Dynakit Mark IV Oscillation

[denny9167]

I purchased a Dynakit Mark IV a about a month and half ago,put it together,the sound was marvelous,about two or three weeks ago my nephew was listening to some of his music,most of it being tracks with heavy bass material,I told him not to play it very loud,but since He was using His iPod to play from,He on accident turn the volume all the way up so He says,and the amp after that point was distorting a huge amount. I powered down,tested the output tubes,they tested OK,put them back in powered up,and all I was getting was LF oscillation,even with the iPod unplugged,I'm stumped. Any help appreciated.

[SY]

Is the LF oscillation sort of a, "phut, phut, phut" noise? Both channels?

[denny9167]

Originally Posted by SY

Is the LF oscillation sort of a, "phut, phut, phut" noise? Both channels?

I could have been mistaken in calling it oscillation,it is more of a hum,or even more like a squeelling buzzing noise,I do know the woofer is fluttering erratically.

[SY]

Both channels are the same?

[denny9167]

Originally Posted by SY

Both channels are the same?

It is just one channel.

[SY]

OK, swap the tubes between channels and see if the problem follows the tubes or stays in the same channel.

[denny9167]

Originally Posted by SY

OK, swap the tubes between channels and see if the problem follows the tubes or stays in the same channel.

I went ahead and installed new tubes re-biased and it's doing the same thing.

[SY]

Time to make a DC voltage map. Get out the schematic, *carefully* measure the voltages with respect to ground at the plates, grids, and cathodes of the input/driver tube. If the tube used in contemporary Mk IV is a pentode/triode (like the 7199 in the original), get the screen voltage as well.

[dcgillespie]

By "put it together", do you mean that this was a kit of new components you just built -- or is this an original Mark IV you purchased already built? Is it stock -- or used modified components? Restored -- or otherwise still has original components installed? This information is all very helpful in conveying condition, when trying to diagnose by post. So with nothing more than a speaker connected (no input) and the unit turned on, it will make the noise, correct? If the amplifier will go silent without the driver tube installed, then the problem is most surely associated with the driver board. Certainly try another driver tube if possible. Then take careful voltage readings on that board as Sy instructed. Problems that happen all at once like this can be as much component related as connection related. You might go over all of those on or associated with the board as well to eliminate that possibility.

Dave

[denny9167]

Originally Posted by dcgillespie

By "put it together", do you mean that this was a kit of new components you just built -- or is this an original Mark IV you purchased already built? Is it stock -- or used modified components? Restored -- or otherwise still has original components installed? This information is all very helpful in conveying condition, when trying to diagnose by post. So with nothing more than a speaker connected (no input) and the unit turned on, it will make the noise, correct? If the amplifier will go silent without the driver tube installed, then the problem is most surely associated with the driver board. Certainly try another driver tube if possible. Then take careful voltage readings on that board as Sy instructed. Problems that happen all at once like this can be as much component related as connection related. You might go over all of those on or associated with the board as well to eliminate that possibility.

Dave

I apologize it is a new build, it was working great up until this point.