F5 Turbo Builders Thread

[Rixsta]

So here we go time for power-up

 

[Marra]

Good luck I think you will like it.

 

[Rixsta]

F5T all good, on mini transformer just to test. thought it was a good idea now to wack in big PSU

 

[AudioSan]

good Luck

 

[PKI]

I trimmed the bias in my F5Tv2 up to 400mV across 0R5, oh man, how much flame and smoke this amp can produce! I guess, Nelson was right about a fire extinguisher For guys who are new in amp building be careful and let the amp heat up before adjusting bias high

 

[6L6]

What burnt? Transistors, resistors or gatestoppers?

 

[PKI]

MOSFETS and most sadly the board(forgot the word — contact plates around holes). Feel myself so stupid! :-(

 

[AudioSan]

I trimmed the bias in my F5Tv2 up to 400mV across 0R5, oh man, how much flame and smoke this amp can produce! I guess, Nelson was right about a fire extinguisher For guys who are new in amp building be careful and let the amp heat up before adjusting bias high

bias runnaway at 0.8A pr device? Guess you have the diodes on the main sink, wich is very hot.

 

[PKI]

No, diodes are out of the main think, but no heat thinks on them,but they are cold . I will try again tonight-tomorrow.

 

[AudioSan]

For guys who are new in amp building be careful and let the amp heat up before adjusting bias high

i would say: drop the diodes

 

[AudioSan]

No, diodes are out of the main think, but no heat thinks on them,but they are cold . I will try again tonight-tomorrow.

get some sinks on them. and watch the bias. you can also lower the source resistance, so that you get your bias with lower voltage drop.

 

[buzzforb]

be careful and let the amp heat up before adjusting bias high

For others, this may be the key sentence. These mosfets have a positive tempco, si if you bias them high while cold, when they begin to heat up, the bias will continue to rise. If you are not watching them, you could get in some trouble.

 

[PKI]

Listen to uncle Buzz, he knows the stuff . Only it happened in seconds: oscillations for a half a sec with increasing amplitude and then just up!

 

[buzzforb]

Possible you had just one start to go. Possible you had oscillation, but since it has been playing for some time, i tend to think differently. As I remember, you had one biasing higher than the others. Was this the one that went first?

 

[henryve]

I suppose that PKI is now also part of the select group of Fearless Amplifier Builders.

 

[PKI]

It is the same channel yes. However, I cascoded FE(1/2.2 of rail). I have to say, that after cascoding the amp became much more sensitive to turns of the trimpots. Do you think the cascode could cause th oscillations? But I tried it with 24V PSU and it worked.

 

[PKI]

Henryve, hooray!

 

[WalterW]

I trimmed the bias in my F5Tv2 up to 400mV across 0R5, oh man, how much flame and smoke this amp can produce!

Ooooh, pictures please

 

[buzzforb]

It is the same channel yes. However, I cascoded FE(1/2.2 of rail). I have to say, that after cascoding the amp became much more sensitive to turns of the trimpots. Do you think the cascode could cause th oscillations? But I tried it with 24V PSU and it worked.

Cascodes tend to stabilize, amps rather than make them unstable, as they provide more constant operating conditions. The Cascoded version I most recently built was on the store boards. Everythign checked out Ok.

 

[buzzforb]

It is possible, but the cascodes could have increased the bandwidth of the amp enough to create possiblity of oscillations. I would think you would have seen evidence of it in burned gate resistors, but i guess it could have been small enough to trigger burn. Perhaps ZM or someone has another perspective or opinion. For now, I suspect, too high a bias on startup and it just got away from you.