Ampeg PF-500

[lietuvis]

Hi, is here anybody have ampeg PF-500 preamp schematic. I'm interstef in jfet preamp section and compressor. If it happens that somebody have it please could you send me email to litvanas at gmail.com or post it in the forum. Thanks in advance

[lietuvis]

Hi, is here anybody have ampeg PF-500 preamp schematic. I'm interstef in jfet preamp section and compressor. If it happens that somebody have it please could you send me email to litvanas at gmail.com or post it in the forum. Thanks in advance

Found:
http://service.loudtechinc.com/secur...00-B00-SCH.pdf

[mozwell]

Several years ago i built an SVT2-P preamp into my amp.
Here are details of the inductor i made.
This can be used instead of the ampeg toroidal inductor. I am fairly sure the same inductor is used on all ampeg amps, so if you need to make one, now you can.
It is important to use the correct core & material type, and make sure it is an ungapped core.
You will need to fit a lot of turns on, so you will need to use very small wire, i used some 0.125mm enamel coated transformer wire, but you can use any insulated wire (teflon is ok) as long as all the turns fit into the space on the bobbin (or you wont be able to get the core to fit over the bobbin). I wound mine by hand.

[J M Fahey]

Thanks a lot for your posting .
I bet you spent quite a lot of time developing it.

[lietuvis]

Thanks Mozwell I have inductor custom made for Ampeg specs by SOWTER, it cost more money than to build your own, but it is well spent money,the sound of inductor is amazing by connecting it to simple OP amp circuit on all 5 corner frequencies. BTW I was looking for that fet input stage, which is also have interesting circuitry using OP amps to bias them or so, I still have to find out how they work for this 2 stage fet preamp. As for compressor, it's not optical as on most Ampeg amps, so I don't bother. I have THAT4301 built in pedal. After all I was hoping to simplify the circuit to my needs and build it to stomp box. But now again to get 52VDC for fet stage in a stomp box...I'll see whether it's worth to me to diy.

[J M Fahey]

And why cramp this in a stomp box?
It's meant to be built on a rack cabinet, even if just 1U high, and fed proper voltages by a real PSU, not a puny battery.

PS: the Op Amps are working as DC Servos, measuring current (voltage drop) across source resistors and adjusting bias as needed to keep current at the design value, no matter what Fet you use.
Better than the klunky 1-preset-per-Fet solution and saves them individual Fet matching.

PS2: what I find somewhat crazy is that they designed a discrete Fet front end, obviously to get some flavor of "Tube sound" ... and then they negate it by *strong* signal attenuation before it can "do something".
The first Fet obviously won't be taxed by any regular Bass ... and the 15dB pad takes care of any beefy Active one .
So far, so good, *but* the second one should be driven closer to non linearity or something ... but they added the 7x attenuator (470K/75K), which avoids most of it.
If you build this preamp, I'd suggest you add a "Drive" pot between Q1 and Q2, replacing R17, and raise R22 to 1M (same value as R6), so *you* can control how much "grind " to add.
jm2c

[lietuvis]

Thank you Juan for explanation and suggestions.

[teemuk]

...PS2: what I find somewhat crazy is that they designed a discrete Fet front end, obviously to get some flavor of "Tube sound" ... and then they negate it by *strong* signal attenuation before it can "do something".
The first Fet obviously won't be taxed by any regular Bass ... and the 15dB pad takes care of any beefy Active one .
So far, so good, *but* the second one should be driven closer to non linearity or something ... but they added the 7x attenuator (470K/75K), which avoids most of it.

BUT there is plenty of gain in tap in that one. Way more than in some generic low voltage FET designs.

The gain of the first stage is easily more than 20x and the headroom is limited to about 20 - 25 volt peaks. About 1.5Vpeak signals will overdrive the first stage and strong bass pickups can easily put out 4V - 5V peaks. Heck, even a generic guitar pickup might overdrive that stage!

Yes, there is padding in between the stages ...for a reason. Throw in more gain yet the same headroom as in first stage and the second stage is even more likely to overdrive than the first one. In fact, a SPICE simulation shows clipping occuring at input signals of only about 700mV peak. Nice asymmetric clipping.

The -15dB input pad is there for a very good reason.

There's another great feature in this design: Bass guitarists aren't overly thrilled about excessive clipping and can live with pretty clean tones... BUT they will appreciate the excellent signal-to-noise ratio of a design that about immediately in the very first stages pumps up a somewhat low amplitude signal to quite high amplitudes.

[marsbel]

Hello,

Could someone re-upload the PF500 preamp schematic?
I'm having problems with the tone controls cutting out. I suspect a bad opamp.

Thanks!
Mark

[nickb]

ampeg PF500.zip

[marsbel]

Thanks alot!
But those are all schematics of the power section.
I suspect a problem in the preamp section, so I'd need preamp schematics...

[nickb]

Sorry bud, that's all I have. It looks like loudtech have taken down their service portal. How sad

[marsbel]

I know.
I was hoping someone has it saved on his harddisk.

[Enzo]

This link work?

[marsbel]

Super! Thanks alot! It works.