Uberschall Rev2 KT88 bias?

Talmaci

New member
Hello!

I modded my Uberschall Rev.2 for KT88.
First, I played very old practically dead tubes biased for 19 mA. It sounded FANTASTIC!!!
After that I bought 4 JJ KT88 and tech biased it for about 35mA. It sounds much worse. Less gain, on 60% it sounds like JCM800. I need to set gain about 75% to make it sound like with old tubes.
Then I bought 4 Electro-harmonics KT88. It still sounds not as good as it was. A lot of experienced guys said that KT88s need much more than 35mA.
Can somebody help me please?
 

Ventura

Active member
:confused:

19mA for any tube with the pV of the Rev2 would sound like shit in my opinion. The best 88 I had was actually the EHX 6550 model tube in the Rev.2 - they handled the pV (around 540 if I'm not mistaken) and sounded awesome at around 32mA

How do you mean you MODDED your Rev.2 for 88/6550s (same tube more or less, hence my inclusion thereof).
 

Talmaci

New member
2 tubes from 4 where practically dead, so we decided to set 19 mA just for a test.
I modded it by replacing the resistance.
 

Ventura

Active member
Talmaci":3v5a09gv said:
2 tubes from 4 where practically dead, so we decided to set 19 mA just for a test.
I modded it by replacing the resistance.
Why?

Sorry - I'm just a bit confused - the stock Rev2 would have MORE than enough pV to run 6550s in its slots. Who recommended the resistor change and to what end?

2 tubes were shot so you used 2 of the 4 tubes at 19mA - but this doesn't make sense as the tubes run in pairs, and would still need to be biased correctly to make anything sound okay.
 

Talmaci

New member
Jorg:
R401 is 100K ohms and located on the pcb where the
power tubes are, for KT88 you need to replace it with a 22K or lower
depending on the grade of the KT88's and then bias the amp.
 

CNutz

Active member
Talmaci":qfalt1rn said:
Hello!

I modded my Uberschall Rev.2 for KT88.
First, I played very old practically dead tubes biased for 19 mA. It sounded FANTASTIC!!!
After that I bought 4 JJ KT88 and tech biased it for about 35mA. It sounds much worse. Less gain, on 60% it sounds like JCM800. I need to set gain about 75% to make it sound like with old tubes.
Then I bought 4 Electro-harmonics KT88. It still sounds not as good as it was. A lot of experienced guys said that KT88s need much more than 35mA.
Can somebody help me please?
What is the plate voltage of your amp? @ 500vdc kt88's need to be conducting 49ma at idle.

Anyhow you just put in tubes with more headroom, and they are biased way cold.

Also really kt88's like to see less grid resistance. Usually you have 100k for kt88's as oppose to the 220k's you see on el34's However this will change the sound too.
 
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