About to change tubes in my bass head - opinions?

[toronado]

Hi all,

any tube amp freaks in here?
I'm about to change the tubes (Preamp + Poweramp) in my Sunn 300T head. That would be a bass amplifier.
As for power tubes, I'm pretty much set on a 6 pack (haha) of Svetlana 6550C's.

But I'm not so sure yet about the Preamp section. I need 3x 12AX7/ECC83 for V1-3 and 1x12AT7 for V4.

I heard good things about the JJ tubes, and also the russian made Electro Harmonix. Inexpensive and good sounding, though I am aware that there is not the ultimate tube that works for everything and everyone.
So, I'm looking more for some input, what were your experiences and so on, rather than a masterplan. Thanks for the input.  :icon_smile:

 

[mayfly]

I remember those heads.  I've fixed a few of them and they are built more like old TV sets than amps.  Having said that, the construction was really solid - just very different.

Go with the JJs.  In the past I've gotten these by the dozen and very rarely had a failure.  They all sounded pretty much alike as well.  I'd do those ones.
BTW - if your old pre-amp tubes are working, why change them?  If they are not mechanically upset and are not noisy or microphonic, they will last forever.

Finally, the Svetlana tubes will do you fine.  Please bias them a little on the cold side.

 

[toronado]

Thank you, mayfly.

The correct bias is 100mV according to the manual. Would you rather bias it to only 80mV?

 

[mayfly]

If the 100mV is referring to the voltage across a low value resistor in the path from cathode to ground, then yep, I'd use 80mA or so - but I'd also check it with a scope to ensure I didn't have any crossover distortion.  BTW - that 80mV must be for a pair of them - correct?