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Old 24th August 2011, 09:38 PM   #1
hoagje1 is offline hoagje1  United States
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Join Date: Nov 2010
Default eq in a tiny Directly Heated Cathode amp

Greetings from Tacoma,

I have looked over DHT and Directly Heated Cathode information on the forum, and I don't see a treatment of my question but please direct me there if I have missed something.

I just built the phase splitter and output of a push pull amp using the diode/triode/power amplifier pentode (ID8-GT), a DHC tube with a single heater (1.5 V with negative end tied to ground). Because the 1.5 V is across the cathode, the cathode potential varies and the triode and pentode share the cathode with pentode grid bias of -9 volts. This amp is very quiet, and so I would like to add some bass, almost a caraciture of the Fletch-Munson curve for sound in the -40 to -60 dB range. This would really be a "super loudness control" and I need it at this low volume.

Aside from tone stacks, which I won't have any trouble fooling with, I'd be interested in what could be done at the cathode. An LC notch filter, properly tuned, spanning the voltage crossing the cathode would boost instead of cut the tuned frequencies just like a .68uF cap gives more highs than a 25uF from cathode to ground. There are other places the LC filter could go (battery negative to a new ground -- this might be problematic due to the DC resistance of the inductor and the effect on bias.

I know that I placed this thread in the HiFi section. This is a question for guys who have really tinkered, so consider it a compliment. Yes, guitar on 10 it is pure metal, guitar on 8 it is good for blues, and guitar on 6 it cleans up completely (I mostly use it on 6). I am very interested in getting your thoughts on bass boost. Ultimately the volume knob might control several things so all thoughts are welcome.


John Hoagland-Scher

Last edited by hoagje1; 24th August 2011 at 09:47 PM. Reason: left out truth-telling
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Old 24th August 2011, 09:52 PM   #2
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Join Date: Sep 2003
Location: Midland, Michigan
I don't think that an LC or LCR notch on the filament/cathode would work for you.
As I understand your post, the triode section is a phase splitter.
Since the filament/cathode are common to both sections, adding an LC to the filament will create some serious phase shifts which will prevent the triode section of the tube from performing the phase splitter function properly.
You may be able to add an additional stage and design a filament/cathode LCR notch filter on that stage.
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Old 25th August 2011, 06:07 AM   #3
hoagje1 is offline hoagje1  United States
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Join Date: Nov 2010
Thanks Frank,

Designing with these directly heated tubes is all new to me. I'll take your suggestion.

John Hoagland-Scher
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