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KT120 Oddblocks
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Author:  gofar99 [ 27 Apr 2018, 16:05 ]
Post subject:  Re: KT120 Oddblocks

Hi, 50 volts is a bit on the low side, the target is 62-65. Up to about 75-80 is fine too. I would up the one resistor to about 75K to fix it. What you want to be sure of is that the difference between any cathode and any heater is within about 100 volts. New tubes are supposed to take up to about 200v between them, but my experience is that as they age the value is less. I have seen tubes fail at 150. Something to remember is that the cathodes at the mid point might not be at 1/2 the top anode voltage. That requires that each half of the tube have exactly the same gain. This seldom occurs. I have seen as much as a 25% deviation.

Good listening
Bruce

Author:  TIJ-NIKI [ 27 Apr 2018, 16:37 ]
Post subject:  Re: KT120 Oddblocks

Hi, thanks both of you for the answers

I am pretty sure the problem is not the voltage divider but something with the lr8k regulator circuit. I get 370v instead of 400 in the output pin. I will recheck with a decent multimeter tomorow. I broke my amprobe in the shop today ;( i am stuck for a bit with this glorified toy

Bruce, I understand the theory behind elevated heaters but why i don't see it more often? Especially in old radios and such. Is it something mandatory for a reliable working circuit or a measure just to increase the life span of the tubes and avoid possible internal shorts?

Author:  gofar99 [ 27 Apr 2018, 16:39 ]
Post subject:  Re: KT120 Oddblocks

Hi, the output should be 300 v, not 400.

Edit. The elevated heaters are not usually needed in circuits that have all the cathodes at or near ground potential. Virtually all well designed "totem pole" designs will have either the fixed raised votlage like in these amps or floating designs. I prefer the fixed design as it seems more stable. Sometimes it is also used to reduce internal tube noise. If the heater is at a higher positive potential than the cathode it will tend to scavange stray electrons that don't head off toward the anode.

Good listening
Bruce

Author:  TIJ-NIKI [ 27 Apr 2018, 16:43 ]
Post subject:  Re: KT120 Oddblocks

brainfart. I meant i have 270v instead of 300v

Author:  gofar99 [ 27 Apr 2018, 16:49 ]
Post subject:  Re: KT120 Oddblocks

Hi, Check the values of the resistors that program it. Be sure it is getting at least 25 volts more on the input side. Beware that when you check it they are sensitive to sudden load changes (like accidentally shorting the probe to ground on the adjust or output terminal) and will fail immediately. They are great little regulators, but not tolerant of that sort of fault.

Good listening
Bruce

Author:  TIJ-NIKI [ 27 Apr 2018, 16:55 ]
Post subject:  Re: KT120 Oddblocks

Thank you for the headsup, I will change that resistor with something with a tighter tolerance

Author:  ILoveHiFi [ 28 Apr 2018, 17:23 ]
Post subject:  Re: KT120 Oddblocks

TIJ-NIKI wrote:
Bruce, I understand the theory behind elevated heaters but why i don't see it more often? Especially in old radios and such. Is it something mandatory for a reliable working circuit or a measure just to increase the life span of the tubes and avoid possible internal shorts?

Some designs have the cathode directly grounded for more gain, in this case the cathode is zero volts so heater biasing is not required.
Raidos tend to feel like lower quality right, so they want to get more gain with less valves so they directly ground the cathode.

Some guys directly ground the center tapp of the heater to ground, this prevents all ac humm on heaters, however dc noise will appear from the diffrential in cathode to heater.
Grounding heater with a large capacitor like 100+uF with dc biasing had given me the best results lowest noise.

Heater is more about the noise in the circuit, heater noise leaks into the cathode. Most valves have 150+v diffrential in heater most circuits have much less than 150v at cathode.

http://www.valvewizard.co.uk/heater.html

Author:  Juancho [ 03 May 2018, 14:06 ]
Post subject:  Re: KT120 Oddblocks

I'm trying to remember: does the LR8 regulator output have to be checked with load connected?
Thanks

Author:  gofar99 [ 03 May 2018, 14:37 ]
Post subject:  Re: KT120 Oddblocks

Hi, A good question. I am not sure, but I always do. If you have it out of circuit then a small load ought to be fine. Calculate what a 1ma load would be and use that value resistance. Be careful not to short it out when testing as they have a tendency (like about 90% of the time) to fail immediately without warning.

Good listening
Bruce

Author:  Juancho [ 03 May 2018, 17:33 ]
Post subject:  Re: KT120 Oddblocks

Thanks. I'm sure I normally have tested with no load and not connected until sure the voltage output was correct but today I installed three and all apear to be passing the voltage through for the loss of 4 volts. There have been no shorts or anything else untoward........

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