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 Post subject: Modeling the EH Scott Bass Boost 6 Pole Filter.
PostPosted: Jun Thu 11, 2015 1:09 am 
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Location: Montvale NJ, 07645
There was some talk about the capacitor values in this bass boost circuit on the Scott forum recently, in particular, if the capacitor values deviated from the original design (0.02uf and 0.025uf), the filter could boost 60Hz hum.

So this peaked my interest to learn what kind of tolerances we were dealing with here.

Here is the original circuit:
Image

EH Scott's description of this circuit is as follows:
The dual bass circuit is connected in the plate circuit of the 6L7G tubes and consists of two 175 henry chokes tuned by 0.02 mfd shunting condensers and coupled by means of .025 mfd. condensers. Bass control is obtained by means of a dual potentiometer control, the one megaohm section of which is shunted across the choke circuit and the 10,000 ohm section is connected back to the inverter grid input through a 100,000 ohm fixed resistor. This system provides a wide degree of bass variation and also gives a high boost "peak" at 70 and 45 cycles, but a decided dip at 60 cycles and it also falls rapidly at 120 cycles. This feature reduces 60 cycle power line hum and station hum that is often picked up.

Modeling the circuit as shown on the original schematic produces the following:
Image

As you can see, the notch is quite a bit off from 60 Hz.

So I measured the inductors with a recent production, lab quality LCR meter and came up with new values. The effective value of iron core inductors can vary with magnetization and signal level, so I used these values as being close.

I now get the following response:

Image

Here is how it looks with all 0.022uf caps:

Image

Not too bad.

Circuit is looking much better with the measured values. But wait, I have 3 of these bass choke assemblies available to me, so I take a look at the values of all 3. All were different, so I re-did the model with a different set of chokes.

Image

As you can see, this takes the notch a bit further away from 60Hz.

So let's see what values of capacitors correct that notch:

Image

It looks like 0.018uf and 0.022uf bring that notch back to 60 Hz.

So the conclusion is that the iron core inductors used were not high tolerance parts and choosing the correct value of capacitors is going to depend on what set of iron you have in your radio.

Next step is going to be to test this on the bench when I get some time. We will see how the models stand up to the bench testing.


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 Post subject: Re: Modeling the EH Scott Bass Boost 6 Pole Filter.
PostPosted: Jun Thu 11, 2015 2:11 am 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 3498
Location: Redding, CA
Great research and input! So if objectionable hum is encountered after rebuilding a Scott Philharmonic chassis we should consider adjusting the values of capacitors in the trap. But before going to the extended effort of capacitor value assessment (due to location inside a difficult to access metal shield can) I recommend trying different 6L7 expander tubes and possibly other tubes earlier in the circuit because the hum of concern is due to filament cathode leakage (60-hz) rather than from the B+ circuit (120-hz). I have a recollection of Scott service advice to use spiral filament tubes to minimize 60-hz hum. Failing to find tubes producing less hum, is there a reasonably simple way to determine if the capacitor values are in need of revision? Were the tests conducted with the shield in place and mounted on a steel chassis? I look forward to your further test results.

Norman

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"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems


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 Post subject: Re: Modeling the EH Scott Bass Boost 6 Pole Filter.
PostPosted: Jun Thu 11, 2015 3:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1258
Location: Bloomington, IL USA
Scott,
Nice analysis, thanks for sharing.
What modeling software are you using?
Joe


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 Post subject: Re: Modeling the EH Scott Bass Boost 6 Pole Filter.
PostPosted: Jun Thu 11, 2015 11:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5939
Location: Montvale NJ, 07645
Joe, the modeling software is LTSPICE.

Norm, the only items that were bench tested were the inductors. All the rest is software modeling.
I am going to put this audio section of my Philharmonic together first and give it a thorough bench test. I want to see how the model stands up to what I see on the bench. It might take me a little while to get there, but stay tuned.


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 Post subject: Re: Modeling the EH Scott Bass Boost 6 Pole Filter.
PostPosted: Jun Thu 11, 2015 11:33 pm 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 3498
Location: Redding, CA
Scott:

Thanks. I'm looking forward to your findings.

Norman

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"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems


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