EICO HF-20 volume attenuating feedback circuit dilemma

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EICO HF-20 mono amplifier volume attenuating feedback circuit dilemma. In the 1950’s I assembled an EICO HF-20 from a kit. It worked fine but I was disappointed at the time with low volume. Normal listening volume occurred only with the level and loudness set above the ¾ setting. Recently I acquired a used factory assembled HF-20 and got it in running order with the help of an experienced tube amp expert. The same low volume problem was apparent. The problem was traced to an apparent original design error in a feedback circuit of capacitor .25 mfd C13 and 330k R19 from the cathode plate to the grid of the first stage of the V2 12AU7 loudness preamplifier. Removing this feedback loop resulted in about a ten-fold increase in amplitude on the scope resulting in normal listening volume at ½ loudness and ¼ turn on level as it should be, solving the dilemma 50 years after it was discovered. Does anyone know why this apparent unnecessary feedback capacitor/resistor was in the original circuit at V2 and persisted in the circuit diagrams and was also incorporated into the factory assembled models.
 
E- The original specification for the HF-20 have an input sensitivity of .40 vac to produce maximum power output with the level and loudness controls at max, and the tone controls centered. As long as the unit operates to this specification, it is operating as designed with the plate to grid FB loop included.

It appears that what you don't like is the position of the controls to achieve a reasonable sound level. This could be because your source signal is lower than required, but also is certainly a product of the Eico design: the more the level control is advanced, the more plate to grid feedback is applied. This in effect changes the characteristics of how the control reacts -- beyond the basic taper of the control itself. The FB loop was included to minimize distortion of the stage, and to make the stage present a (necessary) low driving impedance to the tone control network, so that the controls do in fact represent a flat setting when they are mechanically centered. If you have a scope, you might want to check that the controls still represent a flat setting when centered with the FB loop removed.

Finally, realize that the tube you are referring to is supposed to be a 12AX7, not a 12AU7 as you indicate. If you are in fact using a 12AU7 in that position, that will also contribute to your low gain issues.

Dave
 
Dave,
Thank you for the clear and interesting explanation. Yes V2 is the 12AX7 tube. The HF-20 instructions call for setting the uncompensated LEVEL to control the program material and use the Fletcher-Munson compensated LOUDNESS for adjusting the listening volume which makes sense. It still seems that the LEVEL needs to be set at the high end which does not leave much room at the top end compared to the typical volume control, but as you suggest perhaps this is a characteristic of the HF-20 amplifier design. With your explanation I will reconnect the plate to grid FB loop to assure that the frequency response remains flat. Would an increase in the value of the 330K R13 in the FB loop be an acceptable compromise or would it just again alter the frequency response. In an unrelated question: is the .03 mfd C28 from 120v power line to ground necessary or in fact inconsistent with current safety code.
Thanks again
Richard
 
You could probably increase the FB resistor to as much as 680K with little effect on the tone control action. This would effectively double the gain of the first half of V2. Since the plate load resistor for this tube (33K) is rather small, it also is helping to reduce the output impedance of this stage as well.

The basic power amplifier section of the HF-20 (which starts at the wiper of the loudness control) requires a level of 1.0 vac to achieve full rated power output. The second half of V2 actually operates at a slight loss when the tone controls are accurately centered, so all the gain required to raise a high level input source to the needed 1.0 vac to drive the power amp must be produced in the first half of V2. Accounting for a slight loss through the second half of V2, and based on the original sensitivity specification, the first half of V3 (from a high level input jack to the input of the tone control network) must be providing an overall gain of about 3.

Modern day input sensitivity levels have been reduced since the days the HF-20 was designed (1953), so that now, most equipment requires about 1/2 the signal the HF-20 does to produce full rated power output. Therefore, I have no doubt you have to advance the controls significantly for a reasonable level. But there is another component you can address to wring out a little more gain.

The 56K input isolation resistor connected to the top of the level control is also part of the FB network as well. Its primary purpose is to prevent the source impedance of the input signal from shorting out the FB around the first half of V2 when the level control is well advanced. Reducing it to about 22K will also effectively reduce the FB level by as much as one half as well, since it is the dominate element in the bottom ladder of the FB circuit.

If you raise the 330K FB resistor, and lower the 56K isolation resistor, you would effectively increase the gain for the first half of V2 by a factor of as much as 4 with the level control at maximum. This would bring the overall gain of the stage (as defined above) to about 12, which would increase the overall sensitivity of the unit to requiring just less than .10 vac for full power output. This may be too much compensation, so all you can do is see what values work best for your application. Even at this level however, rough calculations indicate that the input stage would still be operating with at least about 10 db NFB around it. This along with the low value plate load resistor should keep the driving impedance of this stage low enough to minimize any impact on the tone control networks. The fact that the level control still has an influence on the amount of FB around the input stage however, means it will still throw much of the gain in the upper level of the control's rotation. These changes may make enough impact however to make the settings more reasonable for you. Realize too that any increase in gain will also make a commencerate increase in noise as well, so be mindful of that as you make your adjustments.

The AC ground cap always presents problems in older equipment: Without it, hum will likely increase. With it, a shock hazard exists if the polarity of the plug is such that the chassis becomes hot if the cap fails. Even if it doesn't fail, it can cause lower AC potentials to exist on the chassis. Many will say to rip it out and install a modern green wire ground line cord to maximize safety, yet that often produces ground loops with other grounded equipment (cable boxes, etc.) to make the amplifier all but unusable. The best compromise I have found by far is to replace the cap with a modern safety rated cap, and install a polarized AC plug so that the cap is always connected to the neutral side of the line. A regular three wire line cord could accomplish this as well with the green wire simply disconnected.

Students of modern electronic technology will likely cringe at this thought, citing what if the power transformer shorts internally to the chassis. Likely they have never known of such an incident, always citing the theoretical possibility. It is there to be sure. But perils in life exist everyday. History shows you likely have more danger of being taken out by a runaway bus than this theoretical short ever happening. Enjoy your amps!

Dave
 
Great info. I am about to work on an HF20 and this will come in very handy. Thanks!


RWood
 
DON'T use a 3-wire cord unless you ground it! It IS possible to add a "ground breaker" consisting of a pair of 25A or larger diodes, reversed parallel (I've used a packaged bridge) in series with the ground lead. If you use a 2-wire polarized cord, connect the cap to the neutral side (large pin on cord)

Modern electrical codes generally limit ground leakage current to 0.7 mA max - this implies a capacitor of 0.015 uF or less. It should be a type designed and approved for this use ('Y' capacitor)
 
Thanks for the input Tom. How effective is the ground breaker approach in replacing the cap with regards to reducing hum?

Dave
 
Dave, Tom, John

Great tutorial and dialog. Thanks, I have learned a lot.

Dave, Increasing the 330K R13 (R3 in some diagrams) to 680K or 510K in FB loop from V2 plate to grid, makes sense as a good compromise to get a gain of up to 2. However, changing the 56K R18 (R1) to 22K at the level pot may not be worth while because at the midpoint of the 500K level control it would be of negligible effect and as you point out only effective at adding gain when the level is near max.

Tom, removing the .033 mfd cap at 120 v power line and if hum develops adding back .01mfd on the neutral seems safer. When I look at the wiring and globs of solder in some of these 60 year old amps, especially the kits (although sometimes the kits were wired better than the factory), I see all sorts of opportunity for high voltage shorts to chassis and I like the idea of chassis ground plugs.

Dave, not to guild a lily, but what is your opinion of replacing the R48 350 ohm output stage anode resistor with say a 300 ohm + 150 ohm high power rheostat in order to allow the grid bias on the push pull 6L6’s to be adjustable? Should the grid bias be adjusted to -35v to -40v or calculated such that the 6L6 plate current is 50 ma each?

