Fresh Start or anyone , I have one final question on the OT on my AA764 build

[catnine]

"The OT is a big part of any amp's sound too.  Have you told us what the OT is?  The voltage drop across the OT secondary is a little bit higher than "spec" which might indicate a higher secondary winding resistance.  Whatever it is, the secondary winding resistance is greater than 1K.  We know that because the voltage drop across the OT is greater than the voltage drop across the 1K dropping resistor going to the screen grid supply."

 I see what you mean if I look at the voltages on the fender AA764 schematic , the plate and screen are both 350 VDC.

 So to understand this . they only state on the OT the primary resistence of the OT and these are the leads that go to the B+ which is the red lead and the blue lead goes to the plate on the 6V6 . Yet that resistance really has nothing to do with the secondary resistance directly does it , it's the relationship of the two that can't be measured with an ohm meter . Inductance or impedance right? So basically what I have is not a high screen but a low plate VDC based on the fact that the OT is dropping more voltage to the plate and less voltage across the 1k ohm screen resister.

 I am not even certain since I never tried that if one can measure the primary OT resistance which is 7K ohms .

 This OT does have a 4 and 8 ohm tap and since I use the 8 ohm tap with an 8 ohm speaker this means more windings in the secondary , could this result in the OT bringing down the Plate voltage more than the 1k ohm screen resister brings down the screen voltage or I'm I way off base? I am just rying to understand this .

[Fresh_Start]

First off, the OT "primary" is the power tube side.  IOW red/ blue leads in your case.  The secondary winding(s) are connected to the speaker jack(s).  Sorry - my bad for referring to the power tube side as the secondary.  You can measure the DC resistance of the primary winding on the OT, but you can't really measure the inductance which is what matters for AC signal.  You also can't measure the relationship between the primary & secondary windings.  You just have to trust the manufacturer's specs.

Secondly, you're working way to hard to find a data point to measure.  Tube amps are organic instruments.  No two are exactly the same, because no two tubes are identical if nothing else.  That's why we like them!  The amp has too much high end according to your ears.  Your ears are  totally analog and very subjective.  Trust them.  Tweak the amp to get the sound the way you want it.  Either work with the OT you've got, or try something else (preferably something on hand).  Try every speaker you can.  We've given you all kinds of circuit tweaks.  Try some.

Respectfully,

Chip

[catnine]

 I will try the suggestions . I just wanted to try to understand how the resistance of the OT primaries being higher than the 1k ohm screen resister would have the possible affect of lowering the the plate voltage compared to the screen voltage . I realize it's not so simple or can it be applied to common sense . I do appreciate the tweak suggestions .

[Fresh_Start]

I will try the suggestions . I just wanted to try to understand how the resistance of the OT primaries being higher than the 1k ohm screen resister would have the possible affect of lowering the the plate voltage compared to the screen voltage . I realize it's not so simple or can it be applied to common sense . I do appreciate the tweak suggestions .

Sorry - what I said before was misleading at best because the current passing through the OT is higher than the current drawn by the rest of the amp.

V = I * R

Take a look at Ohm's law formula wheel here:
http://www.sengpielaudio.com/calculator-ohm.htm

We know that there is a greater voltage drop across the OT primary than there is across the 1K dropping resistor feeding the screen grid and the preamp, right?  We also know the total current going through the power tube cathode (I = V/R).  That current includes the screen grid current, so we need to calculate the screen grid current separately. 

Measure the voltage drop across your screen grid resistor (470 ohm?), then calculate the idle screen grid current (I = V/R).  Subtract the screen grid current from total power tube cathode current and you'll get the actual plate current.  FWIW that will also give you a more precise, although not necessarily more useful, data point to calculate actual plate dissipation (wattage).

You know what the voltage drop across the OT is, and now you know the DC current passing through the OT primary winding.  It is equal to the plate current.
V / I = R
That give you the DC resistance of the OT primary.

Where I was wrong is that the voltage drop across the OT may be a multiple of the the voltage drop across that 1K resistor, but the greater voltage drop is more likely due to greater current rather than greater resistance.

Where I was right is that it doesn't matter.  It's OK to have the screen grid voltage a little bit higher than the plate voltage in this circuit.  Actually, it's normal.  That's not what is causing your amp to produce more treble frequencies than you like.

