Follow-up to my previous post. These are the differences that I know of between the early and late Thunderbolt.
When the PA grid bias is disconnected in "Linear" mode, strange things happen with the screen current (e.g. off-scale high). Besides making the tubes Not Happy, this is a good way to fry the screen supply. This can happen quite easily; switching to "Linear" with the bias control plug disconnected will do it, and it always happens momentarily whenever the T/R relay changes from blocking to "on" bias. While I was reading my T-bolt manual (which covers the later production) I noticed that the Johnson folks must have realized the error of their ways; a 270K 1W resistor is shown on the later schematic, connected from the output of the grid supply filter cap to the grid supply line leaving the "Mode" switch. In practice, this resistor is simply connected across two terminals of the "Mode" switch. This essentially ensures that, in the absence of anything else, the tubes are cut off. Three easy steps to add the 270K resistor: 1. Secure one (ea.) type 270 K ohm, 1 watt resistor (I'm using a 2 watt part because it is On Hand); 2. Connect aforementioned resistor across contacts 5 and 9 of the Mode switch. 3. Rest easy, knowing that your tubes will never be without bias again. Here are the other design changes noted between the two schematics: - R3, the 350 ohm Globar grid resistor, is replaced with three 1K non-ind. resistors of unknown wattage. - V109 was a VR75/0A3, now is a VR90/0B3 probably to accommodate the different bias requirements of the PL-175s (sorry, should have typed "PL-175" instead of PL-172 in my earlier post). R116 changed from 2K to 5.6K, as appropriate. - SW102, the "Plate" toggle switch, is changed from SPST to a DPST unit; the second pole is wired to switch the other (previously unswitched) side of the AC line to the plate transformer. (NOTE: this is a very good mod to make to your early Thunderbolt in the interest of safety). - C101, the 8 uF 2 kV oil cap, is replaced with a series string of six 80 uF 450V electrolytics, each bridged with a 120K 2W resistor. All of this is mounted on a circuit board located in the space vacated by the oil cap. - R101 - 106 are changed from 4K to 5K (probably to accommodate the approx. 3 mA current being eaten by the cap bank resistors). - C105, a .01 uF disc ceramic, is connected between T101 secondary center tap and ground. The cap is actually located on the back of the LV filter/bleeder circuit board. If you can come up with an explanation for this, please let me know... - T103 is now corectly drawn on the schematic with a tapped primary (0 / 110V / 120V). NOTE: if yours was assembled with the 110V tap connected, you might want to change it to the 120V tap. That's all I noticed. HTH. regards, Mahlon - K4OQ
