New ODS Build - Sounding WAY Better!... Little help please?

[RJ Guitars]

Hello All,

I've completed the initial stages of my ODS build and have moved on to the test and tweak stage. My amp is based upon the D'Lite with the biggest differences being that I'm using a Stancor PT and a Hammond OPT and my own chassis.

This thing is sounding really awful - Bright, Brittle, and Brassy, until you hit the Bright switch... then it's even more piercing.

As if that isn't enough, when I go to the OD setting it adds farty to the overly bright tone and then for a super duper bonus, if you turn up the trimmer pot or the volume gain it goes into a really horrendous shrieking squeal...

I've twisted the knobs a bit and I can find a couple spots where it's actually O.K. sounding, especially clean, but I know it's altogether just not right.

Overall it's a relatively neat build... not art work but decent craftsmanship and technique. I added shielded wire in the obvious spots but it made no difference.

Voltages are:
v1a = 198
v1b = 204
v2a = 195
v2b = 212
v3a = 325
v3b = 338
v4 = 456 (6l6GC) bias 38ma
v5 = 456 (6l6GC) bias 36ma

I know the PI voltages are above ideal, but I think I have bigger problems at this point. (I might have written down the PI voltages in reverse order... but the values are correct).

I've checked wiring, moved wires around, check & re-soldered all the obvious stuff... changed tubes... checked bias ...

It seems no matter what tube I use v2 is a little microphonic... i assumed that was because of the high gain?

Any glaringly stupid things to check out or an obvious place to start sorting out the various issues?

rj

[ampdork]

Try reversing the OPT primary leads. Your symptoms sound like you may be creating positive feedback instead of negative in the feedback loop.
Looking into the amp with the rear facing you the lead on the left is almost always blue, the one on the right brown. If they are not in that order I am sure this is your problem.
I would think running the OPT secondaries against the chassis would be better too but also kinda doubt that is your problem.

[bcook]

RJ,
Try the OT wire swap per ampdork's post. In addition, whack all of your wires with a meter lead, if you find one that makes noise you're close to a bad solder joint - it happens. You're welcome to bring it down also.
-Bob

[Funkalicousgroove]

HOLD THE PHONE!!!!!!!!!!!!!!!!!!!!!!!

If your OT primaries were reversed all you would get is howl and oscillation, you wouldn't be able to play the amp- so that's not your problem.

Post some more pics, my guess is that you have a wrong value resistor somewhere, or something is mis-wired.

[RJ Guitars]

Thanks guys...

I swapped the OPT around and maybe it helped... this thing still sounds awful although the clean channel is sounding better. Reminds me of one of those old overly bright Fenders. I think I can work with this by swapping some cap values and taming the brightness.

I can only get non-buzzy tone when I run the presence all the way up. I hear a squeal also when it's below about 8.

One thing I did notice is that if I run the master volume down really really low, it looses the low end tone and sounds bad... it is a loud amp!

The OD seems to be causing all the grief. Overall, I'd call it a shrill tone with a lot of buzz and quack... it's glassy - but the real brittle kind with nothing sweet and singing about it.

I moved wires and punched around with a chopstick... just get some ring off off that 2nd tube but nothing that seems loose or iffy. I'll keep looking and probing for more...

I can still drive it into oscillation with the trimmer pot or volume knob past 3... and here is another clue. I can also drive it into oscillation or at least change the frequency by changing the values on the tone pots.

I'll get in there and take some close up shots if you think that will help.

Send your thoughts... this is all new ground to me. I've never done an overdrive circuit or even a fixed bias amp before. Lots for me to learn here.

Also, I don't even really know how the thing works... that is, are the clean and OD channel supposed to be interactive or entirely separate? Is the level the "master volume" for the OD channel? I could use a beginners lesson in how Dumbles work.

rj

[dogears]

double post edit

[dogears]

RJ,

Check your presence circuit. The D'Lite layout is wrong. Make sure your presence circuit is wired the same as Dumble did it. Do not take the third lug of the presence pot to ground. Only ground the cap and tail resistor. Also, use the 4.7K feedback resistor.

My advice stands. Do not use modified schematics unless you are cloning a specific amp. The D'Lite values are based on Norm's preferences for a 22 watt 6V6 amp, with certain transformers. That is not what you are building.

You should baseling to known good Dumble values and tweak from there.

The miswired presence will make the amp a whole bunch more raw.....

[Structo]

So was this a scratch build?
What layout or schematic did you follow?

Yes some close ups of the interior would probably help find something if it is obviously wrong.

[tubedogsmith]

do you have your speaker jack grounded?

[odourboy]

I like tubedog's idea of checking to see if your OT speaker ground is tied into the amp ground. And while you're poking around, it might be worth while checking and posting your cathode voltages.

[Funkalicousgroove]

try clipping the bright cap off your master volume.

[RJ Guitars]

try clipping the bright cap off your master volume.

Thanks Funk... this really helped the brightness problem. I was keeping the master down low and the highs were swamping the sound.

[RJ Guitars]

do you have your speaker jack grounded?

Well no it wasn't... I thought this would be the fix, but actually I couldn't tell the difference... rats. It was a good catch though.

[RJ Guitars]

So was this a scratch build?
What layout or schematic did you follow?

Yes some close ups of the interior would probably help find something if it is obviously wrong.

Yes it was scratch... I did follow the D'Lite for the most part... by the way I am very grateful to Normster and Moss for their work. I am actually pleased to have it this far along and it would have been much harder without the D'Lite for a template.

I still need more gut shots... but here is one look. I had a fit with the macro lens and I'll try again

rj

[dogears]

Is your presence circuit correct?