KT88 Screen and Bias resistor values

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MiniMajor
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KT88 Screen and Bias resistor values

Post by MiniMajor » Sat Mar 04, 2017 8:35 pm

Hey all,

Putting together a 50 watt with KT88's. Convention says, 1K Screens and 150K bias resistors. But, have any of you experimented with other values getting good results? 1K5 or 2K2 screens?

I love the sound of 220K's in the bias (like a Park 75!), but would be afraid to turn it up and leave it wide open. I've seen 82K, 100K, 120K, 150K, and even regular 220K's with 6550/KT88's.

What's worked for you guys? I'd appreciate your thoughts!

Thanks!
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bill bokey
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Re: KT88 Screen and Bias resistor values

Post by bill bokey » Sun Mar 05, 2017 3:36 am

6550s, biased close to class B, 220K grid leak resistors, 2K2 screen resistors work fine for me :

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Re: KT88 Screen and Bias resistor values

Post by Roe » Mon Mar 06, 2017 4:27 am

what's the screen and plate voltages? higher voltages require larger screen resistors.
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Re: KT88 Screen and Bias resistor values

Post by MiniMajor » Mon Mar 06, 2017 6:33 am

Damn! Bill that sound fantastic! How do you have the PPIMV set in that video?

Roe, I haven't fired up the build yet. Still wiring up the chassis. Using the high voltage winding of a Marstran 118. Wall voltage reads 120.6v from the wall. I'm anticipating 420v~430v on the plates. Let me know if I'm way off on this?

Look forward to your replies.

Thanks!
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Re: KT88 Screen and Bias resistor values

Post by Roe » Tue Mar 07, 2017 5:52 am

the marstran should give you higher voltages if you use a ss rectifier.
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

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Re: KT88 Screen and Bias resistor values

Post by MiniMajor » Wed Mar 08, 2017 2:15 pm

Thanks Roe. Waiting on the postman for the last order of components before I can fire up this build. I'm using the bridge recto from Valvestorm. Should i expect something nearer ~460v on the plates?
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Re: KT88 Screen and Bias resistor values

Post by Roe » Thu Mar 09, 2017 2:50 am

I'm getting ca 450v with el34s. It will be lower with kt88s
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Super 100 amps: 1202-119 & 1202-84
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Re: KT88 Screen and Bias resistor values

Post by MiniMajor » Tue Mar 14, 2017 6:14 am

Just fired it up!

Bias circuit is all good getting -40v on V4 and V5. Heaters read 3.20v~ per side. But I can't figure out the voltage across the dropping string. I get 463v at the mains filter and 456v at pin 1 on V1, like the dropping string isn't doing anything at all. I unsoldered the first two 2w 8K2 resistor and they both read fine, but I gotta sort this out. Where else should I be looking?


P.S. I can post some photos in the morning when the sun is back up!
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Re: KT88 Screen and Bias resistor values

Post by ol foot » Tue Mar 14, 2017 7:25 am

Are your tubes in yet? Voltage will drop once they're installed.

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Re: KT88 Screen and Bias resistor values

Post by bill bokey » Tue Mar 14, 2017 9:02 am

MiniMajor wrote:Damn! Bill that sound fantastic! How do you have the PPIMV set in that video?
thanks !
The PPIMV was on 10.

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Re: KT88 Screen and Bias resistor values

Post by MiniMajor » Sun Mar 19, 2017 2:19 pm

Thanks all!

I got this amp squared away this past Wednesday, and I've been busy playing it all week! Ended up with 1K5 screens and 220K grid leak R's. I keep going back and forth between 88's and 34's. Although they both do a different thing, I can't decide which I like better!

Appreciate all the help!


I think I'm going to rebuild my '97 BBRI next. This is addictive!
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