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Messages - Loudthud

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The Newcomer's Forum / Re: Battery Draining When Amp Turned Off
« on: January 19, 2020, 12:50:16 AM »
Unsnap one side of the 9V battery and rotate the connector so that the unsnapped side isn't touching the battery terminal. Now, set your multimeter to measure current (sometimes you have to move the red test probe lead to a different jack on the meter) and touch one lead to the unconnected side of the battery and the other lead to unconnected side of the battery connector. You are now measuring the current flowing out of the battery. You may have to change the range setting on the meter, expect a current between 1mA and 10mA.


The NuTube can definitely add some even harmonic sweetness that tubes are known for. There are other ways to do the same that don't cost $50 bucks USD. (One cost estimate I heard.)  Although the gain is low, so is the B+ limit. So headroom refereed to the input is about the same.

Can anyone verify the price ?

Guitar News / Re: Vox Cambridge50 - Nutube at NAMM 2020
« on: January 08, 2020, 01:47:12 AM »
They make the NuTube seem like the greatest thing since sliced bread, but take a look at this: (you may have to join the forum to see the attachments).

All the crap it takes to make the thing work, and some of them are microphonic. Since there is no cathode, they're also a PITA to work with.

Amplifier Discussion / Re: 1988 Randall RG100ES
« on: January 02, 2020, 06:10:10 AM »
Have you checked the low value resistors R55, R56, R57 and R58 ?

Amplifier Discussion / Re: 1988 Randall RG100ES
« on: December 31, 2019, 04:32:09 PM »
On the schematic you posted, what is the meaning of the note #7 ? Is that leg of the resistor grounded ? Look at the Bases of Q8 and Q9. The signals should look very similar. That tells us that the amp is basically working, but there is some problem in the feedback network or the amp just isn't getting enough signal to begin with. Is the signal at Q8's Base nearly the same as it is at the Effects Send jack ?

Amplifier Discussion / Re: 1988 Randall RG100ES
« on: December 30, 2019, 08:59:06 PM »
The two center output transistors are getting slightly warm to touch but outer two are cold
This is normal for this circuit.


There is a 0.18 Ohm 10W resistor (R63) between the low side of the speaker and ground. Have you checked this for continuity ?

Amplifier Discussion / Re: 1988 Randall RG100ES
« on: December 30, 2019, 07:37:54 PM »
Have you tried bypassing the Effects loop by connecting a patch cord from Effects Send to Effects Return ?

Preamps and Effects / Re: So you need a preamp - here are your options
« on: November 06, 2019, 11:37:21 PM »
The J109 would make on OK low level amplifier, but is a challenge to bias and has almost zero chance of working in any J201 or MPF102 circuit without substantial changes. Notice that there is no maximum given for Idss. The parts will melt or explode if you try to measure Idss with DC. This is typical on switching type JFETs. They depend more on R(on), channel resistance.

The J175-77 are P channel. I've used them where I needed a P channel, but they are weird.

Preamps and Effects / Re: So you need a preamp - here are your options
« on: November 04, 2019, 07:48:17 PM »
The first thing you need to know about JFETs is that the specs can vary quite allot. So much so, that it is hard to design a circuit that will operate for any JFET that meets the specs you find on a data sheet. The usual fix for this is one or two trim pots.

On the vast majority of JFETs, you can reverse connections to the Drain and Source with no change in performance.

The first thing to look at is Gate breakdown Voltage. BV(GSS) is a term you might see, it means Breakdown Voltage, Gate to Source with Drain shorted to Source. For a J201, the number is 40V. For the MPF102 the number is 25V. Most circuits don't provide any input protection and a big negative input Voltage will damage the JFET. Sorry, that's the way it is.

The next thing to look at is called I(DSS). It means Drain Current with Gate and Source shorted (to ground). It's usually specified at some Drain Voltage like 5 or 10 Volts. For the J201 the spec is 0.2 to 1.0 mA. For the MPF102 the spec is 2.0 to 20 mA. You have to look at the circuit the part will operate in, but generally try to keep the minimum I(DSS) of the replacement at or above the minimum of the part to be replaced. Try to keep the max I(DSS) less than twice the max value of the part to be replaced.

The orientation of the pins may be different for the replacement part. You will have to check this. J201 and MPF102 have the Gate at one end of the package. Same place the Collector would be on a 2N3904. Most Euro parts have the Gate in the middle. Keep in mind that you can swap Drain and Source. This can be a real PITA if the layout uses inline leads instead of a triangular pattern.

There are lots of other specs like gain and leakage, don't lose any sleep over them, just get some parts to try and be done with it. Beware of fake parts from China, but they might actually work.

With a push-pull circuit the same triode produces almost no even order harmonic distortion at all, as usual.
That is true, until you start drawing grid current on the input side like when you overdrive a power amp. Depending on the source impedance, odd harmonics will drop out.

See experiment with TDA2040 here:

Amplifier Discussion / Re: Z-MATCHER 100W IMPEDANCE MATCHER
« on: September 16, 2019, 04:42:44 PM »
I bought a couple of the Weber 50W (component level) auto-transformers for use on the output of a 50W @ 4 Ohm power amp. Worked OK didn't seem to change the sound much when driving an 8 or 16 Ohm load. Last I checked, the 50W was discontinued leaving only the 100W version. I ordered a pair of those, they weren't much bigger than the 50W units. Fit the same chassis holes.

I'm not sure the Wal-Mart units do the same thing, they look more like constant Voltage distribution transformers with big L-pads.

Amplifier Discussion / Re: Zener vs IC regulator for the preamp section
« on: August 30, 2019, 07:11:44 PM »
If you want organic and touch sensitive, you don't want opamps and regulated power supplies. Those things are mutually exclusive.  Even a tube preamp can be cold and lifeless with a regulated or isolated power supply. To keep the hum out of a preamp, start with a well laid out ground and adequate filtering.

The Newcomer's Forum / Re: Fender Stage Lead Issues
« on: July 07, 2019, 01:40:02 AM »
If the amp has an effects loop, connect a cable from Send to Return and see if that corrects the problem. Other than that, a good guess would be a fractured solder connection between pots, jacks and switches on the front panel and the PCB that holds them.

Preamps and Effects / Re: Rechargeable 9v batteries for fx pedals
« on: July 07, 2019, 01:29:11 AM »
How old are the batteries ? Are they Ni-Cad or some other chemistry ?

Some rechargeables can be problematic. They might not charge up to rated Voltage or hold rated maH. Many times one cell will be weak and may take on a reverse charge as the battery discharges.

Preamps and Effects / Re: implementing presets
« on: May 05, 2019, 02:39:50 AM »
Programmable pots open up a lot of possibilities. You could even make some of your stompboxes programmable. Beware that many programmable pots don't like signals that go beyond the rails so you need +/- supplies when the pot has an AC signal on it.

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