Son of Ampzilla - Rebuild & Restoration

Hello,
Another question...what is working temperature of this amp? Let say, that its powered up for 4h and have speaker load of 8 ohms and without any input signal? Im asking because both channels warming up same...aprox. 50-60 degree celsious (120-140F). Output DC is 12mV on both channels.


Mine runs hot. Haven't measured but your temps don't seem unreasonable. I have to hook mine up and double check.
 
Sorry for late reply...
@ny pheonix
Im measuring on left/right heatsink, but after 3-4h of beeing switched on, doesn't matter where to measure as all case have the same temperature...side heatsinks, top cover,... Anyway, i checked again bias current and dc offset and reset again and now i have 40C degree (104F) which i think is OK.
 
Resurrecting an old thread.

I had started this thread on rebuild/restoration of one of the GAS Ampzilla Son amplifiers a few(!) years ago when I got interrupted by change at work, life changes and other projects. After getting tired of seeing the poor "in-process" Ampzilla Son waiting patiently on the “will-get-to-it-one-day” project rack, I finally decided to complete it.

The amplifier is now almost complete and is just waiting on the parts to finish remaining work on the VU meters & LED lamps.

With all the new components already in place, DC offset and Bias reset - the amplifier really sounds exceptional. I strongly recommend restoring these units - they are definitely keepers :thumbsup:

I will be posting some pictures shortly.

Rgds,

Mayank
 
I hope you replaced that melted EVOX cap on the upper left of the bottom pic in post #23.

Good eye!

Yes, even though the EVOX 1uF/100V capacitors tested good on both the boards, I replaced them with new Kemet MMK series polyester capacitors for cheap insurance along with the two 30 ohm/2W resistors which apparently generated the damaging heat.

Rgds
Mayank
 
Here is how the rear of the GAS Ampzilla Son looks now:

IMG_0584.JPG

The original design did not have a power switch on the front panel (wonder why :idea:). I had planned on replacing the power cord with an IEC socket. In the place of the original fuse & mains cord, I installed a N5L8 Sodial 10A/250V IEC Power Socket which came with a fuse and neon lighted switch.

The speaker terminals were replaced with two pairs of nice audio grade copper binding posts which I had as leftovers from a tube amp project. The corroded input jacks were replaced by gold plated chassis mount RCA sockets. Important: The RCA jacks need to be grounded to the chassis.

A significant improvement from its initial condition (see post #1)

Rgds
Mayank
 
I noticed an occasional "chirp" from one of the channels :(

Using a signal generator and scope to trace the issue, I found one transistor in the two input differential pairs of (now obsolete) Motorola transistors MPS8099/MPS8599 had been replaced by an incorrect substitute 2N4126.

I need 2 matched pairs of MPS8099/MPS8599 to replace the input differential pairs. Any leads on where I can get these?

Rgds
Mayank

PS The schematic has these as 2N5401/2N5551 which are still available at Mouser, but I believe the MPS pairs were the preferred devices of GAS.
 
I noticed an occasional "chirp" from one of the channels :(

Using a signal generator and scope to trace the issue, I found one transistor in the two input differential pairs of (now obsolete) Motorola transistors MPS8099/MPS8599 had been replaced by an incorrect substitute 2N4126.

I need 2 matched pairs of MPS8099/MPS8599 to replace the input differential pairs. Any leads on where I can get these?

Rgds
Mayank

PS The schematic has these as 2N5401/2N5551 which are still available at Mouser, but I believe the MPS pairs were the preferred devices of GAS.
Did you check with woodyzine. I think that's the name. He has all of Jim Borngiorno's parts and supplies. He lists on eBay.
 
Did you check with woodyzine. I think that's the name. He has all of Jim Borngiorno's parts and supplies. He lists on eBay.

Thanks for the lead. I've just sent him a message.
 
