Good idea to run the MK IV at Full Power from time to time ?

[screengrid]

Hi all,

since 1991 I like to use my MK IV combo always with the Tweed Power mode, mostly Triode/Class A, somtimes also Simul Class.
Yesterday I switched the amp to Full Power, just to check out if all is still o.K.
I noticed some kind of crackling noise coming out of the speaker. The noise disappeared again after approx. 30 s of warming up. The noise was not loud, I had to hold my ears near to the speaker, but it was there.
Today I had time to repeat this On / Off procedure test wise, running the amp still in Full Power mode. During the tests I tried different power tubes in Class A mode, turning all volume pot's down and pulling the output level ( silent mode ) to make sure, that the preamp section is completely out of the signal path. Therefore I also took out the phase inverter tube V5. The noise was still there, but from test to test the crackling noise was decreasing more and more and now it is completely away again!
So far so good, but it seems to be a good idea, to run the MK IV in Full Power mode from time to time in order to avoid such problems as described. I don't know what the failure was or perhaps still is at time. To check out this, I surely have to take out the chassis on to my electronic work bench.
I know, the amp is already 16 years old, may be the filter caps need to be changed in near future, or some resistors like the screengrid resistors of the Class A section produce the crackling noise during the warm up phase. I know for sure from repairing old tube radios, that old resistors like to produce such kind off crackling and popping noise, especially anode or cathode resistors of the power tubes. But it could also be a problem in the power supply section for example. I surely have to watch my MK IV during the next time.
Anybody here had similar experiences with the MK IV?

best regards

 

[Boogiebabies]

When the transformer voltage drops in tweed mode, all the voltage to the preamp and power tubes drops as well. It may have been a pre tube seeing 250V for the first time in a while. To make sure the caps are O.K.
I run mine in full power at least once a month.

 

[screengrid]

Hi boogiebabies,

thanks for your reply. I think you are right, it`s a good idea to run the MK IV in Fullpower once a month.
Maybe the caps in the MK IV got too used to the reduced voltage in Tweed mode over the years and the suddenly higher voltages in Full power mode caused some stress to the caps. Perhaps over the several power up and downs the caps became reformed to some degree.

Nevertheless, in near future I will take the chassis out (btw for the first time since 16 years ) to check the caps and everything else inside.

regards

 

[RussB]

I crank up the amps whenever I get the chance...at least once a week!

Question,

When switching to "Tweed" mode, does the heater voltage drop as well?

 

[Boogiebabies]

The tap for the heater voltage is not effected. Just the HV taps. The bias tap is not effected either.

 

[brianf]

For whatever reason I have always felt I should do this too. It sure won't hurt.

Wie geht es Ihnen? I was born in Wuppertahl and came to Canada in 1962.

Bis spater.

brianf

 

[screengrid]

Hi / Hallo brianf

danke, mir geht es gut! Hier in Wesel am Niederrhein fühle ich mich sehr wohl.
Ja, ich kenne Wupperthal, die technische Akademie dort habe ich berufsbedingt ( Elektrotechnik ) schon öfter besucht. Auch in dem Zoo war ich schon 2-3 mal.

Ich hoffe das es auch Ihnen u. Ihrer Familie gut geht.

Viele Grüsse / kind regards