need advice again with strange noise

[dscottguitars]

Hi,  I just finished putting this together and I have strange noise on the distortion path.  On the schematic it's the upper part of the preamp.  What happens is when I turn the gain knob between 20% and 80% a squealing strange noise comes out, the notes are farty/crackly and dissipate quickly.  But below or above those points it's fine, long sustain and distortion above 80% and cleaner below 20%. 


The squealing noise happens when I first get to that 20-30% of the gain knob and then goes away-or gets quieter up to 80%, it's not too loud just there. You can hear a definite change at one point around 80%.  There is also a lot of hiss turning up the volume knob.  I figure this is carbon comp resistor noise and need to add a resistor to the network to bring the gain down.


In the clean mode, it sounds really good, just a light hiss at full volume and gain, but no weird sounds in between like the distortion mode.


What could cause that kind of response in the gain knob in distortion mode?


Thanks,


Daniel

[PRR]

Ultrasonic squeal.

The hiss is not carbon-comp, it is over-excited tubes.

You have *three* gain stages looping through relays. If you bring an output back close to its input, squeal happens.

Huh. You also have V2-a and V2-B with V3 gain in the middle, more ways for V2-B output to sneak back to V2-A input.

Given that general plan, I think the quick-trick is to put 470pFd from grid to ground at V2-A, V3, and V2-B

[dscottguitars]

Thanks, I'll try that.  So you know, V2a and V2b are not in the same path, V2a goes to V3 and then to the phase inverter, or maybe you know that and I don't understand fully what you mean.


How can the output of V2b go into V2a when V2a is grounded during that phase of the relay switching?


And, what is over excited tubes?


Looking at my drawing, I could connect the output of V3 directly to the tone stack.  I didn't realize that the relays would bring it too close.  It's only there for convenience.

[jojokeo]

I remember first seeing this before and wondering if there any oscillations or parasitic issues to be had anywhere? Very ambitious design with that layout sandwiched in like that. Grounding, layout, and lead dress is all crucial here. But, your amp "looks" very nice indeed, well done! PRR is suggesting ways to reduce ultra high frequencies which can address oscillation issues early in the preamp stages. But, you may notice a slight loss of your high end with too high of values or used in too many places? Sometimes when troubleshooting something like this you have to accept or give something in order to gain something? You don't always hear these issues but you can tell they are there because it will negatively affect your sustain on either many notes within a range or only on a few? It can also be heard in different ways such as a cabinet rattle, a buzzing like a gnat or bee, slight or severe, smooth or coarse, etc... As for feeding back into itself, read up on ways that the signal can accomplish this (high & low impedance wires, signal wires and/or circuit crossing over itself, wiring proximity to itself & other components especially where higher current & voltages are concerned, ground loops, etc.). Also, oscillations usually are frequency dependent and why through a certain part of the dial rotation you are experiencing this. There's just too many things to try to describe and explain which is why the advice about reading further (not trying to be a dick). It would be time very well spent. You can either search topics regarding oscillations and their cure or get a few good books like Merlin's. It just takes up too much space and time of others to go through everything thoroughly and then back and forth with you trying this and doing that, when this subject has been covered many many times already. Good luck and don't get frustrated, it's how we grow and learn through our failures which make us better. And if it was at first easy then everyone would be doing it.

[dscottguitars]

I added the 470pF caps on the grids to ground on V1, 2a, 2b and 3.  That took away the oscillation.  But I still have that loud hiss and now I can't even play a note.  All I get is a bad farting, putt putt sound with each pluck of the string.  If I turn up the volume it does it without plucking a string.  This is in the distortion path.  I've used a similar circuit before but had separate channels to control it.  I wanted to get away from having an extra gain, volume and tone stack so I came up with the relay setup I have.  It's hard for me to believe that my relay setup is causing this but I don't know for sure.  There is no input signal wire coming any closer to an output wire than the input output pins on the sockets.  Unless, it's in the relay switch itself.


What is meant by over excited tubes?  Is it too much gain?  I can drop that with a voltage divider but I've had similar in another amp and it works.


Channel still works great, I did lose some highs but I can make up some of that in the coupling cap by changing it to a 0.01uF.


What I tried to do here is have the distortion mode not go through the tone stack first.  I found out awhile back that I get better distortion this way.  And , I wanted to be able to tailor each mode to be more or less the same volume when switched just change in clean or distortion. I don't want a boost here because to get the amount of distortion I want it will have to boost too much. 


Here is a schematic of my last amp that worked.  I had a 1/2 tube switch in as a boost for either clean or distortion channel and never got any of the noise like I'm getting here.  I'm going to try unhooking some things like the clean tube, one of the distortion tubes to see if any of that works.  I've got a lot of money invested in this and I don't want to settle for a clean amp OR a distortion amp.

