Concert lead question

[Joe]

I just found an old Concert Lead head which sounds great. I opened it, and it seems to have no obvious repairs. However, there are only 4 output transistors
on the back of the amp in two separate heat sink arrays (2 on each one). The back photo of the Concert
lead on this site shows it with 5 output transistors
along one long heat sink array. Was mine altered or did they come this way? Mine had a sticker inside indicating it was tested on April 5, 1972. Any info greatly appreciated!

[Joe]

: I just found an old Concert Lead head which sounds great. I opened it, and it seems to have no obvious repairs. However, there are only 4 output transistors
: on the back of the amp in two separate heat sink arrays (2 on each one). The back photo of the Concert
: lead on this site shows it with 5 output transistors
: along one long heat sink array. Was mine altered or did they come this way? Mine had a sticker inside indicating it was tested on April 5, 1972. Any info greatly appreciated!

All Concert (leads, basses, etc) have four *output* transistors.  The mystery fifth transistor is actually the driver... which drives a transformer primary... for the other four which are driven off of individual secondary windings.

All five transistors are 2N3055's in the to3 "power case" style, and the driver... like the outputs, appears in the schematic to be off the PC board.

This doesn't mean that it *has* to be physically located with the four outputs, although it probably draws enough current to require some heatsinking.

I'm not specifically aware, but there could have been some production change that mounted it elsewhere (my concert lead has it on back with the other four).  

On the power amplifier printed circuit board, find the lug terminals numbered 10 and 11.  Follow the wires off of these lugs to find where the "missing" transistor is hiding... they both lead to it.

Hope this helps!

[Joe]

: : I just found an old Concert Lead head which sounds great. I opened it, and it seems to have no obvious repairs. However, there are only 4 output transistors
: : on the back of the amp in two separate heat sink arrays (2 on each one). The back photo of the Concert
: : lead on this site shows it with 5 output transistors
: : along one long heat sink array. Was mine altered or did they come this way? Mine had a sticker inside indicating it was tested on April 5, 1972. Any info greatly appreciated!

: All Concert (leads, basses, etc) have four *output* transistors.  The mystery fifth transistor is actually the driver... which drives a transformer primary... for the other four which are driven off of individual secondary windings.

: All five transistors are 2N3055's in the to3 "power case" style, and the driver... like the outputs, appears in the schematic to be off the PC board.

: This doesn't mean that it *has* to be physically located with the four outputs, although it probably draws enough current to require some heatsinking.

: I'm not specifically aware, but there could have been some production change that mounted it elsewhere (my concert lead has it on back with the other four).  

: On the power amplifier printed circuit board, find the lug terminals numbered 10 and 11.  Follow the wires off of these lugs to find where the "missing" transistor is hiding... they both lead to it.

: Hope this helps!

Hey man-thanks for the info! I DID locate the fifth transistor mounted inside the chassis. This amp seems to be old; it has the distort knob rather than the distort switch. Maybe they made a few production changes early on. Also- any clue
why this head sounds so great on bass through a bass cab? There doesnt appear to be any huge transformers or caps, yet it pumps out amazing tone and bottom end. Whats up with that? Also, does anyone know the official power rating at 8 ohms, 4 ohms, etc??Thanks...

[Joe]

: Hey man-thanks for the info! I DID locate the fifth transistor mounted inside the chassis. This amp seems to be old; it has the distort knob rather than the distort switch. Maybe they made a few production changes early on. Also- any clue
: why this head sounds so great on bass through a bass cab? There doesnt appear to be any huge transformers or caps, yet it pumps out amazing tone and bottom end. Whats up with that? Also, does anyone know the official power rating at 8 ohms, 4 ohms, etc??Thanks...

You must have a footswitch for your head.  Without the footswitch, at least on the earlier models, the "distort" knob essentially did nothing, and the "boost" was activated.  The distort and boost switch of the later models, a single channel concert lead released in the mid 70's, simply allowed you to turn these two features on/off with a front panel switch... and so those without a footswitch weren't "stuck".

I don't know the official power rating... It's probably close to 150 watts into 4 ohms... which would make it somewhere between 80-100 into 8.

The older two channel lead... and it's same year twin for bass were almost identical.  Only one resistor changed in the power supply to make up for the lack of reverb channel.  

I'm not sure what size transformer/caps you're expecting in the power supply, but I consider it pretty hefty.  Plus on the older models, there are actually 3 secondary "power" supplies... One of these is a dedicated winding, rectifier and cap (3450mfd) to provide 70 volts only to the output transistor stages.  The later model "reverted" to a single secondary center-tapped winding power supply, providing +/- 34 volts at the two primary filter caps, and it was used to derive voltages for the entire amp.