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KT88
Nov 21, 2023 17:19:04 GMT -5
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Post by vanlifeson on Nov 21, 2023 17:19:04 GMT -5
Is it a 100 percent no no to use a KT88 as the modulator tube in a Tram 201? Thanks
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KT88
Nov 22, 2023 12:55:49 GMT -5
Post by cbrown on Nov 22, 2023 12:55:49 GMT -5
A buddy who owns a D201 used a KT88 as the modulator tube in his unit and saw no problems. Been running it for 4 years like that.
Disclaimer - I'm not a Tram guy, just telling you what I've seen
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KT88
Nov 26, 2023 0:27:12 GMT -5
cbrown likes this
Post by 2600 on Nov 26, 2023 0:27:12 GMT -5
You'll find that this tube draws more current. We adapt the D201 for that tube by taking the 'cold' end of R637 loose from ground. Slicing the foil trace between the two sides will accomplish this. A 1k 1/4w resistor goes across this cut in the foil. A 1000uf 25 Volt cap goes across the 1k resistor, positive side to ground. A 1N4002 or higher voltage type gets the cathode (banded) end connected to pin 7 of V603, the audio power tube socket. The other end with NO band (anode) goes to the junction of R637, the new 1k resistor and negative side of the new cap. This places about 8 Volts negative bias onto the tube's control grid . Brings the current down to make the original cathode-bias resistors read the correct 31 Volts DC (more or less) at pin 8 of V603.
This assumes that the two cathode-bias resistors R638 and R639 are properly in place on the plug-in BA board. A 40-channel radio that has jumper wires on the BA board where those resistors would go is a lot less common, but the mod is totally different.
Ran a D201 as the everyday bench radio here for ten years with a Sovtek "6550WD" tube for V603. It didn't have to work quite so hard as a 6L6 would do.
73
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KT88
Feb 21, 2024 14:07:04 GMT -5
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Post by vanlifeson on Feb 21, 2024 14:07:04 GMT -5
You'll find that this tube draws more current. We adapt the D201 for that tube by taking the 'cold' end of R637 loose from ground. Slicing the foil trace between the two sides will accomplish this. A 1k 1/4w resistor goes across this cut in the foil. A 1000uf 25 Volt cap goes across the 1k resistor, positive side to ground. A 1N4002 or higher voltage type gets the cathode (banded) end connected to pin 7 of V603, the audio power tube socket. The other end with NO band (anode) goes to the junction of R637, the new 1k resistor and negative side of the new cap. This places about 8 Volts negative bias onto the tube's control grid . Brings the current down to make the original cathode-bias resistors read the correct 31 Volts DC (more or less) at pin 8 of V603. This assumes that the two cathode-bias resistors R638 and R639 are properly in place on the plug-in BA board. A 40-channel radio that has jumper wires on the BA board where those resistors would go is a lot less common, but the mod is totally different. Ran a D201 as the everyday bench radio here for ten years with a Sovtek "6550WD" tube for V603. It didn't have to work quite so hard as a 6L6 would do. 73 I probably should have specified that this is a hand wired 201. Are there any modifications to the hand wired that need to be done to run the KT 88? Thank you
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KT88
Feb 24, 2024 1:13:52 GMT -5
Post by 2600 on Feb 24, 2024 1:13:52 GMT -5
The same mod applies. But the physical location of everything is different. You'll have to find R637, a 220k half-Watt connected to V603 pin 5. Take the resistor's ground end loose, insert a 1k 1 Watt resistor between the now-loose end of R637 and ground. The junction point between them now gets two more parts attached. The negative end of a 1000uf 25 Volt electrolytic cap. The cap's positive end goes to ground. A 1N4001 or higher rectifier gets the cathode (banded) end connected to pin 2 of V603. The anode end goes to the junction of R637 and the new 1k resistor/ cap.
This will add about 8 Volts of negative grid bias to the tube, in addition to the cathode bias supplied by R638/639 and R640. You should show a DC voltage not over 32 or 33 Volts on pin 8 of the KT88.
This worked for our "bench" D201A. Used it as the workbench base for about ten years with the same audio tube.
73
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KT88
Mar 15, 2024 13:49:27 GMT -5
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Post by vanlifeson on Mar 15, 2024 13:49:27 GMT -5
The same mod applies. But the physical location of everything is different. You'll have to find R637, a 220k half-Watt connected to V603 pin 5. Take the resistor's ground end loose, insert a 1k 1 Watt resistor between the now-loose end of R637 and ground. The junction point between them now gets two more parts attached. The negative end of a 1000uf 25 Volt electrolytic cap. The cap's positive end goes to ground. A 1N4001 or higher rectifier gets the cathode (banded) end connected to pin 2 of V603. The anode end goes to the junction of R637 and the new 1k resistor/ cap. This will add about 8 Volts of negative grid bias to the tube, in addition to the cathode bias supplied by R638/639 and R640. You should show a DC voltage not over 32 or 33 Volts on pin 8 of the KT88. This worked for our "bench" D201A. Used it as the workbench base for about ten years with the same audio tube. 73 What would happen if this mod was not performed? Thanks.
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KT88
Mar 15, 2024 23:31:56 GMT -5
Post by 2600 on Mar 15, 2024 23:31:56 GMT -5
The tube will run hot, along with all the other components in the circuit in line with it.
73
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