Bass Amp eating power tubes, remedies?

[Jonas]

Hi Everyone,
I have a bass amp (home built Marshall Major) that is eating up power tubes.  Power tubes are shorting out, causing artificial ground (100 ohm resistors) to open. 

Does anyone have any remedies?  Suggestions on stabilizing the amp?  Would using pedals increase the likelihood of this event?

Thanks

Jonas

[sluckey]

Try biasing a little cooler.

I'm guessing the tubes are shorting between pin 2 and pin 3? If so, be sure the sockets are clean and don't have any carbon arc traces between those pins. If the socket shows signs of arcing it's best to replace the socket. A typical cause for a short between pins 2 and 3 would be incorrect speaker impedance or an open speaker load, ie, speaker cab not plugged in.

If tubes are shorting some other way tell us which pins are shorting.

[Jonas]

Thanks Sluckey,

Will bias a little colder

Yes, I confirm that shorts are from plate (3) to heater (2).  I get sporadic ohm reading between pins 2 and 3 during bench continuity check on know bad tube.  Open reading on known good tube.

Also, I do not have a grid resistor at the input.  I haven't been installing input grid resisters as of late since most communication is now digital.  Would it be a good idea to install input grid resistor to condition the signal some?

Jonas

[PRR]

> I haven't been installing input grid resisters as of late since most communication is now digital.

How does that make any difference?? Surely the tube amp input is analog?

Blow the 12 cents.

[sluckey]

Pull all power tubes. Measure resistance between pin 2 and 3 of all power tube sockets. Reading must be open for each socket.

[Jonas]

thanks everyone.

Resistor in at V1 grid. 

Pulled power tubes, double checked ohm reading between pins 2 and 3 - all good
I have the amp up and running, but will need to tweak the bias circuit a little; when bias control pot is 100% CCW, I am still reading 50mA. 


I don't like the minimal adjustment range on the stock Marshall Major bias circuit.  Do you have a better bias circuit that I you could recommend I try?

Jonas

[sluckey]

How are you measuring 50mA?

Does your bias circuit look like the one in the linked schematic? If so, increase the size of the 10K resistor to 15K or 22K to get a cooler bias range.

     https://el34world.com/charts/Schematics/files/Marshall/Marshall_major_1978u_bass_200w.pdf

[Ritchie200]

Jonas,


What voltages are you running in this clone? Is the arcing occuring in the the tube or on the socket? If at the socket, as sluckey said, you would typically have to replace it as it is near impossible to clean enough considering the high voltages that like to dance (if you are running typical Major voltages). Speaking of sockets, ceramic? What OP transformer are you using? Did you reproduce the Major circuit with UL output? For that matter, did you reproduce the Major circuit exactly? If not, need to see schematic. Do you lose the tubes when they are humping? Or just general use? Are you boosting the signal in - slamming V1? Do you lose a fuse when the smoke comes out?


Most important, let me ask you this - what do you think, or have been told, the bias should be set at? Sluckey, I think something is not right on his bias circuit.


Jim

[2deaf]

The RMS voltage of your bias winding would be a handy thing to know.

[Jonas]

thanks everyone for the help

Sluckey, I am measuring 50mV of voltage drop across a 1 ohm resistor tied from power tube cathode to gnd = 50mA
I am currently using the bias circuit from this Marshall Major schematic (follow link)
https://drtube.com/schematics/marshall/200w.gif

PT is a Mercury Magnetics MP100-67 (197V-0-197V winding but has a voltage voltage doubler.  The bias winding is listed at 90V on the data sheet.  Bias winding does not have a center tap.

Jim

tube sockets are BELTON plastic sockets
OP is Mercury magnetics with NO UL tap
Trying to use KT-120 output tubes, recently biased at 50mA.  Not much research out there on bias; will try to set at 35mA
One power tubes will always go down during incident, and no longer good after incident.  Also lose the (2) 100 ohm artificial ground resistors, sometimes the fuse
My friend uses Pedals and is slamming V1, also his Bass guitars are made of aluminum with aluminum neck which I am also assuming is slamming V1
B+ and plate voltages are typical Marshall Major voltages

To say the least, I have had a difficult time and have been struggling with the bias circuit.

Thanks again for helping me out!

Jonas

[2deaf]

Did you try this bias circuit?

[Jonas]

thanks 2deaf for the bias circuit

Would I need to make any changes to this bias circuit if implementing in an amp with 4 power tubes?

thanks - Jonas

[pdf64]

Missing vital info about all this, eg schematic, voltage survey.
Without that, potential for useful suggestions is hobbled.

[shooter]

One power tubes will always go down during incident

unless I missed something, you don't need tubes to "set bias".  what "range" do you have without tubes?

[Ritchie200]

Missing vital info about all this, eg schematic, voltage survey.
Without that, potential for useful suggestions is hobbled.

+1, From your description there are too many differences from a Major so that schematic is useless.  If it is a homebrew based on some of the Major topology, that's fine.  But we need to see what is different.  Even a hand drawn schematic would be good.  Please ignore my questions about driving the front end as I thought you had an original Major (sorry I didn't read close...).  Those questions have absolutely no bearing on your amp.  You mentioned it has typical voltages of a Major.  What are they?  I can do KT88's, KT90's, KT120's, and 6550's and there is plenty of swing in my or any Major bias circuit to accommodate without modifications.  As Mr. Maguire said, "Help me, help you!"  As Shooter also said, what range do you have now?

Jim

Jim