Amp question... about a tranny really.

[digi2t]

Hi all,

Not a pedal question, but looking for advice....

Bit of a pickle here. I have an Ampeg G212 amp from the 70's, which started getting all distorted when playing through it. It's a 120W combo (2X12). I got the service manual, started poking and prodding, and found that a transistor didn't pass the diode test. It's an RCA transistor, here's a pic of it;



And here's the schematic of that shows the power module section, the transistor in question is Q302, at the top of the page;



There is no listed manufacturers number in the manual insofar as these trannies (there are 2 of them, one NPN, and one PNP) are concerned, only an AMPEG part number. And the number on the tranny is probably an RCA house number, because I've scoured the net, and come up with nada on the numbers the tranny is wearing.

So, my question is this...

I know there are a bunch of folks here much more gifted than me, so can anyone give me a clue as to what I can sub in here? I'd greatly appreciate it. It's a great amp, loud as hell, and it would be nice to actually use the amp part sometimes, instead as strictly a cab as it is now.

Many thanks in advance,
Dino

[Govmnt_Lacky]

Hey Dino!

Any chance you can list the numbers that are stamped on the tab of the transistor? I doubt they might help but ya' never know 

[digi2t]

I FOUND THE PROBLEM!!!!!!!! 

When I was testing, I was probing the turret board. After I yanked out the tranny, I tested again (thank GOD!!), and it tested OK. Solder joint was bad  . Amp is back to it's roaring self again  . YIPPIE!!!!! These amps are just so bloody loud, that I guess any solder that's not just right, bears the brunt of the vibration, and eventually causes problems like this.

I would like to once again thank this forum for all that I have learned so far. This amp has been down for about 2 years now, but with all the info, knowledge sharing, trial and error, and kindness of the folks here, I finally had the courage to tackle something like this with some sense that I had a clue to what I was doing.

Thanks for checking in on me G_L. Much appreciated. I'd still like to get the bottom of what this sucker is though. On the tab, we can see a "3" and a "1", as well as the "B" and "E", which denotes the base and emmiter. The number between them is "274". I managed to find the RCA Transistor Manual, but there is no mention of these numbers anywhere. It's a 60's manual, but I've read that there is a 70's edition as well. I'm still on the hunt.

[Mike Burgundy]

I always like these little treasure hunts as an excuse not to walk the dog in the rain ;P
BD262? That is a power PNP. Wildly discontinued though ;P

[digi2t]

You know guys, it's always good to step away from something some times. After working at it all day, I went back to the service manual, and lo and behold, there's a list of cross-referenced parts... including Q302 (this one), and Q303 (it's PNP counterpart on the other side of the power module.

Sound of hand slapping forehead!

Ampeg lists 2N5682 as the suitable replacement for this baby, and 2N5680 for the PNP counterpart. Only problem is that the alternates aren't TO-220 style, like these ones. Rather, they're TO-39, so I'm not sure if they would hold up insofar as heat dissipation is concerned. I guess one could use one of these;



But, at least I've got the spec's, so I guess I can match up something with the same package style now.

Guess we're gonna have to walk the dogs Mike 

[PRR]

> amp from the 70's

Late 1977. "7732" is the date-code, 32nd week of 1977. Assuming the transistor is original and Ampeg didn't keep tons of parts around.

> alternates ..., they're TO-39

TO-39 will barely-work on a test-bench, NOT in a real world of intense musicians and spike-heels on speaker wires.

TIP33C would be the shade-tree replacement. It's a little slow, slower than the 30MHz 2N5682, but the Ampeg is not a fussy amp.

[digi2t]

> amp from the 70's

Late 1977. "7732" is the date-code, 32nd week of 1977. Assuming the transistor is original and Ampeg didn't keep tons of parts around.

> alternates ..., they're TO-39

TO-39 will barely-work on a test-bench, NOT in a real world of intense musicians and spike-heels on speaker wires.