Seriously, understanding Ohm's law is essential.

Hope this helps.  I apologize for misleading you earlier - it was done with the best intentions...

Chip

[catnine]

"Measure the voltage drop across your screen grid resistor (470 ohm?), then calculate the idle screen grid current (I = V/R).

 I measured this just to check the screen grid current at idle . I put my RAC 6V6GTA in thier just to see what diff that made . This may seem and probably is a really stupid question ( please insert laughing ) into the answer.

 I measured the votage drop across the actual 462 ohm screen resister and it was .86 VDC and came out with .00186 which would be .019 mA draw on the screen .

 Either I missed something or did the math wrong.

[DummyLoad]

I measured the votage drop across the actual 462 ohm screen resister and it was .86 VDC and came out with .00186 which would be .019 mA draw on the screen .

rounded up it is 1.9 millamps. 

.001 amp = 1 milliamp

[catnine]

I measured the votage drop across the actual 462 ohm screen resister and it was .86 VDC and came out with .00186 which would be .019 mA draw on the screen .

rounded up it is 1.9 millamps. 

.001 amp = 1 milliamp

 Thanks , that's right , I was rounding off then for some odd reason I was looking at the math for the cathode resister which was .035545 which would be rounded off 35.5mA and with that in my head I moved the (.) 1 0 over . I was checking the amp with the RCA 6V6GTA instead of the JJ 6V6 with the figures all on paper to compare.

[catnine]

I will try the suggestions . I just wanted to try to understand how the resistance of the OT primaries being higher than the 1k ohm screen resister would have the possible affect of lowering the the plate voltage compared to the screen voltage . I realize it's not so simple or can it be applied to common sense . I do appreciate the tweak suggestions .

Sorry - what I said before was misleading at best because the current passing through the OT is higher than the current drawn by the rest of the amp.

V = I * R

Take a look at Ohm's law formula wheel here:
http://www.sengpielaudio.com/calculator-ohm.htm

We know that there is a greater voltage drop across the OT primary than there is across the 1K dropping resistor feeding the screen grid and the preamp, right?  We also know the total current going through the power tube cathode (I = V/R).  That current includes the screen grid current, so we need to calculate the screen grid current separately.  

Measure the voltage drop across your screen grid resistor (470 ohm?), then calculate the idle screen grid current (I = V/R).  Subtract the screen grid current from total power tube cathode current and you'll get the actual plate current.  FWIW that will also give you a more precise, although not necessarily more useful, data point to calculate actual plate dissipation (wattage).

You know what the voltage drop across the OT is, and now you know the DC current passing through the OT primary winding.  It is equal to the plate current.
V / I = R
That give you the DC resistance of the OT primary.

Where I was wrong is that the voltage drop across the OT may be a multiple of the the voltage drop across that 1K resistor, but the greater voltage drop is more likely due to greater current rather than greater resistance.

Where I was right is that it doesn't matter.  It's OK to have the screen grid voltage a little bit higher than the plate voltage in this circuit.  Actually, it's normal.  That's not what is causing your amp to produce more treble frequencies than you like.

Seriously, understanding Ohm's law is essential.

Hope this helps.  I apologize for misleading you earlier - it was done with the best intentions...

Chip

You didn't miss lead me . at this point it does appear the the OT is drawing down more current than the screen on the 6V6 and the 12ax plates . Rather than try all sorts of tweaks I will try the other speaker I do have and if that does not do it then the heck with it. It is what it is and it works and has not fried anything yet . The treb pot does work and cuts the highs where it is fine . I am the one who should be sorry for asking all sorts of questions . Since I built this amp over two years ago one thing I do recall is when I installed the weber sig 10 s ceramic and took out the worn jensen C10R I was at first disappointed because the weber seemed sterile .

[tubeswell]

Seriously, understanding Ohm's law is essential.

What Chip said.

(just to keep digressing ;-) here is a simple way of visualising Ohm's law E = I x R, E/I = R, E/R = I in a spatial triangle with E at the top. You divide E by either I or R, in order to get R or I correspondingly, or you multiply I x R to get E at the top. Hope that helps. I sure found that is picture helped me with the memorising)