Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Orange Bias filter and other ?'s

11/8/1997 4:31 AM
Orange Bias filter and other ?'s
would there be much of a difference in noise or performance if I increase the bias caps in an Orange 120 from 10mfd to 47mfd? Will it slow down the bias from reaching it's stable point? The Amp I'm looking at is Biased at 43v.The schematic in pittman's book shows 37v. Any opinions on which is better? Last but not least is it normal for the F.A.C. switch to pop when it is adjusted? TIA for all ideas and guidance.
11/8/1997 5:52 AM

Marc I don't think there will be a big difference  
in noise.  
Don't worry so much about the bias voltage ...  
find out what the bias current is instead.  
-43 volts will be a little safer and run abit  
cooler then -37. How does it sound biased up there?  
You'll have better and more stable bias supply voltage with bigger caps in the bias filter PI network.  
I don't use anything bigger then 100uF but a pair of 47uF work very well too.  
On my Marshall clone amps using no seperate bias winding, I find it take about 90 seconds for the bias voltage to come up to -45 volts,  
I would bet it would be different with different  
value caps too but the biggest difference I see is if I steal the bias voltage from one side of the  
Hi volt secondary versus a real 50 vac bias tap/  
The resistances required to get 350vac down to  
a reasonable level at such a low low current draw  
dramaticly increases the charge up time constant.  
It has been so long since I have heard the Orange  
goofey tone switching thing-a-majig, I can't comment on the noise.  
If the caps are in the EQ section and the EQ section is not DC blocked ahead of it, I bet it would make some noise as you increased or decreased the cap on the rotary switch.
11/9/1997 3:32 AM
Thanks for the reply. Ireplaced the bias caps w/ 47mfd units since the original caps had a '73 date code and were swelling a bit. you were right about the difference. The Bias is set at 44.4 now- ala pittman's book- the owner has a plug in meter that a local tube guy made. I measured the current @149ma (4 EL34's) w/ 44.4V, I also did a couple other settings to recommend. btw, at -39v it was pulling better than 210ma. hot.  
the tone section is weird- a baxandall type followed by the cap in series switch. After the first sweep thru it is quieter.I'm guessing the caps charge up and don't pop as much then. I didn't find anymore than .003v AC after the switch so I guess it's caps aren't leaky.  
It's nice to see a 25 year old amp in such good shape....
11/9/1997 4:45 PM

Hi Marc,  
I have a homebrew push pull EL34 amp that I was mad at and threw in the corner of my shop.  
It originaly used a Fender type tone circuit but  
for fun I replaced it with the Baxandall type. I really didn't care for the tone as much as I had hoped. It just was not punchy or ...something!  
It didn't add that sound I am acustom to.  
No matter how I set the two controls, I never could get a tone that jazzed me up.  
I'll have to look in Pittman's book for the Orange amp and see how it is configured because I took the same type layout as my EL34 amp but used a pair of 6V6 tubes with a cathode follower ahead of Baxandall and it sounded much, much better.  
The tone circuit was very EFFECTIVE.  
Although it seemed like the controls needed to be set in an unusual way to get a cool mid-range grunt and squawk.  
By the way my Marshall/Traynor clones sound the best with about 34ma to 38ma for bias.  
Pretty close to what you have.  
BY the way Marc the capacitors WILL pass AC. You should have been looking for DC... typo?  
To be safe, I always put a .1 to .33 600volt cap in front the tone section to block any DC from the EQ components.  
So far I have not had any problems with phase distortion because of the additional series  
capacitance. Well, if there is any I like it.  
Describe the noise you are trying to filter out.  
11/10/1997 8:43 AM
Marc - Bruce is right. Don't even measure the bias voltage, unless you suspect that the bias supply is not making enough voltage and you need to debug it. The tube current is what matters. Try reading the biasing section of the Tube Amp FAQ at for some how-to's on biasing.
11/11/1997 2:13 AM
Thanks, Bruce and R.G.  
I DID mean DC. and did measure a little comming off the .068 cap before the F.A.C. control- replaced it and the popping is almost all gone.I also replaced the cathode bypass caps- they had drifted up in value and I suspect they were contributing to the flabby bottom end....  
I also tacked in a temporary 10 ohm resistor and fiddled with different bias settings so I could give the Orange's owner an idea of different settings. He is more of a player than a DIYer...  
As for the tube FAQ - I have read it, printed it(thanks R.G.) and would like to credit the earlier version with inspiring me to shelve my solid state tinkering in favor of vacuum tubes. I am very appriciative of all the folks here who are so helpful and patient with my novice questions...
   Page 1 of 1