HARTKE Systems Model 3500

hartke 3500

First Impressions

When it first landed on my bench, this Hartke wasn't a pretty sight. Not only was the amplifier dead, all pots at the front were damaged. Some even had their shafts broken off.

Upon removing the top cover, I was relieved to find the innards intact. On closer inspection, there were no burnt marks on the preamp and amplifier board, which is encouraging.

Power Supply

The power supply filter caps for the amplifier section are two pieces of 4700uF/100V SAMHWA.

Their outputs measured at +/- 88Vdc.

Generally, with such high rail voltages, we are looking at an amplifier output of about 320W into 8Ω.

I thought it was a bit odd too that only four pairs of power transistors were used. I decided to look up the specs of this model 3500.

hartke 3500 capacitors

Dynamic Headroom

According to their manual, the 3500 is rated for 240W/8Ω and 350W/4Ω. Now I begin to see why the psu is such. It has to do with what is known as "dynamic headroom".

For normal use, the Hartke 3500 is guaranteed up to 240W/8Ω. However, it is capable of up to 320W in very short burst. The catch phrase is "short burst". If one attempts to play with 320W continuously, the amplifier will first distort, then followed by failure.

An amplifier with a power supply of this nature is said to have a 1.5dB dynamic headroom. The 1.5dB refers to the 120W in reserve power (120W is half of 240W, therefore it's 1.5dB).

In practical terms, what all these technical jargon means is a guitarist can play as loud as 240W continuously without the amplifier failing. Should he decide to strike a string hard, he can expect it to sound louder because there's a "reserve" of 120W.

However, he must be careful in using this reserve. Playing at "reserve" level all the time will lead to amplifier failure.

Next > Re-Working the amp module

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