A hum I can live without - need help!

[drpastorbob]

When I posted about a week ago, we just got the amp up and running - no smoke and out came the sound (I posted a link to a video). Since then we have been trying to track down a low-frequency hum. I've checked all the connections, the grounds, and cleaned up some of the connections to caps, etc. to V1 and V2. I have not yet changed resistors to get V1=V2=200V. Any ideas to help me narrow it down? I also seem to hear some low-level "motorboating".

Thanks in advance.

[heisthl]

Easy way to divide and conquer and no scope needed:
Clip lead the PI input cap to ground - gone = preamp NoChange=poweramp or filtering. If preamp and hums at idle, work from input to PI and one at a time cliplead to ground any point that doesn't have DC til you stop the hum (try a chopstick on the lead dress around the preamp rubes). If the Hum only occurs when playing use a 0.1 600v cap on the cliplead instead of going direct to ground - this allows you to connect to anything.
If poweramp cliplead a 0.1 between the PI plates. NoChange means problem is Filtering. Cliplead an additional filter cap in parallel to the B+ cap. Also don't forget the relay supply as a source of low hum - try additional filtering there too. A poorly regulated/filtered relay supply can be misleading, no hum when grounding the grid of V2b (or the V2b plate through a cap to ground) might make you think it's an audio problem when it's really a relay supply problem.

[drpastorbob]

heisthl,

Thanks for the advice. The noise is on the preamp side and hums at idle. If I turn the volume up fairly high it goes into a low-frequency driving oscillation (A resonance?).

I checked all the jumpers on the main board for continuity, cleaned up some of the V2/V3 connections, as well as reheated solder connections on the main board. I have it wired up without the relay board so that we could first make sure everything is working before adding that.

I have not yet tried the sequential shorting as you suggested, but will do that next. Any other suggestions? Have I messed something up badly?

[heisthl]

Assuming the amp is set to clean mode does removing the OD tube(v2) get rid of the hum?

If you really screwed something up the smoke detector would be going off....

[drpastorbob]

I pulled both V1 and V2 and it still hums.

With all tubes in place I have the following voltages:
Voltages are:
v1a = 217
v1b = 216
v2a = 215
v2b = 225
v3a = 320
v3b = 308

[drpastorbob]

Sad to say but I missed the wire to ground on the bleed resistor on the power supply board! Everything quieted right up after we added it.

Thanks for all your help though, I really appreciate it.

The amp now sounds much more dumble like. I'm waiting on tubes now to do the final tune.

[RJ Guitars]

I worked with drpastorbob (DPB) on this tonight (he only lives a block away from me) and it was a quick and fruitful trouble shooting event... One missing ground wire was hummin' a tune that wasn't all that musical. His amp is now sounding great.

A few months back I talked to him about building our own ODS amps, and much to my surprise he said count me in. We both finished up the amps a couple weekends back and have been sorting out the issues and learning about dialing them in since then...

Tonight is the turning point in history from building amps into playing through them...Dueling Dumbles and some guitar jammin' comin up... and these amps go to 11.

rj

[LooseChange]

Nice work.
I'll guess that they both were built with the exact same parts.
Do they sound the same?

[RJ Guitars]

Nice work.
I'll guess that they both were built with the exact same parts.
Do they sound the same?

Well they have nearly identical parts except the transformers. DPB's has the heyboer set and I (RJ) have a Stancor PT and a Hammond OPT.

I had the chassis' and faceplates made at a local machine shop (I have more if anybody is interested). I made the circuit boards myself and generally followed and learned from the D'Lite layout.

They are both brand new and I haven't had a chance to play mine much yet... After we cured the the hum in DPB's I sorta thought it sounded better than mine... but I'm still training my ears and exploring the amp. In either case I think they have very useable tone and lot of ways to dial it in....

I'll start buggin' DPB to do some sound clips with me and let you compare for yourself...

rj

[Structo]

That's probably fine to ground that to the stud but usually it's taken to ground with a lug on the chassis right below the OD level pot.

Wow that is a huge OT on the amp to the right.

[ayan]

I worked with drpastorbob (DPB) on this tonight (he only lives a block away from me) and it was a quick and fruitful trouble shooting event... One missing ground wire was hummin' a tune that wasn't all that musical. His amp is now sounding great.

A few months back I talked to him about building our own ODS amps, and much to my surprise he said count me in. We both finished up the amps a couple weekends back and have been sorting out the issues and learning about dialing them in since then...

Tonight is the turning point in history from building amps into playing through them...Dueling Dumbles and some guitar jammin' comin up... and these amps go to 11.

rj

Just one comment, since you posted pictures. You may want to consider grounding that green wire somewhere else for two reasons:

1. Current "flow wise," there is a chance that ground point might be better off located somewhere else. Is that a 150K FET "emulator" resistor? If so, grounding it by the input jack would probably be adviseable.

2. I am not a big fan -- although I am completely guilty of doing so in the past myself -- of using ground points where there are mechanical connections that serve another purpose, in this case holding a loaded board in place. Granted, the likelyhood of that thing coming loose is extremely low, especially if you use nuts with locking inserts, or lock nuts. But, dedicated ground lugs with little mechanical loads of them are always good practice, especially when there are high voltages involved (which would be the case if that resistor floated for some reason).

My two modest cents,

Cheers,

Gil

[odourboy]

1. Current "flow wise," there is a chance that ground point might be better off located somewhere else. Is that a 150K FET "emulator" resistor? If so, grounding it by the input jack would probably be adviseable.

Gil - you've piqued my curiosity with that remark - what advantage is there in grounding the 'FET emulator' resistor to the input jack?

[heisthl]

I can answer that - On a D style grounding scheme - ground V1 and V2 circuits and filtering on the pot buss (also the input jack). The FET simulator is the last thing on the dropping string so it should also be grounded there. The V3 supply gets grounded at the rear panel preout pwrin jacks (the reason you want the presence ground off the pot buss) the screen supply is grounded near the output tubes and the main B+ is grounded where the transformer center tap is grounded.

[odourboy]

Thanks Heisthl - makes sense.

[ayan]

I can answer that

Yep, you sure can and did.

Gil