New build issue w/ HRM OD (solved)

[mlp-mx6]

I searched for this issue, in vain.

So I decided to gut and rebuild my first Dumblish build. (I've learned a lot since that build!) It has 5 preamp tubes, so I did a TripleDrive kind of build. Clean/OD HRM build on one side, pure Fender clean on the other. It also has a built-in D'lator.

The Dumble side follows the HRM-102 schematic. My issue is with the OD master (1M). The OD has a big-time 120Hz buzz *unless* I turn the OD master almost all the way up. Once I do that the buzz goes away. I have the HRM board ground going to that pot so that they share a common ground point (trying to eliminate any ground loops).

The other thing I'm seeing here is that the mid control makes no audible difference to the tone. I used a .01 mid cap (as per the schematic), and the mid control is isolated from the bass & treble controls (again as per the schematic).

Finally, I have the "thin PAB" thing that others have mentioned, but I'm waiting to solve that until I can get this ground buzz thing fixed.

Thanks in advance for any advice you can offer.

Michael

[Bob-I]

Post some detailed pics and a schematic on how you did the switching for the additional channel. I had some difficulty with an amp I did years ago with similar topology.

[bluesfendermanblues]

Gutting it sounds pretty drastic. Sounds like a you only have to move a couple of wires.

FWIW my HRM board is grounded to the OD cathode grounds that - together with the CL cathodes - are grounded on the bottom of the chassis close to the input jack.

The OD master is grounded to the brass bar the is connected to the other grounded pots on the frontplate. The brass bar is also connected to the same plase as the above mentioned preamp tube cathodes.

Sounds pretty complicated in writing, but I guess its the standard grounding scheme that I've seen posted by IC-racer and tony VVT.

Works like a charm, no circuit hum.

[Structo]

Make sure you are grounding your ground buss wire at the input end only.
Study how #124 is grounded and follow that.

Again look at #124 layout to see how the mid cap is wired.

Do you have your clean master and OD master volumes wired correctly?

[mlp-mx6]

Post some detailed pics and a schematic on how you did the switching for the additional channel. I had some difficulty with an amp I did years ago with similar topology.

I'll post some pics this evening. Not my neatest build, but there is a lot going on in this chassis.
I can describe the switching much more quickly than I can provide a schematic. One DPDT relay is the Dumble clean/OD switching. The output of that goes to the other DPDT relay, which selects either the output of that relay or the "Fender clean" signal. The output of this goes to the D'lator input. I do not think there is an issue with the switching - but perhaps I should check that yet again as well. The buzz/hum is only in OD mode, but again, if I turn the OD Master all the way up it goes away. It's LOUD, but there is no extraneous buzz. THAT is what has me mystified.

Gutting it sounds pretty drastic. Sounds like a you only have to move a couple of wires.

I gutted it from the prior build, not due to this issue. The prior build used a cathode follower on the first tube to drive the 2 tone stacks. It worked okay, but just sharing the input signal across the 2 input grids allowed me to have a topology more like I wanted (straight Fender clean, Dumble clean/OD). Plus I was able to lay out the board more like an ODS - whereas I did my own layout with no knowledge of ODS internals the first time.

Make sure you are grounding your ground buss wire at the input end only.

Done. Always been like that.

Again look at #124 layout to see how the mid cap is wired.

Done, matches exactly. It couldn't be simpler - from the mid cap to the wiper, ground the pot. It is a totally separate circuit from the rest of the tone stack.

Do you have your clean master and OD master volumes wired correctly

I've checked this a dozen times. I'm as sure as I can be that they are both correct.

[Bob-I]

Done, matches exactly. It couldn't be simpler - from the mid cap to the wiper, ground the pot. It is a totally separate circuit from the rest of the tone stack.

Then you MUST have a bad component or solder joint.

Like Sherlock said, if you eliminate all but one possibility, the one possibility, no matter how unlikely, must be true.

