kevzep
Its all about the Music
Some of you may have been following my Hot-Rodded G-7700, with the output section I had to fabricate from, well, nothing.
I thought i was home free with this amp, but NO, boy was I in for a surprise.
I had the Toshiba 2SC 32381/2SA 1302 OP's which came my way for free after I blew up the NOS Sankens when I fired it up with the trimpots on the VBE multiplier set the wrong way, and the Multimeters measuring the bias current both set to AC instead of DC.
I went with the VBE multiplier because I thought it would be an elegant way to regulate the bis current.
I went with the MJE15032/33 drivers as I assumed the original ones were toast.
I set bias, ran the amp up a few times, and reported the insane amount of power coming out of this amp on a thread recently.
I had the amp running for a few weeks mostly at low levels, but then the other day I decided to play some loud music, when I did I put my hand on the heatsink and just about received 3rd degree burns, holy crap the thing was running hot, I could have fried my dinner on it.
So I thought okay somethings not right here. I put the amp on the bench again to study the bias current. I had mounted my VBE transistor on the heatsink for thermal tracking, so I couldnt understand why the amp was running hot. The idle current was a couple of mV above the manufacturers spec but it still didnt explain what was going on.
Okay, so I put an 8 ohm load on the amp, with a 1k sine wave and wound it up. To my horror the bias voltage just sky rocketed, up to over 200mV, and took out 2 emitter resistors, one OP, fried a base resistor on one of the class A pre-drivers, blew open a base resistor on the OP which blew.
Holy Crap.
There was a fairly impressive mushroom cloud from that meltdown!!!
I fixed everything and tested everything in the driver stage, to my surprise the driver transistors were okay.
So we were up and running again, now this time when I was setting the bias I noticed some high frequency oscillation following the sine wave, I dont know how I missed it before but when I got to the optimum bias current it was reduced. I put a 1000pf cap across the Base and Collector of my VBE transistor thinking it may help with the HF Oscillation, but no such luck.
Next thing I did was to check the thermal tracking of the bias, I took the vbe off the heatsink and while monitoring the bias current, pinched the transistor between my fingers, and as soon as I did I could see the current go down, great news I thought, because I suspected this was not happening.
But what the hell with this skyrocketing current and the HF oscillation??
I went through everything over and over and over and couldnt come up with an answer.
Was it these OP's? I didnt think so..........
I improved my thermal management in the heatsink department, by adding a large alloy plate which I bolted over the top of the OP's with thermal paste, to help dissipate more heat.
The other thing happening was the horrendous crossover distortion, which required way more idle current to correct than it should have.
Clutching at straws I decided to test the original 2sa968/2sc2238 driver transistors, to my surprise they tested okay, so i put them in.
Hey bloody Presto, the idle current was majorly reduced and very stable. Also the HF oscillation was gone.
Even at a few milliamps there was NO crossover distortion........
So I set the Idle current and DC offset, I went at 17mA which is about what the original spec is I believe.
I ran both channels at almost clip, and yes this was about 41VRMS for about 10mins.
The OP's were warm, not hot and the bias current was stable.
I put it back in to service............
So those driver transistors may not be a suitable sub, I know guys have used them, I used them on the recommendation of a couple of members but it looks like there could be a incompatibility issue there. It could be something to do with the OP's I used perhaps.
I have been listening to this amp for the last couple of days, and it is by far my favorite, this thing has BALLS and crystal clear BALLS at that.
I actually cant get over how good this thing sounds, Im running it through my JBL ES90's, I even pulled the faceplate off deoxited the switches and pots cleaned her up.
The VBE multiplier is a very good way to regulate the bias current, its very easy to do. All you need is a MJE350 a 1K5 resistor, a 470ohm resistor and a 1K trimmer and you're off. It's a bit twitchy with the fine adjustment, but I think I could change some of the resistor values and make it a bit less sensitive, but its not as twitchy as setting the DC offset, and I didnt have any trouble achieving the current I wanted so Ive left it.
I will scan something in the way of a schematic next few days and post it if anyone's interested.
Now I need something else to resurrect!! Man did I blow some **** up doing this but it was all in the interest of research and development, and reviving an awesome piece of history!!
I hope what Ive learned will be of use to other members one day.
