Tube question - Heathkit IO-12 'scope

[Paul Moir]

Lately I've been playing around with my old Heathkit IO-12 oscilloscope; replacing capacitors and drifted resistors, adding a probe calibrator.  Not doing a restoration but rather having some fun and hoping to learn something.
While everything is working pretty well, I've run into a problem with the vertical amplifier.  While the bandwidth of these oscilloscopes is pretty horrific, +-1dB to 2.5MHz then 5dB down at 5MHz according to the O-12 manual, mine seems even worse.  I'm starting to roll off before 1MHz and down below 10dB at 5MHz.  The problem seems to be in the 2nd and 3rd stage amplifier, which use a 6AN8A tube as it's active component.  I'm loosing about 50% of my signal from 1-5MHz in the 2nd stage and the rest in the 3rd. The losses are moderate in the first stage and seemingly very small in the plate driver.
So my question is:  can an old tube loose bandwidth, or is there something else I'm overlooking?  The plate voltages are a little high (+10V) as is the supply (112V).  The 40uF cap is new and the resistors are in tolerance.  Neither .1uF cap seems to leak at all, though the pin 2 grid voltage is a little high (0.73V).  The only other seemingly relevant observation I made is this stage has some distortion at the top and when driven hard clips at the top far before it distorts at the bottom.

[singapol]

Tube specs. detoriate with age, if that happens it's time for a new tube.

PS Normal consumer tubes are good for 2000 hrs. You may need to substitute 12BY7 with 12BH7 but compare pinouts and specs. first.

[Ian.M]

The voltages being off would all point to the tube having low emission.  It looks like 6AN8A is still obtainable N.O.S.  However the other tubes are probably headed in the same direction, so (assuming it has other loads) the +DD rail is likely to stay a bit high.   It may be worth arranging a HV Zener string to clamp the rail at slightly over 100V to see if that makes any difference.

[Paul Moir]

Thank you for your advice.  Ian.M yes, the DD rail only supplies this circuit; the others are pretty good.  While everything is showing some wear, this 'scope is definitely moving into it's golden years.  It won't be used more than a couple hours a year.
I have a new... well another 6AN8A coming.  We'll see how it does when it gets here.

One thing I always wanted when this was my only scope was a vertical amplifier that did DC.  Might be more fun than a tube swap?