Peavey 400 Bass Amp

[CJ]

in the operating table today is a solid state amp, one of the 15 volt regs blew and took down about half the parts on the pwr board so we replaced all the transistors in case some were partially damaged,

Peavey is helpful and they send parts with free shipping, ebay picks up the slack on the generic stuff for cheaper,

this design is easy to service, one preamp board and one pwr board, not too many connectors, and plenty of space between parts,

we soldered the xfmr wires to the board as they carry a lot of current,

and we added a bigger chunk of metal to the new +/- 15 regs,

 

[CJ]

there is one part that is a PITA, an Intersil (remember them?) OTA, reads operational trans amp,

nobody makes this chip anymore, fewer people know what it does,

oh well, at least they stayed away from the Harris popcorn CMOS chips,
don't even get me started on the 7612, 

http://mysite.du.edu/~etuttle/electron/elect22.htm

Intersil dude>

Jean Amédée Hoerni (September 26, 1924 - January 12, 1997) was a silicon transistor pioneer and a member of the 'traitorous eight'. He developed the ground breaking planar process for reliably manufacturing semiconductor devices such as transistors.

Hoerni was born on September 26, 1924 in Geneva, Switzerland.[1] He received a Doctor of Philosophy (Ph.D.) from the University of Cambridge and another Ph.D. from the University of Geneva.

In 1952, he moved to the United States to work at the California Institute of Technology, where he became acquainted with William Shockley, the "father of the transistor."

A few years later, Shockley recruited Hoerni to work with him at the newly founded Shockley Semiconductor Laboratory division of Beckman Instruments in Mountain View, California. But Shockley's strange behavior compelled the so-called 'traitorous eight': Hoerni, Julius Blank, Victor Grinich, Eugene Kleiner, Jay Last, Gordon Moore, Robert Noyce and Sheldon Roberts, to leave his laboratory and create the Fairchild Semiconductor corporation. At Fairchild Hoerni would go on to invent the planar process which was critical in the invention of Silicon Integrated circuit by Robert Noyce. Jack Kilby from Texas Instruments is usually credited, with Noyce, with the invention of the integrated circuit, but Kilby's IC was based on Germanium, and as it turns out, Silicon ICs have numerous advantages over germanium. The name "Silicon Valley" refers to this silicon.

Along with the 'traitorous eight' alumni Jay Last and Sheldon Roberts, Hoerni founded Amelco (known now as Teledyne) in 1961.

In 1964, he founded Union Carbide Electronics, and in 1967 Intersil."

CA 3094 is the p/n

one guy even went so far as to build a discrete version, they get used in a few stomp boxes also,

we bought 5 on evilbay so we can fix any more amps that come in with this chip,

it sits on the FB loop of a TL074,

it is circled in red on this pwr amp shot>

 

[CJ]

power transistors were all over the place, probably from years of getting baked,

so we matched up a new set,

 

[CJ]

big iron on his hip,

those TO-3's and large traces can suck the heat, so we hit and git with 120 watts,

 

[CJ]

newer parts gonna oscillate due to increased bandwidth? we will be on the lookout,

check those beefy diodes out,

 

[JohnRoberts]

there is one part that is a PITA, an Intersil (remember them?) OTA, reads operational trans amp,

Operational "transconductance" amp....  effectively a variable gain amp, used in the DDT (clip limiter).

nobody makes this chip anymore, fewer people know what it does,

The CA3094 is just a CA3080 OTA with a darlington buffer hanging off the back end.

oh well, at least they stayed away from the Harris popcorn CMOS chips,
don't even get me started on the 7612, 

http://mysite.du.edu/~etuttle/electron/elect22.htm

Intersil dude>

Jean Amédée Hoerni (September 26, 1924 - January 12, 1997) was a silicon transistor pioneer and a member of the 'traitorous eight'. He developed the ground breaking planar process for reliably manufacturing semiconductor devices such as transistors.

Hoerni was born on September 26, 1924 in Geneva, Switzerland.[1] He received a Doctor of Philosophy (Ph.D.) from the University of Cambridge and another Ph.D. from the University of Geneva.

In 1952, he moved to the United States to work at the California Institute of Technology, where he became acquainted with William Shockley, the "father of the transistor."

