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Old 23rd February 2007, 08:10 PM   #1
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Default Modified Fender Deluxe - Constant Current Phase Splitter

I recently completed a modified Fender Deluxe tube guitar amp per the attached schematic. The main distinguishing feature of the mod is the phase splitter. Rather than using the traditional Schmidt long tail, I used a constant current source (Vishay J505 1.0 ma) splitter which provides considerable gain and an absolute pefect split. The Schmidt splitter provides unity gain and does a poor job as a splitter. Due to the extra gain, you will need a master volume between the recovery tube and the constant current splitter. Sounds great, give it a try.
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Old 24th February 2007, 12:40 AM   #2
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Your attachment was probably slightly too big and didn't make it through.
I'm definitely always interested in seeing good mods to classic Fender amps, and my next project may be something along these lines.
Try posting it again.
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Old 24th February 2007, 01:03 AM   #3
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Here it is.
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File Type: pdf modified fender deluxe.pdf (77.4 KB, 171 views)
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Old 24th February 2007, 01:22 AM   #4
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Quote:
The Schmidt splitter provides unity gain
Who told you that? It provides about half the gain of having a grounded cathode stage in each half. The difference adding a CCS makes is to provide excellent balance, as you say, but the gain remains unchanged.
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Old 24th February 2007, 01:59 AM   #5
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Thanks for your interest in this topic. I heard the schmidt splitter provides unity gain by more than one source. One of the sources is a gent that actually ran a local company that made guitar tube amps back in the 70's and 80's. He designed every one of their amps. I originally built the stock Deluxe with the schmidt splitter. I had problems with a loud oscillation and took it to this gent and we put it on the scope. It turned out to be a defective output transformer (which was new by the way, I always had the speaker plugged in so it wasn't my fault). While at his place, we checked out the stock schmidt splitter which didn't look that good but is okay for guitar amps. Anyways, he talked me into using a constant current source phase splitter. He said it provides more gain that the schmidt. I can can get the amp into some pretty crunchy distortion which a'int happenning with your stock Deluxe.
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Old 24th February 2007, 05:42 AM   #6
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This link at Aiken Amps will tell you all about LTP design for guitar amps and will explain the gain issue and the need for a high resistance or a CCS in the tail.
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Old 26th February 2007, 07:22 PM   #7
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Thanks for the reference. The article is very good and helped me get my head around what a CCS does in the splitter versus a tail resitor. I now agree with your reply that the gain will be the same for both the Schmidt and the CCS variation. The only diference between the two is that the bias current will remain rock steady no matter what the AC signal delivers at the input. This will give a better combined output wave form. For guitar amps, this doesn't really matter though.

I have also discovered that the schematic I posted is dead wrong for this CCS and the Fender amp. The amp works with the J505, but only provides 1 ma of bias current whereas the Fender design calls for 3.2 ma which will give more headroom.

The revised CCS schematic for a +325 V supply using a 12AT7 is attached. This uses a transitor rather than the J505 current diode.
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File Type: pdf revised ccs fender deluxe.pdf (85.7 KB, 93 views)
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