Acoustic 164 Guitar Amp + OPT Tear Down

CJ · Feb 4, 2017

this amp is similar to a Mesa Boogie Mark 4, that is what people do in this industry, in a lot of industries actually,

it has some unique features, and we thought it had a bad transformer,

100/60 watt switch, Fet or Tube preamp choice, nice EV 12 inch ceramic magnet speaker,

has it's share of problems, some say don't buy one, nuthin but trouble, a real money pit, but people don't care in this town, the weirder, the better, don't ask how many Sunn Beta Leads and Bass amps have come through, that darn Kurt Cobain started that deal,

so who get's stuck with an endless stream of weird amps to fix? but hey, what could be more fun than that, after a million Fenders and Marshalls, we need a challenge now and then, even if we do starve to death from endless weeks of hassle,

here is the hassle of the week>

CJ · Feb 4, 2017

here is the fet/tube preamp, this fet sounds dull compared to the tube circuit, but there might be something wrong with it, others say they like the fet,

CJ · Feb 4, 2017

here is the boost/normal circuit, the blue line follows the boost signal, red is normal,

this is old school series boosting, now days, they use parallel boost circuits so you can have tone controls for both channels, some people like the sound of the series boost, old 4 hole Marshall amps use series boost depending on what input you use,

this is actually the schematic from a 165, the 164 schematics on the web are a bit tough to read,

so there are some differences in the master vol switching, notice this amp has both masters doing the same thing while in boost mode.

CJ · Feb 4, 2017

inverter looks standard, has some feedback tone tweaking,

CJ · Feb 4, 2017

eq and pwr supply, and ink splotch,

somebody had put in a 15 volt zener in the bias supply, this made for some very interesting effects, like a loud popping noise and tubes arcing out, they might have been trying to fix the conductive circuit board which is what we think is going on here as the OPT is ok, and we already rewound the choke,

there is some in and out crackling noise that appears at random, the popping tubes have been fixed with a 1N4007,

CJ · Feb 4, 2017

two Shumacher's in a row, wtf?

this OPT is exactly like the Carvin OPT we just did only different,

it lacks the 16 ohm wind but has the same primary turns and structure,

flat and very stable from 10 to 60 K,

inspected by #39, actually probably a bin number,

CJ · Feb 4, 2017

we got some cool looking blue tape holding down the poly wrapper between pri-sec sections,

CJ · Feb 4, 2017

the band saw guy got it right this time and left a margin so the HV primary does not arc out to the core as compared to the Carvin OPT,

CJ · Feb 4, 2017

bi fi secondary, uses red wire for both winds this time, that gap could be a hi-freq tweak but probably just winder's discretion,

CJ · Feb 4, 2017

schematic for the OPT, pri DCR is a bit lower than the Carvin OPT, there is more winding room without the two winds for the yellow-orange 16 ohm section, so they might have bumped the wire up a 1/2 gauge or 1 gauge,

CJ · Feb 4, 2017

only difference in winding structure is the lack of the 16 ohm winds,

CJ · Feb 4, 2017

we are going to throw in the 10 H choke at no extra charge,

Gus · Feb 4, 2017

CJ

Does the print of the jfet circuit match the amp?

The constant current device in parallel with the source resistor is what is a little puzzling.

Does it have the 1uf going to the gate? With 1 meg gate to ground you can use a lower value film etc cap.

Voltage measurements at the source, drain and "top" of the 22K drain resistor should help make sense of this circuit

PRR · Feb 4, 2017

> a 15 volt zener in the bias supply

Stupid, stupid.

> constant current device in parallel with the source resistor

Attempt to stabilize the wide variation of operating point. This could instead be done with a large resistor to a negative voltage. Then you expect the 1K to have a series cap so it does not spoil the constant-current intent. Which may explain why some like 'em and some don't: randomly different op-points.

Gus · Feb 4, 2017

I asked about the jfet circuit voltages because the jfet number and power supply voltage is not shown.

If we know the voltage at the "top" of the drain resistor and at the drain we know the drain current

If we know the voltage at the source we know the current in the 1.5K so if we subtract the 1.5K current from the drain current we know the CC device current

If you don't bypass the CC device you will not get any gain

A CC device in the source leg can set the drain voltage without selection of jfets

So if you add the 1.5K in parallel with the CC device(CC should be high resistance)you set the gain by the drain resistor and source resistor and gain etc of the jfet math

I was wondering why they set it up this way maybe to set gain and input level the jfet can handle

You could use a CC or quasi constant current(like in one of the Fender Havard amp preamp section) in the source with cap and gain setting resistor in series in parallel with the CC device

So could it be to use one less part?

CJ · Feb 4, 2017

looks like they did change the circuit, here is another version>

CJ · Feb 4, 2017

see here>

CJ · Feb 4, 2017

here is the whole Mary Ann right here>

PRR · Feb 5, 2017

> here is another version

Making less sense with each picture.

As Gus says, there's no supply voltage for the first JFET circuit and no supply at all for the Graphic EQ. Both should be positive voltages, but the low-volt part of the supply is all negative. (Did they buy plans from Boogie?)

Then this "another" shows an NPN to switch two relays, but there is a JFET part number PAINTed on.

The Mary Ann close-up is frightful, and the long-shot is no more comforting.

Get paid well. You deserve it.

Gus · Feb 5, 2017

From a picture it looks like the 1k, 10uf and CC diode were added after

I would guess there was only the 27K drain resistor and the 1k and 10uf were added later. This could be because there was too much power supply noise with the 27k connected to what ever supply they used.

Maybe the CC diode was added to help get the drain voltage and drain current closer for good jfet operating points.

CJ can you measures the voltages around the jfet and the jfet power supply voltage?

I think we can adjust this jfet circuit

Acoustic 164 Guitar Amp + OPT Tear Down

Help Support GroupDIY Audio Forum:

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Well-known member

Join the conversation!

Join today and get all the highlights of this community direct to your inbox. It's FREE!