[Bob-I]

I can describe the switching much more quickly than I can provide a schematic. One DPDT relay is the Dumble clean/OD switching. The output of that goes to the other DPDT relay, which selects either the output of that relay or the "Fender clean" signal. The output of this goes to the D'lator input. I do not think there is an issue with the switching - but perhaps I should check that yet again as well. The buzz/hum is only in OD mode, but again, if I turn the OD Master all the way up it goes away. It's LOUD, but there is no extraneous buzz. THAT is what has me mystified.

Sounds like what I tried the first time as well. I had nothing but problems with buzz but I don't remember if it was in OD or clean mode.

I ended up moving the Fender/Dumble switch to the input, making sure to ground out the stage that wasn't being used, buzz solved.

[mlp-mx6]

Color me stumped. *sigh*

The mid control thing was my dumb mistake letting the tie wire for the input and wiper touch the pot shell. Oops. An easy fix once I saw the problem.

Though I 'solved' the buzz issue I am unclear as to why/how. To recap, if I turned the OD master all the way up the buzz went away. Below about 4 o'clock on the control, however, buzz city. I was playing around with things and discovered that the bright switch on the D'lator send would kill the buzz too if I turned it on. I could also kill the buzz by turning up the D'lator send pot to about 4 o'clock as well. Just bizarre.

I then learned that if I touched the send jack and the chassis it would kill the buzz. So I wired a fresh ground wire to the D'lator and on the 3rd try it killed the buzz. The first two re-groundings did not kill the buzz. I have no idea why the 3rd try worked and the first two did not.

Clearly there was some issue w/ the ground connection and the D'lator. Why it only mattered in OD and not for *either* clean tone I cannot explain.

If anyone has any theories, I'm listening...

Thanks for following along.
Michael

[talbany]

Color me stumped. *sigh*

The mid control thing was my dumb mistake letting the tie wire for the input and wiper touch the pot shell. Oops. An easy fix once I saw the problem.

Though I 'solved' the buzz issue I am unclear as to why/how. To recap, if I turned the OD master all the way up the buzz went away. Below about 4 o'clock on the control, however, buzz city. I was playing around with things and discovered that the bright switch on the D'lator send would kill the buzz too if I turned it on. I could also kill the buzz by turning up the D'lator send pot to about 4 o'clock as well. Just bizarre.

I then learned that if I touched the send jack and the chassis it would kill the buzz. So I wired a fresh ground wire to the D'lator and on the 3rd try it killed the buzz. The first two re-groundings did not kill the buzz. I have no idea why the 3rd try worked and the first two did not.

Clearly there was some issue w/ the ground connection and the D'lator. Why it only mattered in OD and not for *either* clean tone I cannot explain.

If anyone has any theories, I'm listening...

Thanks for following along.
Michael

I just had a similar issue with a Lator as well.. Buzzed badly obviously a ground issue.. I found that if I isolated the ground for the send jack at the transformer bolts buzz went away.. On the chassis buzzed like nuts.. Too weird.. never had one do that before..The chassis was small 5X7

Tony

[boldaslove6789]

Pics for enlightenment? This amp sounds interesting.

[mlp-mx6]

Tony - I had to ground this one at the amp input!

I'll take some pics and post them later. Thanks.

[Tonegeek]

2 things that I found out when I had a similar buzz:

#1 My power strip lost the ground lug connection to the wall. This did really weird things to the amp. Buzz would come and go with certain settings. Once I figured out about the power strip it all cleared up.

#2 I was getting a buzz when I would turn certain pots all the way down. I think it was the mid pot or master, can't remember. I ended up putting a small resistance right before the .05 input cap to the built in dlator. This fixed it all. There has since been some discussion about strange things like that happening when the input to the cathode follower is shorted to ground (which happens when the master is all the way down).

[Structo]

Weird.
I have the same issue with my d'lator.
If I turn the return pot all the way down it buzzes badly.
I figured it was a bad pot since I used the cheap Alpha's on it.

Haven't looked much more into it because it works fine otherwise.

[mlp-mx6]

I have pics, but cannot attach any due to some "Sorry, but the maximum filesize for all Attachments is reached" error. I'll try again some other time.

[boldaslove6789]

I have pics, but cannot attach any due to some "Sorry, but the maximum filesize for all Attachments is reached" error. I'll try again some other time.

Try Picasa