Dandolin wrote:Pretty sure an expert will jump in to nullify what I've said
Nah, you got it. Though I would also argue I am not an expert.
Blood_mountain wrote:Yeah, I was wondering about that. I have just read that some folks swapped out the original tube and liked the sound better. I found a vintage Telefunken 12ax7 from an Ampex console for $60 CAD ($46ish USD), so I might try that out just to see if it makes a difference.
Those can be pricey tubes, if you can carry that cost as pure experiment it may be well worth doing so. Do note, if the white paint exterior lettering is intact do whatever you can to AVOID TOUCHING IT. That stuff will wipe off like fog on a mirror, and if you're lucky enough to get undisturbed printing the considered "collector value" is greatly increased...
Anyway, back to the reply I was about to send on:
I have done my share of tube swapping, and am of the opinion that some circuits benefit from it more than others. I have no experience with your preamp, so, I'm not going to weigh in on that; but I do see they claim 200 volts on the anode, so it's probably worth the effort to experiment. It looks like it's mono, so balance between halves is not as paramount a concern as it is with hi-fi & amp stages.
That said, I have not done a lot of shoot-outs between old stock and more modern tubes, because, well... I don't want to spend cash on modern tubes that I do not at present need. I have done preamp tube substitutions in stuff ranging from studio gear to hi-fi, and I tend to think there is greater impact with respect to hi-fi, as at that point the valve is affecting the entirety of signal at the "introduction to room" stage.
You're going to get beneficial differences with the lower gain (12AU7/ECC82 & 12AT7/ECC81) tubes primarily where current capability (say, driving a transformer connected to a reverb tank - or driving a tank through a cap) is desired. They do offer lower gains (60 & 40 respectively against the 100 of a 12AX7 - IF MEMORY SERVES), which can prevent distortion in *following circuits*.
There are tons of tubes that will drop into that slot (200 volts is fairly mild, and should pose no restrictions), 7025 is the industrial designator (MIL-SPEC or JAN use both, if memory serves).
NOS tubes in the common types have been an item of widespread interest for several decades now. This means they've largely been picked clean and what passes for NOS these days is either not, or expensive (or both). Furthermore, it's my understanding that by and large any available JAN tubes are probably soft rejects and are therefore undeserving of complete adoration.
Am I sounding like a downer? I don't mean to - I just mean to say DO NOT OVERPAY unless you have tried said tube (as in, that exact tube) in your application and have observed first hand any benefits.
I swear, some things just sound good no matter what you stuff in them.
Without further ado:
Flat (or smooth) plate Telefunken are generally considered excellent. I've seen them branded Fisher (or other audio gear companies as well). Telefunken are easily identifiable by the diamond shape cast in the glass envelope between the pins.
Ribbed plate Telefunken are also good, gaining traction when clean flat plate examples started bypassing the $200 mark.
Amperex (Bugle Boy) are also excellent tubes, I've seen these in test gear before.
Mullard are, of course, another generally excellent breed, but I feel like we're starting to veer into "voicing" territory a bit (read as: consistency is not as prevalent). This is, of course, a completely subjective missive on my part..
RCA "black plate" are another highly considered tube, but in my opinion at this point we can just dip into the ENTIRE spectrum of U.S. consumer based tubes because it seems there are good and not so good examples of tubes across the board. I may be guilty of not giving the domestics enough consideration, but I am generally happy planting a GE, Sylvania, RCA, or any of the OEM labelled variants willy nilly.
I have some NEC tubes that are excellent as well...
But as with anything 50+ years old, there are more and more duds out there all the time, so use your ears.
TL;DR: try and find pull jobs from the HAM radio community, accrue a small collection, and give anything that costs more than a decent meal a listen to first whenever possible.
