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Featured VT: The 6BQ7

Posted 19th January 2013 at 07:01 PM by Miles Prower

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This is a small signal dual triode in the nine pin mini format. From the spec sheet:
Quote:
The 6BQ7A is a miniature, medium-u, twin triode primarily designed for use as a cascode radio frequency amplifier in very high frequency TV tuners. In this application, its performance is characterized by low noise figures and high gain.
From the piccie, it's obvious that this is indeed a high frequency triode. It has the additional advantage of a high gm, made possible by the comparatively enormous cathodes. These are much larger than those of other small signal types like the 12AU7, 6FQ7, 6SN7, 6C4. The 6BQ7 also has a u-Factor that falls nicely between types like the 12AU7 or 6FQ7, and the 12AT7.

The spec sheet even includes a composite plate characteristic for cascode operation. What you don't find is any mention of any sort of audio applications. The RCA Receiving Tube Manual does make a concession...
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Featured VT: The 6J5

Posted 19th January 2013 at 02:37 AM by Miles Prower

This type is a singleton triode with an octal base. As nine pin mini types proliferated during the early 1950s, few singleton triodes were made, and the few that you do find are almost always VHF amp types intended for 400MHz+, running as grounded grid amps. Most triodes were made in pairs, such as the 12A*7 series, or the 6FQ7, 6DJ8, etc. If you need a singleton triode in the seven or nine pin mini format, you will either have to go with a type like the 6C4 (a type that is not recommended as an audio triode, but rather a low power, Class C RF driver/final) or make a pseudotriode from a small signal pentode. (The 6AU6 works nicely for this, and it has a u-factor that fits nicely between the medium-u triodes like the 12AU7 and the high-u types like the 12AT7 or 12AX7).

The 6J5 appeared with the Octal base, and metal envelope. There is also a glass Octal as well, but is much harder to find, and more costly to acquire. Regardless of packaging, the type is a small signal, medium-u,...
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Featured VT: The 6BQ6

Posted 18th January 2013 at 01:45 AM by Miles Prower

The 6BQ6 as an audio final came to my attention as a chance result of finding
an old article from a Portuguese (Brazilian? -- it was written in Portuguese)
ham magazine. This described a simple AM plate modulator that claimed an output
of 30W (fixed bias) or 25W (cathode bias) that used a push-pull pair of these
VTs. You would expect to see 6L6s or 807s used in this particular application.
Why 6BQ6s, and what were they?

The 6BQ6 is a large signal beam former. It has no audio pedigree
whatsoever, and the spec sheet makes no mention of its use as an audio final.
During the 1950s, screen sizes and deflection angles increased, giving a larger
viewing area, and a shorter CRT for more compact TV sets. This development meant
that the usual audio finals and RF types became increasingly unsatisfactory for
horizontal deflection duty. New types more suited to the task were developed,
and one such type was...
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Featured Vacuum Tube: The 807

Posted 15th January 2013 at 10:05 PM by Miles Prower

Featured VTs: The 807

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807

The first of the beam formers is also one of the most enduring types: the 6L6. Developed by RCA in the mid-1930s, this type was originally intended for use as an audio final. It included other, then new, features besides the elimination of an actual, physical suppressor grid required to smooth out the screen grid "kinks". This included the now standard Octal base (up to eight pins possible, and with a keyed base for proper socket alignment) and a metal envelope. The latter was made in one of two ways: a glass envelope VT slipped into a metal shield can, or using the shield can as the envelope, with a glass base to bring out the connections. Other improvements was to give the control grid and screen grid the same pitch and wire diameter. By overlaying these two grids, the negative control grid serves to "shadow" the screen, thereby reducing the useless...
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Cathode Follower

Posted 14th May 2010 at 11:40 PM by Miles Prower

AC Coupled CF

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In this schemo, Rk is the normal cathode bias resistor. Rl represents the tail load in parallel with the load impedance. Rg is the control grid DC return. The CF gives excellent high frequency performance since Miller Effect is absent, and the Cgk sees very little current since the grid and cathode are always at nearly the same potential. This makes the Cgk effectively smaller than its static value. The main component of input capacitance will be the reverse transfer capacitance: Cgp. With small signal triodes, it is easy to present a Hi-Z, Lo-C load to the driving stage. This isn't just helpful at RF.

This is another circuit which has lately come under unjustified criticism within certain audiophile circles. Much of this is unjustified on the basis that the CF is a negative feedback circuit. This view that all NFB is all bad does have a basis in fact. It has...
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Demystifying the Cascode

Posted 14th May 2010 at 11:25 PM by Miles Prower

Basic Cascode

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This is what the schemo of the cascode looks like. Rk serves to establish the Q-Point bias for the lower triode, and Rg is its DC grid return. Rp is the passive plate load. The voltage divider connected to the grid of the upper triode establishes its Q-Point bias.

So why would you want to do this? What you have here is a cascade of a grounded cathode stage driving a grounded grid stage. The GC topology has the advantage of a Hi-Z input. However, its high frequency performance is impacted by a high Cmiller that only grows worse with increasing voltage gain.

The GG topology avoids Cmiller for excellent high frequency performance, but it suffers from a Lo-Z input. It's not very often that a Lo-Z input is desirable. However, you can combine the two in a manner that work together. The Lo-Z of the GG stage loads down the plate of the GC stage, reducing its gain...
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