It seemed like such a simple job. By now, I've installed three of the four basic EFB(tm) configurations into three different 6BQ5/7189 designs, and all with superb outcomes: output stage distortion was significantly reduced (by at least 50%), and output tube quiescent dissipation was lowered to typically 75% of Design Center ratings, or 65% of Design Maximum ratings for the tubes -- as opposed to so many designs that operate these tubes at 90% or more of their Design Maximum ratings. Most importantly, these reduced dissipation operating levels now represent the lowest distortion operating point for the tubes, producing significantly less distortion than the stock design did at much higher dissipation levels. Couple these performance improvements with the significant effect this has on tube and component life, and its easy to see why those who have installed the EFB modification in their equipment to date have heartily endorsed it.
I've always subscribed to the notion that a job worth doing is worth doing well. I wanted to install EFB in a 7591/7868 application, because soooo many pieces of equipment use those tubes, and many of those designs operate these tubes very hot as well. Therefore, a design with these tubes became the next logical target for EFB, and my own Fisher 400 became the unit of choice to take aim at. Except, I've come to find that the 400 is kind of like the proverbial thread that just keeps coming when you pull on it.
First up, was identifying what I had. My unit is a latter unit with an extra FM limiter stage installed, and a revised bias circuit to cool the output stage down. Except, that when I was making my stock performance measurements to develop a base line for comparison, I found the stock unit could not meet the original published specifications for the 400. What gives? Some quick checks showed that with the turns ratio in the OPTs installed in my unit (original Fisher devices), it could not possibly produce the power output claimed by Fisher -- even on a sunny day with matched NOS tubes and a high line voltage. Checking the Service Manuals showed different part numbers for OPTs in the two basic versions, and searching further turned up yet more clues pointing to an OPT change as well. The point, and bottom line of this information, is that the base information and the EFB operating points I've developed in this exercise potentially only relate to the latter unit. I wish it could relate to all of the versions, but based on all available information, those wascally Fisher engineers apparently decided to make a change in the output stage -- and one that would appear to be big enough so as to require different EFB operating points for proper operation in each version. Make no mistake, both versions can benefit greatly from EFB, but the operating parameters would be different in each application.
So what have I got with my "version II" of the 400? Basically, a unit that produces 25 watts RMS per channel maximum (or 22.5 watts RMS both channels driven), from 30 Hz to 20 kHz. This is not as powerful as the specifications state, or as possibly the original version of the 400 is, but is still a very nice power niche to be in with efficient speakers. Distortion is pretty typical with single channel performance at 25 watts returning 3.75% THD @30 Hz, .5% THD @1 kHz, and 1.90% THD @20kHz. With both channels driven at 22.5 watts, distortion at 1kHz doubled to just over 1%, while distortion at the frequency extremes more than doubled. This is all as driven through the Aux/Tape input with the tone and balance controls accurately centered.
Actually, I'm ahead of myself however, because to deliver even these performance results, I had to deal with the rather unique phase inverter circuit, so there is a lot to tell with this tale. As a result, this effort really amounts to a redo of the power amplifier circuits in the 400 version II, but as a teaser, some trial results in developing EFB for the modified power amplifiers returned a 1 kHz THD of just .22% with both channels driven, and a 20 kHz THD of only .80%. This is nearly an 80% drop in distortion at mid frequencies and over a 70% drop at high frequencies, while at 30 Hz, distortion fell to about 2.5% with both channels driven at 22.5 watts output each. The transformers themselves are the main factor at this frequency in terms of delivering a low distortion output.
This was also produced from a very good and well matched (+/- 1 ma) quad of NOS 7868 tubes from Sylvania, of late 70s origin. I should also say that with the unit operating at 117.0 VAC, I found tubes that would idle as low as 26 ma, and some as high as 40 ma in this unit, with the vast majority of tubes idling at about 34 ma, which I believe was the target quiescent current draw by Fisher for this version. This is the current level each tube in my NOS quad drew in the stock design, and the current draw the base measurements were made at. This results in a stock dissipation of just under 14 watts per tube, which is hardly a bad place to be for plate dissipation in these tubes -- except for how closely they are physically spaced together on the 400's chassis, and particularly so when a cabinet is used. Overall however, in the Version II design, plate dissipation is not nearly the concern with these tubes that another factor is.
In the Version II design, because of an apparent change in OPTs, the screens of the output tubes get particularly pummeled at elevated power output levels -- much more so than than they already do with more traditional 7591/7868 operating parameters. Indeed, at full power output, they show themselves to glow quite well, and with even the slightest overload, they illuminate the inside of the bulb. Fisher was obviously aware of this, which resulted in the goofy phase inverter design that they used in many of their 7591/7868 designs. In this unit however, the screens are of particular concern, so the measures taken with the phase inverter are particularly severe. Fortunately however, since this is a pentode design, the screen control portion of the EFB bias supply regulator can easily deal with the screen issue in a proper way, allowing the operating parameters of the phase splitter to be altered to a linear portion of its operating curve. For all of the performance figures quoted here, this change had already been made. All of this will be dealt with over the course of this thread, and will take some time to lay out, but I hope those of you interested in the 400, or EFB in general, will follow along.
Oh yeah, initial results from applying EFB to the Verson II 400 yielded the exact same amount of power output, but with the significantly lower distortion levels mentioned above, and no screen heating of any kind -- even with severe overload. And the quiescent plate dissipation? Just 8.5 watts per tube, with each tube only drawing 22 ma total cathode current each. That's just under 45% of the Design Maximum rating. The tubes gotta love that! When finished, this should prove to be a very worthwhile modification in deed.
Some pics:
1. The candidate.
2. The output stage as Avery wired it. I got started and thought I better snap a pic before I got the first connection completely disconnected.
