Tube preamp build inspired by the EH 12AY7 Mic Preamp with LL1636 on top...

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tonedude

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I have two LL1636 left over from a preamp rebuild. First I thought I should put one of them in my Electro Harmonix 12ay7 Mic Preamp, but after some thought I have decided to build a stereo preamp from scratch using both of the transformers and put the build in a 19 inch rack using the circuit from the EH preamp. I'm gonna use the circuit of the EH 12AY7 preamp as inspiration, or more exactly, I will mostly clone it, but I'll make at least two changes. I will move the gain from before the first tube stage, and put it between the 12AY7 and the 12AU7 tube, and I will also put a master volume/attenuator after the 12AU7 tube. Otherwise i will mostly use the EH circuit as it is, first tube stage is fixed at 50db of gain, with the LL1636 in 5:1 configuration in front of it. The circuit will then have a total of 63dB gain.

I need some help before ordering parts.

Whats the smartest way to implement the attenuators between and after the respective tube stages (... the “gain and master” pots)?

Could I use an H-pad design?

... I know, the build will have some obvious similarities with another well known tube preamp...
 

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ruffrecords

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To be honest, I am not sure the circuit you posted lends itself to the modifications you have proposed. The 12AY7 is set up in grounded grid configuration with the input cathodes driven by FETs. The second stage is simply a dc coupled cathode follower.

Cheers

Ian
 

tonedude

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Here is a description of the circuit from the designer...


I can't see that there would be a problem using some kind of pad at the output, after the 12AU7, maybe a U-Pad after the output capacitors, and use 300ohms at R1/2 and as R2 a potentiometer with 20k ohms, or something like that. Trial and error will tell what could be useful... This pad doesn't have to offer full attenuation, just offer a wide enough range so that it would be possible to tailor and attenuate the sound for different flavours.


But I'm more concerned about what "pad design" that would be useful/functional between the two tubes. The circuit is balanced throughout, so the pad/attenuator/"gain pot", has to be something that doesn't mess with cmrr...
 
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ruffrecords

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A U pad at the output depends on the drive capability of the output stage. If it can drive a 600 ohm load then adding a pair of 300 othm build out resistors plus a pot between them to create a U pad will be fine. The original circuit uses 33K cathode resistors for the 12AU7 cathode follower (for some reason you have changed them to 330K in your schematic). If the voltage across each 33K resistor is 155V as indicated in your schematic then the quiescent current through each cathode follower is about 4.7mA. That is about enough to drive about 18mW into a 600 ohm load which is about +12dBm. As you have opposing outputs you get an extra 6dB giving a total of +18dBm. How much distortion a 12AU7 will give with this heavy a load is anyone's guess but I suspect there will be plenty of colour so in theroy your U pad should work. I expect the designer only anticipated a 10K load would be used so I would be tempted to increase the build out resistors to 1K each.

As I said in my previous post the output stage is directly coupled to the previous one so a pad between them would be carrying dc which is definitely not a good idea.

Cheers

Ian
 

tonedude

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Thank you for your tips and thoughts Ian!

I did post the first schematic because its easier to read and "follow" the circuit (it's from a modding thread at Gearslutz... ). Im not using the values from that schematic, I'll be using the original values, or try the upgrade tips that the designer proposed in his description of the circuit.

In its original form the circuit is designed to be capable of driving a 600ohms load, but It'll not do it with ease... but I guess thats the purpose of the U-Pad in this case. I'll try different values for the buildout resistors.

I guess that amount of DC on a potentiometer on a pad between the tube stages is a bad thing, in many ways, safety being one. Hmmm...
 
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