Improving a Fisher 500B Power Amp Section – Advice?

Ok,I'll calibrate the Commander as best I can and then test it against the generator. I knew I'd kept the old guy for a reason. I've always preferred analog meters as well.

Are those HPs chock full of unobtainium ICs or are they old enough to be reasonably service friendly?
 
I've had caps in my HP 339A THD Distortion Analyzer test set to out, but never any of the SS devices. They are basically built like a tank........

Dave
 
Sidebar for some meter comparisons. I have to imagine, that like me, many AKers are happily sailing through our hobby, secure in the dependability of our favorite multimeters. Especially those of us who plonked down good money for those pretty yellow Flukes. I was blissfully happy too - until Dave started beating up on my meters. :tears:

As part of the frequency testing of this amplifier, I need to be able to accurately measure AC voltage / DB at frequencies from 1khz to 50khz. When my data looked suspicious, Dave gently pointed me at the specs for my meters. I was horrified to discover that most of them specify AC measurement accuracy to only 400 or 500 hz - I need 50Khz! (Yep, even the Flukes). I posted the specs for my meters above in post 79.

Today I calibrated my old Millivolt Commander (since it had the most promising spec) and started checking it against my Tektronix SG502 sine generator. The output amplitude spec of the SG502 is +/- 0.3 DB over its entire range of 5hz to 500khz. Immediately the readings I was getting smelled bad. :screwy:

Time to get serious! I hooked five meters up to the SG502 simultaneously (hopefully they don't interact?). I set the generator output to max at just over 5vac. The results are pretty interesting:

Freq.......Micronta......Commander......Fluke 111.....Fluke 27.....Tek 505
500hz........5.0................5.1................5.4.............5.36..........5.37
1khz..........5.0................4.9................5.3.............5.36..........5.36
5khz..........5.0................2.5................4.5.............5.33..........5.35
10khz........5.0................1.4................3.7.............5.33..........5.35
20khz........5.0................0.6................3.0.............5.30..........5.35
30khz........5.0................0.2................2.8.............5.28..........5.36
40khz........5.0................0.1................2.7.............5.25..........5.37
50khz........5.0................0.05..............2.7.............5.21..........5.38

Clearly the accuracy of the Commander drops like a stone at higher frequencies and the true RMS Fluke 111 isn't far behind. The old analog Radio Shack Micronta 22-211A was rock steady thoughout the entire range and the Tek 505 wasn't bad either. At least these two look usable. Additional tests at other amplitudes followed the same pattern.

So there ya have it: Test your test equipment before you test anything else! :yes:
 
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Sometimes, the old non-electronic (as in no active amplifiers) multimeters, consisting only of a meter, rectifier, and various scaling networks, will have the best response of all. Can you offer a description of the Micronta meter? It, as well as the Fluke 27 and Tek meters look pretty good within the range tested!

Dave
 
Here's a picture of the Rat Shack Micronta. It's old and a little crusty (lives in the garage) but works great. I haven't been able to find any accuracy specs for it.
 

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Yup -- exactly as I thought -- it's a non electronic piece containing just a meter, rectifier, and scaling networks. It presents problems using it to measure response in other circuits (such as in a preamplifier for example) due to its very low input impedance. But for your purposes with this project, it will do fine!

Dave
 
Now that the meter situation is sorted out, I reran the HF response test. The test started with a reference sine wave at 1khz at 4VAC output (1 watt) and worked up the khz scale from there - one channel driven. I used the Tek DM505 for the VAC measurements and the Micronta 22-211A for the DB scale (being small, it was not easy to read accurately). In the chart below, "VAC" and "Calc DB" are the voltage readings and calculated DB from the Tek: "Meas DB" is the direct reading from the Micronta.

Results:
Freq____VAC___Calc DB____Meas DB
1khz____4.00____0.0_________0.0
5khz____3.98____0.0_________0.0
10khz___3.89____-0.2________-0.2
15khz___3.78____-0.4________-0.5
20khz___3.67____-0.7________-0.9
30khz___3.39____-1.4________-1.5
40khz___3.14____-2.1________-2.1
50khz___2.82____-3.0________-3.0

Both sets of measurements correlate quite well. The HF is only down about half a DB at 15khz which is the point at which my hearing stops. :(
 
Nice job with the testing! In fact, a (basically) 1 db roll off at 20 kHz is pretty standard fair for stock Fisher gear in the power amp section. It was the price paid to achieve the HF stability they did. Better to have the roll off than instability! I like to see response down no more than .1 db at 20 kHz, because the HF information you can't directly hear very much can affect the frequencies you do directly hear!!

Dave
 
Thanks Dave - I couldn't have got this far without your guidance. :thmbsp:

Now to see if Mouser is having a Black Friday sale. Did you get a chance to look at your EFB spec? Am I good to go with the original parts list?

Have a great Thanksgiving!

Rich
 
Don't those analog Micronta VOM's use an internal capacitor in the output jack circuit for testing audio AC? Last year I refurbished a couple of my meters (circa 1968) and found the capacitors had physically leaked.
Any effect for this test?
 
The Millivolt Commander has a couple of caps in the input circuit that I suppose could be leaky. (It has been retired again). I don't know about the Micronta? I have another Tek meter headed my way that includes a DB capability. It will be interesting to how that one performs.
 
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Rich -- I need a series of questions answered before I can adjust the basic EFB circuit as I applied it to the late model 400 Receivers. If you can answer them, it will help me from chasing down a lot of stuff you likely already have at your finger tips:

1. What was the original Fisher specified B+ level feeding the CT of the OPTs?

2. What was the original Fisher specified B+ level feeding the output tube screen grids?

3. What is the B+ YOUR unit produces at the CT of the OPTs when operating from the AC line source you will be using? If you intend on using any of the popular methods to adjust the AC power applied to the primary of the power transformer (i.e. CL devices, buck transformers, variac, etc.), these should already be in place when this determination is made. Also, it should be made when the unit is fully warmed up with the output tubes drawing normal current flow.

4. What system of bias distribution do you plan on using? Stock? IBAM? or the new approach I added into the mix? It doesn't matter which you use, I just need to know.

5. Finally, when your output tubes are biased to a normal average quiescent current as the unit now stands, what level of negative bias voltage is being applied to the output tubes?

Basic things that is already known you will need to change from the published circuit:

A. The 1.8K 4 watt Drain resistor becomes 750 ohms at 5 watts.

B. The 22 uF 350 volt Source cap becomes 22 uF at 500 volts.

C. You will need two more diodes in the negative voltage doubler circuit than shown. This is because that circuit actually relies on two of the diodes in the original bridge diode rectifier powering the DC heater and original bias circuit to operate properly. Since the DC heater supply will not be used, and the EFB bias regulator will source its voltage from the new voltage doubler circuit, then the original bridge rectifier package will likely be dispensed with. As a result, you will need two additional rectifiers to fulfill the function of the two that were used in the original bridge device.

Once I know the answer to the other questions, I can then provide the other values that will need adjusting.

Dave
 
Dave: I didn't realize that I was asking you to rejig the circuit. I thought it was mainly what you have listed under basic changes. So please, no rush - only when and if you feel like it.

To answer the questions:

1. Original B+ spec at CT: 395V

2. Original B+ spec to output screens: 348V

3. Warmed up at 116.5V mains (variac), B+ at OPT CT = 400V. I intend to install two CL-80 thermistors on the mains input which typically brings my wall voltage down to 116.5V.

4. Planning to use your newer Bias and Balance approach for bias distribution. It seems very similar to the Scott systems that I am used to.

5. Current tube set is biased at 27, 29, 34, 32 ma and is drawing -18.6V at pin 6.

Current schematic is attached.
 

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Dave, this may be as good a time as any to interject a question here. I've read several of your other threads where you "developed EFB parameters" for those builds. Can you describe the process by which you develop EFB parameters? I'm going to want to replicate this process when I add EFB to my build.

Once EFB parameters are selected, I think I do understand how to tweak the EFB regulator to deliver the needed voltages, I just don't quite understand the larger process of deciding what are the best parameters to use.

I'm guessing it involves something like picking the lowest distortion operating point at some known frequency at at some known output power and then 'setting' the EFB controlled negative bias and screen voltages that support that lowest distortion operating point.
 
Rich -- With a little more time now, let me backtrack on statement I made in my last post, regarding the original 1.8K drain resistor: scratch the value I stated in that post.

OK. Based on the EFB(tm) model as published for late model 400 receiver operation, with revised parameters including a main B+ of 400 vdc, a screen voltage of 350 vdc, and the other basic parameters of the output stage in your application (load impedance of 6500 ohms, and ~ -17.25 vdc grid bias required for the quiescent operating point you are targeting), and based on the output of the EFB grid bias regulator driving the DC bias/balance circuit I published as installed in my Fisher 800C, here are the values you will need to revise the 400's EFB installation for operation in your 500B power amp project:

1. The 1.8K 5 watt resistor become a 500 Ohm 5 watt unit.

2. The 100K Gate resistor becomes 39K @ .5 watts.

3. The Gate 270K 1 watt resistor and .47 uF 600 volt cap remain unchanged.

4. As stated before, the 22 uf Source cap becomes a 500 volt device.

5. The 1 meg coupling resistor to the EFB grid bias regulator remains unchanged, as does the .22 uf cap located at the base of the inverter transistor.

6. The collector to base feedback resistor for the inverter transistor should become 75K @ .5 watt. Alternately, you can make this resistor a 68K in series with a 10K pot to provide a trim adjustment on the output of the grid bias regulator if so desired.

7. The 22K collector load resistor for the inverter transistor remains unchanged.

8. The 4.7 uF cap at the emitter of the buffer transistor remained unchanged as well.

9. The 10K resistor at the emitter of the buffer transistor is eliminated.

The SS devices remain the same as with the 400's installation. The power mosfet will need to be heat sunk to the chassis, dissipating just under 1 watt under all operating conditions (from quiescent, to full sustained power output in both channels). The specified part is a plastic piece, allowing for easy mounting to the chassis without the need for an insulating package. Just use some grease, and your good to go.

