Cool old tubes for tone gurus

[nsubulysses]

Hi I have encountered something kind of funny and I'm curious how some others would interpet it. I feel like replacing these tubes I would guess must be the original 12BH7s that came with the amp in the phase inverter of this old SVT. Transformers say 1973.

But when compared to new 12AU7s (I'd have to order BH7s) the amp performs the same, clipping at the same point for power output and also at the same volume output from my signal gen.

I don't think they're so cool they must stay. it's just that my test equipment says otherwise. I'm more tempted to bow them out and thank them for 40 years of service enduring 300W rumble.

??

[Leo_Gnardo]

AU's may work - in a pinch - but over the long haul I think you'll find the BH's "built to last." All that extra plate & glass lets them dissipate heat that will cook your AU's. Next time you need some other tubes also order up some BH's and enjoy decades of trouble free SVT rumble .

[Chuck H]

Obviously the BH7 doesn't come up much WRT modern guitar amps. I just looked up the specs (not for the first time, but the first time in a while ). What a tube! Like an AU7 on steroids! Way more tolerant of high plate volts, 3/4 watt more plate dissipation, lower primary Z requirements. The AU7 has higher "peak" specs, but it's pretty clear which one was intended as either a signal OR power tube in a pinch. As good a tube as I've found it to be, the AU7 seems second to the BH7 in this regard.

[Justin Thomas]

Maybe could be used as the output tube in a "medium-micro" amp, like the ones that use an AU7? Would this be the equivalent of putting a 6L6 in a Champ? :P

Justin

[Chuck H]

Maybe could be used as the output tube in a "medium-micro" amp, like the ones that use an AU7? Would this be the equivalent of putting a 6L6 in a Champ? :P

Justin

My thoughts exactly. I have a weird little PP OT a little bigger than a 9V battery that I pulled from an old hammerloid finished "thing" from the 50's. Some kind of two frequency signal generator with a headphone output for ??? I was going to use a single 12au7 in push pull, but now...

[Justin Thomas]

Obviously played through a 4x12"! And probably will sound friggin AMAZING!

Justin

[Chuck H]

Obviously played through a 4x12"! And probably will sound friggin AMAZING!

Justin

A 2x12"... I don't have a 4x12" anymore Just as well really. I don't need it for the bedroom and if I ever DO play a gig I'm old enough now that I would hrumph about moving a 4x12"

[Chuck H]

I just posted on a thread about the Blackstar HT5R which uses... Wait for it... A 12bh7 in push pull for a power tube! BING BING BING BING BING! So it's been done. And that amp is said to be 5 watts. Maybe a bit much for my purposes as well as possibly stressing the little transformer. Just as well since I have about a bucket full of old 12au7's I pulled from an organ about twenty five years ago. Most are microphonic to some degree but test good and should still work as power tubes.

[g1]

The BH7 is the driver tube. From what I understand, the new SVT's use AU7's instead of BH7's.
On this forum alone, I read a fair bit about guy's needing to replace the AU7's in the newer SVT's.
I could be way off the mark, but this implies to me that the BH7 is more reliable and long lived as a driver in these amps.

[nsubulysses]

So this tube seems like it's about 90% towards not being a vacuum any more since almost all the getter flash has turned clear/white. Should I let the good times roll, is what I'm sayin.

I want to replace them so his SVT can be reliable since he is in kind of a decently known touring band. However it has been 40 years so maybe I'm insane to doubt the reliability of these old BH7s

I just used AU7s becasue that's what I had on hand. If I were to replace I would order new BH7s

[Leo_Gnardo]

So this tube seems like it's about 90% towards not being a vacuum any more since almost all the getter flash has turned clear/white.

IOW you reckon the inside of the tube is at 90% of normal air pressure?

If that was the case the tube definitely would not be working at all. The filament would have oxidized and could no longer light up. Well-worn and in need of replacement, yes, but you wouldn't get anything at all out of it was full of air. Even 90% full.

I want to replace them so his SVT can be reliable since he is in kind of a decently known touring band. - - - If I were to replace I would order new BH7s

Tough enough to keep SVT's working on the road, yes do install 12BH7's. I'd suggest order a couple extra in case there's any duds, regardless of any guarantees or testing offered by the vendor. It's also not cheap to keep SVT's going. My observation is, they're like collecting locomotives. Expensive and time consuming to maintain, but so magnificent when they're working.

[nsubulysses]

I'm just saying that it looks like ~90% of the getter flash has been used and there's not much left, so maybe the era of this tube being a vacuum is nearly over.

[g1]

I just used AU7s becasue that's what I had on hand. If I were to replace I would order new BH7s

Sounds good, and I suggest you do replace them. Not sure if anyone is making them or whether you have to go NOS.
Sorry I misunderstood your question. I thought you were wanting to replace them with AU7's.

[Justin Thomas]

EH is making them, ~$17/each... NOS can be had for about double. NOS might be worth it...

Tubedepot.com

Justin

[nsubulysses]

Thank you everyone for helping me make my decision before 5PM so I can place my parts order and have it ship same day

I was really on the fence. And didn't explain my question well. I will throw a few BH7s on the order.

Also thank you G1 for helping me realize the 12DW7 is actually the phase inverter

Lots of help on here. Sometimes I need it..........you guys rule.