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**Leo_Gnardo** · 04-04-2016, 03:20 PM

On your power amp, have you replaced your C311? You have a single-ended power supply, the signal leaves the amp "floating" on half the power supply voltage, C311's function is to strip the DC component. If it's leaky, you'll have some DC offset at your speaker. It's not unusual to have the condition you see, a large offset at switch-on, quickly diminishing to zero. For folks who are bothered by this I've installed a "standby" switch, which keeps the speaker disconnected until switched on, and dissipates the DC offset thru a resistor until the speaker is connected. A 1K 5W resistor would work fine, give it 30 seconds or so to settle then you're good to go.

Given the age of the amp it wouldn't hurt to replace that cap C311 if you already haven't. And it's OK to use one with a higher rated voltage.

**Jazz P Bass** · 04-04-2016, 03:59 PM

My feeling is that you will not see the offset IF a speaker is hooked up.

Try it with an 8 ohm dummy load.

**J M Fahey** · 04-04-2016, 04:12 PM

You-do-not-have-a-problem

Congratulations on being cautious on a new untested amplifier, but in this case, it is normal to have a *single* DC pulse (not *continuous* ) at the speaker terminals.

As leognardo pointed, this old amp has "single supply" power, only one about +75V rail, and output sits at half that, so around +36V.
To block that DC but pass audio (AC) they add a huge cap there , C311.

What's confusing to many is that they think in black/white ideas, such as "caps do NOT pass any DC, period" so "if I find any DC at the other end it must certainly be bad".

Fact is that Caps DO pass DC ... for a very short time, until they fully charge, and then yes, they fully block it.

How long is that time?

It's called "time constant" and is calculated; to avoid Math there are online calculators:
RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin
C311: 1000uF
R: speakers, so 4 ohms
Time constant (check the online calculator I linked): 0.004 seconds (4 milli seconds)
so yes, under normal use, every time you turn amp on you will have 36V across it ... for 4 thousandths of a second.
Will you damage it? ... no way.
Will you hear something? ... definitely, the famous SS amps "turn on thump" .
If annoying, you may add an extra switch as suggested to connect speaker a few seconds later; large rack PA or Bass power amps include a relay with a few seconds delay to do the same, automatically.

Now, if you do NOT connect a speaker, just your meter which has around 1 Megohm input impedance, C will be the same but R infinitely higher , so having DC for 30 seconds or more after turn on, under that circumstance, is perfectly normal.

I would solder a 2W 100 ohms resistor in parallel with speaker jacks (I'm amazed Acoustic didn't do that themselves, it's something very common) and try again, I bet in a few seconds there is no DC present (or only a few millivolts).

FWIW the time constant for 1000uF and 100 ohms is very reasonable: 0.1 second, so I bet a couple seconds late you will measure essentially 0.

If not, if you have continuous , say, 0.5V or more, then C311 is leaky and should be replaced.

Please test and post results.

**TigerAmps** · 04-07-2016, 01:39 AM

Update:

Powered it up with a speaker and it was fine (less than between 1/2mV and 1mV on the speaker) and the "pop" was not too bad.

However: I have a low volume hum.
a) I disconnected the pre-amp boards and no change.
b) I substituted a new main power cap (10,000uF/75V) and no change.
c) Volume controls had no effect on it.

This leads me to think something on the power board.
After leaving it on for a few minutes (no signal - just hum):
a) the power transistors (4 new mj15003) were cool
b) The 560 ohm resistor was cool.
c) The two 400 ohm resistors were slightly warm (not hot)

All the electrolytics on the power board were replaced so I doubt it is a cap problem.

Any ideas on how to isolate/trouble shoot?

**TomCarlos** · 04-07-2016, 03:18 AM

Tiger....

I had an acoustic 150B head. It too had some hum on the output. Just about every Acoustic amp like this one (that I heard) had some hum on the output. So... you have hum and low volume. Does that change if you increase the volume on both channels or adjust the bass and treble controls?

Do a google search on "Acoustic 150 amp hum" - there are some interesting posts out there.

Tom

**TigerAmps** · 04-07-2016, 10:10 AM

Sorry - I forgot to mention that I have no output from the amp other then hum. (no signal output with an input plugged into the amp).

**TomCarlos** · 04-07-2016, 02:04 PM

I should also add you can find several threads here on MEF related to this amp. The Acoustic 150, 150B, 134, and 135 all use the same power amp and the input card that has Volume, Treble, and Bass. Do a search on this keywords.

For my 150B, I followed the suggestions from members and was then ready to use a scope and follow a signal path. I disconnected one of the input cards and starting tracing the other through the amp. Once that was working, I then swapped out the inputs (It is a matter of unsoldering one wire that connects to the power amp card) and repeated the process.

