Home Brew GS-31B HF Linear Amplifier

[W3DRK]

Greetings! This is my first post on this forum and I thought I would share my GS-31B linear and perhaps ask a few questions as well since there seems to be some builders on this forum with quite a bit of experience using the Russian triodes.

This is my first home-brew amplifier...I caught the bug after retrofitting a GI-7B into my GLA-1000 so I wanted to try building one from scratch.

The goal of this amplifier is to have a 10-160 meter amplifier that would safely achieve 1500 watts output on all bands and fit nicely on the desk at my operating position.

The triode is a single GS-31B and the plate transformer is a custom 80lbs 5KVA unit which will eventually be enclosed in a separate cabinet under the desk. Depending on which primary tap I use I can apply anywhere from 3600 to 4300 volts @ 1.5 amps continuous on the plate.

The amp is monitored and protected by a WD7S Triode Control Board and I also used his HV-2 rectifier/filter board as well as his TU-6B tuned input board, all of which I built from PCB blanks I purchased from him.

The amp was completely hand built by me, all 150-something screw holes hand tapped out in the garage. Some of the more complex pieces like the stainless shaft couplings were made at work on a lathe and a milling machine was used to mill the chassis openings for the meters, switches, and the opening on the bottom to access the cathode/tuned input.

I just recently got the thing up and on the air and so far it's been working really well, but I've noticed except for 160 meters the GS-31B seems to struggle to maintain 1500 watts output. I know the official specifications for this tube state a 1000 plate dissipation rating which I am exceeding, but some web sites mention in amateur service 1500 watts pdis should be achievable. I'm currently running about 4000 volts on the plate and grid current rarely climbs above about 220ma.

The problem I'm experiencing is after about 15 seconds of flat-out 1400-1500 watts CW output into a dummy load, the grid current will begin to steadily drop as well as the output. If I un-key and give the tube 10 or 20 seconds to recover it will return to normal Ig until I keep it steadily keyed again. I never leave it keyed long enough to see what eventually happens since I'd like to avoid long-term damage or catastrophic failure of the tube. Is this due to heat buildup in the tube perhaps temporary distorting the grid structure?

I'd also like to note that varying the plate voltage seems to have little effect on output power above 7 MHz...anything from 3600-4300 volts and 1400-1500 watts is about the most I'll get...and it tops out at about 600-700mA Ip. On the other hand, down on 160 meters it'll do >1amp on the plate and peg the watt-meter at 2KW.

I am considering the purchase of a GS-35B to see if it will be better suited for QRO use. Even though laying on the key isn't a practical test of capability, I'd feel more comfortable knowing the tube isn't being run on ragged edge and have plenty of headroom rather than thrash on this poor GS-31B.

Anyway here are a few cell phone pictures of the amp. Ignore the Vp scale on the plate meter...I need to re-scale it. Also, after these pictures were taken I've eliminated the temporary parasitic suppressor since this amp doesn't appear to need one and I've also added gas arc protection devices from B- and cathode to ground as a preventive measure.

Questions and comments are welcome!!

Thanks and 73

[NQ6U]

Way over my head. This looks like a job for...Super Amp Man!

That's Bob, VK3ZL; he builds a new amp whenever he's got nothing else to do and every one of them is a work of art.

[K7SGJ]

Greetings! This is my first post on this forum and I thought I would share my GS-31B linear and perhaps ask a few questions as well since there seems to be some builders on this forum with quite a bit of experience using the Russian triodes.


This is my first home-brew amplifier...I caught the bug after retrofitting a GI-7B into my GLA-1000 and wanted to try building one from scratch.


The goal of this amplifier is to have a 10-160 meter amplifier that would safely achieve 1500 watts output on all bands and fit nicely on the desk at my operating position.


The triode is a single GS-31B and the plate transformer is a custom 80lbs 5KVA unit which will eventually be enclosed in a separate cabinet under the desk. Depending on which primary tap I use I can apply anywhere from 3600 to 4300 volts @ 1.5 amps continuous on the plate.


