ACOUSTIC 165 sounds bad

[plook]

I am trying to repair an ACOUSTIC 165 from 1981 for a friend, it sounds really bad, horrible distortion, farty and the reverb is very faint. i have swapped out all tubes and put a new reverb pan in without improvement. i have managed to get a schematic but have no voltages to work with, so i don't know what he B+ should be.

How can i diagnose where the problem is likely to be in regards to faulty or weak coupling caps or bad filter caps. Has anyone got pictures of inside the chassis or has anyone worked on one of these and perhaps made a note of voltages and suggest parts to swap out.
Thanks

[JHow]

Is it this one?

[plook]

Yes, that is the one. I have the schematic but trying to find the source of the problems seems to be another matter. I have done some reading on 'farty' and nasty distortion sounding amps and filter caps and leaking coupling caps are mentioned. I don't seem to be able to find out how to test a cap for leakage/failure though and i don't want to change out unnecessary components if they are good and can be retained for the vintage vibe. Having the voltages for various points of the amp might work and some pictures might work too, especially of the el. caps around the bias pot - they all seem to have the + side going to ground.

[Jazz P Bass]

I don't seem to be able to find out how to test a cap for leakage/failure though .

A coupling caps job is to block the plate Vdcas it passes the varying plate voltage to the next stage, usually a grid.
They can be tested by reading the Vdc on the "grid" side of the cap.
Depending on the specific circuit you would expect to see zero volts.
If the cap is leaking, it will pass the plates dc voltage.
This is what you test for.
Is the cap blocking the Vdc.

[plook]

Thanks for that. I will check it out. if a coupling cap is leaking, what would the resulting tone be like?

[52 Bill]

Depending upon where it is in the circuit, it can cause loss of power, distortion, etc.

As for your voltages, this circuit is nearly identical to any number of other amps out there, so look at the schematic for an early Marshall or Fender amp and you will get an idea as to what kind of voltages to expect in the different stages.

Don't worry about the reverb until the amp is working. Have you cleaned the preamp out/power amp in jacks? Plug a signal into the power amp in jack. How does it sound? Take the signal from the preamp out and send it into another amp how does it sound? If you isolate the problem it will be easier to find and fix.

[plook]

Fender Bassman / Marshall JTM45?

Many thanks for your tips, i'll get into this tonight.

[plook]

There are 4 preamp type tubes in this amp, 3 x 12AX7 and 1 x 12AT7 (reverb i guess). Looking at the schematic you attached, page 1 shows 6 circles representing small tubes. How many of them are actually divided into 2? I'm trying to match the tubes up to the layout of the amp.

[JHow]

The schemo identified the 12AT7 on page two in the phase inverter section. It is a twin-triode, so the triode sections shown are in one 12aT7 envelope, I would presume. The first page has six triode sections from three 12ax7, and include the reverb send and receive, plus the pre-amp ones. The rest of the tubes on page 2 are your output tubes. Since you have the amp in hand, you can trace the connections coming off the 12at7 socket and verify.

[plook]

Thanks for that. I did manage to identify the tubes using the schematics and tubes in relation to caps and resistors, so i found the PI and noticed that it should be a 12AT7 and this amp had a 12AX7 in there. It's too late at night for me to test it properly but i put a 12AT7 in and if i am not mistaken it just might have brought it under control. I'll crank it in the morning and we'll see. Could a 12AX7 in the PI position be responsible for the amp sounding hideous?

When i first looked in this amp it was a bit bewildering, but sitting down with the schematic and looking in the amp is starting to make sense. If the amp still sounds bad tomorrow though i'll have to go looking for leaky coupling caps. I might record the voltages on the tubes too. i have set the bias at -52v, it was -57v.

[JHow]

Pinout is the same, obviously, 12AT7 handles more current and might sound a little cleaner in that spot than if a 12ax7 was there. At any rate, that is the spot for it.

[plook]

Well, that 12AT7 didn't cure it and it's not the coupling caps as even though they were reading 2mV i swapped them out for new ones with the same results - channel one okish until turned up, channel 2, the drive channel, nasty as soon as you get any drive on it and a really horrible distortion as you wind up the master volume. I'm at a loss.

The Bass pot is a push-pull pot and when you pull it out it's a BOOST which sends the amp over the top into the most horrendous squalling, flapping-farting noise.

[Jazz P Bass]

To nail this, I think that you will rquire some test equipment.
Barring that thought, have you measured any voltages.
A good place to start is the preamp tube voltages.
Plate.
Cathode.
Grid.
No signal.
Write them down on the schematic.
If all of the static voltages look good, then it is a dynamic problem.
That will require a scope & a signal generator at the least.

[plook]

I finally got around to measuring some voltages. Here they are: P=pin

PREAMP TUBES
V1 (12AX7) P1:207v P2:-3.5mV P3:1.45v P4:2.7v P5:2.7v P6:212v P7:0.3-2.2 (oscillating) P8:1.4v P9:2.7v
V2 (12AX7) P1:86v P2:-21mV P3:0.8v P4:2.7v P5:2.7v P6:220v P7:0.3-2.2 (oscillating) P8:1.7v P9:2.7v
V3 (12AX7) P1:237v P2:-0.5mV P3:1.5v P4:2.7v P5:2.7v P6:321v P7:0.3-2.2 (oscillating) P8:2.1v P9:2.7v
V4 P.I. (12AX7) P1:243v P2:105V P3:115v P4:2.7v P5:2.7v P6:236v P7:107v P8:116v P9:2.7v

POWER TUBES
V5 (6L6) P1:0 P2:2.7V P3:470v P4:472v P5:52.1v P6:0 P7:2.7v P8:69v
V6 (6L6) P1:0.2 P2:2.7V P3:469v P4:470v P5:52.1v P6:0 P7:2.7v P8:0v
V7 (6L6) P1:0.2 P2:2.7V P3:469v P4:470v P5:52.1v P6:0 P7:2.7v P8:0v
V8 (6L6) P1:0.2 P2:2.7V P3:465v P4:471v P5:52.1v P6:0 P7:2.7v P8:69v

If you can see anything wrong here i would be very grateful for comments/suggestions.

I changed cathode bi-pass caps and resistors just to be sure, made no difference.

[Gtr_tech]

Note that you have it set to 60w (half power) mode, so you'll see the cathodes of one pair of output tubes with voltage present 'cause they are floating (gnd lifted via the sw).