Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: TIMBO on November 08, 2015, 12:40:25 am
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Hi guys, another side project.
These amp weren't TOOOOOOOOOOOOOO good. http://www.ozvalveamps.org/savage.htm (http://www.ozvalveamps.org/savage.htm)
So I thought I'd try the right the wrongs.
First of all the power amp only lasted as long as it took the everything to warn up and BANG, DEAD SAVAGE.
I have sourced a power amp kit that has done a great job in replacing the old (beyond help)
My next problem is the preamp.
I have circled the two 1k/1w resistors, these were well cooked, one was open and the other measured 1.1k, these are connected in parallel. The value of the resistors are a guess as the markings have burnt off. I can make out the third colour as RED, but the other two look to be BLACK, but that doesn't work, so I can assume that the first is BROWN and the second is BLACK, being 1k.
Replacing these resistors with new 1w resistors have them cooking themselves pretty well.
With the way is was with the chard resistors, and the SUSTAIN on, the LED would glow and decay with the decaying of the played note.
Also when removing some parts the Q3 broke off as it was affected by the heat of the cooked resistors, I removed another transistor (same) from the dead power amp.
With the replacement transistor and resistors I don't get that decay in the brightness of the LED, it just stays bright.
I have changed the power supply for the new power amp that has +- 40v rails, where the old had a single rail that I measured to be about 70v. The preamp at "A" was connected to the 70 volts off the rectifier.From what I know this was the major problem to start with.
I have used the +40v side to power the preamp at "A" and works fine.
I replaced the parallel 1k resistors with a 560r/5w and still this gets very hot
I do NOT understand the idea behind the LDR/LED of the circuit, SO please have a stab. Thanks
(http://i761.photobucket.com/albums/xx254/Timbo-08/005%202_3.jpg) (http://s761.photobucket.com/user/Timbo-08/media/005%202_3.jpg.html)
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Your schematic is keeping me up at nites! what is the switch "labeled" on the faceplate that connects to Q3?
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Hey shooter, I took me awhile to work that switch out. Its on the back of the SUSTAIN pot.
When the switch is engaged Q3 functions. When a note is struck the LED brightens and then decays with the decaying of the struck note.
The LDR measured about 7k dark and light about 1k, this must be enough to pass more signal to create the SUSTAIN. :think1:
The original PN3568 was working as described but for what ever reason it was cooking the two 1k resistors and luck would have it, that transistor broke off as it was next to the cooked resistors and was affected by the heat.
I scrounged another from the power amp board, the problem was this time the LED would NOT decay as described and the new resistors got extremely hot. :dontknow:
I found a replacement transistor BC639 that was a good match and took a stab it would work, other than a different pinout it works a treat and the resistors don't even get warn.
I can only say that the original circuit was running on more voltage than the spec for these components and has FRIED them.
I am now in the process of putting it all back together. :icon_biggrin:
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it works a treat
glad to hear you got it. I was leaning toward Q3 or the steering diodes, I'm still pondering Q4's purpose in life :dontknow:
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:dontknow:
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I'm happy to say that I think I have saved this amp from being landfill.
I have freshened up a some other parts, pots, Ecaps and she's sounding good.
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Nice! love the color scheme
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Hi guys, Put some finishing touches on the cab and added a vent to help things stay cool.