AB763 not loud at all

[zfiles1701]

looks like my ab763 is only putting out 12 watts-rough est. Rough est method is put my DMM on speaker and get voltage, then use speaker imp or ohms (did both) and calc power.  My ax84 calcs to about 11 watts, which sounds right to me for a class a el34.   And comparing it to my ax84 p1x, I found the ax84 is louder clean and dirty.  Which leads me to believe something is wrong, but what? 
Have tried the following:  swapped power tubes with my plexi (6l6) which were loud as all get out in the plexi.  NO effect.
swapped preamp tubes NO effect.  beefed up PS cap to 2X47uf so its got 94 uf in the first cap.   Using the same speakers for both amps to elimiante them to.  Cranked up this thing should be loud and put out at least 30 watts.  It's not.  Any help would be appreciated.
Thanks

[zfiles1701]

And I checked all my voltages and cathodes.  they all seem good.  In the range of the other builds.  One thing.  I had to use two 10k in parallel to get 5k for the last dropping resistor to the input tubes plates.  The schem call for 4.7k.  I don't think that would drop the voltage enough to make the gain that weak.

[FYL]

If voltages are OK, check
http://www.geofex.com/ampdbug/lowpower.htm
http://www.geofex.com/ampdbug/distort.htm

[samato]

Is that an acceptable way to estimate power?  I haven't heard of it but I'm not very experienced.

What I've learned to do to calculate static dissipation (idle wattage) is to multiply plate voltage by the voltage across the 1 ohm resistors from the cathodes to ground.  Of course, if you don't have those resistors...

[zfiles1701]

Voltages on PI kinda funny.  90vdc on cathodes and 60vdc on the grids for the fender in question.  Another amp that I compared the fender to, the plexi that I measered in 37vdc cath and 22vdc on the grids. does this sound right?

[Fresh_Start]

First off, comparing voltages at specific points in the circuit between a Marshall Plexi and a Fender AB763 does not compute.  Look at an AB763 schematic (see link at the top of the page for Hoffman's schematic library).

Secondly, which AB763 are you working with?  Voltages in the PI of the Super Reverb are very different from the Deluxe Reverb.  You mentioned 6L6s... 

Is this an old amp that used to work fine, or did you build this amp from scratch and it's not working as expected upon start-up?

How are the power tubes biased?  Doesn't matter how you measure plate current, but we need to know what you've got (and verify you're using 6L6s).

Does the output transformer's primary/secondary ratio make sense for a 6L6 amp with whatever speaker(s) you are using?  Deluxe expects to "see" 8 ohm load.  Super Reverb expects to "see" a 2 ohm load.

Sorry for all of the questions, but the comments about PI voltages made me think we need to start at the beginning.

Respectfully,

Chip

[jjasilli]

Is that an acceptable way to estimate power?  I haven't heard of it but I'm not very experienced.

What I've learned to do to calculate static dissipation (idle wattage) is to multiply plate voltage by the voltage across the 1 ohm resistors from the cathodes to ground.  Of course, if you don't have those resistors...

Samato: that calculates plate dissipation in watts.  This is important to set bias, but is not the "output wattage" of the power amp.  For that use a variation of Ohm's Law:  Watts = V² / Impedance.  You can use either:  a) at idle: [Plate Voltage]² / nominal impedance of the OT primary; or b) on the secondary side of the OT:  [Speaker Voltage]² / nominal impedance of the Speaker Load, measured at full clean volume.

[sluckey]

Voltages on PI kinda funny.  90vdc on cathodes and 60vdc on the grids for the fender in question.

That's probably OK. You can't trust grid voltage readings for a LTP PI because your meter will load down the real readings. A better voltage reading would be just measure the voltage at the junction of the grid resistors and the cathode 'tail' resistor. Or measure the voltage between cathode and grid. Plate voltages should be about the same.

I know you said voltage readings look good but exactly what are the readings? What is the voltage reading on pin 5 of the 6L6s? Can you post voltage readings for ALL tubes and ALL pins?

[zfiles1701]

Not with amp right now.  will have to get back home to check what I wrote down.  will post it tonight.  Appreciat the input.
roughly I think my PT plates are about 420 VDC.  bias about 35mA.  PT grids about -40 VDC.  cant rememeber anything else.  and yes 6L6 hoffman ab763 build 8 Ohm speaker tap with 8 ohm speaker. 

[samato]

Is that an acceptable way to estimate power?  I haven't heard of it but I'm not very experienced.

