Hartke 3500 BJT

[Jazz P Bass]

On a Hartke 3500 , with the 2SC5200/2SA1943 transistors, does anyone know what the AC current draw from the mains is.
No signal, no load.
I have one on the bench that is pulling 3.5 amps.
I disconnected the +- 80 volts to the power amp. Removed the 12AX&7 tube.
The only thing left is 11 opamps & the + - 15 volt regulators.
+ - 15 volts is steady. No ripple. No diode shorts. No hot opamps
I guess it's time to start yanking opamps.

[Enzo]

I know that 400 watts at idle is silly.

The draw remains with the power connector removed from the power amp? OK.

If it were an op amp trying to draw 400 watts from those 15v rails, they wouold not be sitting at 15v any longer. And 400 watts should make an op amp IC pretty toasty, and your fingertip should be able to find the hot one.

Your power transformer secondaries connect to the main board. Unplug them ALL. If the 80 has a separate bridge rectifier (I don't rememebr) pull the AC wires off it too. Now your transformer has ONLY the primary wiring connected. Apply mains power. Still drawing 400 watts? That points to a bad transformer.

Falls back to normal? Then start reconnecting things one at a time.

[Jazz P Bass]

Hartke HA 3500 power transformer

Thanks Enzo.
As you were typing I did just that.
The power transformer is the culprit.
Kind of silly that I was looking at everything incorrectly.
Check the power first, right?
A very humbling repair.

[Enzo]

As long as you get where you are going, that is the bottom line.

Once narrowed down to the transformer, just make sure there is not a bad cap across the mains.

[Jazz P Bass]

And The Answer Is......

Just an update on the Hartke HA 3500 pulling 400 watts.
I ordered a new transformer from Samson.
On testing, the transformer by itself drew 1 watt.
Fully loaded, amp at idle, she drew 52 watts.
This final figure depends greatly on the output idle current.
Tested at 1500Hz, 6.56 volt rms output into 8 ohms, 100 watt draw.
Not too shabby.
Cheers!

[Enzo]

Did you set the idle current?

A cheap and dirty way to set the bias is by monitoring your mains. Set the control to the coldest setting, and watch mains draw as you slowly advance the control. At first there will be a steady low draw. At some point you will see the draw start to climb with the adjustment. FInd that spot and back off just a hair. Your amp is now biased.

[TieDye]

Push pull amp bias

For setting bias on push pull amps, if you have a oscilloscope and a harmonic distortion analyzer: load the amp, provide 1Khz input. set output for 1Vp-p and watch the distortion output with oscilloscope. should show only noise component. Slowly adjust bias control until you see a small notch in the noise. Then back off adjustment to point where noise goes away.
Some folks in high end audio world claim adjusting past point where notch is lost gives warmer sound (amp will produce more heat, even at low volume levels). For folks who repeatedly blow up amp by over driving i have had limited success by leaving a notch. Notch sound may be noticeable to trained ear.