[dscottguitars]

I just unhooked:


the clean tube V2B, no change...


the first distortion tube V2A better but still bad sustain...


I also connected the output of V3 directly to the tone stack, unhooked the lead from the relay and it sounds the same (did this to see if it was in the relay somehow, but it's not  ...


I think I have too much gain and will try to drop that tomorrow evening. 


If anyone can verify that the bad sounding notes are too much gain that would help. 


Would an oscilloscope show me this?  I'm thinking of getting a used one and learning how to use it.

[jojokeo]

Hmm, I never saw where you had a loud "hiss"? That sounds like a grounding issue possibly w/out knowing anything more? Farting could maybe be what you're describing as a type of cross over distortion maybe? Similar circuits in other amps does not guarantee anything in a new amp due to things I described earlier. Yes, extra gain is better controlled at the end of cascading and not before or during with higher gain circuits.

A coupling cap change will not increase your high end or make up for what you're losing from the plate caps. You can get the channels working first then worry about dropping signal levels to match later after you resolve your issue or get your design tweaked properly. Another way is using split plate loads but again, later...

There is no schematic attached. Don't give up or get frustrated so soon. Sometimes you need a good nights' sleep and a break for a day or two. Then think things over with a fresh set of eyes and perspective. Most of us have gone through similar things at one time or another and it can be the simplest/stupidest of things we didn't see before.

[dscottguitars]

PRR described the hiss as over excited tubes, but I don't know what that means.  Cross over distortion, that maybe what I am experiencing.  I'll have to read about that some more... Thanks.


I disagree with you about the coupling caps not changing the high end, or low end.

[HotBluePlates]

Ultrasonic squeal.

Look at the schematic fragment below. Your choice of routing through relays has placed the output of V3 right next to the input of V2-A, as they are on the same relay. If you also take note of polarity, a signal on V2-A's grid is the same polarity as the signal at V3's plate, meaning they are in-phase. This makes a guaranteed oscillator.


I've drawn the path from V3 output to V2-A input; the dashed line shows the small physical gap where the output signal is coupling back into the input through the relay.

I added the 470pF caps on the grids to ground on V1, 2a, 2b and 3.  That took away the oscillation.  But I still have that loud hiss and now I can't even play a note.  All I get is a bad farting, putt putt sound with each pluck of the string.  ...

PRR described the hiss as over excited tubes, but I don't know what that means. 

Oscillation, that's what it means.

PRR had hoped slugging the circuit with a capacitor to ground would kill the squeal, and it may have done that. Or it may have just changed the frequency of the oscillation, because the physical arrangement almost insures instability. Now, instead of being an audible oscillation, the frequency may have gone ultra-sonic, and manifested as a rasp or hiss.


Skillful use of a signal generator and oscilloscope will show oscillation as a high-frequency wave riding on top of the desired signal.


But I think it's easy to say now that you need to break up that uppermost relay into 2 separate relays (if you stick with that method of switching), spaced well-apart.


There could be other issues, but this is the most-certain one at this point, with this layout and symptoms.

I also connected the output of V3 directly to the tone stack, unhooked the lead from the relay and it sounds the same (did this to see if it was in the relay somehow, but it's not  ...

Taking this statement at face-value, are you certain no V3 wiring got anywhere near V2 wiring? You could rule out the relay but not fix the root problem of an output being too close to an input. Keep in mind that your 1MΩ gain pot is a high impedance connected to the input of V2-A, which still qualifies as an "input" even if you move V3 wiring away from the relay.


All this points out why the overwhelming majority of high-gain amp builders use p.c. boards: repeat-ability of layout.


if you have to get all parts and circuit-paths situated just-so to insure a stable high gain able with a lot of switching incorporated, the a p.c. board guarantees each amp has exactly the same parts placement and wire routing.


We might debate other applications for p.c. boards and differing manifestations of amp quality, but the above winds up being an almost inescapable truth.


EDIT: Included omitted attachment

[dscottguitars]

Thanks for that.  If you included a drawing it did not show up.  But, I understand from your description.  I do have V3 output going directly to the tone stack input-3rd pot from the left on the picture.  The line from the relay to the tone stack is disconnected.  There is 2" of space between the V3 output and the relay right now.  But out of the tone stack it goes to the other relay and is close to V2B-the clean 'channel'.  Maybe another problem area?  Who knows at this point.  I am going to unhook everything and start with the distortion, get that working and then add the switching.  I can use the center relay for that and use another for mid boost.  BTW I am using shielded wires too.


How long can my leads from the tone stack to the mid boost relay be?  I could put it between the first and second-drill a couple more holes and move the second one over a bit.  Or move it way off to the side which would be about 8 inches away.


Thanks again for clearing things up for me...


Daniel

[dscottguitars]

here's a quick drawing using another relay switch...


This should keep the inputs and outputs separate, yes?  Unless the wire from the one relay to the other can still pickup the output of V3?

