Repair Log: Ten-Tec Titan 425 Linear Amplifier

A friend recently asked me to take a look at his Ten-Tec Titan 425 Amplifier that had failed while he was using it on the air.

According to him, the 75-meter group he was rag-chewing with reported that his signal strength had dropped significantly.  When he looked at the amplifier's meters, he noticed that the grid-current meter was pegging at full scale!

He assumed that the tubes were ruined and bought a new-old-stock pair to replace them with.  But before he plugged them in, he wanted to ensure that they would not be ruined by whatever fault had caused the original high grid-current reading.

I'm usually very hesitant to work on tube amplifiers -- I would prefer to be no where in the vicinity of high voltage!  But, in this case, I thought I could leave the HV to the amp unconnected and instead start debugging by checking the tubes' cathode voltage.  If there were an issue, that's where I thought it would lie.

Sounded like an interesting problem, and I didn't believe there would be high-voltage involved (and there wasn't), so I agreed to check it out.

Trouble-shooting:

The original tubes had been removed by the owner and no new tubes had been installed.  I turned the amplifier over onto its top and removed the bottom plate that gives access to the tube-socket pins (as well as the Input Bias board).

I found a schematic on-line for the Titan that K9YC had redrawn -- his annotations were a huge help!  Below is the part of the schematic that deals with Cathode voltage and the sensing of Grid current.

I high-lighted the grid-current sensing circuit in yellow.  Note, too, K9YC's notes:  (grid) bias is -28V when the amplifier is unKeyed and -8.5V when the amplifier is Keyed.  Note that the polarity of these voltages means that they are with respect to the Cathode.  This means that, because the grid is grounded, the voltages we would read at the Cathode, with respect to ground, are +28V when the amplifier is unKeyed and +8.5 when the amplifier is Keyed.

A quick note...Grid biasing and the grid current and plate current loops can be better understood using the diagram, below, from an earlier Ten-Tec Titan 425 Manual.  Note that the location of the grid-current sense resistor has sinced changed from being in-series with the collector of Q1 to now being in-series with Q1's emitter, but because collector-current essentially equals emitter current, the grid-current reading would be the same with the resister in either location.

My measurements of the Cathode voltage, with respect to Ground, were:

• Amplifier unKeyed:  28.9V  (close enough to K9YC's 28V).
• Amplifier Keyed:      27.0V  (should be around 8.5V!)

A problem!  A Cathode voltage of 27 volts to ground meant the grid bias was -27 volts -- in other words, the tubes would never leave their Cutoff (i.e. non-conducting) state, even when the amplifier is keyed ON!

Also, I noticed that when I keyed the amplifier, the grid-current meter pegged at full-scale (even though there were no tubes installed).

So there was bad news and good news.

The bad news -- something was screwy in the Cathode biasing circuit and keeping the tubes in cutoff even when the amplifier was keyed ON.

But the good news -- because the tubes were remaining in cutoff, rather than conducting when the amplifier was keyed, meant that the high grid current indicated by the meter did not actually indicate grid over-current in the tubes.

So the original tubes might still be OK!

Looking further into the Cathode-bias/Grid-current circuit (shown below):

Q1's emitter voltage was going up to 20V when I keyed the amplifier.  Given the 4.7 ohm resistor (R17) connected between Q1's emitter and ground, such a high voltage did not seem possible.  I did a quick ohm-meter check and discovered R17 was no where near 4.7 ohms.

I removed the Input Bias board from the chassis and took a closer look at R17.  Hmmm...was the PCB under R17 a bit scorched?

I removed R17 and noticed two things.  First, R17 measured 2300 ohms, not 4.7 ohms, and second, it appeared that the heat damage on the PCB might have happened in the past and was not due to this R17. (By the way, while R17 was removed I did a quick check of Q6 in-circuit with a DVM -- BE and BC junctions looked good and no CE short, so it appeared Q6 did not need replacing).

Could there be some sort of over-heating issue with R17?  The (grid) current through the resistor would have to be about 230 mA for R17 to reach its rated 1/4-watt dissipation. Hard to imagine how it could get this high!

But I figured better to be safe than sorry, so I replaced the single 4.7 ohm resistor with a parallel combination of 10 ohms and 9.1 ohms (4.76 ohms if perfect resistors -- measured to be 4.6 ohms), thereby doubling R17's power-handling capabilities.

I also noticed that the K9YC annotated schematic included a grid 'over-current protection' circuit consisting of transistor Q7 whose base-emitter junction connected across R17 through a 470 ohms resistor.  

This circuit was not on the Input Bias board I was debugging.  It seemed like a good idea, however, so I added it to the PCB, using a 2N3904 for Q7.

Here's the final schematic of my repairs and modifications to the Input Bias board:

And here's a photo of my implementation:

(The orange bit is a piece of wire insulation I added to protect the middle lead (base lead) of the 2N3904 from its collector and emitter leads.  Unfortunately, I used a not very heat-tolerant piece of wire insulation, and it quickly melted while I was soldering the base lead to the 370 ohms resistor).

Final Testing:

With the changes in place, I reinstalled the Input Bias board and remeasured the Cathode-to-ground voltages:

• Amplifier unKeyed:  28.8V
• Amplifier Keyed:        8.2V

Looking good!  And the grid-current meter was no longer pegging!

But there was a final question to be answered...were the original tubes good or bad?

By the way, I don't have 240VAC in my lab, so I temporarily used a 1000VA 120-to-240VAC step-up transformer to power the Titan.  To minimize transformer heating, I limited the amplifier's output power to 500W, max, and minimized the time the amplifier was keyed ON.

With the original tubes reinstalled, here are the test results (testing on 75 meters, Voltage Selector switch set to High, Load = 8, Tune = 6):

• Drive Power:  1.09 W,     Titan Output Power:  29.3W,      Gain:  26.9 (14.3 dB)
• Drive Power:  5.5 W,       Titan Output Power:  152W,       Gain:  27.6 (14.4 dB)
• Drive Power:  14.6 W,     Titan Output Power:  458W,       Gain:  30.8 (14.9 dB)

I don't have a Titan for comparison, but Google's AI tells me that the gain of a Titan should be 13 to 14 dB, so it appears to me that the original tubes are still good.  Hurray!

Here's the test setup (using my homebrew FPGA SDR transceiver as the driving source):

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.