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
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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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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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Re: KT88 Screen and Bias resistor values

Post by revolver1 » Tue Aug 28, 2018 2:33 am

Hi minimajor,

what resistors did you go with in the bias circuit to get it in range for the KT88?

I'm talking about the one before the diode and the one off the trim pot.

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