Adding EQ Stage to Berlant Concertone Tube Preamp

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Mar 4, 2023
Maine, USA
Hi Y'all,

I'm fixing up and modifying a '54-56ish Berlant Concertone custom into a stereo tube preamp. I'll start off by saying that I'm a pretty hefty noob at all this, and this would be my first foré into the modding world. I've stripped out all the tape machine control circuitry since this one didn't come with a control unit, and I'm duplicating the existing preamp design, modifying a couple of the existing tube slots. Based on the design this should be relatively fluid since the tape machine control circuitry has very similar power needs and there's should be plenty of juice to go round, and the heater circuit can be essentially unchanged. I'm also taking the power supply off the chassis to reduce buzz and noise since these units have some issues with that, I'll also be arranging the grounds to a star formation. In doing all this moving and swapping I keep ruminating on the possibility of adding a simple eq stage, but I'm worried mostly about messing up the existing impedance structure. Here's a couple ways I've thought about doing it.

1. Adding an inductor mid high band or mid low band, this would have the ideal sound but given that it increases impedance significantly and would be tricky to source good inductors this probably isn't in the cards.

2. Adding a baxendall NFB stage in between the 12ax7 first stage triode grids. This could be a decent option for usefulness and sound, but I'm not sure about polarity shifts, and again how much it would increase the impedance of the circuit. These preamps sound great and I also wonder how much I could just mess it up by trying to do something with NFB on the existing tube circuit, so a passive option may be better.

3. Adding a tone stack similar to guitar amps, this perhaps is the most feasible but like to produce the wonkiest response curve, it may be possible to do this after the 12ax7 input tube stage, but obviously it would color the sound less at that point, or rather have the eq influence how the sound is amplified much less.

4. Least colored and simplest approach would be some basic HP and LP filters on the output then use a higher impedance output traffo.

From what I can deduce from the schematic and reading, the impedance the first 12ax7 stage wants to see is about 1megohm, 2nd stage states it is a 200k load (this is where the bridging input comes in) but there's 2 100k resistors that seem to set this and likely it's hitting something different by the time it gets to the tube, and then it seems like progressively less and less throughout ending in a 15k or 8k output transformer (depending on the model) so probably the impedance is existing at about 150k at that point given berlant's tendency towards safely underloading things.

I have plenty of pot real estate on the front panel so it would be cool to set up some adjustable frequency choices, perhaps switching to different caps or resistors in the eq stage. For all the parts i've tried to source parts from the era or those that create similar effect (PIO tone caps, it came with bumblebees originally, Silver mica, and carbon comp resistors.)

Let me know any thoughts or advice you have on this! Obviously safest choice would be to not do this at all haha, but it would be sweet to add some options, and make a cool sounding eq stage for this.

Schematic attached, redded out sections are tape control related, green highlights are where eq stages seem possible.



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    berlant eq questions.png
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I have 6 of these that I use often with Coles 4038 and RCA 44. They are really nice and have a lot of gain with the correct input transformer.
I only have one RCA OP-6 so I often use the Berlants for stereo room mics.
Just making sure you know about the DC series heater string. Make sure you don't pull an associated tube from your Red areas.
Best wishes!

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