First Tube Microphone Preamp Build Occilation.

[cory ponzo]

Hi everyone this is my first post.

I built the Jensen Cascode Mic pre cathode follower. I uses Edcor 1:8 input and 1:5 output. It works and sounds beautiful. Some of my best vocal sounds to date. The power supply is two nodes. 22uf 10k, 22uf 22k and 44uf on the b+

but..... if I turn the pot down to 9pm it starts occilating at 2 cps. Like pop pop pop sending the speakers to full boom. I tried a capacitor between the middle wiper and the first stage it stopped. I lost all my gain. I tried different internal cable routes but this made the occilation happen at all gain settings. I tried different caps on the wiper but all messed with gain.

Luckily I don't use it at low gain so it's not the end of the world but I'd like to get it sorted.

Here's a pic of the schematic and the build. I have changed the output transformer from step down to step up since this pic was taken. I have also sorted the grounds as it had a small hum. I know my 48v is overkill but it's what I had at .5 amps it is fully adjustable supply.

Thanks for any help.

 

[cory ponzo]

Hi everyone this is my first post.

I built the Jensen Cascode Mic pre cathode follower. I uses Edcor 1:8 input and 1:5 output. It works and sounds beautiful. Some of my best vocal sounds to date. The power supply is two nodes. 22uf 10k, 22uf 22k and 44uf on the b+View attachment 126543View attachment 126544

but..... if I turn the pot down to 9pm it starts occilating at 2 cps. Like pop pop pop sending the speakers to full boom. I tried a capacitor between the middle wiper and the first stage it stopped. I lost all my gain. I tried different internal cable routes but this made the occilation happen at all gain settings. I tried different caps on the wiper but all messed with gain.

Luckily I don't use it at low gain so it's not the end of the world but I'd like to get it sorted.

Here's a pic of the schematic and the build. I have changed the output transformer from step down to step up since this pic was taken. I have also sorted the grounds as it had a small hum. I know my 48v is overkill but it's what I had at .5 amps it is fully adjustable supply.

Thanks for any help.

Here's the layout

 

[rock soderstrom]

Hi everyone this is my first post.

Welcome Cory!

but..... if I turn the pot down to 9pm it starts occilating at 2 cps. Like pop pop pop sending the speakers to full boom.

Oscillation can have various reasons, your cable dressing is certainly worth optimizing, I would put some more love into it. Your wires are pretty long!

In general, you should make sure that the inputs and outputs of amplifier stages do not come too close to each other.

What is also noticeable is that your two signal transformers are un-shielded, which can also lead to problems, especially if they are so close together. The input transformer is more problematic than the OPT. I would vary the position, distance and rotation of them to see if your situation changes.

What is the noise floor of your amp? A shielded IPT would be better in this respect, too. Can you hear any hum? I would also give the power supply unit a little more ripple filtering as a test. Does that change anything?

Just a few ideas, see if it changes anything. Good luck.


Avoid magnetic crosstalk.

Edit: Do you use the case as a ground bus for your signals? If so, I wouldn't do that. I don't see any PE contact, is the housing grounded?

 

[Tubetec]

Normally you will orientate mains and signal transformers for minimum interaction ,
thats usually with the cores at 90 degrees to one another , seperated by a distance , preferably with metal sheilding between .

Sounds like you have way to much resistance in the HT ,maybe its best to include the power supply schematic just for clarity . How do circuit voltages compare to the drawing ?
Adding a filter stage between the 12AU7 and 12AX7 anodes supply might be a good plan .

lead dress is important , wires crossing over each other creates unwanted capacitances than can cause oscillation .

 

[zamproject]

I'm not 100% sure with the pictures resolution, but look like at IN trafo the load resistor leg pass trough a pin which seem primary CT ?!?

 

[cory ponzo]

Yea that resistor going through the center tap is an optical illusion. This is my first build from a schematic the only other build has been a champ so it all the work was done for me. I was getting so much harsh high end through my rode nt2 and Urei jfet preamp. So I built this and it most definitely softened the high end. It really sounds good. I had 60 cycle hum and redid the grounding it dropped lower. You can still still hear the 60 cycle hum as the noise floor. It is so low I can not hear it on recorded vocal tracks. I definitely hear it when tracking. I bet I will hear 60 cycle hum on quite sources.

I will definitely redo some stuff. I built the circuit on a champ board cut in half. As I had it already. I could rebuild the circuit on terminal strips and build it point to point right on the sockets. Also I will build some sheet metal shields for the power transformer and audio transformer. Today I'm going to make the heaters dc.

I am also going to replace the gain pot with a 100k. The first half of the 250k pot is unusable anyway. Thanks for the help guys. I like to build my stuff in large boxes so I have room to work but that might have caused to problem. I'll check back it when I get it sorted.

 

[Brian Roth]

I am also going to replace the gain pot with a 100k. The first half of the 250k pot is unusable anyway.

Curious if you're using an audio taper pot for that control....

Bri

 

[RoadrunnerOZ]

Your rear panel socket appears to have only 2 wire connections coming from the transformer? How is that connected up?
Also can’t seem to see any ground connection for the input transformer secondary/pot bottom.

 

[Cqwet Dbdfte]

Low frequency oscillation, aka "motor boating" is a feedback phenomenon thru a soft B+.
With 74dB gain, wiring layout is a concern.
You need to work on making the wiring shorter.

 

[bepone]

first tube and output tube needs decoupling..

 

[RoadrunnerOZ]

C3 470nF decoupling is in place in the schematic.

 

[bepone]

no, separate RC element from second to the first tube, with such of hi gain (but also lower ofcourse) you cannot supply both tubes from the same node.

 

[bepone]

this is how needs to be

 

[bepone]

also here is how the grounds needs to be connected and where

 

[Cqwet Dbdfte]

Cascoded tubes needs an elevated heater supply.

 

[gyraf]

..perhaps the grid snubbers R3-R4-R10-R12 (on right half of schematic) should go directly onto the tube sockets, without long (=inductive) wire in between?

/Jakob E.

 

[Cqwet Dbdfte]

Watch out for birds nesting in there....

 

[Tubetec]

Here each of the high gain stages has a dual RC filter ,
that greatly reduces unwanted interactions via the PSU rail .

 

[cory ponzo]

Hi it's been a long week. I've been messing with this for a while.

I though it was the input circuit so I shortened all the braided cables. What I've noticed is when the input circuit is disconnected the motor boating is not as loud but still happening.

When it is motorboats I can't even get a dc reading on the b+ the meter just bounces around.

My power supply is basically the same as a 5f1 champ only all 22uf caps. With this transformer it is 230v dc. I may order some more parts to get it to 250 as the schematic states.

Pin 6 of valve 1 has 150v on the schematic. When I measure here it bouces from 159v dc to 170ish dc. Pin 8 valve 2 is high 132vdc. Valve 1 pin 8 is 69vdc so it looks low.

What is weird is if I adjust the pot so it stops motorboating I can find faint occilation throughout the whole circuit signal flow For some reason doesn't show up on the output.

When I use it as a di with a styth it functions as it should as long as the pot isn't all the way low or high. When using as a vocal preamp it sounds great but has no headroom I assume this is from the internal occilation.

the input is as so. Pin 2 and 3 are through the input circuitry Balanced into the mic transformer pin 1 ground. The secondary of the mic transformer has one side grounded the other side goes to the pot. The output goes to the output transformer with one side grounded. 1:5 then balanced to pi s 2 and with pin one grounded.

thanks for your help I'm going to the 22uf caps a member left a diagram of.

 

[cory ponzo]

Yes the pot is log