Help with tube swapping

[aaronchoo]

Hi guys!

I'm actually very new to tube amps so I need some help here.
Im using a blackstar HT-5 head and am thinking of swapping the tubes for better clean headroom. Which tube should I be changing, the preamp tube or the power tube?

The stock tubes are the EH12BH7 and SOVTEK12AX7.
And which tubes do you recommend that I get to replace them?
Any additional servicing needed after replacing the tubes? Rebiasing etc.

Thanks in advance!!

 

[dodgethis]

Clean headroom is normally associated with the power section of the amp. From what I understand, the 12BH7 works in the power section of the amp. For what purpose, I am not too sure. Maybe the same as Vox's Valvetronix. If the 12BH7 is the equivalent of the the 12AU7 (19% of a 12AX7), it has the lowest gain in the entire 12**7 series of tubes. Changing that to something else will not help much. So much for their claims of this being a full tube amp. Damn thing has no power tubes at all.

What you can do is swap out the 12AX7 in the preamp section to something of a lower gain, like the 12AU7 or 12AT7 (60% of a 12AX7). This is will lower the overall gain of the amp, so it might allow you crank the thing louder at a lower gain level. If you do not want to try this first, lower the gain first in your preamp section and crank the sucker up. If it can blast louder without breaking up, you might want to go ahead with the tube swap. (This whole paragraph is a big MAYBE.)

As for what tubes are good, no one can say for sure. What may sound good to someone may sound like shit to another. Here is a general review on what each 12AX7 tube may give you. While swapping out other power and preamp section tubes give a change to tone, the tube swap in the preamp section would make a bigger change, as the pre-amp is where the tone shaping takes place.

Where to buy? Burlington Square, Adelphi and maybe Sim Lim Tower.

Biasing is not needed since you are just replacing preamp type tubes. Biasing is only needed for power tube swaps, though if you have a tube power section, changing the phase inverter valve may require you to do a rebiasing, just in case.

 

[lazy3yed]

The 12BH7 is actually the power tube (common in audio applications). I believe there is a trim pot inside to adjust the bias. You will need to rebias if you change the 12BH7 power tube, else it may sound like crap.

Think the biggest improvement you can make is to change the preamp tube to a lower gain one : 12AT7 or 12AU7.

 

[aaronchoo]

Thanks guys for the input really appreciate it!