Ask Bob Heil - interview from June 29th, 2008
Bob Heil - an American sound and radio engineer most well known for creating the template for modern rock sound systems. He founded the company Heil Sound in 1966, which went on to create unique touring sound systems for many famous bands.
He was using PL400 and PL700 amps for his sound PA setups too:
Heil Sound flyer
He also invented "Powered Talk Box" device. David Gilmour of Pink Floyd used the talk box on "Pigs (Three Different Ones)" from their 1977 Animals album, and on "Keep Talking" from 1994's The Division Bell.
[...]
Q:
With regards to the Phase Linear amps. Occasionally I have other engineers come here to work. I use, a Phase linear 400 amp for my main monitors. An engineer once told me that they nicknamed them "flame linear" because when they blow, they take out the spkrs too. My fingers are crossed, but has anyone ever heard of this before?
A (Bob Heil):
Your friends are correct. The Phase Linear amplifiers sounded super and produced a lot of output. They sounded exceptional because they had NO protection circuit. So many of the early high powered amplifiers (especially first generation) had a simple 'crowbar' protection circuit across the output and when the amplifiers were right at clipping, that dang circuit would produced a 'crunchy' or 'clicky' sound to me just as the crowbar was about to reduce the power OR - if way too much output, would turn the amplifier off. I always hated that but what could you do? Bob Carver just removed it and you just had to be very careful not to run the baby any more than about + 4dB into it's calibrated VU meter.
HOWEVER - if you continued to run it higher than that, the output devices would short placing ALL of it's +50 volts DC on your speaker line. Go back and read the story that Bill Mueller told about a Humble Pie show that this happened and we had an additional fireworks show from one of our left side speakers! The crowd thought that was cool....we didn't as we lost an amplifier and a pair of JBL 2204 JBL speakers.
One thing you can do, Chance is to measure the current of your speaker line when you are in the 'hump' mode....as loud as you would ever crank things. Place an in line fuse in that speaker lead and fuse it about 1/2 amp UNDER that limit. This should help you.
We also built a 'manifold' from fiberglass that fit perfectly over the entire heat sink. This manifold had a 5" square opening that expanded out to the edges of the heat sink. The manifold was pop riveted onto the Phase Linear and a 5" box fan was mounted on that opening. This REALLY helped to tame the Phase Linears a lot.
Piece of trivia.....go back early in this session. There is a picture of the original purple Talk Box. That was made from the mold of this manifold. When Joe, Krinkle (Richard Kruzekamp) came up with this consumer Talk Box, I need an enclosure - FAST, so my fiberglass guy took that manifold, closed up the 5" opening. Made a 1.5" hole for the screw on driver and LA LA - we had the Heil Talk Box - at least the first 50. I then had our metal shop build the metal enclosure that has been used over these many decades.
[...]
Source:
Ask Bob Heil