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homebrew OR120 problem!

4/6/2005 3:27 PM
homebrew OR120 problem!
I've just build my homebrew Orange OR120 amp using post '74 scheme found  
here: [link:].  
The amp works quite well and is very quiet (no hum), but has too much  
gain, it sounds dirty at any volume level, i think the problem is the  
preamp bias. I found on the net the original voltages of the two 12ax7:  
Pin 1- 190V  
Pin 3- 1.5V  
Pin 6- 182V  
Pin 8- 1.5V  
Pin 1- 287V  
Pin 3- 97V  
Pin 6- 96V  
Pin 8- 0.5V  
mine are less:  
Pin 1- 153V  
Pin 3- 1.58V  
Pin 6- 153V  
Pin 8- 1.5V  
Pin 1- 246V  
Pin 3- 90V  
Pin 6- 83V  
Pin 8- 0.47V  
Can someone help me ?
4/7/2005 3:20 AM
Don Symes

What plate voltages do you have further up the chain?  
I'm wondering if your choke may be too resistive.  
You can try dialing down the gain, too - until you get the voltages figured out. Another 220k in parallel with V1's Rp could help a bit.  
4/7/2005 6:58 AM

looking at the post'74 scheme B+ rail is:  
465V - L1 - 462V - R32 - 357V - R15 - 308V  
I wonder if i have to change r32 to have more voltage or change the bias on each tube.  
4/8/2005 2:26 AM
Don Symes

What are YOUR voltages at those points?  
I'm thinking something around V1 is sucking a lot of extra current.
4/8/2005 7:02 AM

those which i wtote before were mine. the reference to the scheme was only in order to show you the components sorry for misunderstanding! So are:  
465V - L1(20H/100mA) - 462V - R32(33K) - 357V - R15(33K) - 308V  
thanks again.
4/9/2005 4:01 AM
Don Symes

I'm well out of my comfort zone here, but until one of the real talents shows up, I'm willing to keep trying.  
Those supply voltages and drops aren't out of line, so I'm not really sure where to go with the triode bias question. There's no confusion about 2K2 meaning 2.2k or 2200 ohms, right?  
Did you mention that you've tried swapping/subbing tubes (preferably known-good ones)?  
Do the controls do what they're supposed to?  
Have you tried disconnecting the cap from the Boots control's wiper?  
If you have another amp with a pre-out/power-in of FX loop setup, try to isolate the excess distortion to V1 or V2/power amp.
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