Hartke 3500 Bad Tube Preamp Output

[g1]

It will work, though the 2068 is described as 'low noise' where the 4558 is 'general purpose'. I doubt you would hear the difference in this circuit.

[x-pro]

Hartke 3500.
It's a bass guitar amp. There is an order of magnitude more noise from the guitar and from the tube than in the chip.

[misterc57]

These are what I have available

4558

TL081

UA741

TL082

TL072

NE5532P

[Jon Snell]

is probably the quietest.

[misterc57]

Replaced IC101 with the TL072. This area keeps being problematic. I no longer have any tube driven output. Strong signal going to pin 3 of IC101 and a very weak signal at pin 1. I would expect pin 1 signal to be amplified and strong/louder than pin 3. ???

IC101 has +/- 18 VDC at pins 8 and 4.

Do not think the board can take another IC swap at that spot. Many of the connections have lost some of the via. I'm about ready to consider this a solid-state "only" preamp section.

[g1]

IC101A is configured as a unity gain buffer, so you should have the same signal at pin1 as pin3. Check that pin1 and 2 are shorted together, check DC volts at pins 1,2, and 3.

[misterc57]

IC101A is configured as a unity gain buffer, so you should have the same signal at pin1 as pin3. Check that pin1 and 2 are shorted together, check DC volts at pins 1,2, and 3.

Pins 1 and 2 are shorted.

IC101 PIN 1 = -16.6 VDC
IC101 PIN 2 = -16.6 VDC
IC101 PIN 3 = -16.4 VDC
IC101 PIN 4 = -18 VDC

[x-pro]

This is not good. Is there a 1M resistor on pin 3?
And is there +18V supply on pin 8 ?

[misterc57]

This is not good. Is there a 1M resistor on pin 3?
And is there +18V supply on pin 8 ?

Yes, pin 3 is 1M to ground through R109

Yes, pin 8 is +18 VDC

[x-pro]

Unsolder IC101 and re-measure the voltage at the right lead of capacitor C104.
At +/-18V supply, with capacitor C104, pin 3 should be zero relative to common.
You can just unsolder C104. Keep the chip, but you need to measure at pin 3.

[misterc57]

Removed C104. Still have the same -16 VDC readings on pins 1,2,3.

Same reading on the chip side of R109.

[Helmholtz]

What voltage do you measure directly at and between the leads of R109?

R109 might be open or have a bad connection.

[misterc57]

What voltage do you measure directly at and between the leads of R109?

R109 might be open or have a bad connection.

I had -16.3 on the chip side of R109 and just a few mV on the ground side.

16.3 V across R109

[Helmholtz]

R109 could still be open or have a missing ground connection.
What resistance do measure from pin 3 to ground?

[x-pro]

May be 072 is bad?