TIP33C would be the shade-tree replacement. It's a little slow, slower than the 30MHz 2N5682, but the Ampeg is not a fussy amp.

Thanks Paul! Much appreciated. I was hoping you would chrip in here, and you didn't disappoint  . Insofar as the speed is concerned, could I go with something along the lines of an MJE15028/30(NPN), and MJE15029/31(PNP)? They seem to be in the zone voltage/amperage-wise, higher wattage rating, and the speed is right up there at 30MHz.

[R.G.]

I would certainly go with a modern (much) higher voltage and faster part. There are some Toshiba parts which are TO-220 and intended specifically for driving outputs, although I wouldn't hesitate to try the MJ15xxx series drivers.

There is some chance that this will give you minor oscillation problems in some amps. You ought to check it for parasitic oscillation after you're done. Of course, this is something you ought to do when you do anything to an output stage. *Slower* devices have as much or more risk of pushing it over the edge.

While you're in there, put some 3A/200V or more diodes from output to power supplies so they're reverse biased normally; one with cathode to V+ and another with anode to V-, in parallel with C209 and C208. This protects the outputs from sudden death in certain conditions and is very cheap insurance.

[digi2t]

Thanks for the advice R.G.

I took a look at the MJ15xxx series, but those are TO-3 packages, and I don't feel like modding anything. MJE15xxx series is a TO-220 package, which would fit right in here. I should look into finding some suitable replacements for the power transistors as well, Q304, 305, 306, and 307. Not that they need to be changed, but just to have on hand as spares... in case. I've got the numbers for these though, so should be easy to find. I really love this amp. It's super loud, but super clean, and that's the way I like my amps.

Great suggestion on the diodes. If I understand correctly, one parallel to C209, with the cathode toward connection point 10 side (number "10" in a circle). The other parallel  to C208, anode towards connection point 1 (number "1" in a circle). Basically, same orientation as D209, and D210?

Now I just have to confection an 800mH inductor, for the mid EQ control on channel 1 (someone removed it for some reason ), and this baby will be completely fully functional 

[R.G.]

Digikey has Toshiba 2SA1962/2SC5242 pairs for about $3.50 each. These are in "plastic TO-3" packages, and they're rated for 230V/15A/130W each. They are *highly* regarded in the amp industry. For drivers, the 2SA1837 and 2SC4793 in TO220 are 230V/1A/20W for $0.85 and $0.73 each.

Toshiba transistors are heavily counterfeited because they are so desireable. DigiKey is one of the few places I'd trust to have the real thing. At the price of under $7.00 a pair for output transistors, I'd put three pairs on the outputs in parallel. This should be very, very rugged indeed. These Toshiba devices are so-called "sustained-beta" devices with much higher current gain at high currents, so they're easier to drive than the older devices. They're also much easier to mount if you have a flat place on a heat sink.

Great suggestion on the diodes. If I understand correctly, one parallel to C209, with the cathode toward connection point 10 side (number "10" in a circle). The other parallel  to C208, anode towards connection point 1 (number "1" in a circle). Basically, same orientation as D209, and D210?

DOH!! D209 and D210 do what I meant. I didn't see them there inside the output stage. Yes - same orientation and connection, and everything. In fact, never mind trying to add them. They're there. 

Now I just have to confection an 800mH inductor, for the mid EQ control on channel 1 (someone removed it for some reason ), and this baby will be completely fully functional

I would look at the circuit and see if there wasn't a cap I could change so I could use a 500mH wah inductor.

[digi2t]

DOH!! D209 and D210 do what I meant. I didn't see them there inside the output stage. Yes - same orientation and connection, and everything. In fact, never mind trying to add them. They're there.

No problemo. I was wondering about them, but I didn't say anything, thinking that there was obviously something else I didn't know 

I'm going to look into those transistors you mentioned.

As for the inductor, I have enough room to make a small board, and string 8 100mH inductors together. Should be fine.