A few years later, Shockley recruited Hoerni to work with him at the newly founded Shockley Semiconductor Laboratory division of Beckman Instruments in Mountain View, California. But Shockley's strange behavior compelled the so-called 'traitorous eight': Hoerni, Julius Blank, Victor Grinich, Eugene Kleiner, Jay Last, Gordon Moore, Robert Noyce and Sheldon Roberts, to leave his laboratory and create the Fairchild Semiconductor corporation. At Fairchild Hoerni would go on to invent the planar process which was critical in the invention of Silicon Integrated circuit by Robert Noyce. Jack Kilby from Texas Instruments is usually credited, with Noyce, with the invention of the integrated circuit, but Kilby's IC was based on Germanium, and as it turns out, Silicon ICs have numerous advantages over germanium. The name "Silicon Valley" refers to this silicon.

Along with the 'traitorous eight' alumni Jay Last and Sheldon Roberts, Hoerni founded Amelco (known now as Teledyne) in 1961.

In 1964, he founded Union Carbide Electronics, and in 1967 Intersil."

CA 3094 is the p/n

one guy even went so far as to build a discrete version, they get used in a few stomp boxes also,

I would be vary about a discrete 3094 OTA even being usable, Input LTP matching is very important. You could hang a darlington off the output of a 3080 to make a serviceable 3094.

we bought 5 on evilbay so we can fix any more amps that come in with this chip,

it sits on the FB loop of a TL074,

it is circled in red on this pwr amp shot>

I can't read any detail off that schematic... does it actually say 3094?  full Peavey PN would then probably be something like 70403094... The DDT circuit doesn't need the 3094's output buffer, the OTA output before the buffer is available, so a 3094 "could" be used in place of a 3080. While I don't know why Peavey would do that. I don't recall the 3094 being in the system back then, and very unlikely they used enough to get a special house number selected version.

Peavey had their own version of CA3080 selected for lower control voltage feed through (you don't want DC thumps during gain limiting), IIRC the Peavey special 3080 used house number 7478 or full system number 70407478  (? from my old tired memory).

If pin  1 is shorted to pin 6 and pin 8 is floating, a 3094 could drop in a 3080 socket and work... (I think).

Does Peavey service confirm the ca3094 PN?

JR

 

[rob_gould]

Your job looks like fun CJ!

 

[CJ]

fun job, yes! 

great fun with this acoustic 120 that the guy is demanding for tonight's gig,

can't go to shows in town anymore, afraid the band's stuff that i fix will blow up whilce i'm there,

good memory John!

here is a link for the PV cross ref pdf>

http://matthieu.benoit.free.fr/cross/competitive/Peavey_Transistor_Cross.pdf

 

[JohnRoberts]

fun job, yes! 

great fun with this acoustic 120 that the guy is demanding for tonight's gig,

can't go to shows in town anymore, afraid the band's stuff that i fix will blow up whilce i'm there,

good memory John!

here is a link for the PV cross ref pdf>

http://matthieu.benoit.free.fr/cross/competitive/Peavey_Transistor_Cross.pdf

The 70487478 (I was close , not bad memory for 20 years ago). Is a 3080 AFAIK... I have no idea why they also have 3080s in the system, I guess the old school engineers that knew what is what are long gone, and some junior engineers needed an OTA.

I suspect that is/was/should be a 70487478 in that bass amp socket... If the PCB has a short across the output device base-emitter the 3094 can work in place of the 3080. There is a remote chance that the 70487478 was a 3094 from day one (well before my time there 85-00), I never used one in an original design, but used many amp modules that had the standard DDT design using them already in place. 

JR

PS: If i was still inside Peavey I'd look up the spec file for that part.

 

[Andy Peters]

But Shockley's strange behavior

To call his behavior "strange" is being charitable, at best.

-a

 

[PRR]

> CA3094 is just a CA3080 OTA with a darlington buffer

Then it is a half a LM13600/LM13700.

That too is out of production, but faded more recently, thus more available.

Small Bear Electronics, ICs, CoolAudio--- new-production clones of LM13700, about a buck-twenty.

NOS or new-run CA3094E, over 8 bucks:
smallbear-electronics.mybigcommerce.com/ic-ca3094e/

Steve's stuff WILL be good, unlike many eBay and even some name-brand vendors. He has large customers who know their chit and won't take crap. He has resorted to funding new-production of a few classic chips.