The Owner's Manual gives you some test point voltages (for DC) and ac waves that should be visible. My suggestion is to start there- verify the dc voltages in the amp.

**TigerAmps** · 04-09-2016, 04:04 PM

OK - Got it working ...

When I replaced the driver transistor (2N5322), I used a "top hat" head sink on the transistor.
The original transistor had the square metal heat sink attached and the heat sink was tied to the collector of the transistor.
So - the original heat sink also functioned as a trace/jumper to another trace.
By NOT using the original heat sink, part of the circuit was disconnected.
I fixed this by soldering a small jumper onto the board to complete the circuit and the amp now works (outputs signal).

Still have a little more work to do, but it is coming along ...

Thanks for all the help ...

**TomCarlos** · 04-09-2016, 04:05 PM

::: High Five ::: I am going to make a note on my 150 schematic!! Good info to know.

**TigerAmps** · 04-11-2016, 01:32 PM

Additional update and thoughts:

I appears that one of the sources of hum was the some of the connections made by rivets.
In particular the "death cap" which I eliminated with a 3 prong cord and grounded to the chassis near the large power supply cap.
I also used contact cleaner to clean the circuit boards where the connectors attach. This got rid of most of the noise.

The only hum I have is coming from the pre-amp board closest to the AC power switch.
It is from the bass control. When the bass control is set at the middle, there is no hum.
As you move it off the center in either direction, the hum increases. I am pretty sure it is coming from the inductor (which id an original inductor).

I saw this when I replaced the inductor with a Xicon transformer in an Acoustic 126; it behaved the same way.
So not sure if the inductor is "bad" (though the bass control does work) and/or the proximity to the AC).
I will note that in the 126, the EQ board is at the opposite and of the amp from the AC and power supply,
so I don't think the AC is really the issue (at least in the 126).

Anyone seen this before or have any thoughts?
Is a Gyrator the real answer to replacing the inductors?

I'm willing to experiment, so all thoughts are welcome ...

**TomCarlos** · 04-11-2016, 02:03 PM

Tiger....

I had a 150 Head and 135 Combo back in the day. I remember that audible hum did change when I adjusted the bass and treble- it was annoying. But... a friend's amp did the same so I considered this a design issue.

When I was working on the 150B head, I was studying "star grounding" and I did some experimenting with ground connections and placement of wires. It did seem to make a difference. Using the chassis as ground (for preamp or amp boards) didn't make sense to me back then. So I made sure I separated hi and low ground and took that back to a single grounding point.

As for experimenting with altering the preamp design, that would be interesting. I will follow your thread.

I am not so experienced that I can compare the Acoustic design to any other amp. But to this day, I still do not understand the usage of the transformer in the power amp section - that feeds the output devices. I have not seen that on any other amp!

Tom

**R.G.** · 04-11-2016, 03:01 PM

Originally posted by TigerAmps View Post

The only hum I have is coming from the pre-amp board closest to the AC power switch.
It is from the bass control. When the bass control is set at the middle, there is no hum.
As you move it off the center in either direction, the hum increases. I am pretty sure it is coming from the inductor (which id an original inductor).

Yep, it's probably coming from the inductor. I have been re-engineering the Thomas Vox line of amps, and they share a lot of design points with this amp. There is an inductor used in the bass circuit much like this, and if you don't use the original toroidal inductor, they get sensitive to hum. The toroids are pretty quiet. There are some inductors from Wilco that are about the right value that are wound in a completely-ferrite-encased core that I suspect would be quiet too.

I saw this when I replaced the inductor with a Xicon transformer in an Acoustic 126; it behaved the same way.

Yep. A normal transformer will pick up hum more. You might be able to help this some by wrapping a copper band around the whole transformer. Maybe.

So not sure if the inductor is "bad" (though the bass control does work) and/or the proximity to the AC).

No, the inductor's OK if the bass control works. It's magnetic pickup.

I will note that in the 126, the EQ board is at the opposite and of the amp from the AC and power supply,
so I don't think the AC is really the issue (at least in the 126).

It is. In the Thomas amps, they put the inductor in a separate sheet metal chassis, but this is hung right next to the PT in the other chassis. Sigh.

Is a Gyrator the real answer to replacing the inductors?

Gyrators have other problems, like high frequency hiss from the active devices. Might be OK here, and the circuit even makes it easy to experiment. Cobble up a 1.5H gyrator on a bit of perfboard, disconnect the inductor and hack the gyrator in. I'd be interested in what you get.

