[Amps] TenTec 425 Plate Current at Standby

[Robert Paluf]

Dear Gentlemen,

I have been copying the mail on all of the discussions about amplifiers for a long time.  I am now the new owner of an older TenTec Titan HF amplifier, model 425.  I picked it up from a private party a few weeks ago from the second owner.  The Titan was purchased new in 1993.  I have done a thorough covers off inspection, careful vacuuming, and clean-up of both the RF deck and the very heavy power supply that sits on the floor.  All of the components look good.  The finals are a pair of 3CX800A7 triodes, and the power supply electrolytic capacitors are all clean, smooth, without leaking or swelling, the look new.  The power transformer looks new.  Now to the purpose of my posting is for some advice.  I put the equipment back together and made up a 240 VAC power cord.  I wanted to fire up the equipment first with the high voltage cable removed 9 (it is a separate braided cable) and verify the filaments, power supply, and the RF deck front panel comes to life.  Secondly, I planned to connect the high voltage cable and observe the equipment warming up fully in standby, without a dummy load or antenna connected and confirm quiescent operation.

The first step without the plate voltage went well ( actually boring).  The power supply came to life and the RF Deck lit up, and after 3 minutes the “wait” lamp went out indicating that the filaments had warmed up the tubes.

So I shut down, and connected the high voltage cable from the power supply to the RF deck.  At power on all was well, I was in selected low power mode (1800 VDC final plate voltage), the blower responded to selecting either low speed or high speed.  I checked the plate voltage meter at low and high power (2400 VDC) for metering and power supply integrity.  All is well.  After about two minutes of warm up, the “wait” lamp was still illuminated.  I notices the plate current come off the zero reading and start to very slowly climb up to 200 milliamperes.  I then shut down the power, disconnected the power, and read the manual again.  Yup, the plate current should not be rising to 200 ma.  It should have remained at zero.  I opened up the RF Deck and reseated all of the connectors and inspected the components and wires on the PA bias control board.  All looked very good, almost new.  No smells, burns, broken parts, or bad solder joints that I could see.  I repeated the power up with high voltage two more times, both the 1800 and 2400 voltage levels responded as expected in the standby mode.  But, the plate current rose to 200 ma each time exactly like the first time, no change, predictable and consistent problem.

You all probably are smiling as you know what is happening to me.  Unfortunately, I am 66 years old, licensed since I was 14, and this is my first HF Amplifier, so I have zero experience with big power amplifiers.  My first question that came to mind is it necessary to have a 50 ohm dummy load connected even in standby to stabilize the PA tubes?  Another observation, both of the 3CX800A7’s are separately exhausted through the top cover, one of them gets warmer than the other, but both get warm blowing 300+ watts of dissipation.  The manual theory of operation says that in standby the bias is set at +28VDC cathode to grid voltage to push the tubes well into the cutoff region, so no plate current should be flowing while in standby.  The normal operating voltage for grid to cathode voltage is 8.5 VDC, set by a Zener diode.

I suppose I have to poke around carefully making measurements with the covers off, plate voltage applied, and the safety interlocks bypassed.  I need to measure the grid to cathode voltage in standby to see if it is 28 VDC or if it changes as the tubes warm up.  Is there something simple that I am overlooking, due to my inexperience?

Do you think both of the 3CX800A7 tubes are damaged and must be replaced?  That is my biggest fear.

The prior owner told me that he used the amplifier as recently as 3 months ago without any problems, at the full rated power of 1500 watts output.  He told me that there were no problems that he was aware of with the Titan Amplifier.  He did not mention standby plate current flowing of 200 milliamperes.

Any advice, comments, or questions are welcome.

73,
Bob, KV4JO


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