78 Crate CR-1R, 72 Gibson G20, 2006 Randall RG 100 G3 100 watt valve dynamic

[ParthaD]

Hi there... happy with the success of the RG300 project I've been on the lookout on ebay and craig's list for more amps needing work. Found a 1978 CR-1R which was the FIRST Crate amp... all that was wrong with that one was a 1N914 diode that had busted! That amp sounds great with a 412 cab!

Also found a 1972 G20 (ss) Gibson that had a significant amount of white noise unaffected by volume control, i.e. present even at zero volume, not noticeable at all when playing - that one also had the ground power plug prong broken off. Tested and confirmed that the noise was unaffected by grounding. However found that there was some green gooeey mess on the PCB in one location (pic posted), probable cap leak at some point that had been replaced but the mess not cleaned. Anyway cleaned that up and that reduced the noise signifcantly. Also came across some shielding posts on MEF while researching this and used aluminum foil to really shield it well on its 3 open sides when putting it back in the cabinet and that just about took all the remaining noise out!

Brings me to this 2006 Randall 100 G3 Valvedynamic amp (uses a 12AT7 in the POWER amp, unlike some of the hybrids using a tube in the preamp). Need your help on that one. The seller had described it as having come to him damaged from some music store, the cabinet and speaker were gone so all I have is the chassis. Wrote to Randall and got the schematics (posted here). The send jack was fully responsive and working fine on both channels. Connecting the send and return with a patch cable produced no sound. Connecting my Jamman to the guitar input produces reasonably loud but very distorted (not as in sustaining but as in busted speakers) sound - was afraid I'll bust my speakers so didn't turn it too loud. Both channels were like that.

Suspected some blown transistor etc... but then noticed that the board had been cannibalized at some point, 6 transistors (I think MOSFET type) ones were missing! TR1-6 were all missing (look at the lower edge of the posted pic), only reason T7 was spared was probably because it was in the middle. The schematic describes TR1-3 as "1058" and TR4-6 as "162". When I tried looking up "Transistor 1058" at Mouser I found 2 hits, only one in stock but it was the SC-89 packaging not the 3 leg TO-262 kind like T7 -which is an IRF820 - that appears to have been cut out. Same thing for the 162 also. Would appreciate someone pointing me to the right part.

Also when it came to me it had a 12AX7 instead of 12AT7 which has a gain of 100 compared to 60 that the board is designed for. Is it OK to use a 12AX7 even when the schematic asks for the 12AT7?

Thanks in advance!

[J M Fahey]

Hi ParthaD
1) Try 2SK1058 , 2SJ162
2) If your amp is a "75" instead of a "300", maybe not all power transistors were fitted or needed.
Some of the pads look virginal.

[ParthaD]

Now the heatsink??

Thanks Juan.. got those Hitachi replacement parts from Tayda Electronics (Mouser does not have them) for about $22 and also the datasheets by googling. The amp says 150 watts @4 ohms on the chassis. However, those MOSFETs will burn out pretty quick won't it since there is NO heatsink to attach it to, just a big hole where I guess it used to be, it was gone when I got it. So now how can I get a similar heatsink? Since those MOSFETs are rated at a *dissipation* of 100 watts each, the actual audio power delivered would be about 50 for each right? Also I will need mica pieces right - to go between the mosfet and the heat sink assuming I find it, I already have some heatsink compound. I'll write to Randall and see if they can send me a heat sink that will work.