Mark Reccord said:The 12DW7 is back in production.
Nice to know, but still not really a good reason to change the design back, other than for sentimental reasons . . . . but no harm if they did (unless the 12DW7 submarines again . . . . )
- Tim
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Ampeg SVT-VR
- Thread starter gwx014
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Nice to know, but still not really a good reason to change the design back, other than for sentimental reasons . . . . but no harm if they did (unless the 12DW7 submarines again . . . . )
- Tim
tadawson said:
You're totally right with respect to the new SVTs. At least we know that there'll be a pretty much guaranteed supply of 12AX7s and 12AU7s.
I was pretty happy when they started making 12DW7s again because I have a 70s SVT and I didn't really want to mod it for 12AX7s.
And to use a single tube instead of two! where the svp-cl replicates the 12dw7tadawson said:
but, maybe im the one being silly here . . .
My recollection of the conversion is that the tube count does not change at all. Since there is more than one 12DW7 in the SVT, each 12AX7/12AU7 pair will replace two 12DW7s. Since the SVT has three 12DW7's in the preamp, and one in the power amp (for a total of four) then two 12AU7s and two 12AX7s will do the conversion in the same number of sockets.
Actually, the Ampeg published procedure for the SVT was even easier than that, since they didn't use the 12AU7 at all . . . R25 and R6 were changed in value, and the tubes merely swapped. (The 12AU7 and 12AX7 are very subtle in their differences, and most of the old substitution manuals indicate that they were considered pretty much interchangeable in most circuits. And yes, I do have those old manuals . . . .)
- Tim
Actually, the Ampeg published procedure for the SVT was even easier than that, since they didn't use the 12AU7 at all . . . R25 and R6 were changed in value, and the tubes merely swapped. (The 12AU7 and 12AX7 are very subtle in their differences, and most of the old substitution manuals indicate that they were considered pretty much interchangeable in most circuits. And yes, I do have those old manuals . . . .)
- Tim
- Mar 20, 2005
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- I endorse & use Tech 21 pedals, Eminence loaded cabs, EMG pickups, Jim Dunlop picks & Ernie Ball Strings, BC Rich Basses.
well I heard from a reliable source that the SVT VR is actually going to have 437 tubes in it, and thats just in the preamp... 
tadawson said:
You might want to check those old manuals......
The 12AX7 and 12AU7 are really quite different. They have completely different amplification factor (18 vs 100) different max current, etc. Looking at the plate characteristic curves, there is a nearly 10 to 1 difference.
You may be thinking of the 12AT7, another "high-mu triode".
i_got_a_mohawk said:And thats damn right, who needs a gain control or tube protection circuit!
Isn't the lack of a tube protection circuit one of the reasons some folks prefer the original SVT-II over the IIPro? How does tube protection affect an amp's sound and performance?
Dont know to be honest, was just joining the ranting band wagontightbidness said:
The original SVT-II has a different pre section than that of the 2-Pro, and i do know that the protection circuit was dodgy in the original 2-Pros and CLs, but thats not really a tone thing
Jerrold Tiers said:
The point here is that the 12AX7 is a higher dissipation device, which will typically work in a 12AU7 socket, and that the procedure that I have which was supposedly published by Ampeg to adapt the old 12DW7 SVTs when the 12DW7 went out of production used ONLY 12AX7s . . . so apparently Ampeg thought they worked pretty well in place of the 12AU7 side of the 12DW7 as well. Not a perfect match, but apparently acceptable . . . . . and yes, the 12AT7 is of a similar family as well.
Oh, and just rechecked the manual. It says what I said - the 12AU7 can be used to sub a 12AX7, but not in all circuits. The 12AT7 is NOT listed as a sub!
Interestingly, both the 12AU7 and 12AX7 are listed as subs for the 12DW7, but once again, with the caveat of not in all circuits. Electrically, all three tubes are a twin triode, with identical pinning and maximum plate voltage specs. As you stated, gain varies a bit, as does maximum dissipation, with the 12AU7 being the highest as 2.75 watts, and the 12AT7 second at 2.5, and the 12AX7 at 1 watt, with the gains falling in the opposite order . . . .
Sources are the 1954 RCA receiving tube manual, and "11,000 Direct Tube Substitutes" by H. G. Cisin, 1963.
- Tim
- Tim
tadawson said:
OK.... some more serious tech-talk.....
Of course the tubes are "pluggable-in-able" in the same socket, and they will "function", that is they will usually light up and pass signal.
However, their "substitutability" is lots better in a TV than in any audio amplifier. The substitution book listings must be taken with a shovel or two of salt.