 

[CJ]

used in the Small Stone Phasor (early models) and Mutron Phasor,

came in a can also, seen RCA versions of the can also,

stomp box guys use the LM1300 types also,

 

[JohnRoberts]

> CA3094 is just a CA3080 OTA with a darlington buffer

Then it is a half a LM13600/LM13700.

A subtle distinction but as I recall the ca3080 (RCA) was the grand daddy of OTAs, so the LM13600 was a dual 3094.  I recall seeing and using CA3080s in the early '70s before the cheap one chip companders (NE570) were developed by Signetics (mid 70s).

I actually used the LM1600/700  (mid 80s) in a state variable topology to make the sine wave oscillator for my Loftech TS-1 audio test set. <20Hz to 30kHz with around 0.2% THD. I could have made the distortion lower but the settling time was a concern with that much frequency range on a single sweep knob..

That too is out of production, but faded more recently, thus more available.

Small Bear Electronics, ICs, CoolAudio--- new-production clones of LM13700, about a buck-twenty.

NOS or new-run CA3094E, over 8 bucks:
smallbear-electronics.mybigcommerce.com/ic-ca3094e/

Steve's stuff WILL be good, unlike many eBay and even some name-brand vendors. He has large customers who know their chit and won't take crap. He has resorted to funding new-production of a few classic chips.

The Peavey DDT circuit is using the OTA current output. The CA3080 current output is on pin 6, the CA3094 has current output on pin 1 and buffered (voltage) output on pin 6.  Shorting pin 1 with pin 6 turns off the buffer transistor and would allow both chips to work in that same socket.

JR

PS: RCA also made an improved dual 3080, CA3280. I used that in my CX vinyl NR kit ('80s). So there are several parts that could be made to work, but the 3080 and 3094 are drop ins for that design.

 

[CJ]

this thing again, finally got t working, after replacing every transistor except two fets, because they looked like a diff pair the way they were orientated on the board, back to back, figured why mess with a set of matched transistors, color coded from the factory, what were the odds of having the replacements being matched as good as the factory?

well, they were not a diff pair, they were fets used as switches in the start up circuit, 

these are P channel fets, 2N5461, so when the gate is positive, they do not conduct,

unless they blow up that is, which fets tend to do when they get spiked all the time,

so what happens in the circuit posted below is that the shorted fet puts a positive voltage on the + input of the TL074 opamp, when this happens, it drives the output pin to the + rail, then i believe that the CA3094 OTA tries to servo out the plus voltage and you get a situation where the power transistors are getting slammed off and on real hard, which is what we were seeing,

so after replacing the fets, all the direct coupled voltages went back to what they should be as verified by the test voltages given in the schematic,

 

[CJ]

here is the startup circuit that feeds the gates of the fets, it is hard to wrap your head around as drawn...

it is fed plus voltages from the two different supplies, it reduces startup thump during power on,

 

[CJ]

here is a redraw of the startup circuit,

there is a 470,000 x 2.2 uf = 1 sec time constant,

CR30 will drain the cap during power off, at least down to  0.6 V,  the resistor will do the rest,

but why is the +16 volt rail attached via CR31?

possibly to prevent the cap from charging in case the +16 volt rail goes down, if it goes to zero, then the current thru thru the 470K will get shunted to ground?  the fets would clamp the input signal if they have no gate voltage,

 

[CJ]

one last thing on this amp, it has a built in compressor.

it was keeping the output power clamped at about 125 watts into 4 ohms instead of the advertised back panel label of 200 watts, wtf, over? 

there is a yellow comp led that lights up when you really thump the bass guitar,

well, as it turns out, the circuit shows a sw to disable the compressor, but this sw is not on the amp!

turns out Peavey jumpered the connector for this sw so the comp is always on, so we installed a sw on the back in case the guy wants to blow up the amp or speakers,  ;D

now we are getting mucho wattage and the new pwr transistors are barely getting hot so everybody wins,

we tried changing R3 and R12 to half there value in order to provide less gain in the opamps, and using diodes with slightly higher fwd v drop, but to no anvil, thus, the install of the switch,

here is the schematic, factory  jumper is green, CR5 is the comp LED indicator,
Maybe Peavey wanted this amp to be mega dependable by leaving the compressor on all the time,

looks like Q11 is modulating pin 5 of the CA3094 to provide variable gain for compressing?

