Stromberg Carlson build- low volume

[zapdog]

Hi all, picked up a Stromberg Carlson AU-32 and have been slaving away at it for weeks now.  I have several amps under my belt, but they have all been pretty much "paint-by-numbers" following known schematics (Bassman, Twin, Deluxe).  This is a different beast and it is kicking my butt.  I decided to keep the octal sockets, but replace the 6SC7s for 6SL7s. The circuit is fairly basic, but utilizes a James tone stack, as I only have options for a treb and bass control.  Also, the PI is copied from Aiken's "designing the long tail phase inverter", as he uses a 6SL7 in his example.  When fired up the tone is OK, grainy, edging toward fizzy.  It is full and robust, but the volume just isn't there.  Originally this amp had metal 6L6s, so maybe it would be better suited for 6V6s? 

Biased using the output transformer resistance method:

                           outside 6L6            inside 6l6                                                     
resistance:            135.2ohms           144.8ohms
voltage drop:              8.83v                  9.28
current:                      65ma                 64ma
plate voltage:             332.8v                333.6v
diss:                            21w

[zapdog]

Sorry, hit the wrong button and posted but i was pretty much ready to anyway.  Thanks for looking, and look forward to your input!

[DummyLoad]

based on the schematic with voltages indicated on the LTPI  i'd guess that the phase inverter anode resistors are wired incorrectly. there should be about 100V drop across the 82K load Rs.


--pete

[ALBATROS1234]

Sounds like something is shunting signal to ground. Or else for some reason you are not getting enough voltage on a preamp tube plate . I crossed up some connections and it acted as a voltage divider and took half my voltage to ground so my preamp tube was getting like 40v . Check all your voltages and make sure you have what you would expect.

[zapdog]

 Thanks guys. Time to re-check the wires! I feel the numbers on the cathode follower are wacky as well. With the standby switch on, the voltage on pin 8 of the rectifier is around 517 V.   Is this voltage drop normal, or might it signify  voltage being shunted to ground?

You guys rock!

-Billy

[sluckey]

Measure resistance between V3 pin 3 and chassis. Should be about 48K. What do you have?

[zapdog]

Yep, I have 48.2k ohms.  What about those voltages?  Are they completely wackadoo?

[sluckey]

You say the voltage on V3 plates is same as supply voltage (305v).That indicates the triodes are not conducting. But you also say the voltage on the cathodes is 124v, which indicates the tubes are conducting. It can't be both ways. Either the plate voltage is wrong or the cathode voltage is wrong. The amp cannot make any sound with those high plate voltages. But since you do have sound I suspect you made an error reading the plate voltages. Please recheck the voltages for V3.

[zapdog]

 Checked the numbers-same readings. Swapped out that tube, and still the same readings. Very strange!

[Tony Bones]

Use your DMM to directly measure the voltage drop across each 82K plate resistor in the PI. One probe on each lead of the resistor.

Do the same for the 47K tail resistor.

[sluckey]

Checked the numbers-same readings.

Impossible.

Show us some pics where we can see the plate resistors connected to the tube socket. Need to be able to read resistor values.

[zapdog]

Ahh!  I had wired in 820ohm resistors, not 82K.  nothing like human error.  Thanks for pointing me in the right direction guys.  Sounds about the proper volume now, but needs some fine tuning.  Here are the voltages on V3:

1: 53v
2: 232v
3: 85v
4: 56v
5: 225v
6: 85v

Look better?  Thought I read somewhere that checking voltages on PI would be inaccurate, but I might have just made that up.  One more thing.  As this originally had metal 6L6s and the iron is physically about the same size as the iron in my deluxe build, should I be thinking about using 6V6s?

[sluckey]

Much better.

Thought I read somewhere that checking voltages on PI would be inaccurate, but I might have just made that up.

Checking voltage on a LTP PI grid will be inaccurate but you can trust the plate and cathode voltages.

I would stay with the 6L6s.

[zapdog]

Good to know.  I will stay with the 6L6s.  Thanks for pointing me in the right direction!