[Mldxcc] Help w/T-T Titan 425 amp

[Rick]

>I know there are a few NCCCers who have and use a Ten-Tec Titan 
>model 425 amplifier. This is the first iteration Titan with the 
>3CX800A7 triodes,
>
>I picked up this amp a few years ago from a local fellow, the 
>original owner. I had it setup and was on the air with it during the 
>ARRL CW DX Contest 4 or 5 years ago, operating on 15 meters running 
>JA's. In the middle of a QSO there were 3 loud bangs, sounded like 
>shots from a .38 Special going off. After the third bang, everything went dead.
>
>Of course, the first thing to check was the fuses, and both of the 
>20A fuses in the 240 VAC mains were open. I replaced them and tried 
>to power up again and they blew as soon as I hit the ON switch. I 
>switched out the amp to a spare unit and continued the contest. 
>Later that week I pulled the cover off and found some blacked parts 
>on the meter shunt board and a crispy 10 ohm, 25 watt plate current 
>limiting resistor. I decided to put it back into it's box for a 
>retirement repair project.
>
>Well, retirement is here so I ordered replacements for the fried parts.
>
>I replaced everything on the meter shunt board and when it came time 
>to replace the plate resistor it became obvious that it is just a 
>stupid design issue for the placement of this component. It is stuck 
>on 1" porcelain standoffs in between the rear frame of the Tune 
>capacitor and the body of the squirrel cage fan. After pulling out 
>the old blackened resistor, it was obvious from the carbon burn 
>marks on the capacitor and fan that some serious arcing had taken 
>place. At most, there is a quarter inch of space from the resistor 
>to the cap and to the fan, probably a bit less. I was running on the 
>High B+ side of ~2600 volts when it died on me, the Low B+ is ~1800 volts.
>
>The bad resistor was by a company name of Jaro on it, a name with 
>which I am not familiar. The insulation had been blasted off of it 
>from the arcing and the underlying wire was exposed.
>
>It was replaced with a nice shiny new brown Ohmite resistor, 10 ohms 
>& 25 watts just like the schematic calls for. Once everything was 
>back in place, I hooked up the power supply, put the B+ voltage 
>switch to low and hit the power switch. Lo and behold, it came to 
>life, pilot lights on and fan blowing air. I shut it down and hooked 
>up the exciter, PTT line and antenna and loaded it up on 20 with the 
>same settings as I used years ago. From previous use, I remembered 
>how smooth the tuning was on the unit and it was still the same, it 
>tuned on 160 through 10 ( no 30 M ).
>
>With this success, two days before the ARRL DX Phone contest, I 
>decided to use it doing SOAB to test it out. It worked flawlessly 
>and I made contacts on 6 bands during the test. The settings were 
>smooth and repeatable. After the contest, I used it for DXing and 
>everything was fine. All this time, I had the B+ set to low - 1800 VDC.
>
>Then, one morning I came into the shack and turned it on for the 
>warm up period, went to get a cup of coffee, came back and it was 
>off. Uh-oh! Check the fuses first and again one of the 20 amp line 
>fuses was blown. Replaced it, powered it up and everything was fine. 
>Worked some DX for a few more days. Then, same scenerio one morning, 
>power on, get a cup of coffee, come back and it is dead. Same fuse 
>blown, Replace it, turn it on, got a grunt from the power supply and 
>it died again, same 20A fuse. I got the grunt I believe because the 
>B+ was still set at 1800 VDC and not the 'shot' like before when it 
>was at 2600 VDC.
>
>Taking the cover off, that shiny brown 10 ohm resistor was now a 
>crinkly, crumbly mess, arcing again to the capacitor and fan.
>
>So I figured that there had to be some way to insulate this 
>resistor, crammed into a tiny spot between two large grounded metal 
>components, to keep it from arcing.
>
>What I came up with is Polymide Tape based on a recommendation by 
>K6TA. The stuff I bought is an inch and a half wide. The specs give 
>the voltage rating at 7000 VDC per layer. The frame of the capacitor 
>and the fan cage now each have 10 layers of this tape on them as 
>well as on the chassis underneath the resistor. I also took the tape 
>and wrapped it back onto itself to form an inch and a half square 
>with 10 layers of tape. This was placed over the area when the 
>carbon marks from the arcing are on the fan and cap. 20 layers x 
>7000VDC/layer will hopefully keep the resistor from arcing. So far 
>it has worked for 2 weeks, still on Lo B+ ( except for the YI9PSE CW 
>qso, that was with HI B+ cuz that was a new one on CW ).
>
>So, how have you other folk kept your Titan from arcing? My 
>amplifier is S/N 00737. This one has the fan speed control on the 
>power supply with the SBY/OPR, LO/HI and OFF/ON switches on the RF 
>deck. Has the design of the RF deck changed from early to later 
>units? Did the placement of the plate resistor change to a more 
>logical location? Is there some additional insulating material I 
>should place between the resistor and the cap & fan?
>
>So far I have only been driving the amp with 45-50 watts. The grid 
>meter shows 30 ma of grid current at this drive level and plate 
>current is around 850 ma at 1800 VDC plate voltage. This will give 
>over 7 watts of power dissipation through the 10 ohm resistor, so I 
>don't want to block it from dissipating this heat by wrapping it 
>with additional insulating material.
>
>Any additional advice on keeping this resistor from arcing to ground 
>will be greatly appreciated.
>
>Also, without putting the power supply in the laundry room and 
>running the high voltage lead through the shack wall, how can I 
>dampen the infernal transformer buzz when transmitting?
>
>Thanks for reading through this tome,
>
>73 & KB,
>
>Rick, W6RKC