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turbofeedus wrote:Just found another 1330-1 on ebay and bought it, hopefully this one has a working inductor that I can measure.
I'd also like to measure the trimpot, hopefully it hasn't been messed with

Maybe you could cut open the bad inductor and show what's inside...whether it's a coil of wire, or a coil with some other component in series. Or maybe even not a coil at all.

I've thought about getting another opamp 1330 to do this with to try and verify for my version schematic, they're so cheap! I'd have thought they had more mojo value what with being vintage and having a reputation for Jimmy Page having once plugged into one.

soulsonic wrote:

turbofeedus wrote:Just found another 1330-1 on ebay and bought it, hopefully this one has a working inductor that I can measure.
I'd also like to measure the trimpot, hopefully it hasn't been messed with

Maybe you could cut open the bad inductor and show what's inside...whether it's a coil of wire, or a coil with some other component in series. Or maybe even not a coil at all.

I've thought about getting another opamp 1330 to do this with to try and verify for my version schematic, they're so cheap! I'd have thought they had more mojo value what with being vintage and having a reputation for Jimmy Page having once plugged into one.

All it takes is one JHS video apparently, not that I want to benefit off anything that asswipe does.

Alright, second unit arrived today, with a working inductor.
Inductor: 963.9mH, 411.8Ω
So, basically one these at 1H.

Also got measurements on the transistors:
Q1 (E320):
- D: 6.124V
- S: 0.569V
- G: 0V

Q2 (2N2484):
- E: 4.701V
- B: 5.320V
- C: 9.40V

Q3 (2N4058):
- E: 8.87V
- B: 8.14V
- C: 5.311V

And the trimpot: 3.573KΩ

Updated schematic: Pics of second unit:

turbofeedus wrote: Also got measurements on the transistors:
Q1 (E320):
- D: 6.124V
- S: 0.569V
- G: 0V

Using these voltage readings, can we figure a replacement for that jfet ?

I recalculated the JFet stage and was surprised that I did not get the right values, so I took a closer look at the PCB picture.
The source resistance is not 6.2kOhms as shown in the schematic but 620Ohms, with this value the calculated values are correct now.
The measured values result in a forward transconductance of about 1.6 Milliamps/Volt @ 1Milliamp draincurrent for the JFET.
The 2N4560 would fit as replacement type.

ppluis0 wrote:Using these voltage readings, can we figure a replacement for that jfet ?

It's configured as a source follower (unity gain buffer) almost any N channel FET would work.
Try what you have in stock.

Why the hell is the edit icon not always displayed?

The 2n5460 would fit as a replacement type with the electrical values but not from the pin out.

It's configured as a source follower (unity gain buffer) almost any N channel FET would work.

Take a closer look, it is not a source follower but a source circuit of the trimming potentiometer is the drain resistor,

Manfred wrote:I recalculated the JFet stage and was surprised that I did not get the right values, so I took a closer look at the PCB picture.
The source resistance is not 6.2kOhms as shown in the schematic but 620Ohms, with this value the calculated values are correct now.
The measured values result in a forward transconductance of about 1.6 Milliamps/Volt @ 1Milliamp draincurrent for the JFET.
The 2N4560 would fit as replacement type.

The attachment BarcusSourceResistor.jpg is no longer available

Good catch Manfred. Updated schematic attached.

The 6.8k res on the treb pot, isn't it a 680 Ohm instead?
Also the 220k res, in the last pic looks like a 120k

Hi turbofeedus,

What taper of potentiometers are using this preamps ?

ppluis0 wrote:Hi turbofeedus,

What taper of potentiometers are using this preamps ?

Great question, not sure why I didn't measure before, but I just opened it back up.

The 2Meg "sensitivity" pot is Audio(Log), the rest are linear.

Manfred wrote:

It's configured as a source follower (unity gain buffer) almost any N channel FET would work.

Take a closer look, it is not a source follower but a source circuit of the trimming potentiometer is the drain resistor,

Doh, you're right. I shouldn't go posting after a few Scotches.

deltafred wrote:

Manfred wrote:

It's configured as a source follower (unity gain buffer) almost any N channel FET would work.

Take a closer look, it is not a source follower but a source circuit of the trimming potentiometer is the drain resistor,

Doh, you're right. I shouldn't go posting after a few Scotches.

I'll take partial blame, I drew the schematic kinda weird.

turbofeedus wrote:I'll take partial blame, I drew the schematic kinda weird.

Nah, I take full responsibility.

I should know better than to post (or bid on Ebay, but that's another story) when under the influence!

Intripped wrote:The 6.8k res on the treb pot, isn't it a 680 Ohm instead?
Also the 220k res, in the last pic looks like a 120k

Sorry I missed your comment, it absolutely is 680 ohm, not 6.8K. Good catch.
The 220K is definitely 220K however.
Updated schematic attached.

soulsonic wrote:Found another big discrepancy with that other schematic. All the pots are 100K linear taper. That's measured with them desoldered from the board.
The values in that other schematic are something like what you read if they're still on the board, depending how the controls are set.

Sorry for the late reply. I'll admit I didn't desolder one from the board, but I did test each one and their values were consistent regardless of pot position. I also found this, which is the exact part, same number printed. Note that the pots are different style, the one's on the 1330-1 are the small, sealed AB type with the 1/8 shaft.

Hi turbofeedus,

Is not neccesary to extract those potentiometrs from the board.
To find what taper have each one simply put all the knobs at 50% of rotation and measure the resistance from the wiper to each other end pin.
If the potentiometers were linear, then you will see 25K from center to each end. (1Mohm at the sens pot)
If were audio taper (I assume that the volume pot can be of this type), then you have 5K at one end and 45K at the other, and in the case of sens pot will be 200K and 1,8Mohm respectively.

Of course measurements will be done with the pre-amp de-energized

Cheers,
Jose

Here the adapted layout according to the last circuit change: SprintLayout6.0 - file:

ppluis0 wrote:Hi turbofeedus,

Is not neccesary to extract those potentiometrs from the board.
To find what taper have each one simply put all the knobs at 50% of rotation and measure the resistance from the wiper to each other end pin.
If the potentiometers were linear, then you will see 25K from center to each end. (1Mohm at the sens pot)
If were audio taper (I assume that the volume pot can be of this type), then you have 5K at one end and 45K at the other, and in the case of sens pot will be 200K and 1,8Mohm respectively.

Of course measurements will be done with the pre-amp de-energized

Cheers,
Jose

Thanks, this was tested initially, I'm confident the pots on mine are in fact 50K linear, excluding the 2M input pot.