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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Those diodes protect against gate voltage rising beyond what source current can limit in some odd circumstance. I wouldn't run the amp without them unless you want to punch through all the fet gates again. Not worth it.
Right but wouldn't that only be during application of a signal? I was trying to bias the amp up without them and not play music though it.
Thanks
Thanks
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.
Normally the gate voltage is ~3.5v at quiescent state.
I guess it could oscillate, I did start up briefly not on a scope and nothing blew. I was either lucky or it's fine.
You are the voice of reason right now lol. Thanks I should probably just wait.
I guess it could oscillate, I did start up briefly not on a scope and nothing blew. I was either lucky or it's fine.
You are the voice of reason right now lol. Thanks I should probably just wait.
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I do have 7.5v and 15v zeners but no 10v. I guess I could put the 7.5v in for now and make sure everything else is fine before I order parts.
Basically I just want to only order parts one more time so I want to make sure everything else works and just go ahead and order the zeners.
Basically I just want to only order parts one more time so I want to make sure everything else works and just go ahead and order the zeners.
A lot of MosFet power amps us 15volt zeners to protect their gates, normally at power on and power off is when the gates need to be clamped, to avoid gate damage.
Yeah. Having plenty of driver voltage but no output stage supply is probably the easiest way to fry a gate with no protection in an otherwise good amp.
Ahhh I see, that makes complete sense. Think I will be okay to use my 15v zeners instead? I may take a trip to the Electronics store in Manchester, they may have some Zeners.
Looks like all I got for the time being 6.8V 1watt
Should work to keep them safe on start up and power off no?
Should work to keep them safe on start up and power off no?
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!!
Remember noise is also a signal. And while your amp may be at idle, a loud power up or down thump can be a large signal. A momentary problem with an input cable connection can send a loud noise through an amp. Are you prepared to guarantee to your amp that nothing can possibly happen?
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.
There are many mosfet amps without diodes so unless your amp still has another problem it should be ok running without the zeners.
The question is "Are you feeling lucky?"
I would always power up with a scope on the output to catch any problems quickly.
The question is "Are you feeling lucky?"
I would always power up with a scope on the output to catch any problems quickly.
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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