[Boatanchors] EF Johnson thunderbolt
Rodger Singley
wq9nsc at live.com
Tue Aug 24 17:47:18 EDT 2021
The Thunderbolt also meters on the low side of the HV output so normally the voltage across the meter is very low HOWEVER there is no shunt/safety resistor in the Thunderbolt in parallel with the meter so if the coil does open (or if the grounded terminal connection is broken), then the meter movement does rise to full HV potential.
I also prefer cathode metering. The Tbolt has separate metering of the control and screen grid current so if you want the actual plate current those can be subtracted from cathode current.
Low side HV current metering is only acceptable to me if a dead reliable shunt resistor is part of the metering circuit so that voltage potential across the metering circuit will be limited if the meter movement goes open.
My Johnson Desk uses cathode current metering but did not have separate metering for screen current so you had to guess at the potential of each based upon Johnson’s predicted values. I added a small external screen current meter which solved that problem. Any high power grid driven tetrode amp should have screen current metering since it provides the best indication of proper tuning and loading.
Rodger WQ9E
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From: Walt Cates<mailto:cateswa at msn.com>
Sent: Tuesday, August 24, 2021 4:33 PM
To: Rob Atkinson<mailto:ranchorobbo at gmail.com>; KA2PTE<mailto:ka2pte at 3950.net>
Cc: Boat Anchors List<mailto:boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] EF Johnson thunderbolt
A lot of systems do plate current monitoring on the low side of the high voltage rectifier. The Hallicrafters P2000 is a good example of that.
See: P-2000 restore (wordpress.com)<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwd0gof.files.wordpress.com%2F2021%2F06%2Fp2000-restore-v3.51.pdf&data=04%7C01%7C%7Cfdf9158b163c47074ed408d96746e1ea%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637654376396827452%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=sZwbUZXycVyNuqEuDaSz6hVy1EvblwyVDevn0mNhfNg%3D&reserved=0>
Walt Cates, WD0GOF
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From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of Rob Atkinson <ranchorobbo at gmail.com>
Sent: Tuesday, August 24, 2021 3:09 PM
To: KA2PTE <ka2pte at 3950.net>
Cc: Boat Anchors List <boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] EF Johnson thunderbolt
That sounds like a mistake. What's wrong with measuring cathode
current? It's a lot safer. You can measure the plate current using
the cathode.
Rob
K5UJ
On Mon, Aug 23, 2021 at 8:21 PM KA2PTE <ka2pte at 3950.net> wrote:
>
> Curious if anyone here would know the max KV rating for the stock plate
> meter?
> It looks pretty sturdy, and I am rewiring it so its actually measuring plate
> current, not cathode current as it was modded previously.It will have 3.5Kv
> on it now,
> which is more than the factory 2.4Kv
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