HT Stage 100 pre amp section

[holdercashqtn]

I own a blackstar HT stage 100 and I still love it as much as I did the day I bought it (december 2011.) I had never changed the power tubes or the pre amp tubes until 2 weeks ago I heard a significant volume drop on the dirty channels. I would switch it back to clean and it would be powerful, loud and clear. When I would switch it back the volume just wasn't there. Luckily I cranked the individual channel volumes and got it back up to a gig worthy level.

I figured since the problem was only with the preamp tubes because it didn't lose power in the clean section. I went out to find the tubes locally and found the two 12AX7's easily. But the 12AU7 doesnt seem to be in stock anywhere. I ran out of time before the next gig and replaced all three tubes with 12AX7's. it seems to be working fine. the sound is as good if not better than it ever was.

My question is, why use a 12AU7 in the design of this amp? would a 12AX7 in its place work as a long term replacement? Will it cause damage to the amp? Most preamp designs use 3-5 preamp 12ax7's, I guess I just don't know why this blackstar is different. Thanks for any info you guys can provide.

[holdercashqtn]

Nobody has an opinion on this???

[MikeD]

not really an opinion but when looking to change the ax7 in my boost pedal I came across someone wondering if it would be possible to use an au7 instead and the reply there was that it was possible but with risk of burning out the transformer do to the use of a tube that uses higher wattage. Also when changing tubes in my ht 100 head I was recommended to use one of the "au" standard to "tame the beast" ....as it was put.....my guess is that an AU7 gives a warmer tone and rounds the amp of which enables it not only to be used for metal but for Classic rock.

.....really just guessing...but at least it´s an answer

...here´s a good tube thread...don´t know if it answers just your question but mabye It´ll provide you with some clues

http://www.blackstaramps.com/forum/view ... =28&t=1881

[Hedgehog]

The AU will be the phase inverter valve (only the HT100 has one, all the smaller HTs use a solid-state phase inverter). I don't know for sure if any other type would work, but I'd be inclined to stick with an AU if you can get one, otherwise possibly try an AT rather than an AX. The phase inverter isn't really part of the preamp even though it's a "preamp" type valve - it's technically more part of the power amp section.

Swapping the PREAMP valves for other types works quite nicely to change the gain structure and tone, as has been mentioned. Quite a few people (me included) use a 12AT7 in the overdrive channel preamp - makes it much easier to get a "slightly breaking up" tone, but still allows loads of gain if needed.