The books are entirely generic, and give a lot of weight to pinout and maybe the maximum limit ratings (voltage, current), as opposed to functional specs like gain, etc. You cannot expect more than the unit "probably functioning" with the broad range of subs listed, although many subs listed may be perfectly acceptable, at least in a TV if maybe not in an amplifier.
That old Ampeg sub procedure changes some resistors as well, to adapt the 12AX7 into the SVT preamp. The results sound-wise are NOT the same as with the 12DW7 or with using the 12AU7 where the medium-mu portion of the 12DW7 was used.
It was a compromise because 12DW7s were horrible at the time. Something had to be done, without re-wiring sockets and making major changes.
On the other hand, the use of half of a 12AU7 for the medium-mu portion of the 12DW7 is essentially a perfect match. The differences are primarily related to physical proximity, and not the gross electrical characteristics.
As far as the similarity.... the gross characteristics between 12AX7 and 12AU7 are rather different. That will be reflected in performance.
The 12AX7 has a plate resistance of 62,000 ohms at typical amplifier voltages. The 12AU7 has a plate resistance of around 8,000 ohms.
The current ratings are different, the 12AU7 has much higher current capability.
The plate characteristic curves are very different, with about a 10:1 difference over same range of plate and grid voltages, as you would expect from the drastically different plate resistances.
Quite a difference, and reflects the different intended usage for the two tubes.
Just swapping one for the other may lead to some interesting surprises. Just try it with your guitar player's amp sometime, swapping some 12AU7s in for the 12AX7s, and see what the reaction is !
Which kinda sums up what I have been saying - not a perfect substitute, but at least a functional one . . . . and I did mention the resistor change in the Ampeg procedure in post 46 . . . .
- Tim
- Tim
- Mar 20, 2005
- 794
- 3
- Disclosures
- I endorse & use Tech 21 pedals, Eminence loaded cabs, EMG pickups, Jim Dunlop picks & Ernie Ball Strings, BC Rich Basses.
I have & it is pretty slick. I of course did not have a vintage SVT there to compare it with, but compared to the SVT CL, it just had much more "air" and depth. In the past I a/b'd my vintage SVT with an SVT CL and noticed the same thing. Through the same cab and set flat, the SVT VR sounds like you turned the horn on, but of course the 810 Ampeg has no horn. The VR is just more present than the CL. If I didnt own a '74, I would likely get one. I think the list was $2499, it was quite a bit higher than the CL. The matching cab is the SVT AV 810, which was not changed to match the vintage specs. but the 810 cab takes on a whole new life with this amp. It sounds really good, I was very impressed. Dennis Kager was the consultant for this amp & I think they put a whole lot of work into making this amp right.
The SVT 8 pro is a pretty bitchen new amp too. If I undrstood the description correctly, its the SVT VR's preamp paired with a a 1400 watt or so class D amp that can be run down to 2 ohms. It is the amp you SVT lovers have fantasized about, a 30 amp that sounds like a tube SVT.
They also have a new ported 8 ohm 112 cab with a horn that was pretty friggin fat sounding IMO. Those are the 3 items I spent time with and was duely impressed. Oh yeah, I saw a couple Dan Armstrong lucite guitars hangin on the wall too, but I am not sure if those are new production items.
Sorry for a lame descriptions, its been a long day. But i saw and heard some killer new Ampeg gear today and I am sure you will hear more about it all soon from someone else.
The SVT 8 pro is a pretty bitchen new amp too. If I undrstood the description correctly, its the SVT VR's preamp paired with a a 1400 watt or so class D amp that can be run down to 2 ohms. It is the amp you SVT lovers have fantasized about, a 30 amp that sounds like a tube SVT.
They also have a new ported 8 ohm 112 cab with a horn that was pretty friggin fat sounding IMO. Those are the 3 items I spent time with and was duely impressed. Oh yeah, I saw a couple Dan Armstrong lucite guitars hangin on the wall too, but I am not sure if those are new production items.
Sorry for a lame descriptions, its been a long day. But i saw and heard some killer new Ampeg gear today and I am sure you will hear more about it all soon from someone else.
- Mar 20, 2005
- 794
- 3
- Disclosures
- I endorse & use Tech 21 pedals, Eminence loaded cabs, EMG pickups, Jim Dunlop picks & Ernie Ball Strings, BC Rich Basses.
Fuzzhead said:Sounds way cool...did you happen to notice if there was a pre out/poweramp input on the back, or switchable voltage? That might be considered a nono by vintage folk though...
Hopefully someone took a camera!
i know it has an xlr out. i'll look more closely tomorrow... or shall i say in a couple hours... it looks like the blue line svt, but the power light on the front changes colors when the standby is off and the standyby switch is on the front where the polarity switch is on the vintage. as i said, its clear they spent some time designing this thing.
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