Im a happy Dude!! And listening to Led Zeppelin right now!! And quite loud too!!
I thought i was home free with this amp, but NO, boy was I in for a surprise.
I had the Toshiba 2SC 32381/2SA 1302 OP's which came my way for free after I blew up the NOS Sankens when I fired it up with the trimpots on the VBE multiplier set the wrong way, and the Multimeters measuring the bias current both set to AC instead of DC.
I went with the VBE multiplier because I thought it would be an elegant way to regulate the bis current.
I went with the MJE15032/33 drivers as I assumed the original ones were toast.
I set bias, ran the amp up a few times, and reported the insane amount of power coming out of this amp on a thread recently.
I had the amp running for a few weeks mostly at low levels, but then the other day I decided to play some loud music, when I did I put my hand on the heatsink and just about received 3rd degree burns, holy crap the thing was running hot, I could have fried my dinner on it.
So I thought okay somethings not right here. I put the amp on the bench again to study the bias current. I had mounted my VBE transistor on the heatsink for thermal tracking, so I couldnt understand why the amp was running hot. The idle current was a couple of mV above the manufacturers spec but it still didnt explain what was going on.
Okay, so I put an 8 ohm load on the amp, with a 1k sine wave and wound it up. To my horror the bias voltage just sky rocketed, up to over 200mV, and took out 2 emitter resistors, one OP, fried a base resistor on one of the class A pre-drivers, blew open a base resistor on the OP which blew.
Holy Crap.
There was a fairly impressive mushroom cloud from that meltdown!!!
I fixed everything and tested everything in the driver stage, to my surprise the driver transistors were okay.
So we were up and running again, now this time when I was setting the bias I noticed some high frequency oscillation following the sine wave, I dont know how I missed it before but when I got to the optimum bias current it was reduced. I put a 1000pf cap across the Base and Collector of my VBE transistor thinking it may help with the HF Oscillation, but no such luck.
Next thing I did was to check the thermal tracking of the bias, I took the vbe off the heatsink and while monitoring the bias current, pinched the transistor between my fingers, and as soon as I did I could see the current go down, great news I thought, because I suspected this was not happening.
But what the hell with this skyrocketing current and the HF oscillation??
I went through everything over and over and over and couldnt come up with an answer.
Was it these OP's? I didnt think so..........
I improved my thermal management in the heatsink department, by adding a large alloy plate which I bolted over the top of the OP's with thermal paste, to help dissipate more heat.
The other thing happening was the horrendous crossover distortion, which required way more idle current to correct than it should have.
Clutching at straws I decided to test the original 2sa968/2sc2238 driver transistors, to my surprise they tested okay, so i put them in.
Hey bloody Presto, the idle current was majorly reduced and very stable. Also the HF oscillation was gone.
Even at a few milliamps there was NO crossover distortion........
So I set the Idle current and DC offset, I went at 17mA which is about what the original spec is I believe.
I ran both channels at almost clip, and yes this was about 41VRMS for about 10mins.
The OP's were warm, not hot and the bias current was stable.
I put it back in to service............
So those driver transistors may not be a suitable sub, I know guys have used them, I used them on the recommendation of a couple of members but it looks like there could be a incompatibility issue there. It could be something to do with the OP's I used perhaps.
I have been listening to this amp for the last couple of days, and it is by far my favorite, this thing has BALLS and crystal clear BALLS at that.
I actually cant get over how good this thing sounds, Im running it through my JBL ES90's, I even pulled the faceplate off deoxited the switches and pots cleaned her up.
The VBE multiplier is a very good way to regulate the bias current, its very easy to do. All you need is a MJE350 a 1K5 resistor, a 470ohm resistor and a 1K trimmer and you're off. It's a bit twitchy with the fine adjustment, but I think I could change some of the resistor values and make it a bit less sensitive, but its not as twitchy as setting the DC offset, and I didnt have any trouble achieving the current I wanted so Ive left it.
I will scan something in the way of a schematic next few days and post it if anyone's interested.
Now I need something else to resurrect!! Man did I blow some **** up doing this but it was all in the interest of research and development, and reviving an awesome piece of history!!
I hope what Ive learned will be of use to other members one day.
Im a happy Dude!! And listening to Led Zeppelin right now!! And quite loud too!!
