Little Mark III

[lowell]

It's a Mark II not III. I can't change the title.

Customer brought this in saying it cuts out and goes into protection at gig volumes. After about 5 min run at 180w it indeed goes into protection. I began checking the bias of the mosfets. Vs is on average 11mv. The Rs ballasts in this amp are .33ohm. Ampeg suggests 50ma per IRFP240 in their svt4 so I'm using that as a reference since I cannot figure out where in the datasheet it describes quiescent current.

So
.33 X .05 = 16.5mv

I adjusted bias colder just to see if the amp would cooperate. At 9mv Vs the amp works great. No protection. I'm now realizing that measuring one N channel and 1 P channel isn't sufficient. There may be one that is much hotter and running away.

Just wanted to post for thoughts as I'm home for the night.

I will try to check all 6 tomorrow and report back.

But please confirm my calculation.

[nosaj]

It's a Mark II not III. I can't change the title.

Customer brought this in saying it cuts out and goes into protection at gig volumes. After about 5 min run at 180w it indeed goes into protection. I began checking the bias of the mosfets. Vs is on average 11mv. The Rs ballasts in this amp are .33ohm. Ampeg suggests 50ma per IRFP240 in their svt4 so I'm using that as a reference since I cannot figure out where in the datasheet it describes quiescent current.

So
.33 X .05 = 16.5mv

I adjusted bias colder just to see if the amp would cooperate. At 9mv Vs the amp works great. No protection. I'm now realizing that measuring one N channel and 1 P channel isn't sufficient. There may be one that is much hotter and running away.

Just wanted to post for thoughts as I'm home for the night.

I will try to check all 6 tomorrow and report back.

But please confirm my calculation.

bad pdf link can't download.

nosaj

[Jazz P Bass]

Manual: Mark Bass Parsek LittleMarkII combo.zip

[nickb]

Those FETs are designed for switching use, not linear. Therefore it will be tough to get any numbers for quiescent current.

Do you know which protection circuit is kicking in, current or voltage? Are all six FETs working- check the voltage across each source resistor.

How hot is it when it goes into protect? What about ventilation - is the fan working properly and is all dust free?

[lowell]

Fan is working...lid is on to maintain air across the heatsinks. It's not doing it anymore and the customer came and grabbed it. I would have preferred another couple days with it but alas. Either he'll come back and I can be more thorough or it will work fine.

Of note...I didn't see any crossover distortion using a sine wave and scope at the low 9mv Vs setting. Sorry I should've held onto it.

Nickb the speaker relay was clicking off. It looks to me that the relay will click off when the P channel sources get too high...too negative.

Thanks for the replies.

[nickb]

OH well.. time will tell.

There are two protection circuit that operate the relay. One monitors the AC output current (IC1A) and the other (T19/T20) the output average voltage.