• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Strange pulse of square wave into cap load

Register to hide this ad
That looks all too familiar: LF oscillation. I assume there's global feedback applied encompassing the output transformer secondary. Can be caused by a number of factors: more global feedback than the design can support or lack of power supply decoupling between stages, e g feeding the same power supply node to two consecutive stages. Lack of PS decoupling would show up w/o feedback connected. If you've checked that its not a power supply problem, it might be related to the phase shift of the interstage R/C couplings between stages and/or the bypass caps on cathodes. You can try reducing feedback or you can tinker with the R/C cutoff frequencies.

A rule of thumb I've used is to try and place cutoff frequencies of the R/C couplings inside the feedback loop 8x to 10x apart if possible, with the output stage coupling having the highest cutoff frequency.
 
Thanks for confirming my thoughts. I just never seen this type of oscillation.
I think R/C coupling "spacing" may be an issue. If I did math correctly they are only 3x apart.
1st stage 0.2uf / 1M = 1.25Hz
2nd (output stage) 0.22uf / 270k = 0.373Hz

Am I correct with my math?

upload_2020-3-21_21-54-29.png
 
Have you checked if the oscillation is in sync on both channels? If it isn't, it's probably not the power supply. If it is, the PS is the prime suspect.

Reading the other thread about the feedback- what was changed to increase the feedback from 7 to 12dB?

Regards,
Gordon.
 
GordonW said:
Have you checked if the oscillation is in sync on both channels? If it isn't, it's probably not the power supply. If it is, the PS is the prime suspect.

Reading the other thread about the feedback- what was changed to increase the feedback from 7 to 12dB?

Regards,
Gordon.
Click to expand...

I haven't tried to see if both channels are pulsing in sync. I will tomorrow. Awesome suggestion!

Initially I was using resistor substitution box paralleled to original feedback resistor. Then I added 56k across original 39k for the moment, meaning to replace with single resistor when I'm settled on all values and satisfied with stability. Right now all that is removed and channel "b" is back to original 39k without bypass cap and it's oscillating.
 
I get 0.8Hz for the first stage (.2uf and 1M resistor).

The output tube coupling caps (.22uf and 270K) are indeed 2.68Hz.

Regards,
Gordon.
 
GordonW said:
I get 0.8Hz for the first stage (.2uf and 1M resistor).

The output tube coupling caps (.22uf and 270K) are indeed 2.68Hz.

Regards,
Gordon.
Click to expand...

I redid it several times this morning and got 0.796Hz as well. I don't know what I did wrong last night.

I have 4 x k40y-9 caps in .047uf / 630v. That would put output stage to around 12hz. Is it a good option? If PS isn't doing it, of course. Still need to verify that later today.
 
I'd be reticent to reduce the output coupling caps to less than 0.1uf. You start affecting the bass clarity and response due to loss of LF open-loop gain, in turn reducing feedback, once you get much smaller than that, most likely.

That puts the output stage to about 6Hz, and about a frequency ratio of about 7.4 between the gain stage and output coupling caps. Probably a good point to be.

Regards,
Gordon.
 
Well, It pulses both channels together. Below is the link to a video of both channels on same screen. 20hz into open output.


Here is the section of the PS. All PS caps are new. Some are stuffed into original cans and others are mounted under the chassis.
Original output stage cathode bypass cap was 200uf instead of 400uf per schematic and I replaced it with 220uf.
Diodes are replaced with Schottky pair.
Again, there was no reason for me to believe that anyone ever touched this receiver since it left the factory.

upload_2020-3-22_12-2-31.png
 
Trying to make it unconditionally stable.

I don’t have .1uf / 600v but will get them
Do I still have to address PS decoupling even if I don’t have any instability?
 
I would still go over all the grounds in the power supply. If you've already tacked on caps, then the other possibility that could cause decouping issues would be grounding. Make sure the caps are grounded at the same point as the factory, and that the grounds are all secure...

Regards,
Gordon.
 
Grounds are in their original locations. This receiver has ground bus running across whole width of the chassis. I will recheck all connections anyway.

I dug a little deeper into my parts stash and look what I found!!
Back to work.

upload_2020-3-22_13-14-58.jpeg
 
I'm back! Did you miss me? :p

Mike DeWine just issued new order of lock down for Ohio. Well, guess what? Auto repair is listed as an essential business, so I'm open! Not sure what kind of business I'm gonna get though. :dunno:
Sucks that my wife still has to show up to work few days a week. She is a hospital pharmacist. They are starting work from home rotations few days a week. Acute care rehab facility with capability to have ventilator in every room!
My son in LA is keeping his "survival job" at Umami Burger as a "to go specialist". Those jobs along with Uber eats will be hot for a while.
My 14yo daughter will likely gain some weight sitting at home and with all tennis centers closed.

Now, since that is out of the way, back to this glowing glass bottle thing!

I replaced output stage couplings with 0.1uf and have same thing going. I rechecked all power and ground connections - all is where it was as designed. Even tried adding capacitance to Output cathode bypass. Looking for some more of the experience suggestions and hope not to run out of parts in stock!

Stay healthy everyone and be safe!
 
6DZ7 said:
I get that part. What I'm saying is you have too low of a rolloff point. If you want to keep low frequency oscillation from occuring it needs to be rolled off more if it's there for some reason, so you need to start at a higher frequency for the -3dB point. More like 20hz, not 3hz or 6hz or 12hz. That's a start. Then do you even have an option to seperate the PS into another filter and tap point for decoupling the stages? Adding a resistor and filter cap to a stage will change the plate load and voltages. Probably the best solution though if that is the source of the LF oscillation.

Good luck, stay well.
Click to expand...

This is a little complicated without practical suggestions that I could follow. I think, by design of this amp, without having to re-bias AF stage I'm a little limited with what I can do in terms of RC filter frequency separation.
With PS however, I think I have some extra voltage to work with. I have a inrush limiter on PT primary that drops power supply voltages to more or less appropriate levels but they are still a little higher starting from 320v screen supply that actually reads close to 350v. All voltages below that point are obviously higher too. I can replace thermistor with one that drops less or eliminate it if need to be. I'm just not sure what "decoupling" really means. Are there examples that I could follow?
 
just for yucks, does it stop if you disconnect the power amp from the preamp and feed input direct to the power amp?

decoupling is basically a RC filter stage. If the preamp feeds direct off the screen supply for example that might cause odd issues, but isolating them with an RC stage would probably take care of it.
 
I tried feeding signs into amp section directly with original coupling values and tried again with freshly installed 0.1uf couplings. Still doing it.
It looks like I should try this additional RC stage. 82uf /400v good there? I have to think about resistor value but is there a minimum to provide enough filtering?
 
6DZ7 said:
OK. Decoupling is simply to seperate stages or branches of a circuit with a resistor and a filter cap. You actually have "decoupled" stages in your PS B+ supply nodes. 315v, 300v,... and the others all have a dropping resistor and the filter cap to "decouple" those branches from the AC signal produced in each downstream plate, as well as provide additional PS filtering from the rectifiers. I don't think the stages need any more decoupling. You are creating an unnatural draw on the amp with just a cap on the output. Not expected to ever happen.
Click to expand...

it does it with cap only load
It does it with output open
It does it with cap + 8ohm in series.
Cap + 8ohm in parallel doesn’t bother it.
 
You must log in or register to reply here.
Back
Top Bottom