Trouble in Loop Land

[UltraHookedOnPhonix]

Hello Everyone!

I recently posted my finished Dumblelator in this thread: https://tubeamparchive.com/viewtopic.php?t=4837 and some issues have arisen that puzzle me.

1. First off, it sounded fine and was super quiet but then I noticed that some settings induced a loud hum. I tracked down a ground loop down and solved it.

2. Now there’s a slight hum when engaged (@ unity gain) and when effects are used between the send and return, the noise increases even more. Some effects are unusable (Nova delay, Verbzilla, DD-5 etc). My amp (modded Overtone) is silly quiet and I’d like to retain that quality with the loop connected.

3. The standby switch doesn’t operate like a conventional standby switch. It pulls the voltage down very slowly and after about 4 minutes the PS caps have about 2V each in them. If I crank the controls, there’s still sound coming through although faint.

If I were to compare my loop to Funk’s, the difference is that instead of using a ground bus for pots and jacks, I ground mine through the chassis a la the “real deal” pics, careful not to induce ground loops.

The unit I built has 242V on the plate of the cathode follower and 346V on recovery side (214V before 100K plate resistor). Seems normal…..

I really have no reference as to how quiet a Dumblelator should be like under normal operating conditions, therefore it would be really helpful if other Dlator loop builders out there could chime in. I’d really like to solve this one as the loop is the final puzzle to my tone quest.

Much appreciated!

[odourboy]

Hmmmm... I'm no expert here but it happens I was messing with mine just tonight. I'm testing an improved power supply to get rid of a bit of a hum problem I had. It's extremely quiet now - just a slight hiss at unity gain.

What caught my attention was the effects you mentioned in your post. I'm using a Nova Delay and a Verbzilla in my loop. Sounds great. No issues.

[UltraHookedOnPhonix]

Dang, well I guess that says something. What kind of pwr supply are you running?

[odourboy]

It's a torriod supply carved out of an ElectroHarmonix tube effects pedal. It's powered by a 10VAC wall wart and kicks out about 360VDC after going through a full wave bridge. It also has a second rectifier powered straight off the supply that drives a 12V voltage regulator for the heater power.

The package is really slick and works like a charm, though my D-lator voltages are a touch on the low side compared to the 'spec'.

[angelodp]

I'm out of town at the moment, so my Dlator investigation is quiet. I do wonder what the net effect of lower voltages at the plates is sonically. When I had it up and running it was very quiet and I ran a reverbamate and boss delay through the Dlator & loop in the Dlite and it all seemed fine. I have my bus grounds ( also chassis grounded ) to a common star ground, except for the IEC and power board ( like HAD ). I also added a redundant bus at the pots ( body tabs ) to ground.

ange

[UltraHookedOnPhonix]

Ok, I’ve tried isolating various corners of the chassis to no avail. I’ve also tried isolating the jacks but that makes it a whole lot worse.

Should I try to switch the primaries?

Anyone with experience in this circuit, any comments are more than welcome!

Thanks guys.

[angelodp]

Hey Ultra, sorry to hear that there are still troubles in loop land. I am out of town and will be doing voltage checks and more sound checks upon my return. Where did you locate the ground loop out of curiosity, as my build is a hybrid of Funk's and your design.

Best Ange

[UltraHookedOnPhonix]

Angelodp,

I isolated the front left corner of the Hammond chassis. If I didn't have it isolated, it would hum like a mother with the recovery input cranked.

Now it operates correctly but hums more than I'd like. It also becomes worse when using some effects in the loop. Weird indeed...

[drz400]

Toroid are actually not that great when anywhere withing 8" of a tube, they trhough a healthy field horizontal more than vertical.
They inject hum, you need to A use a mu metal shield around the transformer, B) use a tube shield C) Rotate the toroid, it will have a sweets spot D) use DC heaters E) never use the chassis as a ground path, you cant control it .

[odourboy]

Toroid are actually not that great when anywhere withing 8" of a tube, they trhough a healthy field horizontal more than vertical.
They inject hum, you need to A use a mu metal shield around the transformer...

Curious. I couldn't find anything to support your claim about a toroid being not that great around a tube. On the other and, there were endless claims of the opposite...

I quote Wikipedia "Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth)"

Here's something I found in an article from SoundStage.com. "But putting an EI-frame in a tightly packed multi-channel amp or receiver is just about impossible...there just isn’t enough room to stay away from the radiated magnetic field. In those applications, you will almost always find toroidal power transformers. You can jam a toroid right into the middle of five amplification channels and have no problems from magnetic fields."

