I stand corrected.

Quote from: Fossilshark on June 20, 2016, 11:01:22 PM
Crap i dont have the right tubes, thanks. But since these are diodes could they be used for some kind of weird diode clipping distortion? Or is that something id have to mess with on my own

Maybe? I haven't heard of anyone else doing it.

A quick google search reveals them to be a rough equivalent of a 6AL5 which is a double diode. The ECL82 is a triode and pentode in a single bottle. The two have nothing in common.

Edit: further googling indicates that the 6Ф3П (6F3P) is the Russian equivalent (or near equivalent) of the ECL82 / 6BM8.
http://www.radiomuseum.org/tubes/tube_ecl82.html

I don't see it hurting the tube but you probably wouldn't hear much, if anything at all.

A guitar with humbuckers will put out a signal of about 100mV peak to peak and that's just on the initial attack. The amplitude quickly degrades as the string vibrates and the note decays. I would think you would need a few volts on the grid to get anything audible out the speaker and probably around 30Vpp to drive it to full volume. And that is, of course, exactly what preamps are for.

If you want to test a single amplifier stage, a signal generator of some sort is in order. If you have the cash, a bench-top function generator is your best bet. If you don't, you can build a rudimentary one (which, incidentally is exactly what I've been working on lately). Of course, if you go that way, you then have to test the signal generator circuit which requires an oscilloscope. So you have to spend money either way.

You're probably going to have to either design a circuit yourself or adapt a non-guitar circuit to your needs if you want to use those tubes. Even back in the day when tubes were current technology and there was a cornucopia of tubes for designers to play with the major amp manufacturers generally stuck to a few models of tube which are the same ones still being produced and used in guitar amps to this day.

That's not to say there aren't unusual designs out there but there were so many types of tube made back then that even if you happen upon a whole box of old obscure tubes chances are none have been used in that application. Some won't even be suitable for audio use at all. Some weren't made for audio but can be adapted for audio and almost all of them are power tubes. There are a few interesting obscure preamp tubes but they tend to be rare and just as expensive as the common tubes and I've found it hard to justify using them.

It all depends what you want to do. If you're dead set on using those tubes because you like the challenge of turning trash into treasure (which is how I got into electronics) you've got a lot of research, planning, simulating and failing to do before you're going to get a working amp out of it (but you get a lot of knowledge too!). If you just want to build an amp right now that has a decent chance of working you might as well just put them away and go buy some more traditional "guitar tubes" and build a little Fender clone or some other well established design. We've all got fantasies about turning a box of junk into something nice but you're not going to save money by using obscure tubes. The transformers are where you'll end up spending the bulk of your money. Not to mention small components like capacitors, potentiometers, sockets, etc... all that stuff adds up and you need it no matter what tubes you use. Once you buy all that, a 6V6 and a couple 12AX7s aren't much of an expense.

To answer your question fully, the only tube on that list I've heard of being used in a guitar amp is the 35C5 (actually it's 50v heater cousin the 50C5). These designs come with a big catch, though. The reason they run at such high heater voltages is that they were designed for the heaters to be run in series right off the 120v mains. They also operate at low plate voltages which (kinda sorta) eliminated the need for a power transformer (which was and still is far and away the most expensive part of a tube amp) and therefore they were commonly used in cheap radios and record players of that era. Some makers of super cheapo guitar amplifiers like Kay and Danelectro followed suit and you can find schematics on the net that will help you but they should not be built as is! They have no isolation from the mains and are commonly called "widowmaker" amps because they are dangerous. Nevertheless the preamp and power amp parts of the schematic can be useful. They are usually simple designs and easy for a beginner to understand. It's the rectified mains power supply that is dangerous and you need an isolation transformer to make it safe. Do not skip the power transformer! You have been warned!

Quote from: J M Fahey on February 22, 2016, 08:17:35 AM
Cheap multimeters are VERY unreliable measuring low ohms, so forget them.

The test is very simple: wire a 25-100W bulb in series and plug it into the mains.

Does the bulb light properly?

I bet it does.

With the lamp on, measure AC volts across the switch (be careful, you have mains voltage there) ... I bet you will read 0 or a *very*  low voltage (less than 1 VAC).