Thanks again,
Dino

[digi2t]

OK, everything is puuuuurfect now. As it happens, I had to change a resistor to get it to bias right where the factory spec's at 0.011 volts. On one side of the bias trimmer, the schematic spec's an 18K resistor (R211), but I had a 15K on my board. It looked factory stock as well. With the trimmer right down, I could only get to 0.042v, and the heatsink was always quite warm running the 30 minute test as per the manual. I decided to play with the resistance value of the 15K, figuring that the factory probably did, to get it to bias. Going down in 1K increments, at 11K I finally biased at the 0.011v spec'd, around 2/3's on the bias dial. The 30 minute test has the heatsink at around room temperature now. Whatever voltages that are spec'd in the schematic are also spot on  .

The mid-range missing 800mH mid-range choke is fixed as well. I took 8 (100mH) of these,



and arranged them in a tight circle on a piece of perfboard. I then soldered them in series, connecting 2 wires to the ends, and tapping at the 100mH, and 300mH points (these points were as per what info I found on the net pertaining to these Ampeg inductors). One end connects to point 4, the other to point 1, the 100mH tap to point 3, and the 300mH tap to point 2. Best of all, the new inductor package, once trimmed down, comfortably nestles into the same space that the old torodial inductor occupied  . Just had to drill a hole in the middle of the perf board, use a short stand-off, and it bolts directly onto the pre-amp board where the torodial used to be.

Sound-wise, A-B'ing channel 1 to channel 2, the mid-range adjustment is VERY close. Without a doubt, there are nuances between the two, as you switch and adjust between the three different ranges, but nothing that can't be dealt with by a slight tweak of the mid-range knob.

So there you have it. You could go the Mercury Magnetics way, and get an authentic style Ampeg mid choke, at a 105 bux. But, if that's too rich for your blood, the "12$ for 10 - 100mH inductor on Ebay" way will get you by just fine.

Thanks again to Paul and R.G. for helping me out on this baby. Long live the G-212!

[digi2t]

UPDATE,

Funny enough, the problem returned. I never had the chance to really push the amp before, but I finally had the house to myself the other day, so I really cranked the sucker. I would get about 5 minutes into playing at substantially loud levels, and it would crap out on me. All I would get would be low distorted sound. I went whole hog this time. I decided to go with new MJ15003's, and the Toshiba drivers, as was suggested.

But, I really wanted to get to the bottom of the problem, so I decided to go part by part, testing between changes to find the culprit. It ended up being the same driver that I thought was cold soldered. It seems to me that every time the amp was working hard, the heatsink temperature would probably get above a certain point, and the driver would crap out then come back after about 15 - 20 seconds. Back and forth. I haven't been dabbling in this stuff all that long, and that's the first time I've come across this. I was under the impression that when a tranny decides to go, well, it goes. Seems that this puppy wasn't ready to completely give up the ghost (much to my head-scratching amusement  ). All in all, I'll take it as a good week, went to bed less stooopid again  .

I now have all new trannies in the power module, plus some new caps and diodes. Again, all voltages line up, and it's even a tad quieter now. The MJ15003's, with the  2SA1837 / 2SC4793 combo really rips. It's never sounded better. I've had it cranked up for a solid hour now, no hiccups. Although, I will have to brush up on my lip reading  .

As always, the intermittant problems are always the worse one. Catch me if you can.

[Cliff Schecht]

These Toshiba devices are so-called "sustained-beta" devices with much higher current gain at high currents, so they're easier to drive than the older devices.

I've never heard of these. How do they maintain beta under higher current operation? Is there a special process they are using to achieve this?

[PRR]

> How do they maintain beta under higher current operation?

Why does beta fall-off at high current? Without the heavy theory: because the crystal clogs-up with charge. It becomes "space charge limited" like a vacuum tube. In a tube, if it's full of electrons, it discourages more electrons.

How do they...? I'm guessing some advantage with geometry and doping, and perhaps just using a larger die so the beta-droop happens off the far end of the graph.