Quote from: Bajaguy on July 28, 2014, 11:40:20 AM
One thing to keep in mind when you are matching a preamp to a chip amp is matching the input impedance of the chip and the output impedance of the preamp. Not paying attention to this will cause some nasty oscillations...... ask me how I know. :grr

Ok, how do you know this? Were you exceeding the input common mode range of the chip? Looking at the ST data sheet for the TDA2030A, the input common mode range is not specified. Ill bet strange things happen when you slam the input rail to rail or try to operate with a non-inverting gain of one.

Thanks to JM for the kind words!

I don't know of any equasion to calculate the base resistor. Since most equasions make the assumption that the circuit is acting as a linear system, they stop working when things become nonlinear. LTSpice takes many nonlinearities into account so I would expect it to do a good job in this case. The easiest thing to do is try a resistor in simulation and adjust as necessary until the desired result is obtained. The resistor will change the gain somewhat along with the bandwidth and it shouldn't matter much which side of the cap it is on.

In figure 3 the low impedance of the generator is feeding signal through the base-emitter junction (which is acting like a diode) and pulling up on the emitter resistor. This subtracts from the voltage available to the collector resistor. The transistor is saturated which means the collector to base junction is also forward biased so signal also feeds directly from base to collector without the usual phase inversion. C1 takes on a charge different for the quiescent no signal condition which makes the output wave shape change over the first few cycles. To minimize this effect you need a resistor in series with the signal on one side of the capacitor. Solid state circuits sometimes act badly when severly overdriven. The usual remedy (bandaide) is diode clipping to limit signal amplitude.

Toroidal transformers have large inrush currents. There may have been an engineering change that upgraded the inrush limiter. I would contact the factory for advise on a replacement since this is a safety critical component.

Bridge rectifiers like what you installed are typically only rated at 1 Amp. That's a little too low. See if you can find one rated at 4 Amps or more. For voltage rating, anything over 50V is fine.

I had a heatsink with a 2N3055 and MJ2955 mounted so I just sky-wired the rest of the output stage, Q8, Q9, Q10. The beta of the 2N3055 was too low, the amp only made about 20W at 4 Ohms and about 18W into 8 Ohms. Your output stage seems to be working so finding replacement transistors won't be a problem.

I'm worried about those output transistors. My data says the 2N2148 is Germanium rated at 12.5W. It won't look good on most DVM diode test functions. I have no idea on the TA2577. There isn't much drive current available to Q10 so it needs high beta.

Roly, Please look at the schematic. This amp is a single rail design with an output capacitor. The 3V on the output is just leakage from the output cap.

Brian, check the voltages around those output transistors and compare to the schematic.

Adobe Reader XI gave me an error and wouldn't open the file.

Check continunity on the reverb pan transducers. I've heard that the wires between the jack and coil are prone to breakage.

Quote from: JHow on May 14, 2014, 04:32:09 PM
I have a 1983 Randall rg80/100 es.   I can take some shots tonight if that helps.  The heat sink is as JMF described, the whole chassis itself.  It's very thick aluminum.

What we need is a shot of the PCB that shows what is mounted on the chassis, the driver transistors, diodes and/or thermistor. I looked around on ebay but all I could see was that the TO-3s were mounted on the chassis. I found the operator's manual. It says the output damping factor is 2 to 4.

Link: http://el34world.com/charts/Schematics/files/randall/Randall-RG80_100%20guit%20amp.pdf

I would be beneficial to see a photo of the Randall amp guts. This will give a general idea of the size of the heatsink and what parts are mounted on it. It would also help to find out how Randall suggests the bias be set. Q15 and Q16 operate in class C (this is unusual) and only turn on when the load demands more than about 1 Amp. Q13 and Q14 are class AB outputs that probably have a bias current of 50mA or less, more than likely 10mA.

Another odd thing about this design is the high source impedance of the output. Voltage feedback only sets the quiescent DC output voltage but signal feedback comes from output current sensed by R63. The stability of this design is unknown.

The device you are wondering about is in parallel with D7. It is probably a thermister, a temperature dependant resistor. It is included to regulate the bias current in the output transistors as the heat sink temperature increases.

One of the challenges in designing a power amp like this is keeping the thing from self destructing. The bias current in the power transistors will tend to increase as they warm up in a vicious circle until they get too hot and fail. Some means of thermal feedback is required that will maintain or slightly decrease the bias current as the power transistors warm up. The 1N914 diodes might be sufficient if they can be mounted on the heatsink. I like to use a TO-92 transistor held face down on the heatsink with some kind of metal clamp.

In practice you build the amp and get it working. Set the bias when the amp is cold, run the temperature up by running the amp into a dummy load until the heatsink get nice and hot and measure the bias current again. If the bias current is within reason, the design is done.

So far the only thing I have on the TIS58 says it's an N channel JFET and the J305 is a possible replacement.

A JFET has three terminals, Gate Source and Drain. The Gate acts like a anode of a diode (in an N channel JFET) and the Source and/or Drain will act like the cathode. In most cases the Drain and Source can be interchanged, the part will operate normally if you reverse those two terminals. Your meter should show a valid diode indication from the Gate to Drain or Gate to Source in both directions when the JFET is removed from the circuit. A damaged JFET often shows a low resistance in both directions on the Gate. Between Drain and Source your meter should see a resistance, in the case of the TIS58 (I'm guessing) 100 to 200 ohms. Some JFETs used for analog switches will measure lower between Drain and Source.

Pinout: Without a data sheet I can only guess but on most USA designed N Channel JFETs the terminal that would be the Collector of a TO-92 bipolar transistor like the 2N3904 would be the Gate. Occasionally it's the middle terminal, and that is usual on Pro Electron parts (2SK and 2SJ).

An N channel JFET acts like a triode vacuum tube. The Gate will be the most negative terminal, usually at ground potential. The channel terminal acting as the Source will be at perhaps 1V or 2V above that acting like the Cathode of a triode and the Drain will be 10V to 25V above that acting like the Plate of a triode.

It's common for the very first input JFET to be damaged. Excessively negative Gate voltage like from a static discharge will damage the gate unless the circuit has protection diodes.

I don't know of a DIY Beta preamp. There are one or two clones built into stompboxes that can be found. The biggest problem with the ones made by Sunn is that they need to be recapped. There are about a dozen 2uF non-polar electrolytics that need to be replaced to get the preamp running well.

If you can find one that is non-operational for a good price, buy it. The -15V regulator frequently fails and the amp goes completely silent, even the power amp stops working. There is a hard to find chip that manages the channel select and footswitch logic. The fix for that is a little piggyback module that will plug into the original PCB found here: https://forum.sunnstillshines.online/index.php?topic=6864.msg29374#msg29374

Edit: The Channel select LEDs on the front of the amp will stop working in both cases above. Normally if you plug into a channel, that channel's LED will come on, both will come on if you plug into the Both input. When the channel select logic goes bad, the power amp will still work if you plug into the Master Effects Return on the back of the amp but the Channel LEDs on the front won't work properly. Sometimes one channel may still work. The power amp has internal fuses that will blow if the power amp goes bad. In that case nothing will work except the light in the power switch. This usually indicates blown transistors in the power amp.

Edit: Jan 7, 2021 updated link to Sunn forum

Quote from: Roly on February 22, 2014, 12:15:48 AM
{willya lookit all those furshlugger trim pots!   :duh   }

It probably cost more for the trim pots than the JFETs. But the upside is the endless possibilities for tweeking. Nice to have one of those little plastic screwdrivers made for tweeking pots.