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Post by ajlove on Nov 26, 2006 20:20:42 GMT -7
On the website Dr. Z says that the phase inverter is "fixed bias".... does this mean that if I change the 5751 V2 tube I need to adjust the bias? I'm a little confused by that and just wanted to make sure before I change that tube...
I understand that the Ghia is self-biasing in terms of the EL-84's, and in my experience one can change pre-amp tubes without needing to adjust bias, but the Fixed Bias Phase Inverter is a unique aspect of the Ghia...
More info would be greatly appreciated.
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Post by Curt on Nov 26, 2006 20:48:04 GMT -7
swap away, no bias adjustment needed, 12BH7's add a lil head room in V-2 and I like that tube in mine, a nice NOS balanced 5751 is also a good choice.
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Post by ajlove on Nov 26, 2006 22:36:07 GMT -7
thanks tele62, I appreciate it! Do you know why? Just curious/wanting to understand amps better. Why would a fixed bias phase inverter not need to be re-biased when swapping tubes?
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Post by DRZ on Nov 28, 2006 4:35:58 GMT -7
On the website Dr. Z says that the phase inverter is "fixed bias".... does this mean that if I change the 5751 V2 tube I need to adjust the bias? I'm a little confused by that and just wanted to make sure before I change that tube... I understand that the Ghia is self-biasing in terms of the EL-84's, and in my experience one can change pre-amp tubes without needing to adjust bias, but the Fixed Bias Phase Inverter is a unique aspect of the Ghia... More info would be greatly appreciated. OK AJ you ask for it, it took a couple of days for me to think about a response. Understand this, the Ghia has three very UNIQUE circuits, circuits that you will not find in other amps, that is till the release of the Ghia. after the Ghia was released Fender did use the Conjunctive filter on the blues Jr. a direct copy of the Ghia though values were altered. But getting back to the fixed bias Phase Inverter. To my knowledge this has NEVER been used in a guitar amp. It's principles are to set the Cathode and Grid bias DC volts from a fixed source in the power supply. 80 % of Phase Inverters in Guitar amps use the Long tail Cathode follower, which develops it's bias voltage from the resistors in the long tail in a SELF BIASING type design. The gain of the tube determines the currant drawn threw the resistor and hence the voltage developed to bias the tube. In the Ghia I select the bias voltage and the operating parameters of the Phase Inverter a very important and many times overlooked parameter in a two tube pre-amp, (my specialty I might add) . I set the tube to operate dynamically balanced, not just statically balanced. Also for the tube NOT to overdrive into clip which would them drive the EL-84's too hard. If the outputs are driven too hard a harsh, splattered, ripping type of overdrive is developed, something I tried to avoid in the Ghia design. I selected a 5751 type device to use for it's VERY tight MIL spec tolerances, so any 5751 can be dropped in. Also they were very easy to find' well built and cheap tubes some 20 years ago when I designed the Ghia. I used to buy hundreds at a time from new Sensor GE NOS Mil Spec 5751's for less then $7.00 each. Also I liked the hollow rich tone that the 5751 had, which differed from the upper mid response of most garden variety 12AX7's. And yes the bias voltage can be varied AJ. But I spent a long time optimizing and plotting the dynamic operation of the Phase Inverter in the Ghia. A slight adjustment of the power supplies voltage divider will alter the response some, and might improve one of the operating parameters but will effect others in a negative way , Trust Me I'm a Doctor......
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Post by bluzsteel on Nov 28, 2006 7:27:29 GMT -7
"Trust Me I'm a Doctor"......
Classic!!!!!
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Post by Curt on Nov 28, 2006 7:44:49 GMT -7
"Trust Me I'm a Doctor"...... Classic!!!!! LOL +1, Classic !!!! Thanks for the info Mike.
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Post by dock66 on Nov 28, 2006 9:33:09 GMT -7
"Trust Me I'm a Doctor"...... Classic!!!!! Me too,I play one in real life. Thank you for taking the time to explain ,Doc. ;D JB
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Post by ajlove on Nov 28, 2006 12:31:45 GMT -7
Thank you very much for the detailed response, Dr. Z, I really appreciate it.
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