So where did the "100 ohms" go?

On mains rated switches such as those, you will always have a tiny oxidation layer on surfaces ... any real voltage punches through and destroys it.

Just don't use those switches for low level audio and you'll be fine.

I've done this and it works just as you said it would. I get less than 100mV across the switch.

Thanks guys.

Quote from: Enzo on February 11, 2016, 10:21:25 PM
And once more for the umpteenth time, buying parts on ebay is always a crap shoot.

before you throw them out, though, try wiring one into a real circuit and see if the contacts don't just burn themselves clean.  A couple of sparky on/off cycles might just clear away whatever film there is from the manufacturing process.

Good call. I just built a dim bulb tester which comes in handy for testing the switches at mains voltage (which is 120VAC 60Hz here). A 100 watt light bulb in series with the live wire serves to limit the current and is a fairly heavy load. The tube amp I'm building is a small one and I don't expect it to consume that much power so this should function as a good test case to see how well these switches hold up.

As I said before, the resistance would change every time you switched it on and off. Before testing it was anywhere from 1 to 20 ohms. For the first test I left the switch in the on position and used a power strip to switch the mains. After disconnecting the power I again measured the resistance in the on position and it came out to around 0.3 ohms. I toggled the switch on and off a few times disconnected and the resistance jumped around again between 1 and 8 ohms.

For the second test, I reconnected everything and toggled the switch itself while it was pulling current hoping, as Enzo suggested, that arcing inside the switch would burn the contacts clean. That seemed to be the case. When I disconnected the mains and measured the resistance it seemed to be a pretty consistent 0.3 - 0.4 ohms when toggling on and off.

That's still not a great value, about 30 times what was quoted in the datasheet, but not as bad as the readings I was getting suggested. The switch did not heat up at all. Maybe I can salvage these things after all but I think more testing is still in order before I trust these things in my amp circuit. It didn't occur to me to measure the voltage across the switch while it was on so I'll probably do that next. That way I can calculate the resistance of the contacts while it's in a live circuit.

Could be the lubricant. The resistance seems to change when I switch them on and off. Maybe the lubricant could cause that? In any case it looks like I have to buy new switches.

So I'm working on a tube amp project and I bought some E-Ten 1222 DPST switches on eBay for the power and standby switches. According to the website, they're supposed to have a contact resistance of < 10 milliohm but when I measure my switches (I bought 4) with my multimeter I get much more. Most are in the ballpark of 10 - 30 ohms or so and one of them measured more than 100 ohms! That's going to make for some significant losses when I put 120v across them.

To make sure I wasn't going nuts I tested some other switches in my parts bin. None of them tested over 50 milliohms in the on position.

So are these switches junk or are they usable?

And yes, I know that you get what you pay for with cheap Chinese parts but switches are fairly simple devices and I figured it would be hard to screw up a switch. Maybe not...

Roly, I appreciate the effort you must have gone through with that last post, but like I said, I'm new. I didn't understand most of it, unfortunately.

But point taken on the breadboard. It's a good idea. I think I'll put the pre-amp project aside for a while and design a test bed. I've got a transformer, plenty of caps, rectifier diodes and an LM317 voltage regulator so I could easily whip up a passable power supply. I also have a small speaker and an LM386 chip amp so that should make it easy to swap in gain stages and test them out.

Thanks.

EDIT: I know I won't get 10v p-p on the first stage. It was the second or third stage I was trying to simulate and it was just easier to set the signal generator to 10v than to build two or three stages before it.

I thought it was the source resistor that sets the bias on a JFET.

Thanks for the reply, Phatt. I guess I should have mentioned that I am a real newcomer and that circuit is (for now) over my head.

I'm not really trying to recreate the valve sound with transistors per se (I'm not a competent enough electrical engineer for that yet) I just wanted to design a simple SS preamp using circuit topologies analogous to the common cathode gain stages used in tube amps (common source, common emitter) as opposed to op-amps and clipping diodes. I was trying to copy the topology really, not the sound (although a nice smooth, bluesy tone was what I was shooting for but I want to be able to have enough gain for soloing too) - kind of like the people who make FET equivalents of tube circuits. Now that I think of it, maybe I should simulate some of those designs and see if they do the same thing.

