Another 811h issue

[edfiero]

Like some others I think I might be victim of an amp damaged in shipment. (I'd like to see the best in people and hope the guy wouldn't have sold me a bad amp)

Anyway here is my issue:

I've replaced all 4 tubes with a never used, matched set.

At Standby, switch in HV, it reads 1700 volts.
At Standby ,switch in Ip, it reads 0
--so far so good.

Keying the transmitter (no drive), I get plate of only about 50ma, not the expected 125. Also at the same time, the grid current meter is showing around 25. (The tune up instructions don't indicate what the grid current should read).

With about 40-50 watts of drive power, the maximum plate current after adjusting load and plate to max is 310 and the maximum grid current is about 120.

The output of the amplifier is only about 180 watts.

Increasing the drive up to 80 watts does not significantly raise the current or the output. Output never goes over 200w and the voltages don't increase.

I haven't operated the Amp with the cover off, but when removing the cover after tune up and then shutting it down, I can feel that 1 of the 4 tubes is significantly cooler than the others. I switched the position of the tubes and now the new tube in the previous problem position is not as hot. So the slot could be a problem not the tube.

Thanks for any troubleshooting info you can provide. Where should I be looking for this sort of problem?

 

[BJ radionut]

First Question are ALL 4 Tubes Lighted...Look through the side panel and verify.
Even with the amp in stand-by position each tube should be glowing.
Let's start there.
All the Best
Gary

PS: What brand of 811A's?

 

[BlowinSmoke]

Make sure D16 is not shorted in the power supply circuit. That can cause you to see grid current in transmit with no drive. If it is shorted you may see the plate and grid meter tracking the same way on the scale. You can only check it with one leg lifted off the board. Replace it if it is shorted. It is a 1N4007. Then try again and see if the grid current goes away when amp is in transmit with no drive. If you still see grid current after you check the diode, then you have a problem tube(s)

 

[edfiero]

Hi Guys, I have an update to this problem. I found out that D16 was shorted, 3 tubes were bad, and 3 resistors (under the tube chassis were blown).

I replaced all 4 tubes. I have replaced the D16 diode as well as performed the AL811 mod from w8ji.com consisting of removing the resistors and grounding the grid. Adding in the 2 gas voltage clamps and neutralizing transformer.

Now, with transmitter keyed, no drive Plate reads 125 in IP mode as it should Grid reads Zero as it should. Improvement over when I first got this thing. (voltage at 1700)

However when applying drive power, however there is no grid current. Meter on 0. Plate reads 325ma which I think could be about normal. Confirmed with Watt meter that the Amp is not outputting any more than is being provided by the radio.

All 4 tubes are now lighting up as I think they should when powered on in Standby.

Any suggestions on where to look for the next problem. What could cause no grid current?