I built a AA764 champ home brew and had one question about the bias

[catnine]

 There is a photo of the chassis with the back off attached. I can't locate a schem right now that I can post . I built it using a narrow panel tweed style chassis that Allen amps used to provide . You could build even a push /pull 40 watt out of this with a diode rect. It came with an eyelet board and I built it to spec just like a AA763 champ. I used F&T filter caps because they fit the narrow board and I used values similar to the 73 silver faced champ I had which used a cap can 40/20/20 MF I used 47/ 22/22 single caps . All I changed was to use the fender style DPDT slide switch that was already punched in the chassis and used it to control the NFB so one way it is stock value 2.7k ohms and the other way it feeds a pot I placed in the un-used 9 pin chassis hole so I could dial in no NFB to more than stock and just flip the switch to stock to save the setting I liked.

 Back to the bias setting . I went by the idea that was floating around years ago to use a 1k ohm 5 watt cathode resister , the amp did sound fine yet the IPD or watts was 10.25 so I kept dropping the value and ended up with a 680 ohm 5 watt resister and had 12.55 watts . One thing I noticed was the highs became more more pronounced , almost ice pick highs at 12.55 watt on a JJ 6V6 or even an old RCA 6V6GT. By B+ and plate voltage did drop by 20 VDC and the B+ was on the high side 390 VDC now its 370 VDC this is afer the higher bias setting which I expected. I used weber transformers for both the PT for a champ 125 Volt primary and the champ OT same size as a fender and has the bells but it provides either a 4 or 8 ohm speaker tap my speaker is a weber sig10 s ceramic 8 ohm . I also have a NOS westinghouse 5Y3 in it. I was afraid of the surge on power up charging the 47 MF fisrt filter cap so I was told by mission amps to install a 47 ohm 3 watt resister between the B+ off the rect to the B+ in series so I did this

 Problem is I can for the most part dial out the highs and used a JJ 12xa7 since it is darker sounding which helped . I am afraid to bias hotter fearing this will make the amp even more bright . Any ideas?

[phsyconoodler]

don't bias it hotter.12.5 watts is fine.14 watts is the upper limit and a couple of watts ain't gonna make much difference.
   That amp should not be ice-picky however,if you are using the stock Fender 25uf preamp bypass caps.I've built AA764 champs with 12" speakers and they sound lovely.Maybe your Weber speaker is the culprit here.The Signature series is not noted for fat sounds at all.

[catnine]

don't bias it hotter.12.5 watts is fine.14 watts is the upper limit and a couple of watts ain't gonna make much difference.
   That amp should not be ice-picky however,if you are using the stock Fender 25uf preamp bypass caps.I've built AA764 champs with 12" speakers and they sound lovely.Maybe your Weber speaker is the culprit here.The Signature series is not noted for fat sounds at all.

 All I have is a Jensen C10R which if memory serves sounded less bright yet they are not known for their lows or warmth either. All of this began when I changed the 1k ohm cathode bais to a 680 ohm . I even changed the mallory 150's and installed xicon brown tone caps , that helped some . I wanted to run the amp closer to the design and 10.25 watts seemed far to low and I wanted some breakup well before the vol was on 7 and I got that now, along with the icepicks that is.

[phsyconoodler]

   Is that the cathode bypass cap to the left of the 680 ohm resistor?
What about the value of the resistor to ground on your bass pot.Measure it and make sure it's right.Too much mids and the amp will be trebly too.
  
What about the treble cap?Lots of stuff inside that amp is interesting but some of it may be where your issues lie.Getting hotter bias almost never equates to more treble.In my experience it makes the amp sound fatter.As you draw down the B+ with more current it lowers the preamp slightly too.That means browner sounds.
  Switching from a normally mellow sounding mallory 150 to an xicon doesn't ring true with me either,unless the actual value is more than the mallory was.
  Any chance you switched from a cheapy guitar cord to a George L's in the meantime?They are much more articulate which translates to more highs.
 Lots of stuff to consider.
   By the way,your build looks pretty awesome.

