TRIO TL911 - the amendment proposed by PA0FRI

[Matadorul]

Kenwood TL-911 – PA0FRI passive-grid modification – output power stops increasing above ~20 W drive​

Hello,

I am working on a Kenwood/Trio TL-911 amplifier with the original five 6LQ6 tubes.

I have modified the input/grid circuit following the PA0FRI TL-911 modification. The amplifier is now operating and the input matching problem I initially had has been solved, but I have one problem that I cannot explain.

The present configuration is:

• Original 5 × 6LQ6 tubes.
• RF drive is applied to G1 through the new passive input matching network.
• G2 circuit remains as in the original TL-911, except that the G2 negative voltage has been adjusted according to the modification instructions.
• G3 remains unchanged.
• Cathodes remain as in the original circuit, grounded for RF through the existing 4.7 nF capacitors.
• The original G1 capacitive trimmer has been removed.
• The original resistor associated with the G1 input coil has been removed.
• A broadband passive matching circuit has been installed between RL2-2 and G1, according to the PA0FRI modification.
• The 4:1 transformer currently installed has been checked separately with a VNA. With a 200 Ω non-inductive load on the high-impedance side, the VNA measures approximately 55 Ω and SWR 1.15 at 6.9 MHz, with good broadband behavior.
• Input SWR remains stable when RF drive is increased.
• No ALC connection is being used.

I have also repeated all the relevant adjustments/calibrations according to the TL-911 service instructions, including grid voltage, static balance and instantaneous protection level.

The strange behavior is the following:

On 7.1 MHz, with a 50 Ω dummy load:

20 W RF drive → approximately 140 W RF output.

Increasing the drive above approximately 20 W does not produce any significant increase in output power. The amplifier seems to enter a very clear gain compression/limiting condition.

The same behavior was previously observed with the antenna/ATU connected, so I repeated the test into a proper 50 Ω dummy load and obtained essentially the same result. Therefore the antenna and ATU can be excluded as the cause.

One additional observation may be important:

With the G2 negative voltage previously adjusted to approximately −78 V, I obtained about 200 W output with 20 W drive.

After changing it to approximately −70 V, as suggested in the modification instructions, the output became approximately 140 W with 20 W drive, but the same limiting behavior remains: increasing RF drive above 20 W does not significantly increase RF output.

At this point I would like to understand whether this behavior is caused by:

• the G1 passive-grid modification/input circuit,
• the G2 operating point,
• the TL-911 protection circuit acting prematurely,
• grid current/loading of the five 6LQ6 tubes,
• or another operating condition that must be changed after converting the input circuit.

Has anyone performed this PA0FRI modification on a TL-911, or worked with a similar passive/swamped-grid conversion using 6LQ6 tubes?

In particular, what measurements would you recommend to determine why RF output stops increasing above approximately 20 W of drive?

I would also be interested in the expected G1/G2 voltages and currents under RF drive after this modification, and whether any other TL-911 adjustment needs to be changed from the original service-manual settings.

73 and thank you for any advice.
DL1YO

 

[nomadradio]

Well dern the bad luck! I went to the current PA0FRI.com/mods page and when you hover the cursor over the link that says "TL911", your address bar will say "TL922". And that's the file you'll see clicking on the link. The 922, not the 911.

Oops. Whoever moved this web site to the new location has flubbed the link to the 911. Thought I'd look at the docs before offering an opinion, but they are a 404.

73

 

[Matadorul]

Well dern the bad luck! I went to the current PA0FRI.com/mods page and when you hover the cursor over the link that says "TL911", your address bar will say "TL922". And that's the file you'll see clicking on the link. The 922, not the 911.

Oops. Whoever moved this web site to the new location has flubbed the link to the 911. Thought I'd look at the docs before offering an opinion, but they are a 404.

73

Thank you for your involvement. If you used this Link : https://www.pa0fri.com/Lineairs/TL911/tl911.htm just know that everything is fine on my end. I hope to get some good advice; I have a few ideas of my own, but I’ll wait to see how the issue is discussed, and then I’ll share my thoughts as well. Thanks again for your good intentions.
73 de DL1YO