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I am getting acquainted with my DR 105 which just returned from being serviced.

I asked the tech to leave the preamp tubes be since I have a stockpile of tubes to fool with.

Somewhere a I saw a spec sheet for the amp or maybe the SC L100. It mentioned there should be 4 12AX7s and one 12AT7. That is the case.

But the 12AT7 is in socket #4. I'd assumed it would be in #5. Is #4 correct?

Thanks!

I'm not really familiar with the layout, but the PI (12AT7) is normally the second-from last tube in both early Hiwatts and Sound City amps. If you want to post a picture of the socket (from the wired side) I can confirm this.

Thank you so much for the fast response. That is where is it seated. A Mullard, no less.

Attached is the shot from underneath with the new caps and such.

If I could trouble you with one more Hiwatt newbie question: on some amps it is said that you only need to put your "best" pre-amp tube in slot #1. But this baby has four inputs. So I assume, there are at least two tone circuits? So perhaps there is a second slot which might benefit from a better tube? Which might be best suited for a 5751 rather than a 12AX7? Not that I'm all that obsessed -- I play bass!

Thanks again!

yes,
the next to the last socket would be the 12AT7/ECC81

from left to right if facing the amp:

V1-12AX7, V2-12AX7, V3-12AX7, V5-PI-12AT7, V4-12AX7

Great! Thank you.

It appears that the Mullard 12AT7 is good. A Sylvania AX7 was microphonic and an RCA seriously compromised.

I put a nice 5751 in V1. Is that sensible? Is there any reason to put a "fancier" 12AX7 in any of the sockets?

Questions, questions, questions ...

Thanks.

happyface wrote: I put a nice 5751 in V1. Is that sensible? Is there any reason to put a "fancier" 12AX7 in any of the sockets?

I run a 5751 in V1 of my 1980 SA112 (to tame its rather hot front end). If your amp has enough gain and you've found a non-microphonic 5751, no harm in using it. Otherwise, try your best 12AX7/ECC83 in that socket.

I haven't owned a Hiwatt with a V5, but I've found V2 and V3 to be less demanding and my amps seem happy with modern production tubes there.

Dave

Thanks!

I agree with Dave, but since inputs 1 & 2 use V1 and 3 & 4 use V2, this amp would be perfectly fine with run of them mill 12AX7s in V3 & V4

Thanks.

That makes sense to me.

Being a bass player, it has been a while since I have used a multi-channel tube amp. And this is my first Hiwatt.

Sorry for the newbie questions. I switched to bass from piano in 1966. Those were tube amp days and I even built some amps from kits back then. But that was just "paint by numbers" stuff = I never learned the theory behind what I was soldering in. Maybe when I retire....?