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Kustom III-Lead Head 1975, PC Board 5065 [message #26104]

Thu, 11 May 2017 18:15

mathias61
Messages: 24
Registered: February 2016
Location: belgium

Junior Member

Hi, In a local shop I've been trying a Kustom III-Lead amp, but it's not working with a cabinet of the usual 4 Ohms. It's well operating with a cabinet of 16 Ohms, and that's a real pity ! The seller of the shop can not figure out what has been happened before with the amp. When opening the head, you could see there is a changement on the PC Board 5065, now there are two Motorola 2N2905A-CG9728 welded with three legs and two 7 0011 RCA-H7710 also welded with three legs. Does anybody knows what are the originals to get back the speaker output impedance of 4 Ohms, suitable for the original Cabinet please ?
Thanks anyway, mathias61

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26105 is a reply to message #26104]

Thu, 11 May 2017 23:30

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

The transistors that you describe are probably replacements for the original driver transistors that have not been available since the 1980's. I would expect that one or more of the output transistors have been replaced as well.

Transistor amps will work with a wide range of speaker impedances, the difference will be in the power that they can generate at the different load levels.

If the amp has been repaired correctly, then it should work just fine with a 4 ohm load instead of the 16 ohm load that it has now. The different transistors have nothing to do with the output load value.

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26106 is a reply to message #26104]

Fri, 12 May 2017 06:06

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

Also note that those driver Transistors need to have slip on Star type heat sinks on them or they will overheat and short out, in this light it's good that the amp is being played on a 16 ohm load as the output stage will run 3 times cooler than normal!

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26110 is a reply to message #26106]

Fri, 12 May 2017 15:08

mathias61
Messages: 24
Registered: February 2016
Location: belgium

Junior Member

Hi, thanks to Chicagobill and Stevem, So if I understand it good, there is no problem for blowing the transfo, connecting a speaker output of the amp at 16 Ohms impedance to a cabinet having an impedance of 4 Ohms ? The only thing will be having less power ? Greetings, mathias61

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26112 is a reply to message #26104]

Fri, 12 May 2017 20:44

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

The amp should work with any load from 16 ohms down to 4 ohms. The amp will probably put out the most power at the 4 ohm load level.

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26113 is a reply to message #26104]

Sat, 13 May 2017 06:05

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

There is no output Transformer to burn up in a Solid state amp like yours, output Transformers are only used in tube amps.
Your Kustom wants to see a 4 ohm load to output its max power and run at the temperature it was designed to.
If you run it at on al lower impeadance ( like 2 ohms ) it will then run twice as hot for any given volume level you play at and will shut itself down with the temperature reaches 185 degrees F.
Your amp will only output some 28 watts of clean power not over 100 watts when run on a 16 ohm load, so it will run cooler then normal which is fine if having only 28 watts is loud enough for your needs.

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Re: Kustom III-Lead Head 1975, PC Board 5065 [message #26114 is a reply to message #26113]

Sat, 13 May 2017 07:36

mathias61
Messages: 24
Registered: February 2016
Location: belgium

Junior Member

To Chicagobill an Stevem, Both of you do have a great knowledge and I am very respectfully !I know I still have much to learn in this matter... Thanks again

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