Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Curranm on September 21, 2017, 10:08:35 am
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1st of all I want to thank the moderators for the wonderful help in getting my channel switching relay to work! I love it!
NOW, I have a new build which is my 1st build with KT88's. A friend of mine recommended 330R screen grid resistors for the power tubes (pin6 to pin4) I am running 480vDC at the B+1 node going into these resistors. I popped a few fuses, and also see one of the tube FLASH super bright blue. I am wondering IF I need to up my screen resistor value to something higher like a 1K or even a 1K5? this is a high gain amp with a nice clean channel, two KT88's running off of a classic tone 8K JTM45 OT. the only reason I am leaning hard on this is that I have built two other amps that have done the same thing and it was all screen grid resistor issues on my end(wired wrong, and NOT there in the other)
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Those screen voltages are high. Screen resistors are definitely a step in the right direction.
According to this datasheet: http://www.triodeel.com/kt8856p1.gif (http://www.triodeel.com/kt8856p1.gif)
Screen current is around 38ma under load (per pair). Assume 50ma peak (for one tube). A 330ohm resistor drops voltage by 16.5v which is not much. 1.5k drops voltage by 75v, which in turn limits screen current and has a meaningful impact on screen dissipation.
Even under load that's still high compared to the datasheet example, but much more likely to be safe.
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(wired wrong, and NOT there in the other)
do you have a 2nd PS tap for screens?
This fixed bias?, where did you set it?
what is the VDC on each side of the 330 you're using?
My current 88 build has no G2 R, it does use an UL OT
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I "shot gunned" some 1K/5watt resistors in place of the 330R/5watts. the issue is gone now. This is my 1st "cobble" amp. pulling the london preamp from Kevin O'conner, marshall PI and output circuit. 100watt power supply 2203 circuit, and a full 12AX7 FX loop from a good good pro amp builder friend of mine, AND adding the relay channel switching from Mr. Hoffman here. This beast has come together quite nicley with all of the mish-mash. my layout is pretty bad and its really crammped in the 17x3x8 chassis BUT everything works, it is quiet, and as soon as I tweak up the gain channel this will be right in line with what I like for heavy metal !!!!
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Glad to hear it's working. Are you using the same tubes that previously had "fireworks" in them? I'm surprised they weren't damaged, and I'd prepare for the possibility that they arc again. This could mean cathode or B+ fuses, which aren't a terrible idea anyways.
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Curranm,
Sorry I did not see this sooner, yes 1K for current manufacture (and even old for that matter) KT88's if you want them to last. They are thanking you now every time you turn it on! Keep an eye on the tube that flashed... The KT88's should give you lots of clean body and will not mush or crunch when you push your cascaded preamp into them. Please post pics and soundclip!
Jim
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480V is high for screens in KT88s (and 6550s). I would tend to run them somewhere between 380 and 420. YMMV.
If you're concerned about excessive sag from upping the B+1 (screen HT) node supply dropping resistance, you can use another form of supply, like a tube-regulated screen supply node.