What I've learned to do to calculate static dissipation (idle wattage) is to multiply plate voltage by the voltage across the 1 ohm resistors from the cathodes to ground.  Of course, if you don't have those resistors...

Samato: that calculates plate dissipation in watts.  This is important to set bias, but is not the "output wattage" of the power amp.  For that use a variation of Ohm's Law:  Watts = V² / Impedance.  You can use either:  a) at idle: [Plate Voltage]² / nominal impedance of the OT primary; or b) on the secondary side of the OT:  [Speaker Voltage]² / nominal impedance of the Speaker Load, measured at full clean volume.

Thanks.  I have some questions about this but I think I'll start another thread for them

[zfiles1701]

First this is a Hoffman, AB763, 2 6L6, only one channel, reverb, and tremelo according to Hoffman.  Here are the voltages:
 (add one to tube number if you look at Hoffman Schem/Layout because no channel one on my build)
Tube    Pins/Values in VDC unless indicated otherwise         
   1         1    267,    2   .1 mV,   3   2.09,  6   262, 7    0,                              8   2.06                                                                 
   2         1    439,    2  .025 mV, 3   8,     6   439,  7   .025 mV, 8                   8   8                                                                                               
   3         1   262,     2   0,         3  2.16,  6   265,  7    0 to 6 mV fluctuating,   8   2.06
   4         1  219,      2   .8 mV,   3  2.36,  6   449,  7    219,                           8    228
   5         1  248,      2   56,       3  95      6   242   7    62,                             8   95
   6                                        3 444,  4  444,  5 to 8  -41,                          8   43 mV
   7                                        3 443,  4  443.  5 to 8  -41                           8   43 mV   

Notes:  Pin 7 on Valve 3 is very noisy when DMM is on it.  All other grids were not noisy like that one.  And pin 2 on valve 2 also has very low voltage on grid as well but not noisy when being probed.
Hope someone can help.  And thanks for looking.                                                                                       
                                                                                                               
                                                                                                               
                                                                                                               
 

[zfiles1701]

just measure the voltage at the junction of the grid resistors and the cathode 'tail' resistor. Or measure the voltage between cathode and grid. Plate voltages should be about the same.

measured between the grid Rs and the Cath R.  93 volts.
And posted the other values for all tubes previously
Thanks

[67polara]

Something wrong with V4 voltages.

Tony

[sluckey]

V4 voltages look fine to me.

[zfiles1701]

What are some of the voltages you guys are seeing on your ab763's with two 6L6's?
Thanks

[sluckey]

I don't see anything with your voltages that indicate a problem. I'm suspecting a wiring error or incorrect component value. Will you measure the cathode resistance to ground for all your little tubes? Put one meter probe directly on the tube socket and the other on chassis.

Lot of people here are very familiar with Doug's AB763. Post some hi rez pics any maybe someone can spot something.

[zfiles1701]

cathode to ground looks good (to spec per schem) on all little tubes.  Only thing I have not done is unsolder and check the cathode bypass caps.  Where can I host pics?  I tried to attatch some hi rez here but file to big.  Gonna go over every component and wiring diagram again.
Thanks

[Fresh_Start]

Here are my Super Reverb voltages.  Please keep in mind that the "Normal" channel is anything but normal.  Also, I had a separate power rail node for the reverb transformer:
http://www.el34world.com/charts/valve/valvedata/ChipAB763.htm

My apologies: V3 & V6 were both 12AT7 tubes, not 12AX7s as shown on that chart.

Chip

[Danskman]

Another BFSR to be compared with yours

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10455

Classic AB763 on fiberboard.
HTH,
Danskman

[zfiles1701]

Shots of the amp
http://s1225.photobucket.com/albums/ee394/zfiles1701/

[zfiles1701]

Better late than never.  the mess in the middle is me switching to different values to get the tremelo where I want it.
Bypassed the effects and still low power-so don't think its the effects when they are desoldered fro the circuit.  Then discon the pre amp and connnected a known good preamp (plexi) into the fender's phase inverter.  Same result, low power.  So at least I know The prob is in the PI PA side.

[zfiles1701]

Well,
Found it by testing the res across all my trans.  The center tap was way off center.  The company I bought from is cool.  Even though out of warranty, they sent me a new one-no charge.  Spent a long time trouble shooting this one-frustrating.  My circuit was good, but I checked, and checked, and checked it.  Disconnected all effects.  Anyway got it going 100%.  Sounds as amazing as I thought it would. AB763, 2 6L6, and 2-10" weber bluepups.  Gettin all kinda sound out of it.  My friend will probably pass out when he hears it.