Troubleshooting a chifi ms12b preamp, and i believe it has a 12au7 cathode follower for the ouput stage. I dont have a tube tester, but checking for a short with multimeter shows all tubes good. In circuit below, bottom tube appears to run away as getting same voltage at cathode as grid... cathode resistor in circuit measures well from pin3 of the tube base to ground, output caps replaced to be safe... kinda has me puzzled...
i guess my question is do tubes short with only higer voltages?
Or could the grid have opened creating the runaway?
edit;
Have new 12au7s on the way, but thought i should double check the preamp before putting new tubes in....
i guess my question is do tubes short with only higer voltages?
Or could the grid have opened creating the runaway?
edit;
Have new 12au7s on the way, but thought i should double check the preamp before putting new tubes in....
Attachments
The upper channel looks worse. Grid is at 160V and cathode is at 7.8V. Current at 7.8/100k is very low even though you have a huge positive grid to cathode voltage. I think bad tube?
Bottom tube is probably OK. It is probably your voltmeter loading down the circuit a bit, giving you slightly false readings.
But the top tube measurement of 7,8V is way off as pinholer said!
But the top tube measurement of 7,8V is way off as pinholer said!
New tubes delivered, glass .020 bigger in diameter than the shield on the 9pin socket 
Socket about 0.760 and tube 0.780
Another delivery to arrive...
What do folks do when this happens, keep buying tubes until you find something to fit?
As the metal is reasonably soft, i hoped to locate a 0.75" pull through mandrel, could not find anything purpose built.. anyone ever cobble something together??
Socket about 0.760 and tube 0.780
Another delivery to arrive...
What do folks do when this happens, keep buying tubes until you find something to fit?
As the metal is reasonably soft, i hoped to locate a 0.75" pull through mandrel, could not find anything purpose built.. anyone ever cobble something together??
Those 7.8V is so much off, tube is toast, C9 or cathode heater could could have been leaking and fried the tube
I would skip the shield. Most likely make no difference.New tubes delivered, glass .020 bigger in diameter than the shield on the 9pin socket
Socket about 0.760 and tube 0.780
Another delivery to arrive...
What do folks do when this happens, keep buying tubes until you find something to fit?
As the metal is reasonably soft, i hoped to locate a 0.75" pull through mandrel, could not find anything purpose built.. anyone ever cobble something together??
I had the same problem once. I just sawed a slot down the side of the shield. It was soft enough then to flex open with fat bottles. You can see it here.New tubes delivered, glass .020 bigger in diameter than the shield on the 9pin socket
Socket about 0.760 and tube 0.780
As the metal is reasonably soft, i hoped to locate a 0.75" pull through mandrel, could not find anything purpose built.. anyone ever cobble something together??
If pin 6 of the upper tube fails to make contact due to corrosion or bad solder joint or broken wire, the 7.8V could be 78uA from grid / cathode vacuum diode.
In which case tube may be o.k.
In which case tube may be o.k.
Spoke to the tube dealer, mentioned that they all vary... I didn't even consider trying the other set shipped in same box.. they fit, although tighter than id like...
Next time the board is out im making the slit! Ty.....
pulled the plug on the amp as soon as I saw 15v on the heater circuit! Chifi 110v pt when 120 is what we have.
Been there before.. Considering 1 20w 8 ohm resistor in the series heater circuit (have a bunch on hand) vs bucking transformer...
And now I understand where my initial thinking was off.. its the cathode driven output via the cathode resistor. Yea, another duh moment.. the 7.8 V was absolutely no Bueno
While i wait for parts for the Skunkie color preamp, i may just fool with this to see where i can get it to go...
Ultimately i plan on the Les Carpenter mods, but its not very high on the priority list....
Next time the board is out im making the slit! Ty.....
pulled the plug on the amp as soon as I saw 15v on the heater circuit! Chifi 110v pt when 120 is what we have.
Been there before.. Considering 1 20w 8 ohm resistor in the series heater circuit (have a bunch on hand) vs bucking transformer...
And now I understand where my initial thinking was off.. its the cathode driven output via the cathode resistor. Yea, another duh moment.. the 7.8 V was absolutely no Bueno
While i wait for parts for the Skunkie color preamp, i may just fool with this to see where i can get it to go...
Ultimately i plan on the Les Carpenter mods, but its not very high on the priority list....
I took voltage measurements quickly, all appeared to be on paper, but definitely not in the black.. didn't want to damage new tubes or even stress them slightly... will adjust heater voltage and re check. I have seen elevated heater voltages affect the operating point in a previous chifi amp.. i know there is a 10% margin but 15v is close to 20% over the 12.6 midpoint