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Messages - Loudthud

The bias seems to be set by Q3 and RV1. Probably not a big deal but that part of the circuit could be improved for better temperature tracking.

It is my understanding that Leo was not involved in the electrical design of these amps.
Tubes and Hybrids / Re: Triode Emulation X-Y plots
December 02, 2011, 07:06:18 PM
The circuit in my first post was just the beginning. I was thinking of a simple JFET source follower input buffer for a 1 Meg input impedance and with a clamp to simulate grid conduction. The problem is that would need to be AC coupled to the opamp. You have to be carefull where the signal is AC coupled to avoid baseline shift where you don't want it. The long range goal is to make a complete guitar amp. The first stop being something like a Tweed Princeton. I have a couple of class A power amps built. One about 5 watts and another about 15 watts. Man, do they get hot. The 5W runs off of a laptop power supply. A good project for the novice. No mains to touch.

I kind of gave up on zener diodes, they just weren't getting the curvature I was looking for. Posted below is where the idea reached a pause. Getting the bend due to grid current had me stumpted for a while, then I just put a clamp on the input and it looked pretty good. I'll need to do some more experiments to see if a resistor is needed in series with the J201s. Maybe two or three 1N4148s will make a better clamp. The Ge diode gives a nice gentle curve from the zero signal point (middle of the screen in the scope photos) and the 1N4148 adds additional curvature near where the tube cuts off. The gain is increased to around 10 which is about as much as can be expected with +/- 15 Volt rails. The signal is a little too linear between zero and about -1V on the input. I might add another Ge diode with a series resistor to give a little bend there.

This circuit is intended as a first stage. Since I would like to hit the second stage as hard the typical tube amp, the input of that stage needs to clip at somewhere around 200mV to 300mV. That will be more challenging.

I like bullet proof inputs so I added a 1N4148 to the -15 rail from the input of the opamp (not shown in the schematic below).

I'm trying to get set up so I can compare tubes and my circuit in real time like Steve Conner did in his thread. The stack of test equipment is getting bigger.
Tubes and Hybrids / Triode Emulation X-Y plots
December 01, 2011, 08:08:29 PM
This is a different take on the emulating triode tube distortion theme. First, a few acknowledgements are in order. Findeton's thread, is some fine work but I find it a little complicated. Certainly the math gives me a headache. I didn't look at Dimitri Danyuk's paper, I just figured it would be more mind numbing math. The Runoff Groove article is interesting but I need to see waveforms, not just equasions. The one problem I found in the article was the statement "The first valve stage of a Fender amp can withstand input voltages up to +/- 2.5V without noticeable clipping." We shall see that isn't true. Steve Conner's thread is more my style. I know Steve from Ampage, he is very knowledgeabe on tubes and solid state design. I hope to be able to present pictures as good as Steve's

The first method I will present is the opamp with diodes in the feedback path. As I mentioned at the end of Findeton's thread, the clipping stage of the TS-9 does not come close because in the non-inverting configuration, the input signal gets added to the distorted signal due to the +1 term in the gain equasion. That leaves inverting opamps and shunt clipping. A significant problem is that diodes clip at pretty low voltages so, to keep from having to attenuate the signal, you would need lots of diodes in series. I have overcome this obstacle with a circuit I call the Diode Multiplier.

When I was in high school, I was told that a silicon diode drop is 0.6 or 0.7 volts. That's true at around one milliamp. With a 100K input resistor, +/- 2.5V input signal, we would like to know what the voltage drop is at +/- 25 microamperes. I have curve tracer photos of a 1N4148, a germanium diode, and the gate of a J201. The germanium diode is interesting because it starts conducting right at zero volts. The J201 gate stays off for a significantly larger voltage than the 1N4148, that could be useful. Low voltage zener diodes are another surprise. They start conducting around 0.4V and don't reach full zener voltage until the test current of 20mA is reached. The 1N5221B is a 500mW zener rated at 2.4V (at 20mA).

The first circuit uses a TL072. One trimpot sets the gain, the other sets the amount of diode multiplication by attenuating the feedback appled to the diodes. This circuit does not address the grid current of a triode issue, that will be added later. The 10K trimpot was set to divide by roughly 17, 10.26K on the top portion and 724 ohms on the bottom portion.
This is an interesting topic but I'd like to offer a different point of view. Although the plate curves of a 12AX7 reveal lots of information, what is really of concern is the transfer function. An X-Y display of input verses output. The attached photo is of a generic Chinese 12AX7, 100K plate resistor, 1.5K cathode resistor with a bypass cap, and a 68K grid stopper. The B+ was 300V.

Note that the diode clipper stage of a TS9 can't emulate the curve correctly (with different diodes for + and -) because the input signal is added to the output by the +1 in the gain equasion. You need to use an inverting stage to get rid of the +1.
Amplifier Discussion / Re: discrete poweramp
November 23, 2011, 11:30:22 AM
An old Class A power amp design that works surprisingly well for guitar. Power isn't very high and you will need a huge heatsink and/or a fan. But, the basic circuit only has four transistors.

Kevin O'Connor's TUT2 book has a chapter on solid state power amps. You can basically roll your own by matching a front end to various power stages. Lots of other info on tube amps and tube/transistor hybrids.

Amplifier Discussion / Re: Speaker impedance matching
November 22, 2011, 09:49:39 PM
Weber sells a couple of impedance matching autoformers, the WZC-50 and WZC-100. They have several taps including some oddball impedances like 5.3 ohms for those 3x12 cabinets. Down near the bottom of the page at: