Jensen Cascode Mic Pre

[raysolinski]

Anybody here build this pre? It is on their site under schematics. I just noticed that I have all the parts for this guy layin' around on the bench. Well, except the input transformer but i think I will order a Jensen to keep it in line with their design..plus i don't have a pre with any Jensen's, just Cinemags and Lundahls.

Cheers,
Ray

 

[skipwave]

I'm intrigued by cascode topology. It's like an exotic food I want to sample, but can't quite justify a whole outing for.

This one (as054) has an interesting note about direct coupling to a 12BH7 output driver stage, shown in as021.

I just pasted the two together, and it looks like a drunken "Ring of Three" seeing all those tube symbols in double.

 

[cory ponzo]

Haha years later, I built this preamp in 2024. Parts from a Akai reel to reel. Edcor Transformers. And Condor 48 volt linear supply for phantom. Has ez81 rectifier dc filaments. And a power supply from a 5f1 Champ.

Sounds amazing. I'm going to build more. My first one isn't the most attractive. The bass DI is Soooo good. I used 1:6 input transformer. 1:4 output. So I used two 12au7 as it had to much gain With v1 12ax7 and was motorboating.

I learned so much. To make it as quiet as possible I tied all grounds to the power transformer base, all except the tube power supply ground and the in/out shield. Also note the orientation of the in and out transformer. It is the lowest noise tube pre I have used.

Definitely build one!

 

[rock soderstrom]

Haha years later, I built this preamp in 2024. Parts from a Akai reel to reel. Edcor Transformers. And Condor 48 volt linear supply for phantom. Has ez81 rectifier dc filaments. And a power supply from a 5f1 Champ.

Sounds amazing. I'm going to build more. My first one isn't the most attractive. The bass DI is Soooo good. I used 1:6 input transformer. 1:4 output. So I used two 12au7 as it had to much gain With v1 12ax7 and was motorboating.

I learned so much. To make it as quiet as possible I tied all grounds to the power transformer base, all except the tube power supply ground and the in/out shield. Also note the orientation of the in and out transformer. It is the lowest noise tube pre I have used.

Definitely build one!

Thanks for info, did you change anything for the 12AU7? Here is the schematic:


PS: the second gain stage is not a "cathode follower" as noted in the screenshot above, it is a SRPP...

Edit: here the original schematic from the Jensen website

 

[cory ponzo]

Haha years later, I built this preamp in 2024. Parts from a Akai reel to reel. Edcor Transformers. And Condor 48 volt linear supply for phantom. Has ez81 rectifier dc filaments. And a power supply from a 5f1 Champ.

Sounds amazing. I'm going to build more. My first one isn't the most attractive. The bass DI is Soooo good. I used 1:6 input transformer. 1:4 output. So I used two 12au7 as it had to much gain With v1 12ax7 and was motorboating.

I learned so much. To make it as quiet as possible I tied all grounds to the power transformer base, all except the tube power supply ground and the in/out shield. Also note the orientation of the in and out transformer. It is the lowest noise tube pre I have used.

Definitely build one!

Oh some pics... lol not the most elegant. I wasn't sure about layout and how to run wires for low noise.

 

[cory ponzo]

Thanks for info, did you change anything for the 12AU7? Here is the schematic:
View attachment 139199

PS: the second gain stage is not a "cathode follower" as noted in the screenshot above, it is a SRPP...

No changes it was motorboating around 12 o'clock. I read in another forum motorboating could be from too much gain. I tried a 12au7 and all was good. So I left it.

 

[rock soderstrom]

Oh some pics... lol not the most elegant. I wasn't sure about layout and how to run wires for low noise.

I don't see a PE pole, is your box connected to the protective ground?

It was certainly not easy to get the amp quiet with the unshielded Edcor transformers, especially the input TX is a challenge with all the gain afterwards!

 

[rock soderstrom]

So I used two 12au7 as it had to much gain With v1 12ax7 and was motorboating.

Motorboating can be caused by the wiring; you may be able to avoid this in your next amp by optimizing it. Keep the inputs of the gain stages away from the outputs. A Mumetal shielded input transformer can also help and would simplify the setup considerably.

 

[Alva Goldbook]

No changes it was motorboating around 12 o'clock. I read in another forum motorboating could be from too much gain. I tried a 12au7 and all was good. So I left it.

motorboating is typically caused by insufficient B+ filtering, or it can be due to low frequency oscillation. All wires emit magnetic fields, so keeping them short and tidy is always a good idea. High current wires like heater supplies should be kept away from sensitive grid wires. In this kind of a circuit, heater elevation would probably be a good idea. This means referencing the heaters at some arbitrary higher DC voltage like 30VDC instead of referencing them to ground (0 volts)

 

[pucho812]

Motorboating could be a polorized cap in backwards that is rated higher voltage then what is passing through it.