The amp is monitored and protected by a WD7S Triode Control Board and I also used his HV-2 rectifier/filter board as well as his TU-6B tuned input board, all of which I built from PCB blanks I purchased from him.


The amp was completely hand built by be, all screw holes drilled and hand tapped tapped out in the garage. Some of the more complex pieces like the stainless shaft couplings were made at work on a lathe and a milling machine was used to mill the chassis openings for the meters, switches, and access to the cathode/tuned input.


I just recently got the thing up on the air and so far it's been working really well but I've noticed except for 160 meters the GS-31B seems to struggle to maintain 1500 watts output. I know the official specifications for this tube state a 1000 plate dissipation rating which I am exceeding, but some web sites mention in amateur service 1500 watts pdis should be achievable. Since I run about 4000 volts on the plate grid current rarely climbs above about 220ma.


What happens is after about 15 seconds of flat-out 1400-1500 watts CW output into a dummy load, the grid current will begin to steadily drop as well as the output. If I un-key and give the tube 10 or 20 seconds to recover it will return to normal Ig until I keep it steadily keyed again. I never leave it keyed long enough to see what eventually happens since I'd to avoid long-term damage or catastrophic failure of the tube. Is this due to heat buildup in the tube perhaps temporary distorting the grid structure?


I'd also like to note that varying the plate voltage seems to have little effect on output power above 7 MHz...anything from 3600-4300 volts and 1400-1500 watts is about the most I'll get...and it tops out at about 600-700mA Ip. On the other hand, down on 160 meters it'll do >1amp on the plate and peg the watt-meter at 2KW.


I am considering the purchase of a GS-35B to see if it will be better suited for QRO use. Even tough laying on the key isn't a practical test of capability, I'd feel more comfortable knowing I'm not on the ragged edge and have plenty of headroom rather than thrash on this poor GS-31B.


Anyway here are a few cell phone pictures of the amp. gnore the Vp scale on the plate meter...I need to re-scale it. Also, after these pictures were taken I've eliminated the temporary parasitic suppressor since this amp doesn't appear to need one and I've also added gas arc protection devices from B- and cathode to ground.


Questions and comments are welcome!!

Thanks and 73

Attachment 10267Attachment 10265Attachment 10266Attachment 10264Attachment 10263

Nice job on the amp. As Carl said, Bob is the man when it comes to homebrew amps. They look nicer than store bought ones, too. I think all is uses are Russian tubes. You guys should be right at home talking amps.

[NQ6U]

Let me state right off that I know very little about this topic. That said, I did a little research and it seems that the GS-31B is very sensitive when it comes to filament voltage. Anything less that the optimal 12.6 volts will result in low output. Have you checked that?

[W3DRK]

Thanks for the complements! And yes, the filament varies between 12.8-13 VAC. Approx 12 ohms in series with the filament transformer primary provides a soft-start for the filament and keeps it right in that range. HV and keying is held-off until after the configured 2-minute warm-up time has passed.

[KG4CGC]

Welcome to the Island!

[W3DRK]

Thanks for the welcome!

I just noticed there was a five picture limit and three of my pics didn't post. Here's the transfer relay enclosure which houses the Jennings vacuum relays and RF detectors that are used for the hot-switching prevention. The 2nd picture is the 50 amp SSRs for the plate transformer step-start. Third is of the massive 80lbs 29M6 EI core plate transformer sitting next to a beer can for size reference. The transformer is wound with GP-200 #8 primary and #18 secondary windings with plenty of kapton insulation used throughout...hi-pot tested to over 9kv with no sign of leakage. After hours of QRO rag chewing it barely gets warm to the touch.

[K7SGJ]

Guinness? Beer? BWHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Just kidding

[NQ6U]

Guinness? Beer? BWHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Just kidding

Guinness is more than mere beer, it's an entire meal.

[W3DRK]

Guinness is more than mere beer, it's an entire meal.

Indeed! I've been living off of it in the evenings after work for the past couple weeks while I tinker with this thing.