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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: 7788 - love the tube!Posted by Romy the Cat on: 2/14/2006

 rafaroxlm wrote:
As you may know the 7788 is a high transconductance pentode which should in theory be able to give higher audio resolution than say 12AX7, 6072 tubes in tube mic pre designs.  On search here the 7788 was mentioned as being used as the first tube in a phono preamp design but no schematic was available.  Woild appreciate assistance from the tube Gurus here in coming up with a design for a 7788 transformer input tube mic pre design.  Have some UTC A-10, LS-10 as well as output trans available.


I’m very much not a guru but I feel that 7788 is a freakishly wonderful tube, probably one of the best out there for the two stages correctors considering the relations of gain to sound quality and to noise. There are quite few ideas of 7788 phonostages around it are all depends what a person might look for. I use 2 phonostages with 7788: one with passive RLC filter (designed by Thorsten) and one with low capacitance feedback to high impedance, identical to what was used in EAR-834P (designed by Dima Kireev)

The idea to use 7788 come to me a few years ago form a guy that I know who used 7788 for almost 30 year for exactly the same purposes that that you intended to use it: the microphone preamps.  After he made me to use this tube I do love the tube a lot:


I have built and used tree 7788-based RIAA correctors that give me some perspective to this tube. Use a few dozen Ohm grid stopper and ferrite bids sitting at the grid entry. Use shock-mounted tube sockets. I do not think that my operation point would be relevant as it would be a property of the specific application, impedance loading and so on.   What is very important is paying super attention to the screen voltage and it’s type.  Usually pentodes sound very idiosyncratic: hard and sharp. Some Morons recognize it as evidence of “quality” but I degree with them. The artistry of using pentodes is to teach the pentodes to sound soft and liquid. It is possible with 7788 and the changing the type of the screen voltage is one of the primary tools to accomplish it.

In the end I would agree with Jim Hagerman and  would propose that the “higher audio resolution” is a bogus concept. Mostly people recognize the “higher audio resolution” as the excessive HF extension and zipping of sound. In reality, there is no such a thing as “audio resolution”. Beside if one would claim that 7788 has higher audio resolution then 12AX7 then a person should propose that 12AX7 has deficiency of “audio resolution”. What would it mean and how a person could judge “audio resolution” (whatever it means) without a context of the specific application and without the specific results? The  7788 is mHz level TV tube and defiantly faster then 12AX7 but the irony is that the 7788 begins to sound “humane” only when it’s UHF bandwidth got killed and what it begin to sound like a triode (12AX7) , but a triode with ultra high gain and ultra low noise.

Romy the caT

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