Hartke 3500 Bad Tube Preamp Output

[misterc57]

Picked up a second Hartke 3500 that had no output. Found a bad IC103 (4558), replaced that and the amp now has output.

The solid state portion of the preamp sounds great. The tube portion sounds terrible, every note has a farting sound.

I have tried swapping the 12AX7 tube with no changes to the issue.

The schematic I am using comes close but most of the component references are off. Examples;

My amp has one side of R109 (1M) connected to IC101 pin 5 and the other side connected to IC101 pin 3
My amp has C101 (474J 100N) connected to the 12AX7 pin 6 plate.

12AX7 voltages are all nominal except the following (I had to read the V with the tube removed).

plate pin 1, 240 VDC
plate pin 6, 238 VDC

heater pin 9, 16 VDC

I cannot find any AC or DC readings on the other heater pins. However the tubes does light up when operating.


I am looking for suggestions and also the correct schematic.

Thank you, Mark

hartke_ha3500_power_amplifier_sch (1).pdf

[x-pro]

here's your diagram(post #2)
https://music-electronics-forum.com/...mb-4005109600a

[Jon Snell]

Check the cathode voltage , pin 3 & pin 8.
One Hartke is the same as another. Chinesium budget stuff but, like Peavey, they work.

[misterc57]

Check the cathode voltage , pin 3 & pin 8.
One Hartke is the same as another. Chinesium budget stuff but, like Peavey, they work.

pin 3 = 0.93 mV
pin 8 = 0.92 mV

Looks like I need to have the tube inserted to see cathode V

R107 (2K) connected to pin 8 = 0.9 VDC
R117 (1.5K) connected to pin 3 = 52 VDC .... YIKES!

[g1]

Yes, tube must be installed to get any meaningful cathode or plate voltages.

[Jon Snell]

Time to get your oscilloscope out now.
With cathode voltages near what I would expect, time to do some real fault finding.
Start from the send and return jacks, if the distortion is there, work backwards through IC403, (you can ignore any tone shaping controls at this time as the send point is before the graphic).

[misterc57]

R107 (2K) connected to pin 8 = 0.9 VDC
R117 (1.5K) connected to pin 3 = 52 VDC

52 VDC at cathode pin 3 is okay?

[misterc57]

here's your diagram(post #2)
https://music-electronics-forum.com/...mb-4005109600a

Seems to be the right schematic. Thank you

[misterc57]

Proper schematic attached.

[misterc57]

Time to get your oscilloscope out now.
With cathode voltages near what I would expect, time to do some real fault finding.
Start from the send and return jacks, if the distortion is there, work backwards through IC403, (you can ignore any tone shaping controls at this time as the send point is before the graphic).

On the scope will I be able to see the distortion with a constant signal being injected?

Thank you

[misterc57]

Progress I think.

Working backwards I saw at IC101A; a clean signal IN (pin 3) and distorted signal OUT (pin 1). At pin 3 was when the signal stopped looking similar to the amp output (distorted) signal and looked clean.

I'm thinking I need to replace IC 101 (2068DD) which I will have to order.

[Jon Snell]

Good detective work.
Exactly how to fault find.

[x-pro]

Думаю, мне нужно заменить ic 101 (2068dd), который мне придется заказать.

rca/ba/jrc/njm 4558/4556/4560/4580/ne5532 !

[misterc57]

rca/ba/jrc/njm 4558/4556/4560/4580/ne5532 !

4558 would work here? I have plenty of those.

[misterc57]

Scope pics. both show amp output signal on top and IC101 pin 1 signal on bottom.

Left pic is solid state, right pic is tube