| Author |
Message |
    
Panda_bear
Junior Member
Username: Panda_bear
Post Number: 11
Registered: 4-2005
| | Posted on Sunday, December 03, 2006 - 6:45 pm: | 
|
can this be done and if so how hard would it be?the only reason i want to change is because one 250 b can do same output with one tube.much cheaper to retube. |
2600
Advanced Member
Username: 2600
Post Number: 578
Registered: 7-2002
| | Posted on Sunday, December 10, 2006 - 2:15 am: |
|
Here's the short version:
Fortunately, the high-voltage transformer in this model is originally wired as a full-wave bridge circuit. Delivers about 900 or 950 Volts DC to the tubes. That's why they used a string of three 350-Volt rated filter caps. Two of the 450-Volt parts in series would not have sufficient voltage rating. This means that the AC voltage on the HV winding is about 600 Volts, more or less.
You can change the HV section from the original full-wave bridge circuit to a full-wave voltage doubler circuit. You'll need a string of six electrolytics for the filter. Parts rated at 350, 400, or 450 Volts would all be suitable. If you use 1kV-rated rectifiers like 1N5408, six of them (two series strings of three) should be enough.
But there's a potential problem. Some Varmint transformers don't have enough additional insulation in them to tolerate the 1800 or so Volts DC this will get you. Some of them break down and arc to (grounded) the transformer core at 1500 Volts or so, way below the (approx) 2 kV you need for this trick.
If you have a HV insulation-breakdown/leakage tester, you can check this before rebuilding the power supply with a pile of expensive new parts. Or, you can rebuild it, turn on the juice and pray it doesn't go Guhhh--Z-Z-Z-Z-A-A-A-A-PPPPPP when you flip on the power switch the first time. In practical terms, bringing up the AC voltage slowly with a Variable-AC supply is safer.
But if your transformer doesn't have the breakdown-voltage rating required, you're out of luck on that transformer.
This leaves only the need for two additional power-supply voltages: Around plus-300 Volts for the screen grid, and around negative-75 Volts for control-grid bias.
This tube requires a blower to pressurize an enclosed compartment under the socket. This is the only way to force enough air through the fins inside the anode ring. They are packed pretty close, and this creates a lot of 'drag' on the airflow. Blade-type fans won't push efficiently against this back pressure. A centrifugal 'squirrell-cage' type blower will be necessary. Figure 30 CFM or more. The factory spec says you can get away with less, but if you'll use it on AM, you'll need all the air you can get.
Now, all you need are new relays, a new keying circuit, a Tune control with twice the air space between the plates, a 2-to-1 or 3-to-1 voltage stepup in the input circuit, swamping resistors and various bypass capacitors, RF chokes, a new plate choke and a ceramic or teflon chimney to force the air through the fins inside the anode ring. Oh, and the parasitic choke will have to be different, too.
But you'll be able to keep the transfomer, power switch, cabinet and power cord. Oh, and the Load control is probably okay to reuse.
By the time all that gets done, 4CX250 tubes will finally have become scarce and expensive like 6KV6 and 8950 tubes.
73 |
Panda_bear
Junior Member
Username: Panda_bear
Post Number: 12
Registered: 4-2005
| | Posted on Sunday, December 10, 2006 - 10:59 am: |
|
ye have little faith.....thanks for the info |
555_bighorn
New member
Username: 555_bighorn
Post Number: 1
Registered: 1-2012
| | Posted on Tuesday, January 31, 2012 - 12:43 pm: |
|
im still buying 250b
how many you want in a Cabinet |
|
