tuner or antenna.

Phil - AC0OB

[K9MB]

This pinet coupling circuit has another interesting factor.
If one wanted to add a neutralizing capacitor, it could be added between the plate of the driver tube and the plate of the 6146 final amp because of the phase shift in the pinet coupling circuit.
Check out the schematic of the later Heathkit Apache TX1 which still used the pinet coupling and an added neutralizing capacitor.
See schematic at this link:

https://usermanual.wiki/Collections/magnet/files/500%20Ham%20Radio%20Manuals/HEATHKIT%20APACHE%20TX-1/HEATHKIT%20APACHE%20TX-1%20SCHEMATIC.pdf.html

They emphasize the lower harmonics with this pinet interstage coupling, but the fixed capacitor also serves to step down feedback capacitance, which can also serve to stabilize the amplifier. It was-however- not considered sufficient in the Apache, however, so they added the neutralizing stub from driver plate to tue 6146 plates to properly neutralize the amp.
It could be easily added to the Eico 720, I think, if instability was an issue.

[WA4WAX]

Yep.......could one connect the neutralizing cap leads after the blocking caps of driver and final?  That would keep HV off the cap to large extent.

[K9MB]

Yep.......could one connect the neutralizing cap leads after the blocking caps of driver and final?  That would keep HV off the cap to large extent.

Yes, just connect to stator of the C12 grid tuning cap.
Then route through to top of chassis near plate of 6146. You could use a flat piece of brass or copper sheet and mounted on a standoff and move closer or further away from 6146 plate.
Just checking the physical layout, you might tap the top of S2D and route through the shield to a vane mounted on the shield just adjacent to the 6146 plate
Or just route the connection to the stator of C12 straight through the chassis to your vane opposite the 6146 in top. Short leads and low stray capacitance important.

[DMOD]

Yep.......could one connect the neutralizing cap leads after the blocking caps of driver and final?  That would keep HV off the cap to large extent.

If there is no indication of parasitics i wouldn't make any mods to a working 720.

The input and output networks have taken care of any potential parasitics.

Phil - AC0B

[K9MB]

Yep.......could one connect the neutralizing cap leads after the blocking caps of driver and final?  That would keep HV off the cap to large extent.

If there is no indication of parasitics i wouldn't make any mods to a working 720.

The input and output networks have taken care of any potential parasitics.

Phil - AC0B

I agree.

The pinet coupling is a clever circuit that lessens the effects of grid-plate feedback by effectively stepping down the voltage by the capacitive divider and it also lowers the impedance so that it is similar to a swamping resistor while still suppressing harmonic feedthrough. I was merely adding a way to neutralize the tube, if needed.