Critique my Layout / lead dress

[cminor9]

I spent considerable time on the board for my 6G3, making sure that all connections were rock solid before soldering. I also tried to make the wiring as neat as possible, and used some very good teflon coated 20awg silver clad copper wire I found at a local surplus.

That said, I am still learning. Would anyone care to critique my lead dress or anything else?

Btw, this is a single channel 6G3 with a presence control. And I Just noticed in those pictures that the solder joints on the eyelets look like solder went everywhere. It did not. The board material I used (fiberboard) sort of sweats when it gets heated even momentarily. Maybe cause it's sitting in my basement.

Looks like file uploads aren't working, so I uploaded them here:
http://ampedia.redbeartrading.com/da...ad_dress_1.jpg
http://ampedia.redbeartrading.com/da...ad_dress_2.jpg
http://ampedia.redbeartrading.com/da...ad_dress_3.jpg

edit: btw, I am getting some 120hx hum here, and the B+ wires are pretty far away from other stuff. Not sure what's up. On my tweed deluxe, I used smaller grid stopper resistors (27k) and it worked fine. I tried the same thing here. Should I go to standard 68k for these?

Other than the buzz, it sounds great.

[NorCalTuna]

The board looks good, and unless there's a mistake, it's not likely the source of the hum; i'm not sure the grid resistors will have much effect here, my understanding is that they tend to filter very high frequency- not the 120 hz you're experiencing.

I'll put this out here: I built an amp for a friend who is a huge garcia fan which could switch out the 68k resistor on the first grid- which results in no appreciable noise (or real gain) increase at all (amp is virtually silent) but does seem to have a bit more harsh tone with a pedal driving it. Food for thought.

with schematic I know you rigorously followed, I don't know if your heaters / lamp have an artificial center tap formed by two 100 ohm resistors to ground. I don't see a center tap coming out of the tranny... but I don't know what that blue wire is either. The fender you based it on had a center tap on it's tranny. This is the most likely possibility.

And where is your bias supply filter cap? that'd HMMM a lot... And if it isn't already, it'd be easy to star ground that amp if that buss ground behind the pots isn't isolated... it could be forming a ground loop....


I'd try, in order:
confirm heater center tap (real easy to do at either end on your layout)

Check bias cap (easy test put your meter on AC and look for ac on the bias supply, any more than a few mv is suspect)

Check ground orientation (lift the buss, star everything else)

Lose / move spare wire. It doesn't have to look perfect, just sound it- dumbles, tworocks, trainwrecks, by far aren't the neatest like a hiwatt or a piece of military gear- or a good blackface fender for that matter- but have tremendous utility nevertheless.

Does the buzz vary with volume? Then it's before that knob. If you shut the amp off without the STBY for a second, cutting the heater AC, does it disappear while the caps discharge and the amp still plays? Thats heater gnd...

[loudthud]

I see you have split the ground on the cap board but I can't really tell where the two wires go. The preamp side should go to the buss bar and the power amp side to the transformer bolt where the HV center tap goes.

[cminor9]

Originally Posted by loudthud

I see you have split the ground on the cap board but I can't really tell where the two wires go. The preamp side should go to the buss bar and the power amp side to the transformer bolt where the HV center tap goes.

thanks for the reply. I have the preamp ground going to the buss, but the other ground doesn't connect to the same lug as the HV CT. I'll give that a whirl. Thanks for the tip!

[cminor9]

Originally Posted by NorCalTuna

with schematic I know you rigorously followed, I don't know if your heaters / lamp have an artificial center tap formed by two 100 ohm resistors to ground. I don't see a center tap coming out of the tranny... but I don't know what that blue wire is either. The fender you based it on had a center tap on it's tranny. This is the most likely possibility.

Thanks for the excellent info, the entire post was useful.

I read and reread your post, and something just hit me: I did the virtual center tap on the filament supply, but with *100k* resistors. But you said 100 ohm...that might explain things! A quick check on el34world.com confirmed that it's 100 ohm. That might be it right there! I don't have any 100 ohm resistors on hand. Think 820 ohm would be ok here? That's the smallest I have in stock.

I will report back once I make the change. But if anyone else sees any problems or possible improvements to my layout/lead dress I'd appreciate it. Even after my amp is working, maybe someone else can learn.

Thanks!

[NorCalTuna]

at 100k they're effectively floating... I'm not sure the 820s would be ideal and being that they're of no audio consequence, you could probably find some sufficient 100 ohm 1/2w res. as locally as radio shack... Sounds like you're on the right track!

good luck!

[cminor9]

Thanks so much for your advice, everyone!

So, I got some 100 ohm resistors in place of the 100k resistors I had on the virtual center tap for the heaters. It is the quietest amp now. I could barely tell it was on until I plugged my guitar in. There is no discernible 120hz hum, and only a faint amount of 60hz hum only once I plug in my guitars (I blame the cables.)

