Drake L-4B Linear Amplifier

[N4GRF]

Hello.  I just acquired a Drake L-4B linear amplifier. This morning, I fired it up; heaters glowed beautifully; plate voltage came up to 2,500 v.  However, the grid current meter showed a negative grid current was negative. I could not get any output from the amplifier. I'm wondering if it's an ALC circuit. I used a ICOM 706II to excite the L-4B. Tell me if I need to rig an ALC connection?

Tom N4GRF

[W9PMZ]

Really don't need ALC, should be able to handle 100W without overdriving the amp.

Negative grid current...  I am sure that someone else knows this fault...  But I'd check for a grid short on the two 3-500Zs...

73,

Carl - W9PMZ

[AH6RR]

Really don't need ALC, should be able to handle 100W without overdriving the amp.

Negative grid current...  I am sure that someone else knows this fault...  But I'd check for a grid short on the two 3-500Zs...

73,

Carl - W9PMZ

My thoughts also is that a tube has a problem. Try pulling both tubes and firing it up if it still has a neg grid current then check the grid circuit if not then plug one tube at a time in until you find the bad tube. BE SURE TO WAIT FOR THE HIGH VOLTAGE TO DROP ALL THE WAY BEFORE DOING ANY OF THE STEPS HV CAN KILL. Your plate voltage looks OK.

Roland AH6RR

[N4GRF]

Carl & Roland: thanks so much for your thoughts here.  I come from the vacuum tube era (licensed in 1966), but I haven't worked tubes since college. The HV on an L-4B is fairly intimidating.

If substituting tubes, one at a time, how much time should elapse before the HV bleeds?  Of course, without the tubes installed (Rolands's suggestion) the bleed should take more time?  I'm hesitant to discharge the HV because I recall doing so is injurious to the filter caps and resistors.

[ZENKI]

If you unplug the L4B and its power supply and let it sit for a few hours it should be safe. To be 100 percent safe,  make a set of shorting sticks. Just get 2 pieces of short PVC pipe, put two bolts on the end. Attach two big 100k resistors in parallel  to one of the sticks and the bolt then run a wire from the end of the resistors to the other bolt on the PVC stick. You can then discharge caps and high voltage safely.

If you remove the L4B's RF deck cover it will short everything to ground, it has a safety switch on the RF deck. Just make sure the power supply is plugged in.

The grid shunt in the L4B is very tough thats unlikely to have opened.

I would also check the cut off bias resistors on top of the power supply they go open circuit.

The power supply module including the rectifiers and filter caps are very easy to remove and check, its all on 1 module.

Anyway you probably have a  bad tube, the Drake L4B's that i have owned have never  had any circuits fail except for the  cutoff resistors.

If you going to change the tubes, I would directly ground the grids  thats about the only urgent mod that it needs.

Carl & Roland: thanks so much for your thoughts here.  I come from the vacuum tube era (licensed in 1966), but I haven't worked tubes since college. The HV on an L-4B is fairly intimidating.

If substituting tubes, one at a time, how much time should elapse before the HV bleeds?  Of course, without the tubes installed (Rolands's suggestion) the bleed should take more time?  I'm hesitant to discharge the HV because I recall doing so is injurious to the filter caps and resistors.

[VR2AX]

Maybe a problem with one or both of the 3-500Z.

What is your idle or standing plate current with the amp in keyed position but no RF drive applied from the transceiver (ie with just the PTT activated)? It should be around 200mA for two tubes, or 100mA approximately per tube. There may be some variation, a bad single tube may only show 50mA standing current, but a tube can also show that and work ok.

Try swopping one tube at a time and check the standing current individually.

It could also be a metering problem in the grid current metering circuit. The Drake uses the second meter to measure Ig, HV and RF output, switchable. maybe someone has reversed the connections to the switch or meter from the Ig metering circuitry (these things do happen..).

It's a good amp, you will enjoy using it once the minor glitch is fixed.

[W7ETA]

There is a device called a Variack(?spelling).  This allows you to start select a voltage between 0 and slightly higher than 120 volts.

I use one to reform electrolytic caps in tube based audio gear that I like to rebuild.  I also use it to check high voltage circuits by sending lower voltage to the circuit, transformers.

73
Bob

[VK4TUX]

This fault indicates a grid - filament short in one or both tubes, usually it is only one.
I myself would replace both tubes, repair metering board grid/plate shunt resistors as required when checked. Add diode protection to the meters (BTB 6A10 diodes fitted across the meter terminals on both). Fit a 50 ohm 50w ohmite glitch resistor in series with B+ HV supply, with a 1 amp hrc fuse also in series. Low Q vhf parasitic suppressor's would be fitted by myself to prevent re-occurrence, but the the other pre mentioned items will save damage in the future.

Adrian ... vk4tux

[W1QJ]

Negative grid current when the amp is not in the transmit mode (standby) usually means a grid to filament short.  Remove tubes one at a time to see which tube has the short.  It is also possible that negative grid current presents when the grid shunt resistor is open but that is usually when in the transmit mode.  I would first check the tubes and see which one has the short.  If you are seeing negative grid current only in transmit, check to see if the grid shunt resistor is opened.

[N4GRF]

Many thanks to all of you who replied on the L-4B question. I am grateful for the analysis/advice/encouragement. 73 and kindest personal regards, Tom N4GRF

[ZENKI]

Dont forget to oil clean the wimpy blower on the L4B. You cant find the exact blower replacement so its well worth looking after the original blower.