note to Peavey: don't use sensitive fets in a place where their failure can cause mega watt spikes to decone speakers, 

customer is probably pissed at the slow turn-around, but will be happy with having twice the power as before, so it is good we did the bypass surgery,

 

[JohnRoberts]

this thing again, finally got t working, after replacing every transistor except two fets, because they looked like a diff pair the way they were orientated on the board, back to back, figured why mess with a set of matched transistors, color coded from the factory, what were the odds of having the replacements being matched as good as the factory?

well, they were not a diff pair, they were fets used as switches in the start up circuit, 

these are P channel fets, 2N5461, so when the gate is positive, they do not conduct,

unless they blow up that is, which fets tend to do when they get spiked all the time,

so what happens in the circuit posted below is that the shorted fet puts a positive voltage on the + input of the TL074 opamp, when this happens, it drives the output pin to the + rail, then i believe that the CA3094 OTA tries to servo out the plus voltage and you get a situation where the power transistors are getting slammed off and on real hard, which is what we were seeing,

so after replacing the fets, all the direct coupled voltages went back to what they should be as verified by the test voltages given in the schematic,

Yup fet looks like a shunt to ground to suppress turn on transients.

JR

 

[JohnRoberts]

here is a redraw of the startup circuit,

there is a 470,000 x 2.2 uf = 1 sec time constant,

CR30 will drain the cap during power off, at least down to  0.6 V,  the resistor will do the rest,

but why is the +16 volt rail attached via CR31?

possibly to prevent the cap from charging in case the +16 volt rail goes down, if it goes to zero, then the current thru thru the 470K will get shunted to ground?  the fets would clamp the input signal if they have no gate voltage,

Generally such circuits mute fast at turn off and turn on slow at power up so the diode with parallel R looks right. Not sure what the second one is about  The 16v rail will collapse sooner than the 15V rail but it seems just using the 16V rail should work. Diodes cost less than a penny so must have been worth a penny to use the extra part.

JR

 

[JohnRoberts]

one last thing on this amp, it has a built in compressor.

it was keeping the output power clamped at about 125 watts into 4 ohms instead of the advertised back panel label of 200 watts, wtf, over? 

Sounds like you are tripping the DDT (clip limiter). This will go into limiting whenever output does not follow input, so voltage clipping, or current limiting, or anything that prevents proper output.

there is a yellow comp led that lights up when you really thump the bass guitar,

well, as it turns out, the circuit shows a sw to disable the compressor, but this sw is not on the amp!

defeat is so amp can make more power during test conditions where amp may be borderline clipping.

turns out Peavey jumpered the connector for this sw so the comp is always on, so we installed a sw on the back in case the guy wants to blow up the amp or speakers,  ;D

In general it is best to leave the DDT operational, but the customer is always right. Peavey didn't make the switch easy for a reason.

now we are getting mucho wattage and the new pwr transistors are barely getting hot so everybody wins,

If the DDT was being triggered the amp may still not be 100%... Did you measure THD?

we tried changing R3 and R12 to half there value in order to provide less gain in the opamps, and using diodes with slightly higher fwd v drop, but to no anvil, thus, the install of the switch,

DDT is a mature design I doubt you will improve it.

here is the schematic, factory  jumper is green, CR5 is the comp LED indicator,
Maybe Peavey wanted this amp to be mega dependable by leaving the compressor on all the time,

looks like Q11 is modulating pin 5 of the CA3094 to provide variable gain for compressing?

note to Peavey: don't use sensitive fets in a place where their failure can cause mega watt spikes to decone speakers, 

customer is probably pissed at the slow turn-around, but will be happy with having twice the power as before, so it is good we did the bypass surgery,

Not sure you fixed it yet.....  DDT is very sensitive and looks at the power amp innards to tell when the output is not what it should be. This obviously limits when it runs out of PS rail voltage, or current limits due to too much output current. This could be anything inside the power amp, section. Broken current limit circuit, or amp still not happy (drive devices), will trigger DDT? 

I can't suggest a single smoking gun, but DDT limiting says the amp is not happy yet. 

JR