Also, unlike the 400s installation, in your application, you will supply the inverter stage B+ dropping resistor from the EFB controlled screen grid source. The new dropping resistor to supply the inverter stage B+ will then be a 5.6K @ .5 watt resistor. You will also definitely want to install 100 ohm screen stability resistors as well (not required with the EFB modified 400).



Kevin -- You have surmised the process well.

Specifically to the 7591 family of tubes, most designs (but not all) set their quiescent operating point to mimic the ideal operating points published by tube manufacturers. With a 6500 ohm load, many such designs employ a 450 volts plate, 400 volt screen, and 38 ma cathode current level (per tube) under quiescent conditions. The problem is, it all goes to pot as increasing power levels are developed, as the plate supply drops somewhat, the screen voltage -- being supplied by a simple dropping resistor -- falls through the basement, while the fixed grid bias voltage basically remains unchanged throughout all of it. Therefore, the ideal operating relationships set up under quiescent conditions gets obliterated as power is increase, resulting in greatly increased distortion, and reduced power output capability. If quiescent current is increased to combat the drop in screen voltage, then the excess current causes the tube to greatly exceed rated dissipation levels under quiescent conditions.

In the 500B example here, things are scaled back somewhat from the example above to 400 volts plate and 350 volts screen, but otherwise, the same train wreck happens as power is elevated.

Setting the parameters for EFB operation in this case however has largely already been done, as was published here: http://www.audiokarma.org/forums/showthread.php?t=539265

With that work, it was determined that maximum performance is in fact achieved with a quiescent current of about 38 ma. What the addition of EFB did in that case is to cause the published performance to be achieved (that is, published by tube manufacturers), that would otherwise have required a quiescent current of over 50 ma per tube to be achieved with typical conventional installations. Or, it allows the original performance to be achieve at reduced quiescent current levels, allowing you to have your cake (the original unit's performance level), and eat it too (with cooler operating temps, longer tube life, etc.).

So for this application, the EFB screen regulator is basically set to scale the screen voltage from its original stock value, and then maintain that relative scaling under all operating conditions. Ditto for the EFB control grid regulator.

In contrast, you might review the thread wherein EFB operation was added to my late model Fisher 400 receiver. Briefly, in that application, the original screen voltage was wholly inappropriate for the operating conditions of the output stage, which included a rather strange move to a 10K plate load. Without any proper readjustment of the screen grid voltage -- or any real ability to do so economically at the time -- the output tube screen grids were really getting cooked bad in that application as power was increased. In that case then, I used the EFB circuitry to not only provide EFB action, but also to readjust the screen voltage to a more appropriate value for the plate load offered in that unit. The result was that the screens then operated as cool as a cucumber under full power conditions, power output was maintained to the same level as before, but the required quiescent current for the low distortion operating point dropped like a rock (to ~ 21 ma per tube) -- as did the distortion itself, while longer tube life and cooler operation were obvious results as well.

So the EFB circuits are doing double duty in that instance. As for determining the best parameters in that case, my Heath regulated bench power supply is my friend. While the Fisher chassis provided the plate voltage, the Heath -- having a variable B+ and C- outputs -- was used to provide the screen voltage and grid bias voltage. The amplifiers were driven with a traditional 1 kHz sine wave to within 1 db of full power output. At that point, the screen and bias voltages could be adjusted for optimum operation, based on the basic drop in B+ displayed by the "plate" supply in the Fisher chassis. With the drive signal removed, the resulting quiescent conditions could then be measured (the screen and bias voltages would be unchanged since the Heath supply is regulated) and checked against tube ratings, and the EFB circuits set up to produce that scaling action from the static B+ supply in the Fisher chassis.

I have also done the same exercise plotting the events with load lines, but frankly, I've become so proficient in obtaining the same results by using the Heath PS, that I find that approach quicker, and ultimately more accurate, since real world results are obtained. For me, I would need to employ the adjustable bench supply approach to verify my on-paper results anyway, so I find that it just saves time to go straight to the bench supply approach to begin with.

I hope this helps!

Dave
 
Helps a lot!! Thanks for taking the time and for providing such a comprehensive explanation. I have read your previous EFB threads but each new thread adds to my understanding.

Now to get that Mouser order going - I'm sure I'll miss something though.

Thanks again. :thmbsp:
 
That's awesome Dave, and indeed it does help in understanding. A regulated high voltage bench supply is one diagnostic tool I do not yet have. But I'm in process of designing and building one (and it is harder than I ever imagined).
 
I could quibble at the Heath in some aspects (low level noise and no CT for the heater power), but all in all, it's a very good design you might check out to use as a basis of your build. Good luck with it!

Dave
 
Mouser order should be arriving in about a week by the 'slow boat' shipping option. EFB transistors are on back order - supposedly 10 days. We'll see.

In the meantime, my "new" true RMS Tek DB meter arrived. It's not analog but demonstrates good accuracy out to 50khz. I like these Tek modules as they can be swapped in and out of the mainframe thus requiring no extra space on the bench. It confirms the previous readings.

Never thought I would prefer these vintage units to my Flukes but they are far more capable at higher frequencies.
 

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