What values do you bias kt77's and kt88's

hmic10 · Post by **hmic10** » Fri May 04, 2012 10:06 am

Got a pair would like to try them out ,
What is the ma's for them?

rblyn · Post by **rblyn** » Fri May 04, 2012 10:17 am

Dont know about the KT77's. I LOVE the KT88's either by themselves or paired with EL34's. I have an awesome tech here in town- He measured plate voltage, check tubes specs etc and told me the keep the KT88's at 47.5ma. I've got a post somewhere in here about that....pretty much says the same thing. Also- If you are going to use these for low volume- they will not sound that great- You have to be in a live situation and push the master a bit. Once they open up, the sound is HUGE, especially through a full stack.

hmic10 · Post by **hmic10** » Fri May 04, 2012 12:32 pm

Thanks for the info !!

Kapo_Polenton · Post by **Kapo_Polenton** » Fri May 04, 2012 7:46 pm

KT77's are supposed to bias in the same range as EL34's and KT88's closer to the 6550 range. That said, in theory you should start with your plate voltage in order to get an idea of what 70% of the tubes operating range will be. I am sure you could find out what Randall's plate voltage is but I would guess somewhere around the 400 range. But generally speaking EL34's will be in that 35-45 mA range without getting too crazy hot and burning out. USe something like this to get an idea: http://www.rogb.dk/tools/bias.html

I have my 6550's in my Marshall running at about 53 mA but plate voltage was 367.

JKD · Post by **JKD** » Fri May 04, 2012 7:51 pm

My RM50 Plate Voltage is usually around 515...but can go up to 525

Kapo_Polenton · Post by **Kapo_Polenton** » Fri May 04, 2012 7:53 pm

Yikes! That's crazy high.. didn't know these were running that high voltage wise. I should get a reading of mine some time and see what it is sitting at..

Rod · Post by **Rod** » Sat Jun 16, 2012 9:19 pm

See next post...sorry

Rod · Post by **Rod** » Sat Jun 16, 2012 9:22 pm

So in an RM 50, what should I be biasing my KT88 and KT77 tubes at?
What your guys favorite brand KT88's?
Should the bias calculater be set at 515 volts?

kc2eeb · Post by **kc2eeb** » Sun Jun 17, 2012 5:52 am

Average plate voltage is 505-525 depending on line voltage.

Rod · Post by **Rod** » Sun Jun 17, 2012 8:21 am

kc2eeb wrote:Average plate voltage is 505-525 depending on line voltage.

Thanks KC2eeb... How are things in Claremont these days?...My RM50's are holding together so far or I'd be out to see you...I keep waiting for that burn trace board thing to happen but ok so far....Should I get the repair done at some time just for good measure?

kc2eeb · Post by **kc2eeb** » Sun Jun 17, 2012 8:43 am

Hi Rod,
Claremont is..........Claremont.
Far as the RM50 goes, "if it ain't broke, don't fix it" applies.
What I would do is take the chassis out and see if it's one that Randall
either cut away the foil from inside the output tube area and filled with epoxy
OR a board that has the foil altered from the factory in China. The latter I've only seen on a "new" RT2/50 power amp board I got from Randall last year.

See:
http://mtsforum.grailtone.com/viewtopic ... m50+repair

Rod · Post by **Rod** » Sun Jun 17, 2012 8:51 am

I'll do that thanks... From what I recall, both my RM50's were made in Korea...

kc2eeb · Post by **kc2eeb** » Sun Jun 17, 2012 9:18 am

Yep,
Both my RM50 and RT2/50 are Korean also. Yours may have been altered already. Also, a very important place to look for any carbonizing is the blade connector strip at the top of the board where the HV comes from the power supply board(white wires) and connect to the output transformer and screen grid resistors.
http://mtsforum.grailtone.com/viewtopic ... m50+repair

Rod · Post by **Rod** » Sun Jun 17, 2012 5:57 pm

Thanks for putting that link up with all the pictures. It's much appreciated!

kc2eeb · Post by **kc2eeb** » Sun Jun 17, 2012 6:03 pm

You're welcome. Pics I took while working on my amp.

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What values do you bias kt77's and kt88's

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