[Funkalicousgroove]

if it did work quietly, and now it doesnt work quietly I'd look at the following:
1 Tube- I've seen them do strange and unpredictable things!

2. Chassis assembly- since you didn't follow my ground scheme, you need to make sure that all your panels are making a good connection-
With my system all the panels are isolated, and YOU control the ground flow- with the system you used there are all kinds of variables for ground flow/loops. Check all your bolts!

3. Conductivity of your shield to chassis ground- also make sure all your jacks are tight.

FWIW, Real Dumbleators are fairly noisy, in comparison, my loops have been quieter across the board, so I'm sure your problem is something simple-

[ayan]

Hello Everyone!

I recently posted my finished Dumblelator in this thread: https://tubeamparchive.com/viewtopic.php?t=4837 and some issues have arisen that puzzle me.

1. First off, it sounded fine and was super quiet but then I noticed that some settings induced a loud hum. I tracked down a ground loop down and solved it.

2. Now there’s a slight hum when engaged (@ unity gain) and when effects are used between the send and return, the noise increases even more. Some effects are unusable (Nova delay, Verbzilla, DD-5 etc). My amp (modded Overtone) is silly quiet and I’d like to retain that quality with the loop connected.

3. The standby switch doesn’t operate like a conventional standby switch. It pulls the voltage down very slowly and after about 4 minutes the PS caps have about 2V each in them. If I crank the controls, there’s still sound coming through although faint.

If I were to compare my loop to Funk’s, the difference is that instead of using a ground bus for pots and jacks, I ground mine through the chassis a la the “real deal” pics, careful not to induce ground loops.

The unit I built has 242V on the plate of the cathode follower and 346V on recovery side (214V before 100K plate resistor). Seems normal…..

I really have no reference as to how quiet a Dumblelator should be like under normal operating conditions, therefore it would be really helpful if other Dlator loop builders out there could chime in. I’d really like to solve this one as the loop is the final puzzle to my tone quest.

Much appreciated!

Hi,

Please see the attached picture and let me point out a few things:

1. I use isolated jacks, with their ground running to a common preamp ground.

2. Pot cases and their grounds also routed to the common preamp ground.

3. I recently started using a choke.

I know very few other people use isolated jacks, and if you do not, I think you're taking unnecessary chances IMNSHO.

Hope this helps,

Gil

[UltraHookedOnPhonix]

Thanks for the responses!

Funk,
I've tried swappin' tubes but same issue remains. I've read in this forum of other similar issues with Dlators humming when the MV is down on the amp etc. Curing it with a small (390R-ish) to big (300K-ish) resistor in-line with the input. Come to think of it, the 220K/250pF the D-man used on 124 activates in-line with the CF input cap (if using a loop in the loop so to speak!). I've also read posts from 67 Plexi, that he employed a 10Ohm resistor to solve an oscillation issue, hung from the Send jack like in the attached Marshall Jubilee loop:

[IMG:600:514]http://i205.photobucket.com/albums/bb17 ... eeLoop.jpg[/img]

I'll quadruple check the things you layed out for me to try.

[UltraHookedOnPhonix]

Thanks Gil!

I'm going to try isolated jacks and a choke and I'll report back to you guys.

[ayan]

Thanks for the responses!

Funk,
I've tried swappin' tubes but same issue remains. I've read in this forum of other similar issues with Dlators humming when the MV is down on the amp etc. Curing it with a small (390R-ish) to big (300K-ish) resistor in-line with the input. Come to think of it, the 220K/250pF the D-man used on 124 activates in-line with the CF input cap (if using a loop in the loop so to speak!). I've also read posts from 67 Plexi, that he employed a 10Ohm resistor to solve an oscillation issue, hung from the Send jack like in the attached Marshall Jubilee loop:

I'll quadruple check the things you layed out for me to try.

I will say this just once and I promise I will shut up afterwards... I suspect your build is riddled with ground loops, and that's a very easy fix as already outlined.

Also, regarding your standby switch: you have a 100K resistor and a 33uF cap at the tail end of the power supply... if you do the math, that is a time constant of approximately 13.4 HOURS. That means, in an ideal world it would take the circuit that long to decrease the voltage amplitue by a factor of 1/e, of course "e" being 2.7 (something referred to as Euler's number). Your build's diagnosis? Normal.

LEFT UNEDITED FOR POSTERITY, BUT I WAS TOO QUICK TO MAKE A HUGE MISTAKE ABOVE. The Time constant is actually RC, not 1/2PiRC, so .33 ms, rather than hours. Besides, wrong caps for this consideration, it's the first two that are the problem. Crow taste great, thanks.
Cheers,

Gil