3. New 10 ohm cathode resistors are installed to know just what the heck is going on.
4. Developing the basic EFB parameters for the Version II. At this point, the unit is developing 25 watts RMS as a single driven channel with under .20% THD.
5. The resulting quiescent current draw at the EFB low distortion point.
Thanks for following along!
Dave
I've always subscribed to the notion that a job worth doing is worth doing well. I wanted to install EFB in a 7591/7868 application, because soooo many pieces of equipment use those tubes, and many of those designs operate these tubes very hot as well. Therefore, a design with these tubes became the next logical target for EFB, and my own Fisher 400 became the unit of choice to take aim at. Except, I've come to find that the 400 is kind of like the proverbial thread that just keeps coming when you pull on it.
First up, was identifying what I had. My unit is a latter unit with an extra FM limiter stage installed, and a revised bias circuit to cool the output stage down. Except, that when I was making my stock performance measurements to develop a base line for comparison, I found the stock unit could not meet the original published specifications for the 400. What gives? Some quick checks showed that with the turns ratio in the OPTs installed in my unit (original Fisher devices), it could not possibly produce the power output claimed by Fisher -- even on a sunny day with matched NOS tubes and a high line voltage. Checking the Service Manuals showed different part numbers for OPTs in the two basic versions, and searching further turned up yet more clues pointing to an OPT change as well. The point, and bottom line of this information, is that the base information and the EFB operating points I've developed in this exercise potentially only relate to the latter unit. I wish it could relate to all of the versions, but based on all available information, those wascally Fisher engineers apparently decided to make a change in the output stage -- and one that would appear to be big enough so as to require different EFB operating points for proper operation in each version. Make no mistake, both versions can benefit greatly from EFB, but the operating parameters would be different in each application.
So what have I got with my "version II" of the 400? Basically, a unit that produces 25 watts RMS per channel maximum (or 22.5 watts RMS both channels driven), from 30 Hz to 20 kHz. This is not as powerful as the specifications state, or as possibly the original version of the 400 is, but is still a very nice power niche to be in with efficient speakers. Distortion is pretty typical with single channel performance at 25 watts returning 3.75% THD @30 Hz, .5% THD @1 kHz, and 1.90% THD @20kHz. With both channels driven at 22.5 watts, distortion at 1kHz doubled to just over 1%, while distortion at the frequency extremes more than doubled. This is all as driven through the Aux/Tape input with the tone and balance controls accurately centered.
Actually, I'm ahead of myself however, because to deliver even these performance results, I had to deal with the rather unique phase inverter circuit, so there is a lot to tell with this tale. As a result, this effort really amounts to a redo of the power amplifier circuits in the 400 version II, but as a teaser, some trial results in developing EFB for the modified power amplifiers returned a 1 kHz THD of just .22% with both channels driven, and a 20 kHz THD of only .80%. This is nearly an 80% drop in distortion at mid frequencies and over a 70% drop at high frequencies, while at 30 Hz, distortion fell to about 2.5% with both channels driven at 22.5 watts output each. The transformers themselves are the main factor at this frequency in terms of delivering a low distortion output.
This was also produced from a very good and well matched (+/- 1 ma) quad of NOS 7868 tubes from Sylvania, of late 70s origin. I should also say that with the unit operating at 117.0 VAC, I found tubes that would idle as low as 26 ma, and some as high as 40 ma in this unit, with the vast majority of tubes idling at about 34 ma, which I believe was the target quiescent current draw by Fisher for this version. This is the current level each tube in my NOS quad drew in the stock design, and the current draw the base measurements were made at. This results in a stock dissipation of just under 14 watts per tube, which is hardly a bad place to be for plate dissipation in these tubes -- except for how closely they are physically spaced together on the 400's chassis, and particularly so when a cabinet is used. Overall however, in the Version II design, plate dissipation is not nearly the concern with these tubes that another factor is.
In the Version II design, because of an apparent change in OPTs, the screens of the output tubes get particularly pummeled at elevated power output levels -- much more so than than they already do with more traditional 7591/7868 operating parameters. Indeed, at full power output, they show themselves to glow quite well, and with even the slightest overload, they illuminate the inside of the bulb. Fisher was obviously aware of this, which resulted in the goofy phase inverter design that they used in many of their 7591/7868 designs. In this unit however, the screens are of particular concern, so the measures taken with the phase inverter are particularly severe. Fortunately however, since this is a pentode design, the screen control portion of the EFB bias supply regulator can easily deal with the screen issue in a proper way, allowing the operating parameters of the phase splitter to be altered to a linear portion of its operating curve. For all of the performance figures quoted here, this change had already been made. All of this will be dealt with over the course of this thread, and will take some time to lay out, but I hope those of you interested in the 400, or EFB in general, will follow along.
Oh yeah, initial results from applying EFB to the Verson II 400 yielded the exact same amount of power output, but with the significantly lower distortion levels mentioned above, and no screen heating of any kind -- even with severe overload. And the quiescent plate dissipation? Just 8.5 watts per tube, with each tube only drawing 22 ma total cathode current each. That's just under 45% of the Design Maximum rating. The tubes gotta love that! When finished, this should prove to be a very worthwhile modification in deed.
Some pics:
1. The candidate.
2. The output stage as Avery wired it. I got started and thought I better snap a pic before I got the first connection completely disconnected.
3. New 10 ohm cathode resistors are installed to know just what the heck is going on.
4. Developing the basic EFB parameters for the Version II. At this point, the unit is developing 25 watts RMS as a single driven channel with under .20% THD.
5. The resulting quiescent current draw at the EFB low distortion point.
Thanks for following along!
Dave
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