KT88 to EL34 in Orange OR80

DragonCrestPC

DragonCrestPC

Well-known member
I just got a mint 1976 Orange OR80 and it's amazing. The current tubes look shot and would like to replace them. Problem is that they're KT88's and i want to put EL34's in there like how it was stock. Am i going to need to take the amp to a tech for a resistor change or component change? Thanks for the feedback!
 
Found this schematic. See the note at the lower left for OR80
http://www.orangefieldguide.com/OFG_SCH ... hem_74.gif
Looks like the original amp would have had 1k screen grids, 2.2k grid stoppers, and 220k grid leak resistors.
If all of this is still in place, and the amp will bias the EL34's, then you're good.
It's possible that the bias circuit was modified to bias the KT88's, and the EL34's will be too cold even at the lowest setting, but you'd have to check. It does look like the bias adjust is a 100k pot, so there may have been enough swing to bias the KT88's without modding the amp.
It's also possible the grid leak resistors have been lowered to something like 100k for the KT88's. EL34's will run fine with a value lower than 220k, but it will change the tone.
Sorry if that doesn't answer your question with a yes or no, but it's impossible to know for sure without having the amp in front of me and checking some voltages.
If you can check all this stuff yourself, and make adjustments where necessary, than fine.
If you have no idea what I'm talking about, take it to a tech.
In any event, it's a minimal amount of work and shouldn't cost very much at all.
Also not a bad idea to have an experienced set of eyes go over a vintage amp. For example, what shape are the electrolytics in?
 
fusedbrain":38wmzaoj said:
Found this schematic. See the note at the lower left for OR80
http://www.orangefieldguide.com/OFG_SCH ... hem_74.gif
Looks like the original amp would have had 1k screen grids, 2.2k grid stoppers, and 220k grid leak resistors.
If all of this is still in place, and the amp will bias the EL34's, then you're good.
It's possible that the bias circuit was modified to bias the KT88's, and the EL34's will be too cold even at the lowest setting, but you'd have to check. It does look like the bias adjust is a 100k pot, so there may have been enough swing to bias the KT88's without modding the amp.
It's also possible the grid leak resistors have been lowered to something like 100k for the KT88's. EL34's will run fine with a value lower than 220k, but it will change the tone.
Sorry if that doesn't answer your question with a yes or no, but it's impossible to know for sure without having the amp in front of me and checking some voltages.
If you can check all this stuff yourself, and make adjustments where necessary, than fine.
If you have no idea what I'm talking about, take it to a tech.
In any event, it's a minimal amount of work and shouldn't cost very much at all.
Also not a bad idea to have an experienced set of eyes go over a vintage amp. For example, what shape are the electrolytics in?
Click to expand...




Thanks good sir! That helps big time!
 
:thumbsup: No problem!
By the way, congrats on the new amp. :rock:
You don't see original 70's Orange amps come up for sale very often.
Those amps are way cool and definitely have their own thing going on.
If I had a chance to grab a minty '70s Orange, I'd snap it up in a heartbeat.
Bet that amp would sound incredible with a nice fuzz hitting the front end.
FYI: the EH Triangle Muff re-issue absolutely kills for not very much $$. :yes:
 
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