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RE: That won't protect the grid.

Posted by Cleantimestream (A) on July 28, 2016 at 14:08:12

In Reply to: RE: That won't protect the grid. posted by dave slagle on July 28, 2016 at 09:39:10:

Am sorry Dave... my fault... lost in translation... My response to Michael Samra was because he made mention catastrophic failure of a direct coupled design was why he did not want to build one. My response was that said direct coupled cathode follower has had thousands of hours on it and remains absolutely constant as the northern star.

So far as the 5 watts... that is what Raymand Bates said... specifically 4.5 watts {that was the 1949 schemo I threw up there and here is the whole dissertation}...Complete schematic diagram of direct-coupled 6V6 cathode-follower amplifier.

"There are two features that give this amplifier its superior performance. One is direct coupling between the plate of the 6SJ7 and the grid of the 6V6. The other is the cathode-follower output from the 6V6.

In addition to its simplicity, direct coupling eliminates the undesirable characteristics that are inherent in ordinary resistance-capacitance coupling, such as short circuiting of weak signals and grid blocking of strong signals.

The cathode-follower output, in addition to its simplicity, provides both improved high and low frequency response, damping out of all the peaks in both the output transformer and speaker, less distortion, and 100 percent degenerative feedback.

In order to determine the circuit values in the schematic, free use was made of the tube manuals and Kirchoff's and Ohm's laws. Commencing with the output stage in the conventional manner, the tube manuals indicate that for a single-stage output, a 6L6, 6V6, 6F6, or 6K6 are likely output tubes. The 6V6 was selected because of its ready availability and relatively lower percentage harmonic distortion rating. The 6SJ7 was chosen mainly because of its high gain and low percent distortion.

In order to keep power requirements down, I chose a 350 volt, center-tapped, 120 mA (53 mA only required) power transformer which is readily available at moderate cost. Allowing for a 15 volt drop in the filter choke and a 250 volt drop from the plate to the cathode of the 6V6, 85 volts is available at the cathode of the 6V6. Since in a direct coupled circuit the grid bias, 12.5 volts in this case, is obtained by the voltage differential between the cathode and grid, approximately 73 volts is required at the grid of the 6V6 (and at the plate of the 6SJ7, inasmuch as these two tube elements are connected directly together). With 73 volts at the plate and 55 volts at the screen of the 6SJ7, a voltage amplification of 115 can be obtained at only 0.8 percent distortion. This means that a 0.1 volt signal at the grid of the 6SJ7 will provide a 11.5 volt signal at the grid of the 6V6 which is considered adequate.

In accordance with Kirchoff's law, the voltage and current distribution around the circuit is indicated in the schematic. Note that there is a 12.5 volt drop through the DC resistance of the primary of the output transformer which was measured to be approximately 250 ohms. The cathode-follower output is obtained simply by connecting the transformer to the cathode of the 6V6 and tying the plate and screen of the 6V6 together to the "B plus" supply, as shown.

The output of the amplifier is approximately 4.5 watts. While this figure may seem low to those accustomed to dealing with amplifiers having output of 20 or more watts, it is entirely adequate for home use when an efficient speaker system is used.

Tests made by various organizations have shown that for listening in the average home living room, an output of less than one watt is generally used. The only reason for providing more power than this is to allow for the peak passages that occur in some classical compositions.

The input sensitivity is such that even the high-quality, low-output crystal pickups will provide sufficient drive.

No provision has been made for the use of variable reluctance pickups, although there is no reason why such units could not be used if a preamplifier stage were added. There have been many satisfactory preamplifiers described in various issues of this magazine.

In the event a preamplifier is used for the magnetic pickups, great care should be exercised in the shielding and placement of parts to avoid hum pickup. The careful selection of the tube used in the preamplifier will aid materially in the reduction of hum.

There is sufficient reserve capacity in the power supply to take care of almost any type of preamplifier without any trouble.

No provision for tone controls has been made, although they could be added with little difficulty. There is a great deal of controversy as to the best type of tone control, and it was felt that they could be added at a later date when the most satisfactory type had been determined by experiment.

It is essential that the primary of the output transformer, T1 have a resistance of approximately 250 ohms, as the resistance of this winding determines the grid bias for the 6V6. A check of the various types of transformers on the dealers' shelves by means of an ohmmeter will be sufficient. The primary impedance of this transformer should be from 5000 to 6000 ohms, with the higher value giving slightly less distortion.

The voice coil winding should be selected to match the speaker in use. Most of the better speakers have an impedance in the vicinity of 8 ohms, and for that reason t his value is specified.

It is not essential that this transformer be of the sealed type, but it should be of good quality so that the full benefit of the amplifier may be realized. Poor transformers are usually deficient in frequency response at the upper and lower frequencies.

The power supply, being conventional, needs no explanation."

I use a 6F6 and calculated .85 watt

For bomb proof design {screen and plate protected} in direct coupled schemos ... see Thorsten Loesch and Jeremy Epstein.
The Mind has No Firewall~ U.S. Army War College.

Edits: 07/28/16 07/28/16 07/28/16

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