I am at the end stages of repairing a blown amplifier. All output devices and drive and bias transistors have been replaced along with their associated diodes and any other blown resistors in the path of destruction.
I forgot to order 1N4740A 10V 1Watt Zeners for the output stages and they are shorted. I was wondering what they do anyway and was curious if I can run and bias the amp without them for a short period of time until I order new ones? It looks like to me they are there as protection against too large AC input signal and so they clamp the AC at 40v?
The Zeners in question are D214-217
Thanks!
I forgot to order 1N4740A 10V 1Watt Zeners for the output stages and they are shorted. I was wondering what they do anyway and was curious if I can run and bias the amp without them for a short period of time until I order new ones? It looks like to me they are there as protection against too large AC input signal and so they clamp the AC at 40v?
The Zeners in question are D214-217
Thanks!
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Hard to say. If it breaks into oscillation for just a microsecond yer done. The only quasi-safe way would be to limit drain voltage to below gate breakdown rating during test for good bias points. That's sketchy to do when the front - end is biased off the output supply rails. You might be fine just throwing the switch. Maybe not.
Yes certainly. I'd hate to order them wait a week, fire it up to find out I need more parts and have to wait another week. So for now they go in and it get's biased up. As soon as it can sit on a bench stable and pass a signal I'll order up the 1n4740 zeners and should have her all finished up by next week
Thanks for all the help guys!!
Which is why I am not biasing it up until there are protection diodes on the output. I figured a lower voltage clamp is safer than a higher one so for temporary biasing and preliminary checks the 1 watt 6.8v diodes were installed.
D114-115 and D214-215 are under the heatsink so I moved D116-117 to where D214-215 are for now so it's easier to just replace D116-117 and D216-217 when the parts arrive. NPN's get 10v and PNP's get 6.8v. Not ideal but MUCH easier to replace when the 1N4740's arrive.
All juiced up. Set the bias and so far everything looks stable
I'll probably pass a signal through and then call it a day and order up those 10v Zeners.
The Mosfets I ordered from the diyaudio store here were much better matched compared to the the factory Mosfets. Night and day difference really to the point where I don't even believe Ampeg goes through the trouble of matching them. No wonder they explode all the time.
I'll probably pass a signal through and then call it a day and order up those 10v Zeners.
The Mosfets I ordered from the diyaudio store here were much better matched compared to the the factory Mosfets. Night and day difference really to the point where I don't even believe Ampeg goes through the trouble of matching them. No wonder they explode all the time.
Except they DON'T explode all the time. These are very popular amps. MANY of them are out there. They will blow up like most amps. I can;t think of a brand that never blows up. WHEN they blow up, someone goes on t he internet for advice. All the thousands that did not blow up, well, no one writes in asking about that.
AMpeg didn't match them, no. If you bought some local, they were probably all from the same batch and so pretty close. But what really matters is that they work together under load, as in high current flow. That is not well tested by my hand held test-o-meter.
AMpeg didn't match them, no. If you bought some local, they were probably all from the same batch and so pretty close. But what really matters is that they work together under load, as in high current flow. That is not well tested by my hand held test-o-meter.
Hey Enzo, don't get me wrong they are really awesome amps. I play guitar and happen to have several bass players as friends. This is the third from just friends that have let out the magic smoke. I know there are lots in service, and the ones that failed did so after many many years of service, not right off the assembly line so it's a testament to their quality. I'm just grumpy because they aren't very service friendly is all
Ask 10 people and you will get 10 responses. One of my bass player friends lives across the country so I can't fix his stuff but his tech said he talked to Ampeg and they said they have a special circuit they match them in and that he needed to matched devices. BUT, I have read elsewhere from people I trust their knowledge that Ampeg doesn't match them and they don't need to be closely matched because of the source resistors. That tech charged him $600 for the repair, still cheaper than a new one and an old one doesn't get wasted. Personally, it wasn't that much more money to get the matched devices from the diyaudio store here so I just went that route, it seemed better to me. It shows at quiescent state, the diyaudio devices are matched nicely, lowest was 20mA and the highest was 27mA. Where as the factory devices the lowest one was 8mA and the highest was 28mA.
I'm a guitar play and use to tubes. Usually when I lose an output tube, (which has only happened to me once ever) it took the screen grid resistor with it and that was it. Very easy and cheap fix. The 1600 watt arc welding Ampeg was a bit tougher to fix but she is doing good now
Sorry if it seemed I was bashing Ampeg, they are great amps hence why all my friends have one. Like I said I was just grumpy they don't make it more service friendly. It would have been nice to remove a bottom cover to get at the underside of the board, instead you have to keep flipping the thing over and out.
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