Update:

These 25+ year old MPS8099/MPS8599 pair used by GAS are no longer manufactured. I was informed by Motorola that they discontinued them in 2003. So if you find any of these listed (cheap) on eBay, they are almost certainly counterfeit - please do NOT buy.

OnSemi lists a modern direct replacement for these transistors as MPSA06/MPSA56. Same pin out and far better matched gain tolerance. This has been verified on a couple of threads at SAEtalk and at DIYAudio. I'm re-stating it here in case someone else is looking to replace these in his/her unit as well.

I've placed an order from Mouser (for the ones made by Central & Rectron).

Rgds
Mayank
 
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It's late ... and tomorrow is a working day, but - I've finally finished the rebuild :banana:

The last zip tie is in place neatening the cables between the boards. A few quick pictures from the test bench

IMG_0214.JPG IMG_0215.JPG

More pictures tomorrow along with a complete list of upgrades. I'll be moving this beauty to our main living room setup replacing an Adcom-555.

Rgds
Mayank
 
Successfully finished the complete rebuild of this great amplifier - and it sounds absolutely wonderful!

All output transistors, drivers, pre-drivers and input differential pairs were replaced with modern devices. All electrolytic capacitors (including in power supply) replaced, other components 100% checked and replaced if out of spec. LED bulbs installed in place of the heat generating incandescent lamps, VU meters rebuilt; speaker terminals, RCA input jacks, IEC power socket with on/off switch, new rubber feet, fresh coat of hammered finish paint ... and the girl is looking as good she was when she first stepped out of the GAS factory.

Enjoy the pictures ... with the learnings under my belt, I'm now starting a similar rebuild restoration of its twin sister.

Rgds
Mayank

IMG_0604.JPG IMG_0603.JPG IMG_0605.JPG IMG_0608.JPG IMG_0610.JPG IMG_0609.JPG
 
It's late ... and tomorrow is a working day, but - I've finally finished the rebuild :banana:

The last zip tie is in place neatening the cables between the boards. A few quick pictures from the test bench

View attachment 1497164 View attachment 1497165

More pictures tomorrow along with a complete list of upgrades. I'll be moving this beauty to our main living room setup replacing an Adcom-555.

Rgds
Mayank
Looks like you reused old trim pots.

Why not replace them as part of a full restoration like this?
 
Looks like you reused old trim pots.
Why not replace them as part of a full restoration like this?

Good eye!
I actually had ordered multi-turn trim pots from Mouser with the objective of replacing the existing single turn ones (1K for DC Offset, 500 ohms for Bias and VI limiters).
Unfortunately the existing holes spacing was very different and I needed to drill new holes in the PCB.
Fine carbide tip drill bits are on order. In the meantime I am enjoying listening to this very fine amplifier - I am only allowed to put a "black box" in the living room which is all "covered" and has "no wires sticking out" :D

Rgds
Mayank
 
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Good eye!
I actually had ordered multi-turn trim pots from Mouser with the objective of replacing the existing single turn ones (1K for DC Offset, 500 ohms for Bias and VI limiters).
Unfortunately the existing holes spacing was very different and I needed to drill new holes in the PCB.
Fine carbide tip drill bits are on order. In the meantime I am enjoying listening to this very fine amplifier - I am only allowed to put a "black box" in the living room which is all "covered" and has "no wires sticking out" :D

Rgds
Mayank

I assume you ordered bourns pots from mouser.

Rather than drill the board, why not solder extensions onto the trim pot leads?

Well, then agian, if the extensions are long enough, adjusting the pot could result in twisting the leads from input torque. So...maybe you should drill.
 
Final picture of the successfully rebuilt SoA now on full time duty in the main living room gear rack

final 3.jpg

Yeah, the right VU meter is a wee bit darker due to the heat from the original incandescent lamps. Of course, now the LEDs run so much cooler with just 20mA current.

I already have started work on its twin SoA.

Thank you all for your support and suggestions in the restoration/rebuild process :thumbsup:

Mayank
 
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