Richard
 
Richard -- Your assessment of the 56K input resistor is correct. While reducing it may not have a significant effect on a typical setting of the level control, it would still none the less increase the ultimate gain of the stage, and improve the sensitivity of the unit. Of course your choice on how to proceed. I just wanted you to be aware of that option as well.

As for making the output stage cathode bias resistor variable, I would suggest not to. If you are trying to address tube life, it will have little impact. The plate dissipation level of the tubes in the original design is about 18.5 watts each, which is well below maximum levels for modern 6L6 class tubes. Also, the quiescent current is 50 ma per tube, which is also well within the operating capability of these tubes when tube life is considered.

In addition, any reduction in quiescent current will have a significant impact on IM distortion within the output stage. This has been well documented in the press of the day for UL 6L6 amplifiers, and I have verified this fact for myself as well.

The best single modification you can make to this amplifier is to replace the 5U4 with a 5V4 rectifier tube. The heater current drawn by the rectifier will be reduced, the turn on B+ surge eliminated, the B+ will be increased, which means that power output will be increased, and distortion will be reduced. The increased voltages will easily be taken by the output stage in stride, and the overall effect on the power transformer will be negligible. If your electrolytic caps can take it, you could even use a GZ34, although output tube dissipation will increase to about 24 watts each, and the power transformer will be handling more watts as well.

Dave
 
Dave

I am reluctant to increase the B+ voltage too much as EICO cuts it pretty fine on the electrolytic can capacitor voltage. On one of my HF-20 pair I have replaced the cans with 525v which should be safe. On the other the original cans are 450v so I will play it safe until I replace the cans. At that point I will try the 5V4 or GZ34 rectifier tube. I presume to avoid the solid state replacement rectifier as the lower internal resistance would generate much higher voltages, especially if run without the load 6L6 tubes in place.

I appreciate all of your valuable advice.

Richard
 
The best single modification you can make to this amplifier is to replace the 5U4 with a 5V4 rectifier tube. The heater current drawn by the rectifier will be reduced, the turn on B+ surge eliminated, the B+ will be increased, which means that power output will be increased, and distortion will be reduced. The increased voltages will easily be taken by the output stage in stride, and the overall effect on the power transformer will be negligible. If your electrolytic caps can take it, you could even use a GZ34, although output tube dissipation will increase to about 24 watts each, and the power transformer will be handling more watts as well.

Dave

Is this just a simple tube substitution, or is there a circuit mod needed?

This might be a moot point, but if you want to make your life easy, just bypass the Preamp all together. I decided to turn my HF-20's into pure power amps by disconnecting the preamp section and running the signal into the tape out jack.

I still run all the preamp tubes because I wasn't sure if pulling the 12AX7's would mess with the filament current....:dunno: I should prolly go back and revisit this issue.....:scratch2:
 
Analog, this is nothing more than a simple rectifier tube substitution. Simply use a 5V4 in place of the original 5U4 to enjoy the benefits it provides.

When Eico designed the HF-20 (1953), the 5V4 was around in its original version (5V4G), but this tube also had a reputation for not being very dependable. By the mid 50s, the 5V4GA was developed which resolved all the short comings of the original tube. Almost all equipment producers switched over to this tube where possible, but for what ever the reason, Eico never did in the HF-20 (or HF-22), but also never used the 5U4 again, either. I imagine that the profit margins were rather thin, and with the 5U4 being the cheaper tube, and the re-printing required to re-designate all the manual information over the new tube, that they just left well enough alone.

Dave
 
Analog addict,
You can run the HF-20 as a power amp through the tape out jack. Pulling the 12AX's should be fine as the filiments are wired in parallel. You can have the shields in place so that it looks orginal. Note that the LOUDNESS control should be in the max position.
Richard
 
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