Also, installed some zeners to lower the voltages down to spec. I played it for a good while, and I have to say: with the proper voltages I like this amp much much better. Played it for a while on Saturday with the band with 440v on the plates of the 6V6s (and biased accordingly) and it was way too loud and harsh. A blackface without the reverb, basically. Which makes a ton of sense!

Not as dynamic as a 5E3, and that may be the most disappointing thing about it. But then again, it's not a 5E3. I probably have to figure out how to use it correctly now.

The trem is awesome on this thing. I like trem. It beats the 5E3 on that

[cminor9]

Now, I have one more very strange thing to debug.

When I turn the intensity knob way up and the trem is off, the amp makes a weird loud clicking sound, almost like a 120hz buzz only louder. Like you stuck a piece of cardboard in a fan. The clicking sound does increase with the volume knob, but on a less steep curve than the volume. The tone knob has no effect. The speed of the clicking doesn't change with the speed knob. It's not a trem "heartbeat."

It never does this with the trem on or with the speed knob all the way up. But with the speed knob backed off, the trem off, and intensity knob full up, it will do this every time. One weird thing, if I back off the intensity knob a bit and switch off the trem, and then hit the guitar real hard, the clicking sound comes back.

I tried using a footswitch I made as well as shorted out the trem at the jack. Both methods produce the same result.

Let's see...more details that may or may not be pertinent. I notice that the intensity pot has no effect beyond about 3 o'clock...which is just about the place where with the trem off it starts clicking. The trem is at its deepest right before about 3 o'clock. Also, I put a presence knob on this amp, and backing off the presence knob with the trem off can cause the problem. But even with the presence circuit removed and the NFB shorted to ground through a 1.5k knob (aka stock) it still happens...so I do not suspect the presence circuit.

I know that's a lot of info, but trying to be comprehensive.

[cminor9]

ok, I did some more research on this one...check this out!

When I triggered the weird loud clicking 120hz sound, I measured some voltages. Voltages went beserk! The voltage coming from the bias board (which should be ~ -26) went nutso high, like 300v. I assume that was negative v, since I had my DMM set up to measure negative voltage. When I saw that I shut it off not wanting to mess anything up. Then I powered it back up, and measured the voltage at the first cap when the weird sound started and it was off the charts.

So is the power supply feeding back into itself or something? Any idea what could cause such a thing?

I am not ruling anything out, but the amp otherwise works. The trem is nice. I am tempted to just not turn up the intensity pot all the way up and deal with it, but the perfectionist in me won't settle for that.

I did notice a "washing machine" sound in pace with the speed if I run the bias hot when the trem is on. The cooler the bias, the less it is there. I can deal with that, I included this just in case it's helpful info.

Sorry for writing a book here, but I am trying to provide as much info as possible.

Please, ideas?

[cminor9]

Did yet some more prodding around here. Found something interesting.

In an attempt to measure more voltages when the weird sound starts, I stumbled upon the fact that holding a probe of my MM to ground and the other to certain places in the circuit (like the wires coming from the caps in the trem circuit or the B+ rail at the cap) the sound wouldn't trigger. If I touch the probes as described above after the sound starts sometimes it would stop, sometimes it wouldn't.

Does this sound like some weird kind of coupling or an oscillation?

One more thing: I have the three wires coming off the board to the trem twisted together cause I read somewhere that's how Fender did it. One of them carries B+, one connects to the intensity pot and carries bias voltage, and the other connects to the speed pot and carries, well, I don't know what. If this is in fact an oscillation, should i untwist the B+ wire?

I am baffled, so I just keep poking around trying crack this problem. Any help is, as always, appreciated.

[txstrat]

Originally Posted by cminor9

One of them carries B+, one connects to the intensity pot and carries bias voltage, and the other connects to the speed pot and carries, well, I don't know what.

In my build I just twisted the wires of the speed and intensity pot together. I usually try to avoid the B+ lines to go too close to other wires.
Have you tried to change the spot where the trem is grounded to switch it off? I mean if it's only there when the trem is off it might be sort of a ground loop or picking up something from other ground wires. In my build the spot where I ground the trem (to switch it off) is right at the jack - not at the star ground of the amp.

Hope this helps

[tubeswell]

Originally Posted by txstrat

In my build the spot where I ground the trem (to switch it off) is right at the jack - not at the star ground of the amp.

Me too with my 5G9 and there was no trouble except when I had it on the wrong OT zed.

Hi cminor 9 - check the resistor and cap values in the LFO circuit again. Do you have an antilog pot for the speed pot? Did you try changing the tube? (You need a good tube in the LFO)

On the biasing - bias vary trem makes the output tubes run hot when the trem is engaged (like typically in excess of 30mA or so for 6V6s), so you need to bias the output tubes on the cold side (I ended up biasing my 5G9 6V6s at 19 or 20 I think in the end, and they still shoot up to 31 when the trem is on).

