GI7B VK3ZL Amplifier not playing the game

[VK3IH]

I have acquired VK3ZL 2 x GI7B amplifier on 40m.

It was the one Bob used himself on his desk.

I have tested low voltage relay switching - all okay
HT okay
Variable Bias increases Anode current.

However PTT to operate and very high SWR and NO OUTPUT - Anode current doesn't change and NO Grid Current

Can't work out what is wrong?
How do I check the GI7B valves?

I have pictures if that helps.

Paul

[ZL1RS]

Hi Paul,

If changing (lowering) the bias increases the anode current, then highly likely the tubes are OK. If there is doubt about one tube or the other, you can remove the anode connection to each one in turn and see if there is still varying anode current when the bias control is varied.

By "very high SWR" I assume you mean high input SWR? If so, that and no increase in anode current/grid current with RF drive indicates that there is no RF drive getting to the cathodes, so check/clean the input relay "TX" contacts and check/replace the RF input tuning components (all the stuff on the right hand side of the tube bases in the first photo).

73,

[VK3HZ]

And check your coax cables for continuity.

[VK3IH]

And check your coax cables for continuity.

Yes done that and the drive power goes through to the dummy load when the amp ptt is off.

I will go over the simple input network again today but the relay is switching through to it and there is continuity as far as the dc blocking caps.

Puzzling,

Paul

[VK3QI]

Paul,

Have you checked to see if the input relay is actually switching the signal through to that blue input capacitor in the picture?

(1) Is the PTT switching voltage a grounding circuit or a positive voltage circuit - knowing Bob, it could have been different to what you normally have on the output of typical transceivers.

(2) Has someone tried to fiddle with the input matching network, thinking they can "improve" it?

Good luck and keep safe with those voltages!

Peter VK3QI

[VK3IH]

okay progress report:

-lifted one plate = half anode current so all probably well with tubes
-found dry solder joint on one side of input network coil (didn't fix high input SWR though)
-found that 12V low voltage circuit for relays was only 10.8V under load and not enough to operate relays reliably
-attached external 13.8V supply relays now operating reliably

-500W key down on 40M is a bit unreliable, easier to tune on 30m - 500W key down

-need to find a small transformer to replace 10.8V output one that is not supplying enough volts for relays

-need to work out why high input SWR on 40 & 30m (the slug in input coil can be seen to be peaking output slightly BUT still high SWR)

[VK3IH]

Progress report:

Found another tap on low volt transformer - relays now operating properly

High input SWR remains on any band tried (30/40/20)
400W key down on 30m won't resonate on 40m but did?????

2 x GI7B should = 800W key down I would have thought? May be one valve down will research..........

[ZL1RS]

Hi Paul,

A pair of GI7B's should do 800 Watts, but only if:
the HT voltage under load is sufficient ... probably 1800 to 2000V is required.
the input is matched (some how or another) ... but it is obviously not at the moment (but see comments below)
the output is tuned/loaded correctly ... can't be far off if you are getting 500W with the existing bad input circuit

First off you mention it is a 40m amplifier, then say you are trying to tune it on 20 and 30m. From the photos there is no band switch apparent on the anode inductor to get the right value for the other bands, and the input circuit appears to be suitable for a single band amplifier (if it has the correct values). In that case probably best to stick with 40m as that was the band the PA was supposedly designed for, and start experimenting with the input matching circuit to get the input SWR down. Don't assume the input circuit in any home brew PA is correct, maybe in the past it was used with a solid state radio that had in internal auto tuner or with a valve TX output radio with wide range pii coupler to achieve the overall amplifier input match?

From the underside photo the input circuit appears to be a pii coupler, if both capacitor values either side of the slug tuned inductor are the same then that is unlikely to provide a match to a 50 ohm solid state radio (unless "lucky" with the valve's input capacitance). Check the input SWR (and general amplifier operation) with one leg of each capacitor alternately lifted (thus becoming a high impedance or low impedance L matching circuit respectively) and see the effect on the input SWR (while adjusting the slug tuned inductor).

GL and 73,

[VK3IH]

Yes tuning the other bands was just a check to see if the amplifier had been modified for other bands without the face plate being changed......

At work at the moment but have had a thought regarding the pi input that I will test tonight.

Thanks for the advice I will report back with successes or otherwise..........

Paul

[VK3IH]

Progress report:

I had a closer look at the input network.....:-)it had NO earth/ground connection at all (advice re being suspicous of home brew circuits noted!) Adding an earth connection lowered swr to 3:1 and it now tunes on 40m and is 800W key down BUT for your amusement - only when on its side and not on its feet...... Another look at input relay and its dodgy at 11.8V and needs a tap to engage, add external 13.8V and it switches nicely so I will have to modify internal low volt supply to give a higher voltage.

Faults so far:

Low voltage issue
Dry solder joint on input coil (no contact)
Dry solder joint output relay (actually just twisted on once with no solder)
No earth for input network ( could it ever have worked without it?)

More to come when I have modified the low voltage supply,

Thanks for all the help so far.......

Paul

[VK3IH]

As an update all now working well, took some rework to get it that way but now operates as it should.

Last thing was to replace 12V '3 terminal regulator' for a variable output version set to 13.2V which now reliable switches the relays.

Thanks for all the help now have an easy 400W SSB or 100W AM.

Paul

[ZL4DK]

Just wondering if your relays are actually made for 24V. A 12V relay normally switches easily with 10 or 11V.

[VK3QI]

Paul,

I go back to my original post - was the relay actually passing signal through to the blue capacitor?

As has been said, a 12 volt relay should easily switch at 10 volts (perhaps not completely reliable).

Also did you check to see what the actual PTT circuit was doing? Grounding the relay coil? Providing a positive voltage to the coil? If the latter, then it is quite possible that there was a larger voltage provided.

Look forward to hearing your new loudenboomer in the JMFD this weekend from VK3ER/p.

Cheers

Peter VK3QI