Basically I wanted to follow these principles:

• Common source / common emitter gain stages
• Clipping against the supply rails instead of diodes
• No negative feedback

When I started reading about electronics I wanted to design tube amps right away and I read a lot of material on tubes. At first I thought I could string a whole bunch of these stages together like a tube amp and call it a day but it didn't seem to work out that way. BJTs saturate and JFETs forward bias and act like closed switches if you drive them hard enough which complicates things somewhat.

Basically what I wanted to know was how fact the gate forward biasing when the input signal goes high enough (and thus clips the input signal) affects your tone. Do you get that strong second harmonic because it clips the input signal or does it not matter since the output of the stage swings from rail to rail?

Would this effect make it unfeasible to use JFETs in such a design?

Hi,

I asked about heavily overdriving a BJT a few days ago and now I have a (probably dumb) question about heavily overdriving a JFET. So I'm simulating this JFET common source amplifier with a voltage gain of 10 (picture below) and I'm driving pretty hard (10v p-p). The green signal is the input waveform, the blue waveform is taken on the gate of the JFET (after the 500k resistor) and the red waveform is the output, taken at the 1 MEG load resistor.



You can see the signal is clipped on the output and partly on the INPUT as well because the voltage forward biases the JFET junction when the signal swings upward (unless I'm mistaken).

Is this a problem? I've been wanting to design a little practice amp for myself and I've been trying to decide whether to use JFETs or BJTs for the preamp and I can't decide whether or not I have a problem with this or not. I basically lose almost the whole top half of the signal. And I've read asymmetrical clipping causes a strong 2nd harmonic and I don't know if that's what I want.

I was hoping to design the preamp by stringing together a bunch of gain stages like this one (common source or common emitter) - like a tube preamp. I realize I could just use an op-amp and clipping diodes and not have to concern myself with this stuff but (for me, anyway) that's no fun. I know it sounds dumb but I like to do things the "old fashioned" way, I guess.

Not to belabor the point or anything but since we're on the subject of collector following, I'm guessing this would be "drain following"?



I won't bother any one for solutions since playing around in the simulator suggests the solution is the same: a gate resistor. Interestingly I did need a much larger resistor to get this circuit to play nice (I used 500k) but it works.

With regards to "hidden traps", I'm aware of them but I guess I've been lucky enough not to run into them on the bench. So far I've designed and built one amplifier from scratch and was lucky enough to have to not need to do a lot of troubleshooting to get it to work. I designed it in LTSPICE and built it on a prototyping board. I even built a little enclosure for it.

I did have problems with oscillations initially but it turns out it was because I had the input jack wired backwards. I simply swapped the polarity and it worked without a hitch.

For the layout, the rule of thumb I followed was that I built the preamp on the right side of the board and the power amp on the left. It was powered off a 9v battery so there wasn't much of a power supply but I would have put that on the right as well, far as possible from the preamp. The idea was to keep the high impedance, high gain circuitry isolated as possible and I think it worked decently.

I designed it like that because of the reasons you noted. When I started in electronics I was only interested in making tube amps so I came across a lot of material about hum and interference, like how to twist the heater wires, adding a DC offset to the heater supply, orienting the output transformer 90 degrees from the power transformer and so on.

Admittedly, I used to be in the tubes good, transistors bad camp but I started building solid state amps to get experience and also because of cost. Building that little amp actually disabused me of much of my tube-snobbery. It actually sounded better than I expected. It had a nice bluesy tone which I used to associate with tube amps. I used to think transistors clipped really hard so any sold state amp I built would have a hard, heavy-metal kind of tone. So I was surprised when I started testing it with my cheap Fender Squire Strat that it sounded pretty anemic. I didn't know how much gain I needed and it turns out my little amp didn't have enough for my liking. I was pretty unhappy with it until I plugged a guitar with humbuckers into it. Then it sounded much hotter!

So now what I'm doing is basically the next revision of that design. I wanted to build it on the same principles but I want the new version with more gain and a little more tone control. Maybe I'll start a thread about it when I get the design fleshed out a little more.