[cminor9]

thanks guys, I'll try untwisting the wires to see if that helps. As much as I hate the idea, I guess I could try replacing the caps in the trem oscillator. I did check them for DC leakage, and they showed none. And they are new, not that that means much.

The speaker jack itself is grounded. I don't use a star ground on this amp, I use a buss to ground the preamp, PI, and trem circuits to. I tried moving the ground apart from the PI and preamp and it made no difference. I have also switched tubes around and that didn't help. Also, the pot is a 3M RA pot and I checked it on my DMM.

Tubeswell, what you say about bias is interesting. I was afraid I was going too cold, but maybe I need to go colder. I am currently have -34v in the bias supply though the schem specs -26v. The plate voltages are almost exactly the same. Popped the fuse on my DMM so I am not sure about the current. It's hard to find a 500mA fuse at a local hardware store! I should have been more careful. Anyway...

So what do you guys think about the fact that when I touch the probes to the wires the weird sound doesn't happen? That seems significant to me, that disturbing the circuit with a probe seems to help. I almost wonder if I have some oscillation that the meter is absorbing, but I could just be barking up the wrong tree.

[tubeswell]

Hi cminor9, did you try changing the output tubes (in case it is a tube related problem)? I ask this because the intermitent ticking might be related to biasing. Also what is the condition of your output tube grid load/bias resistors? (One of them might be overcooked - if they are carbon comps) (- just takin' potshots here)

[cminor9]

I'll try that again. I had a different set in there before, but I have since made some improvements to the amp (checked all components and moved a few things.) Really appreciate your bearing with me! You guys have given me some good suggestions to try out. Of course, anything I learn, I'll report back.

[txstrat]

Originally Posted by cminor9

So what do you guys think about the fact that when I touch the probes to the wires the weird sound doesn't happen? That seems significant to me, that disturbing the circuit with a probe seems to help.

That's why I was asking about your grounding scheme. Some day I found sorta this in one of my builds too and the culprit was a not sufficient grounding (don't remember if it was a cold solder joint or something). The circuit somehow took ground over my DMM.

[cminor9]

Originally Posted by txstrat

That's why I was asking about your grounding scheme. Some day I found sorta this in one of my builds too and the culprit was a not sufficient grounding (don't remember if it was a cold solder joint or something). The circuit somehow took ground over my DMM.

I will certainly do some more experimentation with it, and I'll try moving a few things around on the grounding.

Like I told Tubeswell, I will certainly report back with whatever I try and the result. Thanks for all of your good suggestions!

[cminor9]

I think I have this thing fixed, even if by a hack.

First, I tried switching out tubes. This didn't fix the problem, but interestingly, it changed the tone of the weird sound. It went from being a loud clicking to a more high pitched sound, almost like a car engine revving real high.

I changed some grounds around, and this didn't fix anything.

I took the presence knob out of the loop, since this was an easy mod and not destructive at all, but changing the presence knob would bring on the weird sound. Taking this out had no effect.

Then I think I stumbled across the fix. Now, I can turn the knobs anywhere I like at any volume and it's all good! Check out the attached schematic to see what I added. It's in the trem circuit, and I called out with some big text.

So, it's a hack. I am a software developer. Sometimes hacks in code are needed. In code, though, it's a predictable system where I can usually see why a hack is needed. I am enough of an electronics n00b, though, where I cannot really do this and it's frustrating. Can anyone tell me why such a hack would work? BTW, I zenered my voltages down so that they are within a couple of volts of the spec.

I have attached an updated schematic. Does anyone have any theories on why this fix worked?

[tubeswell]

Originally Posted by cminor9

ICan anyone tell me why such a hack would work? Does anyone have any theories on why this fix worked?

Hi cminor9

Good to see you've worked out a solution (and handy to know what it is - Ta)

My guess is that it must've been that the voltage swing from low frequency waver from the oscillator was too intense causing the bias of the output tubes to be thrown too far out of whack, and you have now 'dampened' it a bit more and brought it into line. Either that or the cap next to it is dodgy, and the new resistor has compensated for it. (Funnily enough - on the 5G9 schematic the cap and resistor are in the opposite order - I guess because the 5G9 trem has a CF feeding the bias and it needs a coupling cap at the cathode.)

I now recall that I did something similar when I first built my 5G9 - and accidentally left out the 1M (which was in a similar location) and I had a similar problem with the intensity control on full, although the trem was deeper. Have you tried maybe experimenting with the value of your new resistor, to get the optimum value (for depth) without getting the ice-picky, chopper-like noise? (or replacing that other cap?)