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I asked about the input cap because a couple of years ago, a friend, VK6OX,had a problem with his dual GI7b 6m amp. It took forever, for VK6OX, VK6JX and myself to track down but it ended up being the input tune cap shorted due to a flashover in one of the tubes.

In the article you quoted, input coupling cap, C1 is between the input socket and the input tune network. In this configuration, if input tuning variable cap C2 is shorted, it will ground the cathode. The grid is already grounded, so you effectively have a grid to cathode short via the capacitor.?

If C1 is on the cathode side of the network, this can't happen. In that configuration, if somehow C2 is short circuit, the tube will behave normally DC wise but your input RF will be shorted to ground.

Anyway. Check that out.

73, Alek VK6APK.

On 3/05/2025 2:01 pm, Philip via wrote:

Hi Alex,

Tnx for your reply. So the changes were not all at once. It was over a long time, one by one to?

try and find the cause

Let me reply to your questions.

Have you hi pot tested the GS35b between cathode and grid? Are you able to swap the old tube back easily?

I tested the Old tube and New tube with Heater On and saw a good Grid to Cathode current as a Ham instructed

me to do. If I remeber correctly he told me a very good valve will have about 8 mA current after about 30 mins, and good

one will have around 4 mA, under 3 mA the vacuum in the tube would have started to go soft and can be repaired with

some prolong running of Heater but not always. New tube is nearly 5 mA, old ones little over 3 mA.

Have you checked the heater feed through cap?

Yes I have changed the feed through caps with new ones same problem.

Also changed HT feed through. The HT Supply is used also for another

GS35b Amp and works 100% OK, so I eliminated the power supply from the equation.

Was the heater transformer a like for like replacement??

The old transformer was changed as it was an Oil Filled one and was leaking

New one was from another GS35 amp and voltage adjusted to 12.6V .

Is there a DC blocking cap between the input network and the cathode? If not, then check the input tune variable capacitor/s for short/s.?

Yes there is 1nF Blocking Door Knob capacitor leading to the input tuning network. I have not checked anything there, I was terminating

the input while applying HT.

Hope I have explained myself well, if anything not clear please let me know. Thanks for the help.

73 Philip

9H1PA

On Sat, 3 May 2025 at 05:23, Alek Petkovic via <vk6apk=[email protected]> wrote:

Seems like you've changed too many things at once.?

A few guesses off the top of my head.

Have you hi pot tested the GS35b between cathode and grid? Are you able to swap the old tube back easily?

Have you checked the heater feed through cap?

Was the heater transformer a like for like replacement?

Is there a DC blocking cap between the input network and the cathode? If not, then check the input tune variable capacitor/s for short/s.

Good luck, Alek VK6APK.

On 2/05/2025 5:07 am, Philip via wrote:

Hi All,

I am new to the Group, a friend of mine suggested to ask my problem here as there are many capable Hams on this group.

I have a GS35 on 2m and has been working for years, I have changed the tube during it's life and blown a bias transistor here and there.

The problem I am having now is one I have never seen. When I apply HT to the Amp usually the Amp is cut-off with the cut-off resistor and no current flows. Now I get Negative Grid current and Anode current starts to flow, and the more I increase the Anode say from 1000v to 1500v the current increases.

The things I have changed in the Amp:

Tube - New tested good tube installed.

Bias Board - tested adjustable from 0 to 55 v

Filament Transformer

HT & Heater feed-thru cap.

Usually when the Grid Bias transistor goes - I am using a BDX66B from DL4MEA design (? ) the amp does not do anything so not sure that Bias is the problem.

I anyone might have come across this negative grid current problem please let me know.

73 Philip

9H1PA