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Old 28th February 2012, 01:18 PM   #111
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I am interested too in the LED bias, will follow closely.
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Old 28th February 2012, 04:28 PM   #112
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Default LED Bias for beginners

Hi KT88 SE community,

I'm running this amp now for about a year.....
....and I just opened it to try some other components.

Can somebody explain to me exactly what to solder where for LED bias....I build this amp with Scotts excellent manual ...but I feel still like bloody beginner when I hear you talking about circuits aso

Thanks

Gregor
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Old 28th February 2012, 08:15 PM   #113
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I have read on the forum that it is possible to switch between triode, ultralinear and pentode mode. Is it possible to implement pentode mode on this design?

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If your transformer has ultralinear taps you can use a 3 way switch to go from triode to UL to pentode mode. I put a switch like this in the guitar amp that ate the previously mentioned KT88's. 38 Watts in pentode, 30 Watts in UL and 18 watts in triode mode with about 400 volts B+.
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Old 1st March 2012, 05:07 AM   #114
iNSuL is offline iNSuL  United States
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Its been a year since I first built the kit so I decided to do a rebuild with some tweaks. Heres some pics of the new version. Tweaks: led bias on driver, cap changes to PIO, and a few resistor changes. Still loving every note that sings from this amp.
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Old 1st March 2012, 09:16 AM   #115
debk is offline debk  United States
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I added LED bias for the 6N1P. I used a pair of HLMP 1302 red leds (Digikey part number 1080-1042-ND) for each cathode. This led has a 2v drop so a pair of them will give about the same bias point the original resistor did.
I bought 10 of the leds and measured the voltage drop across each one and picked the four that were the closest matched. This was probably not necessary, they were all within 0.03v

The led bias made a very noticeable improvement in detail. The music sounded clearer and more accurate. It still very much had the tube warmth, but now with more clarity.
I think it is well worth the upgrade. It is easy to do, and does not cost much (leds $0.46 a piece or $0.33 each if you buy 10).

I think this made enough of a difference that if I were to build this amp again, I would use led bias to start with.

Deb
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Old 1st March 2012, 10:53 AM   #116
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Hello Deb,
Can you post a simple schematic of how to connect the LED? Cathode side to ground? Should the bypass capacitor stay in place?

Thanks!
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Old 1st March 2012, 10:59 AM   #117
debk is offline debk  United States
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It is very simple. Connect two leds in series. then connect the string to the tube with the cathode to ground. Remove both the resistor and bypass capacitor

Debra
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Old 1st March 2012, 12:23 PM   #118
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Easy peasy!


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Old 6th March 2012, 09:58 PM   #119
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Quote:
Originally Posted by iNSuL View Post
Its been a year since I first built the kit so I decided to do a rebuild with some tweaks. Heres some pics of the new version. Tweaks: led bias on driver, cap changes to PIO, and a few resistor changes. Still loving every note that sings from this amp.
Looks great! Are those giant caps in place of the 30uF plate filters?

Scott
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Old 6th March 2012, 10:39 PM   #120
iNSuL is offline iNSuL  United States
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Thanks. They're the driver b+ bypass cap. The 10uF
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