[HotBluePlates]

...  The line from the relay to the tone stack is disconnected.  There is 2" of space between the V3 output and the relay right now.  But out of the tone stack it goes to the other relay and is close to V2B-the clean 'channel'.  ...

How long can my leads from the tone stack to the mid boost relay be?  ...

The problem is there is no single correct answer; it always depends on the particular amp in question.


I did see some forum posts where builders were cloning the Soldano SLO 100 (thread 1, thread 2). I found it very interesting that the approach in that amp was to use Vactrol optoisolators instead of relays. What is critical is the Vactrol is placed by the circuit it switches.


In other words, the switching is placed at the circuit; they do not route the circuit to the switching.

What this means for you is that you may benefit from a different approach to accomplish the switching you have set up.

[dscottguitars]

I'll look into the optoisolators.  I had them in an 80's Fender Concert I had.

[HotBluePlates]

here's a quick drawing using another relay switch...


This should keep the inputs and outputs separate, yes?  Unless the wire from the one relay to the other can still pickup the output of V3?

Maybe, but does it really change things physically? I'm not sure, because I'm guessing you're going to simply rewire what you have in the existing amp, rather than make a fundamental physical change to the circuit to move things.


There's another problem, which is you should try to keep the "loop area" from a tube grid input through to its cathode as short as possible. The "loop area" is the physical path from grid through grid resistor (the Gain pot for V2-A), through ground, back up through the cathode resistor and to the tube's cathode.


The bigger you make that loop (especially for a circuit with a high impedance to ground, like 1MΩ), the more it acts like an aeial antenna to pick up signals radiated fom other stuff.


This issue could apply to every circuit in your amp. So when you look at it through that lens, what do you think about your physical layout? Maybe good, maybe not?


If this isn't a good commercial for a breadboard, where you can test and debug amp layouts before building a final amp, I don't know what is. The more complexity, gain and switching you throw in the mix, the more critical all layout and wiring becomes.

[dscottguitars]

I thought my layout was pretty good considering the Fender I had...see pic.  What could I possibly do to change my layout?  And, do you have any pictures of your amps for an example you can show me?  Thanks...

[jojokeo]

I thought my layout was pretty good considering the Fender I had...see pic.

You cannot compare yours to other amplifiers and simply draw conclusions like that. It isn't a beauty contest in this respect. As it turns out Fender's looks like crap but sounds great, yours looks great but isn't worth crap - YET. Or until you get it fixed and operational. (No offense intended just making a point of your comment)

What could I possibly do to change my layout?

There's a long list of things which could've been done to minimize the possibility of problematic issues. (this is why the reading suggestion earlier - to know ahead of time what to try to prevent and to design for these appropriately)

So, the question is - do you have a layout drawn up that you designed and followed before you put all this together? If so, please provide it so we can better understand what was done. Just like being lost - you have to know where you are before you can find your way where you want to go. There are no shortcuts or free rides for successful designing and building.

[HotBluePlates]

So, the question is - do you have a layout drawn up that you designed and followed before you put all this together?

This thread may be the same amp.

I thought my layout was pretty good considering the Fender I had...see pic. 

The Fender has uglier wiring, but probably has less gain-per-square-inch than your amp. I also see Fender used vactrol switching located at the switched circuit.

I'm not trying to squash your efforts; you're just hitting some snags that are an unfortunate and difficult reality of high-gain amp building.

And, do you have any pictures of your amps for an example you can show me?

They won't help much, because they're low-gain, mostly single-channel and little or no switching. For example, my 25L15 amp. Single channel, so the whole circuit flows left-to-right and no output has a chance to pass near an input.

But that's why I linked the other threads. Yes, they're OCD with some of their wiring, but I thought the switching approach was what would be informative.

[dscottguitars]

Well I got it working but I had to sacrifice one thing.  I decided to keep the distortion channel as it was and use the clean channel going through the tube before the tone stack, which makes it break up a bit more with the gain knob up high.  I am planning on getting a dual pot to use as the gain so I can add a resistor on the clean channel to drop some of the initial gain and then not have one on the end after the tone stack to make up the loudness. 


There was no way I could use the switching scheme I had and have it the way I wanted.  I would need two separate channels and a bigger chassis to make it work.  Maybe some other time.  I did learn quite a bit though given the frustration I had, but that's what it's all about, right??


Thanks for all your input.


I having another problem I will start a new post about concerning using a rectifier tube.  I get way to much current in the tubes when I switch it in and the plate voltage drops a lot too.

[jojokeo]

It takes a lot to "give in" just a tad or revamp your schematic and amp for the better of overall operation. You're making the best of the situation and you get major kudos dscott. You'll do better on the next one I'm sure. It's so easy to be overambitious. It's all part of taking little steps that gets you several BIG steps forward.