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:

(http://www.goodsoundclub.com/TreeItem.aspx?postID=171#171)

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.

Rgs,
Romy the caT

hagtech wrote:

Stu just found a bunch in his stash and asked about them on asylum - totally unrelated to this post.

Did your Stu ever learn that anything that deserves any real attraction or interest and exposed to the asylum’s barbarians get vandalized or juts got misunderstood by the asylum’s Morons?

hagtech wrote:

Gold pins, superb construction, etc.  So it must be something special.  Could be great for your mic pre.  You might want to try both pentode and triode modes.

Pentode, the triode does not sound as good as some other "triodes", like 7721 for instance….

The Cat

hagtech wrote:

My Stu never got a reply by the asylum inmates.  Maybe nobody there ever heard of a 7788.  No help.

http://www.goodsoundclub.com/PDF/7788.pdf

It is very stable by parameters . I have many of them, from different brands, and all of them are identically good. You can take any two of then drop replace and see that gain, nose will be the same and event a plate voltage will drift with no more then 1-5V (I drive them at 150V). 7788 has very neutral sound, not “sticky”, not “glossy” and not “showy”. It might be at “too fast” side if not properly used, but it is only if not properly used. All together it is a phenomenal thing.

Rgs,
The caT

typical line voltage should sent 7788 deep into A2 and in clipping

Romy wrote:

why don’t not you try my 6E5P?

hagtech wrote:

I didn't want to steal any of your ideas.

hagtech wrote:

Seemed to me there were enough 7788 around on eBay and whatever. I am also using it in a DAC output stage. Gain and driver in one. I needed 30dB or so and then a reasonable output drive. The 7788 has both. I like it better than the 12aX7 gain into 12AU7 cathode follower I had previously used.

This is very bogus notion

I do not believe in driver/input stages neither around 12aX7...

hagtech wrote:

Yes, it is an E55L.  I wonder how stable it is.  Interesting to me how many of these "uber-tubes" are designed for RF operation.  Should make a wonderful oscillator.

Go figure not stable are they. All those MHz tubes are very fast and defiantly need a good shock-observant mounting, rid stopper and elevation of anode over screen to minimize possible oscillation. How stable are they after that? I think it is a model by model, brand by brand and in many instances a tube by tube. The 7788 and particularly made by a good brand is very good from this perspective and from my view it has the best transconductance vs. noise ratio.

hagtech wrote:

The other find I came across was a pair of 6CL6.  Makes me want to design all kinds of new amplifiers.  The video amp schematic in the 6CL6 data sheet shows the exact series-shunt inductive peaking I used many years ago, but with transistors

Well, here were we are differents – I never have any desire to design/build “all kinds of new amplifiers”. I know many people do, it is what I call the Lynn Olson Syndrome – I do not suffer from it. I need juts one amps and one speaker and I do not found a pleasure to intermingle again and again and again with the amps and speaker. For the Olson-type of people the ceremony of intelligent BS on the subject is meaningful and self important. I am in contrary found that result is something then more motivates me. So for me that fact that there is another good tube out there or another good driver out there is not the solicitable factor. To me more important the specific problems that I recognize currently with sound and the specific direction where I would like to lead my Sound. To massage everything that is massageable, as Lynn Olson and many others do at his DIYaudio site is too tedious and ignite no interest to me. I am certainly not against the 6CL6 but it would be more useful to me if you say what is wrong with your sound NOW and how you feel the 6CL6 should change it. Well, you use 7788 with 1V of bias at line level input – I know what and why it should be wrong with your Sound now… :-)

I need juts one amps and one speaker

Lucian,

here is where the beauty of 7788 comes - max Gm with absolutely lowest noise ( the area where Russian tubes reportedly can hardly compete). What is particularly cool about 7788 that that are dead quiet one after another. I have near ten different 7788 brands all of them are dead quiet.

What you said is very interesting: “… there is no known correlation between Gm and sound quality…” This is a whole big subject. With transconductance of 55.000 in one stage and with very low signal from MC cartridge or a microphone where would be a virtue of “sound quality”? Will two gain stages of half transconductance be inherently inferior juts because it will have one more active stage? It is hard to say.

I have Dima’s designed 7788-7721 phonostage with 65dB gain in juts two stages. It is very good; would it be the same if it were 3 stages? I doubt. BTW, the 7788 is very sensitive to the “quality” and type of screen voltage, sometimes I will play with this subject more.