Acoustic 470 Guitar and Bass Amp

an old classic,  gets good reviews from people who still use them,

came in after the Acoustic 360 bass amp which is probably the amp that got the company going strong,

Channel A has a distortion knob and reverb,
Channel B has vibrato,

also a switch for guitar or bass on the right,

chassis pic, huge pwr supply cap, 7800uf/100V, would probably be 1/3 that size today,

has line voltage switches, 120/220, and 107/120 for east/west sag out variance,

shoe horned in a reverb pan which makes the amp easy to work on, 

5 band EQ board, brown things might be inductors,

some late additions to the board, common in the early products of a company,

rev 2, rev 3, rev 4, rev 5,  .... tweaked to perfection, (or stability in the case of transistor amps)

rev 6, rev 7, rev 8....

grease days are finally over, gel pads are the way to go,

Brown things *are* inductors, kinda nice for a cheap graphic eq.  These Acoustic amps are kinda cool in their own way.

P.S.  If that big mama cap is bad I have a 10,000uf/100v sitting here that I ordered accidentally!

might take that cap, let me check this one,

schematics for the 470>

http://music-electronics-forum.com/attachments/22973d1366500170-acoustic-470-1-pa.pdf

http://music-electronics-forum.com/attachments/22974d1366500182-acoustic-470-2.pdf

http://music-electronics-forum.com/attachments/22975d1366500190-acoustic-470-3.pdf

http://music-electronics-forum.com/attachments/22976d1366500198-acoustic-470-4.pdf

Acoustic Forum-Duke Aguiar  used to work at Acoustic in the early days  and has a few post here>

http://unofficialacousticcontrolcorpmessageboard.yuku.com/directory

here is some interesting stuff from Duke regarding replacing old power transistors with new ones from

http://unofficialacousticcontrolcorpmessageboard.yuku.com/topic/2491/370-Bias-Wrong?page=2

"TO3 4815 Output transistor could be marked 4815, 480015, 60085. It is a SELECTED RCA 2N3055 HOMETAXIAL type.  RCA made a change about 1976 to an EPI BASE type. Motorola did also make the change. If you use any 2N3055 type it may FAIL. We did a 100% Vce and SOA test on each device. We did not get 100% yield. The newer parts will need additional compensation to have a stable amplifier.

Good Hunting.
Best Regards Duke Aguiar "

and

"The RCA 2N3055 (48-15, 480015 Homotaxial devices) were 100% tested for SOA (80V @ 1.25A to 80-100ºC). Not matched.  The copper wiring of the modules along with the Vbe's of the RCA devices acted as the Emitter Resistors. If some one rewires and or changes the wire size as some repair tech have done, then FLAMES might be seen or the top copper trace on the pcb will open. (Thermal runaway after a short period of time,)

I do remember that both RCA and MOT changed the Mfg process in mid 70's and the parts were still "2N3055" however the stamped flange ID stamps were changed.

New output devices may need some additional emitter resistance; however the real problem will be stability as the outputs today have much greater Ft's. The Bandwidth of the Homotaxial devices were 600-800 KHz the Epitaxial-base are 2-4 MHz. Some of the common output devices today have Ft's to 10-15 MHz. Any one who uses the newer devices will have INSTANT FAILURES and SHORTED OUTPUTS unless you redesign the amplifier. You need to run a Bode plot and verify the phase margin.

Remember to verify any repair and always use an AC INPUT POWER WATTMETER. All of the benches had a variac and wattmeter as part of the basic test equipment. For the turn on test of the repaired amplifier use a LIGHT BULB (100w) in series. This will light bright upon turn on and darken after caps charge. No load on the outputs, then apply signal and the light shall remain dim unless bias is running away or output instability.

During my years @ Acoustic, BGW, JBL, Mackie and Audio Precision I have many short cuts tryed by technicans to repair products. Use yoy Brain.

I hope this helps. Best Regards, Duke"

and again>

"I did go over the OUTPUT DEVICES we used in the power amplifiers.
The newer power semi have much GREATER SOA, and much better THERMAL DIE BONDS that the older 1970 parts.
Be careful about Ft as the new devices will need different loop compensations to make them stable. The new parts Ft will be
4 MHz to 15 MHz, while the old RCA were about 0.8 MHz.
THIS CAN MAKE A BIG POWER OSCILLATOR THAT WILL MAKE SMOKE AND DISTROY PCB, WIRE, FUSES & ALL TRANSISTORS.

DUKE A  2/10/2011"





we checked this 470  and 3 out of  6 power transistors were shorted 470,

ordered some old school NOS RCA 2N3442 off of evilbay, hope they work,
drivers are good which is unusual, i guess the pwr fuse did it's job in time,

here is a cross ref for transistors taken from this thread>

http://unofficialacousticcontrolcorpmessageboard.yuku.com/topic/3030/Transistor-Replacement-Cross-Reference-Counterfeits?page=2

plugged in some NOS RCA stuff and this thing works!

interesting to note that two out of the six power transistors on the back are drivers!

pretty heavy duty drivers, eh? well designed amp, nothing runs hot, no wonder it is still ticking after 40 years of what looks like hard use,

sounds great for a transistor amp, nice reverb, ok vibrato, distortion knob also works,

could not get the push-pull transistors balanced, bias and sym. controls no help,

turns out that when a pair of transistors went down, the heavy current draw changed the value of a 0.1 ohm resistor, so ot is now 0.8 ohms, enuff to unbalance the output stage,

so if you change shorted power transistors, it is probably a good idea to replace the power resistors also,

you really need a 4 wire meter to  measure 0.1 resistors, however, we cheated the system by running  enuff volts thru the resistors to pass about 300 ma thru them, then we stuck a voltmeter across the whole mess and took the mili-volt readings,

so for a  0.1 ohm,  300 ma x .1 = 30 milli-volts, 

the bad resistor will clock in at 300ma x  0.8 ohms = 240 mv,

so we will replace those and then we should be able to get this thing balanced,

I´ve got one of these as a gift from a reheasal room clean up. It didn´t work. Several of the smaller electrolytics were dead. After recapping it worked. Soundwise it was instant 70s. - not exactly what I liked back then.

I really like playing bass through my 360.  It's usually either that or an Ampeg V4. 

I used an Acoustic ss head for bass for ages. 100W box.

And one of them Acoustic 'brownface with eq' ss 2x12 combos.

I loved it. Was my stage amp for the period of my playing 'live' days.

My first teching was in these ss amps of the seventies and early eighties.

From there, I went all fender with tubes ..   

I've been using an Acoustic 450 for bass since 1985. I bought it dead  for $25.00 from a technician who couldn't fix it. Every transistor in the power amp section was shorted or open. It uses 2N3772 / 2N3773  outputs if I recall correctly, and a single ended supply with a big electrolytic speaker coupling cap.

I have the original schematics, and apparently Acoustic Corp. had  many service bulletins issued due to thermal runaway problems. I guess in those times they didn't realize the benefit of using a diode coupled to the heatsink  for the bias circuit. After a few modernization modifications, it's been a reliable amp.