"these go to eleven"  :tu:

Speaking of EQ........there is a "human factors" element in designing EQ controls for people (maybe musicians fall into this category)   I've noticed that many guitar players are confused by parametric EQ front panel controls. Getting through the mix may be more of a function of how familiar a person is with the equipment and how easy it is for them to relate to the hardware and consistently achieve the same results. Of course, actually understanding the the psychoacoustics of sound and some electronics theory doesn't hurt either.

Reliable purchases from American - just ask and Reliable will order what you want. cheap too,

You're going to love this site http://www.americanhandle.com/ 

I have been "trained" to populate proto SMT PCBs using a pre-heater and a heat gun - the paste is dispensed with an old Madell dispenser and manual "pick & place" with an aquarium vacuum pump. I need a microscope to see what I'm doing  Same station does rework

Interesting device but when Semelab discontinues it there is no replacement - the market for dual N/P is DC converter, Load switch, LCD display driver, etc - BVDSS <30V. Better off with "discrete" MOSFETs in TO-220.

Thanks for the links -

QuoteLow output PA impedance lead to hard speaker dumping, that doesn`t allow cabinet to "breathe".
Look at frequency response of transformer output fet PA loaded on active load and cabinet (voltage on PA output).

It seems like an OT is an expensive solution to solve the problem - damping factor of a tube vs SS design. What if the "soft clipping" of the signal occurs not in the PA but in the preamp stage?

BTW I have a very good friend who writes and speaks Russian language (he is from Moscow) if you have more papers/information.

Sorry, joining this discussion late. Why the OT in the MOSFET PA? Gilding the lilly or does the bandpass like response and non-linear transfer function of the OT add to the "tube sound"?

I would love to see a FET (JFET that is) design for the preamp.

I have used this source for JFETs - very good process control, screening available and good product offering http://www.linearsystems.com/

 KMG - any chance you can repost the schematic and/or pictures? I can't load the files in the original post. Cheers

http://www.ixys.com/product_portfolio/products_link.asp?framepage=ixyscolorado_contact.htm&framepage1=http://www.ixysrf.com/products/diodes.html

Probably start to see these in tube amp power supplies soon

I finally have this PA working again but have a question about the AGC circuit. Are Q1/2 some sort of detector used in the AGC loop? How does the AGC work in this design?

This amplifier just about puts out 18Vrms (@2.7 ohms) before the positive part of the input signal starts to degrade. I'm also only getting 60 V out of the secondary at 120 V on the mains. I'm going to assume this was typical ACC production control in 1977.

Replaced all the preamp ICs and now they're in sockets. Can't get a signal into IC1 now - the signal "disappears" at R44 - unless the IC is removed from the socket

I have replaced the OP/checked all the components around the socket/VCC+- are good. What am I missing here? Something is loading the signal down at R44.

Preamp schematic is on the previous posting.

Fixed the power amp but now the problem is the preamp. I can chase a signal to the input of the VCA (SSM2013) but nothing at the output of the VCA. I checked the bias on the VCA and the V+ rail is only 10V. I rebuilt the positive regulator on the PA board but I still can't get +15 rail back up to 15V. The negative rail is okay. I have to assume that some of the other dual OP-amps are damaged and pulling the supply rail down? So now I know why someone wanted to airlock one of these amps

BTW does anyone have a readable copy of the schematics for this amp?

Two shorted transistors in the output section - it also looks like someone "repaired" the +15V regulator transistor. I'm waiting for replacement parts from On Semi.

Funny but this amp was considered unrepairable by the previous tech who worked on it???

Clean it up and check the features/output and it should be ready for battle 

Thanks JM - Double checked and the lead on the DVM is faulty. Must always remember that it is rare to damage a PT.

The schematic shows a 10 amp 3AG fuse though the fuse holder has a (6 AmP) marking
This is the 300 Bass amp circa 1985.

Thanks for the sanity check - now I can start to see what's pulling down the supply. Here's what I know:

When I got this box there was one blown fuse (I think it was the positive rail).
No signs of damaged or overheated components.
Looks like all the original output devices
PS filter caps ESR measure in spec.

So I'll start down the posisitve rail (PNP output devices) and see what I can find. Next step test for over current with only rectifier bridge and filter caps comnnected.....