[catnine]

   Is that the cathode bypass cap to the left of the 680 ohm resistor?
What about the value of the resistor to ground on your bass pot.Measure it and make sure it's right.Too much mids and the amp will be trebly too.
  
What about the treble cap?Lots of stuff inside that amp is interesting but some of it may be where your issues lie.Getting hotter bias almost never equates to more treble.In my experience it makes the amp sound fatter.As you draw down the B+ with more current it lowers the preamp slightly too.That means browner sounds.
  Switching from a normally mellow sounding mallory 150 to an xicon doesn't ring true with me either,unless the actual value is more than the mallory was.
  Any chance you switched from a cheapy guitar cord to a George L's in the meantime?They are much more articulate which translates to more highs.
 Lots of stuff to consider.
   By the way,your build looks pretty awesome.

 Yes to the left of the cathode resister is the cathode bias cap . , the bass pot resister to ground is right on 15k ohms .  there are the three caps on the tone each is almost right on spec  a 250 pf , then .1 mf , then .047mf and I did use a 1 meg for the vol pot and 250 ohms for the treb and bass pots all audio taper. And 100k ramp .

 On that switch and the pot , what it is is this , in one position the switch connects the stock 2.7k ohm NFB as is stock value. If I move the switch in the other position it disconnects the stock 2.7k NFB and connects the pot which is a lin pot which I believe is 1k ohm so I can have no NFB up to 1k NFB but it is not connected to the 2.7k resister. I have to check the pots value but I do know the 2.7k is out of the circuit with the switch in that position. I was just a thing to have an adjustment rather than 2.7k or no NFB . I rarely use it because the champ with no NFB does not sound tight at all. I will check the value of the pot and draw up a wiring diag of how I did this . I didn't add this pot in until afer the icepick highs came into the picture  it was an attempt to add something as many like champs with no NFB . At first looking for options I used to switch to break the ground on the tone stack like some do but hated that sound. In the beginning I built the amp all stock and the switch was just in the punch to fill it . Later I was after ways to smash the icepick highs I will check all volatages and draw up a schem with them .

[catnine]

thses are photo's of the progression of the build

[catnine]

 I went on to add the 4 ohm tap jack and NFB pot. the second photo shows how it is now really a JJ6V6 and new standby and power switches because the ones I had were to big and to stiff. and the added 47 ohn 5 watt resister to hold the 5Y3 from a voltage surge on the 47 mf 1st cap .

[catnine]

The only other thing I can think of is that the OT is one that has a 4 ohm and 8 ohm tap , I use an 8 ohm speaker and was told by Hotplateblues that I should connect the 2.7k ohm NFB to the 4 ohm tap even though the speaker is an 8 ohm speaker on the 8 ohm tap and this would show the OT the proper NFB resistence. At first I just took the lead from the 4 ohm OT secondary tap and wired it direct to the 4.7k NFB resister , after I added the 4 ohm tap jack I still used the 4 ohm OT secondary tap for the NFB resister only off the jack rather than direct off the OT 4 ohm wire. . Champs were 4 ohm only as you know. I found this on Aiken amps.

"The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer.

The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output. Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms.  You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair.  In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap.  Note that the tap voltage is  not linear with respect to the impedance, it varies linearly with the square root of the impedance. That is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap.  It helps if you think of the equation for power: P = V^2/R.  If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have 100W into 8 ohms, the voltage is V = sqrt(100*8) = 28.28V RMS.  If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS.

When designing an amplifier from scratch, it doesn't really matter which transformer tap you use.  What you are trying to do is obtain a certain amount of negative feedback, which is determined by the ratio of the two feedback resistors (the series feedback resistor from the output and the shunt feedback resistor that goes to ground, which form an attenuator).  The tap simply determines the initial amount of voltage applied to the feedback network, which is then adjusted to achieve the desired amount of signal reduction, or negative feedback. "

 Since I have the 2.7k NFB resister on the 4 ohm tap and the speaker on the 8 ohm tap how does the above info apply?