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Help with KT88 Bias

 
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dsmetal14
RM50


Joined: 04 Sep 2014
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Location: Gainesville, TX

PostPosted: Wed Dec 24, 2014 9:12 pm    Post subject: Help with KT88 Bias Reply with quote

Does anyone know what the bias reading is supposed to be on a KT88 in my RM100? Also, can I bias the tubes without a cab plugged in or will that damage it?
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Corium_AZ
Lynch Box


Joined: 15 Aug 2013
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Location: Phoenix, AZ

PostPosted: Wed Dec 24, 2014 9:37 pm    Post subject: Reply with quote

I can't answer the bias question on kt88's, but I'm sure you can find it in searching this forum, or just the net in general. But, no, you should NOT bias without something loading the output. Make sure your connected to your cab or a load box when biasing...
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suphuckers
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PostPosted: Wed Dec 24, 2014 9:47 pm    Post subject: Reply with quote

Actually, (I'm not telling you to do this because if you screw up your amp I'm not responsible but...) I'm pretty sure you technically could bias your tubes without speakers hooked up. You're supposed to do it with the Volume turned all the way down. As long as you're not trying to send a signal to a speaker it wont damage your amp. Even if you did send a signal out, I'm pretty sure nothing is gonna blow up. Someone like Rob should hopefully chime in here, but I think the transformer will be damaged if you repeatedly send a signal out with no load. I don't think 1 time or even 5 times will do it. Unless you have it maxed volume and are hammering on the Low E for several minutes. I'm not positive, but I'm pretty sure I'm right about this.
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kc2eeb
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PostPosted: Wed Dec 24, 2014 10:36 pm    Post subject: Off Topic Reply with quote

http://www2.randallamplifiers.com/images/stories/manuals/RM100-OwnersManual.pdf

You can bias KT88s the same as 6550s.
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suphuckers
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PostPosted: Wed Dec 24, 2014 10:38 pm    Post subject: Reply with quote

I didn't read this, but I'm pretty sure the answer about the speakers being connected is in here Wink

http://peavey.com/monitor/pvpapers/Chapter7.pdf


My guess, the answer is... "you can but you shouldn't"
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Last edited by suphuckers on Wed Dec 24, 2014 10:47 pm; edited 2 times in total
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Corium_AZ
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PostPosted: Wed Dec 24, 2014 10:41 pm    Post subject: Reply with quote

suphuckers wrote:
Actually, (I'm not telling you to do this because if you screw up your amp I'm not responsible but...) I'm pretty sure you technically could bias your tubes without speakers hooked up. You're supposed to do it with the Volume turned all the way down. As long as you're not trying to send a signal to a speaker it wont damage your amp. Even if you did send a signal out, I'm pretty sure nothing is gonna blow up. Someone like Rob should hopefully chime in here, but I think the transformer will be damaged if you repeatedly send a signal out with no load. I don't think 1 time or even 5 times will do it. Unless you have it maxed volume and are hammering on the Low E for several minutes. I'm not positive, but I'm pretty sure I'm right about this.


You are right about it not technically damaging anything. I've powered up my RM plenty of times, even tried playing thru it prolly too long, without being connected to a speaker load, and mine is fine. It's not good practice at all, but I'm sure it'd be fine.

However... I would imagine that your bias would not be quite right if the amp was not connected to a load. Even if you don't have the volume up, I'm pretty sure you want that load there to 'complete the circuit' if you will.

Someone more technically knowledgable about this I'm sure will chime in, but you'd surely have better luck posting this in the 'Egnater/Randall Modular Amps' section of the forum. Might get missed by a lot of folks in the off topic section, and this is definitely not off topic...
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kc2eeb
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PostPosted: Wed Dec 24, 2014 10:55 pm    Post subject: Off Topic Reply with quote

I was answering the bias question. I, personally, always have a speaker (load) connected to the secondary of the output transformer for two reasons.
First, the theory. With the power and standby on, you have around 500 volts DC across the primary of the output transformer. If you suddenly cut the DC
(This is with NO signal) you induce a pulse in the secondary. If it's connected to a speaker it has somewhere to go. Some old Fender amps had shorting speaker jacks for this reason. Better to have it shorted than open.
Second, more practically, we all make mistakes. If somehow a signal is going through the amp, you'll never know if the speaker isn't plugged in.
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Corium_AZ
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PostPosted: Wed Dec 24, 2014 11:32 pm    Post subject: Re: Off Topic Reply with quote

[quote="kc2eeb"]I was answering the bias question. [\quote]

I see that. Sorry, I think we were all writing responses at the same time, lol. Your answer wasn't posted yet.

I too looked @ the manual, but it didn't say anything about KT88's - good to know they can be biased like 6550's. Makes sense...
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dsmetal14
RM50


Joined: 04 Sep 2014
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Location: Gainesville, TX

PostPosted: Fri Dec 26, 2014 10:46 am    Post subject: Reply with quote

I went ahead and hooked the cab just in case. Not worth the risk and it makes sense that it could slightly effect the bias even with the master volume all the way down.

I searched on the net for a good while but was never happy with any answers. From what I read the KT-88 tubes are very similar so I figured they were same 35mv - 45mv I almost went with that and finally called a guy I know that has been playing guitar for a while he said 35mv - 40mv so I went with that. He was very confident in his answer for anyone that may want to know. He seemed to be fairly reliable source.

I am slowly learning this tube stuff. Thanks for all the help and patience!
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Jaded Faith
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PostPosted: Fri Dec 26, 2014 9:11 pm    Post subject: Reply with quote

You should never operate your tube amp without a load or speaker(s) connected. If you do, the result will be a high level of induction voltage that could damage ? or even destroy ? your amp?s output transformer and tubes. A power amp is designed with a specific B+ and primary impedance on the output transformer. The transformer expects to see a load on the secondary side to balance and complete this equation. If you do not, you are straining the primary side and it will fail. Your power tubes will also fail in short order as well.

Also, understand that there are no "correct" bias voltage for any given tube type. Bias is a negative voltage used to set the quiescent point and you need to know what plate voltage a tube sees to calculate the proper bias point. Your friends advice about where to set KT88's may be correct for them in one amp (actually for one seeing about 550V of B+) but not all. Use 42-49mV in the Randall.
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dsmetal14
RM50


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PostPosted: Sat Dec 27, 2014 11:25 am    Post subject: Reply with quote

Thanks Rob! You are a information beast.
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Jaded Faith
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PostPosted: Sat Dec 27, 2014 1:00 pm    Post subject: Reply with quote

Happy to help. This is also the reason you always see a speaker or at least the extension speaker jack drawn into schematics. They are part of the circuit, a critical one at that.
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