ODS troubles when cranked

[jelle]

Hi,

I hope you guys can help me with this one...
I have recently build an ODS reverb combo that is sounding real nice. However, when the master volume is up and a bass note is played hard, the amp starts making a loud noise like the mechanical vibrations from the speaker are causing troubles in the amp. This is a very unmusical sound/oscillation that goes away when the master is turned down.

The amp works flawless when outside the combo or played into a separate cab. Also the three switches on the front are popping. Wiring looks ok and the other amp that I build does not pop. I have opened the amp to search with chopsticks and the critical areas are:
1) the ground buss (!)
2) the preamp board
3) The input grid coax of V1a
4) the reverb transformer wires
Kinda all over the amp...

Replacing the tubes does not solve the problem.

Any suggestions? Cannot post pics right now...

Thanks,

Jelle

[glasman]

How do you have the reverb tank mounted. If you have it rigid to the case you will be in trouble. The case will shake the tank and it can very easily take off on you.

The tank must be allowed to float around a wee bit.

Gary

[heisthl]

Is one of your controls pots loose? this can sound like this especially if its the first volume pot. The chopstick method on the solder lugs should find it....

You may have to divide and conquer by plugging a line level source into your pwr amp in jack while it's mounted in the combo cab. If your lucky it will be in the power amp (less stuff to trace).

[jelle]

Thanks for the replies

Gary, the amp does this even with the reverb tank disconnected. It must be inside the chassis. The noise goes away when the master is turned down so it must be preamp related. I have ordered new 1M vol pots so I can replace the first volume and the master. I will chopstick some more I'll go the divide and conquer route.

BTW just played the amp with a tele through two 4x12 marshall cabs...First one was a G12M lead and it was less than magic but the G12H bass cab...OH MY GOD!

Jelle

[ayan]

Thanks for the replies

Gary, the amp does this even with the reverb tank disconnected. It must be inside the chassis. The noise goes away when the master is turned down so it must be preamp related. I have ordered new 1M vol pots so I can replace the first volume and the master. I will chopstick some more I'll go the divide and conquer route.

BTW just played the amp with a tele through two 4x12 marshall cabs...First one was a G12M lead and it was less than magic but the G12H bass cab...OH MY GOD!

Jelle

I am having trouble actually understanding the problem. Is it that when you turn the amp up you get some low frequency oscillation that only goes away when you turn the master volume down? If so, what are you using for POWER TUBE grid stoppers, and what kind of power tubes are you using?

Gil

[jelle]

Gil,

When the amp is turned up, master 10 and vol on 6...the sound is very nice when played. But on the low notes at this volume all hell breaks loose and the amp starts making noise. Some where between the sound of a jet engine and a tree falling down...if that makes sense. This sound only goes away when the master is turned down. So yes, turning the master down removes the self generated noise. It also reminds me of the sound a television makes when the coax cable is not connected.

Disconnecting NFB does not help....

The power tube input grid stoppers are 5.6k directly connected to the grid pin. The screen grid resistors are 470 ohms CF about 5W and the PI is std ODS. NFB is 390 and a 6.8K. Pres e-cap 1uf. Master wires are coax too. I used sovtek 5881's, sino 6l6GC and this does not change anything. No choke...I tried 220ohms -1K resistor here.
Hope this helps!

With chopsticks inside the chassis, the 70's style ground buss is sensitive.

Jelle

[skyboltone]

A cap crapping out somewhere in the chain? Even a filter cap working hard and failing?

[heisthl]

If it works when not using the internal speaker it has to be vibration dependant. Put your bench amp stand on a board, put a round cylinder under the board (a sturdy tin can works well) and teeter totter the amp while its running to simulate vibration.

[ayan]

Gil,

When the amp is turned up, master 10 and vol on 6...the sound is very nice when played. But on the low notes at this volume all hell breaks loose and the amp starts making noise. Some where between the sound of a jet engine and a tree falling down...if that makes sense. This sound only goes away when the master is turned down. So yes, turning the master down removes the self generated noise. It also reminds me of the sound a television makes when the coax cable is not connected.

Disconnecting NFB does not help....

The power tube input grid stoppers are 5.6k directly connected to the grid pin. The screen grid resistors are 470 ohms CF about 5W and the PI is std ODS. NFB is 390 and a 6.8K. Pres e-cap 1uf. Master wires are coax too. I used sovtek 5881's, sino 6l6GC and this does not change anything. No choke...I tried 220ohms -1K resistor here.
Hope this helps!

With chopsticks inside the chassis, the 70's style ground buss is sensitive.

Jelle

Hmmm... not sure what to make of it, if the gound buss is sensitive you might have a grounding issue. Are you using EL34s?

Gil

[drz400]

Backwards filter cap?
Primary wires on the power tube socket too close to the speaker jacks or secondary?

[BobW]

jelle,

Your ground buss should be quietest thing in you amp. If not, you may have either a loose ground or power return, or you have attached the buss ground in between a power return ground path. If you haven't done so already, try attaching the ground buss to the chassis towards the input (left) of the chassis.

Have you tried swapping preamp tubes? It could be a microphonic tune resonant only at a lower f. hth

[jelle]

Hi Guys,

Thanks for all your replies!

Turned out to be a cold solder joint from the ground buss to the input jack chassis ground...

Problem solved!

Thanks again!

Jelle