Federal AM864 Motorboating

[Deepdark]

Hi guys

Just finished my Fed and it sound really good, exept for the motorboating sound. So far, it has been located in the sidechain circuit (pulling out 6sq5 cancel motorboating). All voltages are good, pots are working. So, my thought is that trouble must be around C5/6, R6/7/8/10/11. For now, I tried moving some wires, moving B+ farther, playing with different setting, but it's still there. Once the unit start to compress, motorboating appear. With threshold off, curent ctrl off, there is a little motorboat, but once I turn it a little it dissapear and there is certain spot were it appear again. Threshold pot at around 12-1 o'clock motorboat appear.

Motorboating being a 5-10hz oscillation, what in that particular unit (or vari mu compressor) can cause motorboating? Is a certain ground scheme can be the culprit? I linked my schematic .

So far, I think about some things that could possibly be the cause. Maybe layout related.

1. I installed R6 on the same board as my psu, at the other extremity of the case. The run to connect screen grid of 6sk7 is pretty long. Could be acting as an antenna?? So moving it on the main board could possibly help, at least reducing the run.

2. Installing C6 straight at the tube socket, instead of runing wires from the tube socket to it and get back.

3. 6SQ5 cathode goes to R8 wiper and then I connect the wiper to R10 pin 1. Maybe I should wire pin 3 to both, so I would have 2 wires comming from pin 3 instead of 1 and having him split at the pot.

4. Connecting R8 ground to the psu ground, instead of the signal ground point.

5. Adding another filter cap at the psu

Other than that, what should I check? Thanks for the helps. Much appreciated.

 

[JohnRoberts]

I am not a tube guy, but check PS decoupling caps or grounds in the area that is misbehaving.


JR

 

[Deepdark]

Thanks for the reply. PS decoupling caps, you mean the filter caps, isn't it? I'll try to put one more caps to see if it helps. I can strap one before R7, too, to cure any ripple before entering the plate of 6sq5. I'll give a look at the ground around the sidechain circuit. From the top of my memory, it has good continuity from all ground point.

 

[Deepdark]

is it always some sort of ripple that flow through the circuit and screw signal or is there other cause of it?

 

[JohnRoberts]

is it always some sort of ripple that flow through the circuit and screw signal or is there other cause of it?

Motor boating is generally a very LF oscillation, which is why it is often associated with power supply, but like i said I am not the tube guy here....

JR

 

[Deepdark]

ok thanks. That's the first thing I'll try the, putting in another filter cap. and put a cap at the jonction of R6 and R8, just to see what it does to the motorboating. That way, I guess I'll be able to know if it was power supply related or trouble is elsewhere

 

[JohnRoberts]

I am the wrong guy to be helping you (trust me) but where the caps connect to ground could matter too.

JR

PS Surely there is a real tube guy around here somewhere?

 

[Deepdark]

where the caps connect to ground could matter too.

Here you speak about hierarchical grounding, isn't it? For now, I did it à la LA2A (one ground taking all psu filter cap to main IEC bolt and the other point take all signal ground connection (everything around V1/21 and V3). For now, I sended the ground side of R8 to the same ground  point as the signal, but I'll change this to strap it at the psu. And I plan making a hierarchical ground scheme with the siganl ground. Screen of tube are strap to the socket mounting bolt to minimise run.

Concerning the tube guru, I'm pretty sure they will give sign of life  ;D

 

[Deepdark]

oh and another thing about the symptoms guys, when cranking threshold, motorboating sound change, as if the put put was accelerating. Maybe it has to do with C1/R1? Like the pulsation/oscillation was accelerating

 

[Matt C]

I don't have a definite answer for you but based on other people's posts in other threads, it sounds like this can be caused by the size of the capacitors in the sidechain/timing circuit.  I would try adjusting C1/R1.

 

[Deepdark]

Actually, i keep thé stock values. And i already tried another matched pair of 6sk7. No luck for now

 

[Deepdark]

allright. After adding another 47uf filter cap in the psu, re-routing some wires, and changing my ground chain for a hierarchical one, I got rid of the major motorboating I had. Now it works like a charm. And while playing with it, I noticed how important balance of the 6sk7 is, and how curent ctrl and threshold are interdependant. I got some little motorboating sometimes with high threshold setting but adjusting a little with curent ctrl pot and I can get rid easily of it. Probably adding a balance pot would help, but I'm pretty sure playing around curent ctrl will basically do the same. Overall, I'm quiet happy with it. No hum at all. A little hiss, but when adjusting workflow accordingly (i strap it across my apogee ensemble, and adjusted in and out at -18db digital, around 0db analog), I got a noise of about -72db on meters of my daw. Not really scientific, but a good guess. This thing is fat on bass, I tried it on acoustic guitar part and it shines a lot. Thanks again.