72 Fender SF Deluxe Reverb (AB868) runs at half power on both channels

[jaster55]

This has been completely gone through... Recap, sockets cleaned and retensioned, cleaned all pots, jacks, & switches, checked tested and swapped tubes...
All voltages match the schematic within 15% except one...
V5 pin 3 is supposed to read 2.1 volts and it reads zero... The other bypass cap and resistor going to V4 pin 3 & 8 reads perfect
All other voltages are perfect...
Turning this amp up to 8 gives you volume you should be getting at 3

Any advice would be appreciated... Thanks

[sluckey]

Try another speaker.

V5 pin 3 is supposed to read 2.1 volts and it reads zero...

Zero would be correct if the footswitch is turned off or not plugged in.

[jaster55]

Thanks for the quick response... That cured the missing voltage problem.
I have it on the bench... I know the speaker is ok.
Could it be the output transformer. I checked voltage and resistance on the primarys when I was checking bias and that was where it should be... Is there a quick test I can do just using my multimeter to check it.

Again... Thanks for the quick response and any other info you can pass on...

[neskor]

https://ampgarage.com/forum/viewtopic.php?t=35771

[Latole]

Many things can cause low volume other than OT,
A signal generator and an oscilloscope will help you determine where the signal (the sound) stops.
Essential tools for anyone working on amps.
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Testing Output transformer are very easy
 
I do this test on few OT with success. I find wich are good, wich are bad.

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Testing Output Transformer and finding Impedance

Amp at OFF, disconnected from wall outlet
Power tubes removed

1- Signal generator set to 1000 hertz AC 0.5 volts

2- Send this signal to OT secondary , check with accurate meter to have 0.5 VAC on OT tap. Or close

3- Read AC voltage at OT primary

4- For exemple if you read 14.35 VAC you divide by 0.5  ( or what you read at 1- )

    14.35 / 0.5 = 28.7

5- Square the result  ; 28.7 X 28.7 = 829.67

6- Multiply by secondary output impedance where you are connected  , say 8 ohms
    829.69 X 8 = 6631

7 - This reading 6631 is the primary input impedance

If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms  for two 6L6 in PP class AB

The reading we got 6631 is on spec
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If transformer is no good your reading will be very far from what we should get .

By experience these test do not work on Super Reverb Transformer or other with 2 ohms Output Transformer impedance

You can do this test on unknow OT to see how / where you can use it

[stratomaster]

You can sometimes get away without a scope.  If you have a way to send a sine wave into the amp (phone app with a 1/8"stereo to 1/4"mono cable) and a meter capable of reading AC voltage RMS then you can measure the AC voltage at the grids and plates of each stage.  The Reissue schematics have reference voltages you can compare to.  But you should be getting 50-60x at each preamp stage.  This can help isolate a bum stage or a signal leak/short to ground that isn't affecting your DC voltages. 

You can even get VacRMS at the speaker to confirm power output with speaker impedance (just be sure you choose a driving frequency in the linear-ish and near nominal-ish impedance range). Compare this to the AC voltages at the plates and the expected turns ratio of the transformer for a quick and dirty OT test.  Latole's test is much better, though.

[jaster55]

Thank you all for the responses
I purchased a scope with built in signal generator last year... I watched a few videos and learned to set it up... I messed around with it a little then life got in the way.
Your responses have me determined to pick up where I left off...
Thanks for the kick in the butt...