Acoustic 165 has low output

[lectricmandan]

After this amp sat for 8 years I decided to check it out . Changed blown fuse , the DC filter cap . The print calls for a 500 v x 40uf/40uf , but I had a 500v 32uf/32uf so I put that in. Once I powered it up I noticed that it had very little power . The voltages were between 5 and 15 volts low on the grid and plate supplies. The phase inverter it working but it doesn't seem to have much gain (if it supposed to ?) . Just looking for suggestions . I did try running a separate amp pre amp out into the effects input but had the same results.

[J M Fahey]

Search for the schematic and post it here.

5 to 15V differences in hundreds of volts supplies are normal so don't worry about them.

What speaker are you using?

That amp, when properly set up is a beast, Acoustic's equivalent of early Mesa Boogies but with a personal twist.

Do you have a scope?
It would help a lot.

If not, just a multimeter and an audio generator (which can be an MP3 played through a notebook/phone/MP3 player) is fine.

[TomCarlos]

Acoustic 165 Schematic

[lectricmandan]

I have a Tektronix scope ,a Fluke 123 scope ,Fluke multimeters ,Eico signal generator and a Hickox 6000 tube tester . The amp has an Electrovoice speaker ,I have tried it through a couple of altec lansings but the out put was the same .I do have several copies of the schematics .

[J M Fahey]

Ok, you are fully equipped, fine.
The question about the schematic was not only you having it but posting it here, so we all speak the same, as in R124 meaning to you and to us.
But that seems to be solved for now (thanks TomCarlos ) , in any case check this one matches so we all use the same.
Will read it and suggest some tests.

[lectricmandan]

There are a few different versions of schematics for this unit . I do have the schematic that Tom Carlos referenced and I also have the G100t schematic that is also close to the unit I have . The main control board on my amp is labeled "G100t" which is why I used this schematic . While neither one is exact the "G100T" is closer to what I have.

[g1]

That explains where those voltages you mentioned came from.
With modern AC line voltage, I'm never surprised to see above normal, but below normal is always a bit of a surprise.
What are you getting for heater voltage? How about plate, grid, and screen of power tubes?

[lectricmandan]

Here is the information you asked for :

AC LINE VOLTAGE = 126

Power Tube 1 : fil = 6.13 , P1-0 , P3 - 462 , P4 - 462 , P5 - 46.6 , P8 - 0
Power Tube 2 : fil = 6.17 , P1-0 , P3 - 461 , P4 - 462 , P5 - 46.6 , P8 - 0
Power Tube 3 : fil = 6.21 , P1-0 , P3 - 461 , P4 - 462 , P5 - 46.6 , P8 - 0
Power Tube 4 : fil = 6.30 , P1-0 , P3 - 462 , P4 - 462 , P5 - 46.6 , P8 - 0

V1 : fil P4+5 to 9 = 6.00 , P1-227 , P2 - 0 , P3 - 1.2 , P6 - 236 , P7 - 00 , P8 - 1.6
V2 : fil P4+5 to 9 = 6.00 , P1-192 , P2 - 0 , P3 - 1.7 , P6 - 236 , P7 - 00 , P8 - 2.5
V3 : fil P4+5 to 9 = 6.00 , P1-335 , P2 - 0 , P3 - 3.0 , P6 - 176 , P7 - 30 , P8 - 40.0

PI : fil P4+5 to 9 = 6.15 , P1-356 , P2 -153 , P3 -181 , P6 - 347 , P7 -144 , P8 -182

Test Points : "A" = 461 , "B" = 458 , "C" = 435 , "D" = 400 , "E" = 61 , "F" = 20

[g1]

Those V3 DC voltages seem a bit odd, can you swap with one of the other tubes and double check the readings?

[lectricmandan]

Substituted a different 12ax7a (took it out of my Twin Reverb) and got these readings :

V3 : fil P4+5 to 9 = 6.00 , P1-329 , P2 - 0 , P3 - 3.0 , P6 - 174 , P7 - 30 , P8 - 42.0

[lectricmandan]

I have been going through this amp component by component ,wire by wire and can still not find anything wrong . I also don't understand where the voltage on V3 p7-30v and p8-42v come from as I don't think there should be anything on them pins . I'm pretty much spinning my wheels now !

[lectricmandan]

After being an Industrial Electrician for 47 yrs, the skills you acquire generally pay off , just stick with it,be regimental and you will find the problem even if electronics are not your forte. Found an open trace in the phase inverter section . Where R24 and C16 tie into R25 /C15 was open . Repaired it and the amp took right off with plenty of power and everything works !!! Also found that whoever worked on this amp previously had put a 500 ohm resistor in series with C16 for whatever reason I don't know . If anyone has an idea why the resistor would be put there or what its function would be please let me know . The amp did have a 12AX7A in the PI but I changed it back to a 12AT7 as the print calls for and took out the resistor. I think that this is my last repair job as I have done a complete restoration on my Hammond B-3 / Leslie 122 ,repaired my Ampeg Jet ,Acoustic 470 , Acoustic 126, Musicman 65 and my Twin Reverb . Thanks to all for your help !!!!!!