Standby short squeel + distorted sound

[TheNixonWayne]

Hi,

I made a SLO based clone and I am facing two problems.

1. When I flip standby switch I can hear low frequency squeal for 2-3 seconds. I tried several things such as replacing inductor with resistor (just for test purpose to see if I am getting flyback voltage in power supply), I replaced electrolytics caps but this error still persist. If I take out PI tube then squeal disappears. I also cut the feedback line to prevent any unwwanted oscillations but without success.

2. When I select clean channel and set it's volume at 0 and it's gain at full I can still hear comming out of speakers distorted sound. It's very low level. If I turn lead channel volume to full it will be even more audible. I took out lead channel valve to see if lead channel has any impact and it seem that lead channel has nothing to do with that. I guess I am facing troubles with some parasitic sound. When I play clean channel at normal volumes I can hear that parasitic sound which is added to clean channel. I replaced optos but problem persists.

I checked my OT primaries. They are running close to the chassis and not under the board. They are twisted.

I would be grateful for any suggestions or ideas.
Thanks

Here is link to youtube video: http://youtu.be/I_2s0fwKXh4

[Structo]

Do you own a scope or can you borrow one?
If not read below for a do it yourself probe.

That is a strange oscillation.
At first I would have suspected positive feedback on the NFB circuit but since you disconnected the NFB it shouldn't be that.

I think that is 120Hz buzz, which comes from the B+ power supply.
Check all grounds, reflow and suspicious ones.

Check power supply capacitors.

Make sure bias supply is negative (Bias cap + goes to ground).

Double check all wiring to make sure there is not a mistake.

Use a meter to double check continuity, like from input jack to first tube.

You can use another amp as a listening amp.
Get a wooden chopstick, or equivalent non-conducting stick, a 1/4" plug, a 3 to 6 foot long length of shielded cable (or just cut a sacrificial instrument cable in half), a good 630V signal capacitor--almost any value will do, an alligator clip, hook up wire, some tape and some shrink.

1. Attach the alligator clip to the end of about 18" of flexible hookup wire.
2. Attach the 1/4" plug to one end of the shielded cable--center conductor to tip, shield to shaft. This is already done if you use a guitar cable.
3. At the other end of the shielded cable, strip a bit of the center conductor, and solder it to one lead of the capacitor.
4. Solder the free end of the clip lead to the shield.
5. Use shrink tubing and electrical tape to affix the cable/capacitor to the chopstick, so the free lead of the cap sticks out past the chopstick by about an inch.

Now you have your probe. The capacitor keeps high volts from the amp from damaging whatever you have the probe plugged into.

Here's how you use it.

OK, so plug a signal from a signal generator or from a cd player or radio headphones, into the input jack. Plug the 1/4" end of the probe into another amp, and clip the alligator clip to the chassis of the amp under test. Now, using the exposed end of the probe, start touching points along the signal's path, starting at the input jack. Do you hear the input sound? If so, move to pin 2 of V1. Still Sound? If so, good, move to pin 1.

WARNING: High Voltage Danger. Don't touch anything inside the chassis.

If you find noise right at the first stage then something is either wired wrong or the power supply has faulty filters or is wired wrong.

Good luck!

Here is a RG Keen article about troubleshooting hum.

http://geofex.com/gtramps.htm

[Blackburn]

I agree with Tom about checking the PS caps regarding the octave thing going on. I had something similar happen when I missed the grounded end of the reservoir cap one time when soldering. The standby oscillation is also familiar to me, but not quite the same as what you're experiencing. Did you wire the reservoir cap on the hot side of the standby when the switch is closed?

[TheNixonWayne]

Thanks for your comprehensive debug hints and tips. I'll go through list and check all that.

I attached schematics of how I've done power supply. In my design it's basically done the same way. Except that now I replaced choke with resistor (for test purpose - to test for voltage flyback).

I did some changes and I don't experience octave higher sound anymore. It seems ok. But now if I turn down all volume controls and clean channel gain all the way up I can still hear some distorted sound coming out of speakers. If I turn non-selected channel volume up this effect becomes more audible. It seem like there is leaking something from one channel to other or maybe even some parasitic sound.

For the squeal I'll try to replace switch as well.

I will report my observations.

[pdf64]

For the squeal, try tacking in a smallish additional reservoir cap on the choke side of the switch, eg 10 or 22uF, so that there's always some decoupling at the plate node.
For the sound 'leak' consider the stray capacitences between all the wiring/circuit elements in the chassis; something carrying a big signal from the clean channel just has to have a tiny bit of stray capacitence linking to a high impedance circuit on the other side of the master volume for this to occur.
Might even happen within the relay itself.
The full scheme and good photos of your build would help.
Pete

[TheNixonWayne]

Here are pictures. Because this is combo version PCB inside must be turned around which makes this amp harder to debug. But I had to make it so because I wanted to keep all controls at same position as in head version. My head version is working OK, but this one has errors.

[TheNixonWayne]

Here are some screenshots of signal measured at speaker jack. That error occurs when switching stdby switch. The frequency of oscillation is 100Hz which is double of 50Hz. I tried to replace PI decoupling cap but without any good results. I also added 100nF/630V cap at choke side but no changes at all. Transient lasts for about 2,5 secs.

[Structo]

Where are you located Aljosa?

Please put your location in your profile so we can better help you.

What is your wall voltage and freq?

For example, 220v 60Hz or 110v 50Hz.

[pdf64]

Crikey that upside down board is a nightmare - deja vu Fender Super 60.
So you've got two builds which are mirror images of each other, head and combo, with the combo being the problem one?
Pete

[TheNixonWayne]

I am from EU where we have 230V @ 50Hz. Yes I have two builds. Head is 100W with EL34 power valves and this one which is combo with 6L6 power valves.

[TheNixonWayne]

Just to add some observations.... I think something is wrong with grounding scheme I used. I tried to connect 360V CT directly to negative pin of electrolytic capacitor. The effect of squeal of 100Hz seems to decrease. I will go through whole design again and check grounding scheme.

[TheNixonWayne]

Just to add some observations.... I think something is wrong with grounding scheme I used. I tried to connect 360V CT directly to negative pin of electrolytic capacitor. The effect of squeal of 100Hz seems to decrease. I will go through whole design again and check grounding scheme.