Originally posted by TomCarlos

I am not so experienced that I can compare the Acoustic design to any other amp. But to this day, I still do not understand the usage of the transformer in the power amp section - that feeds the output devices. I have not seen that on any other amp!

It was common for a few years, from the late 1950s to the mid 1960s. It's a way to use only NPN or only PNP output devices. The transformer is a phase splitter/driver. The transformer primary is driven from a single ended transformer driver circuit, and steps the signal voltage down but the current up, and feeds the base of two output devices with two out-of-phase secondaries. The base-emitter junctions on the output transistors can only accept current in one direction, so current alternates into one, then the other of the outputs as signal swings from positive to negative. Thomas Vox amps used this setup. This power amp's output stage is remarkably similar to the Vox Beatle amps.

**TigerAmps** · 05-11-2016, 04:53 PM

Sorry for the delay; but it's been busy; finally tried the gyrator to replace the inductor ...

Power Supply Resistors:
(1) 120K 1/2 W
(2) 10K 1/2 W

This connected to the +74 Volts and ground provides Vcc = 20V and Vcc/2 = 10V

Resistors connected as follows: +74V -> 120K -> 10K -> 10K -> Ground
Note: Resistors were mounted on a terminal strip; not on the vector board.

Acoustic 150B Inductor= 1.5H

Values for the Gyrator:

C1 = .1uF 50V ceramic
R1 = 100 Ohms 1/4 W
R2 = 150K 1/4 W

Where L = R1 * R2 * C1 so here L = 1.5H

IC = TL071CP or uA741CP (I used the TL071CP chip)
IC Socket: Multicomp SPC15494 8 pin socket
Board Material = Vector8029 (cut it with a hacksaw)

Required 3 attempts:

1) figure out the layout on the vector board.
Learned it easiest to jumper pins 6 and 2 of the IC under the socket
Used 1/2 watt resistors and 200V caps and they were too big.
Never installed this version or tested it

2) used 1/4 resistors and 50V ceramic cap - worked much better - more compact
screwed up the power supply feeding the vector board (long story)
Installed it; but it did nothing because Vcc < 1V (silly I know).

3) Fixed the power supply to get Vcc = 10V and Vcc/2 = 5V and it worked.
Tone wise it performed as good as the original inductor (in the other channel).
It had a little hum at full bass but that could be because I grounded the power supply to the chassis.
I will move the power supply ground to the main power supply cap (where I am pulling the +74V)
and see if that makes a difference. I will also check over the input board for bad solder joints.
The other input board produces very little hum (with the inductor installed).

Note: the fit was tight in that the back of the board butts up against the bass pot.
I put some tape on the side of the pot in case the board moved and hit the pot during testing.
You could use a dab of silicone when you are done testing and ready for final assembly.

Thoughts and comments are welcome ...
I hope some day this may help someone out there fix their Acoustic amp.

**J M Fahey** · 05-11-2016, 10:28 PM

Originally posted by TigerAmps View Post

Sorry for the delay; but it's been busy; finally tried the gyrator to replace the inductor ...

Power Supply Resistors:
(1) 120K 1/2 W
(2) 10K 1/2 W

This connected to the +74 Volts and ground provides Vcc = 20V and Vcc/2 = 10V

Resistors connected as follows: +74V -> 120K -> 10K -> 10K -> Ground
Note: Resistors were mounted on a terminal strip; not on the vector board.

[ATTACH=CONFIG]39084[/ATTACH]

Acoustic 150B Inductor= 1.5H

Values for the Gyrator:

C1 = .1uF 50V ceramic
R1 = 100 Ohms 1/4 W
R2 = 150K 1/4 W

Where L = R1 * R2 * C1 so here L = 1.5H

IC = TL071CP or uA741CP (I used the TL071CP chip)
IC Socket: Multicomp SPC15494 8 pin socket
Board Material = Vector8029 (cut it with a hacksaw)

..........
3) Fixed the power supply to get Vcc = 10V and Vcc/2 = 5V and it worked.
Tone wise it performed as good as the original inductor (in the other channel).
It had a little hum at full bass but that could be because I grounded the power supply to the chassis.
I will move the power supply ground to the main power supply cap (where I am pulling the +74V)
and see if that makes a difference. I will also check over the input board for bad solder joints.
The other input board produces very little hum (with the inductor installed).

Note: the fit was tight in that the back of the board butts up against the bass pot.
I put some tape on the side of the pot in case the board moved and hit the pot during testing.
You could use a dab of silicone when you are done testing and ready for final assembly.

Thoughts and comments are welcome ...
I hope some day this may help someone out there fix their Acoustic amp.

You bet

Thanks for posting

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