Hiss/oscillation on the OD channel of an ODS

[CB Amp]

Hello,

Here is a subject that has been discuted before, but I can't find any precise answer on the forum.
I work with ODS schematics for a while now, and I always get some awfull hiss on OD channel when I turn up the gain and volume controls, even with no signal on the input.
I use this type of schematic : https://ampgarage.com/forum/download/fi ... &mode=view

Edit : I talk about a big oscillation at high frequencies, more than just a hum or soft hiss when the amp is cranked up.

I did a lot of experiments : OD tube voltages, swapping snubber caps, pots, playing with the trim, etc... But I always get hiss at some point, especially at high gain level of course, but also with high volume and mid gain. There is perhaps a "perfect combination" of all that, but I didn't found it (I point out that I use premium components, coax wire, etc).
I read that there is often some hiss with Dumble designs, but I can't believe that Dumble sold amps with hiss, even at high levels. So did I miss something ? Is it a way to have a responsive but quiet OD channel ?

Thanks for your help.
Cyril.

[martin manning]

Have you tried screening for low noise tubes, especially in V1?

[CB Amp]

Have you tried screening for low noise tubes, especially in V1?

Yes, I also swapped different tubes. Some are quieter indeed, but the hiss still comes with high gain and volume.

[sluckey]

There is a direct relationship between gain and noise(hiss). As gain increases noise also increases. No way around that fact.

[norburybrook]

Have you tried screening for low noise tubes, especially in V1?

Yes, I also swapped different tubes. Some are quieter indeed, but the hiss still comes with high gain and volume.

tube amps hiss on high volume/gain.

if you ever stood in front of Brian May's 6 AC30's you wouldn't believe the hiss that comes from them just sat there. Once everyone starts playing it's not an issue

I've not found the Dumble amps OD to be particularly bad as they have a master volume and get their OD from the pre amp. Dimed non master volume maps are more of an issue I find.




MC

[Bombacaototal]

I would think that the correct outside foil orientation of the caps might help?

[Reeltarded]

With stages wide open there is going to be a waterfall. Those settings are WAY beyond anything useful.

Play guitar through the amp. First stage around 4. Tone controls moderated, OD controls OD gain around 6-7 and OD volume to match clean volume (arond 3-4) . Master on 5. That is how these amps are supposed to be used. Cranking every control is like flooring the gas in your car and saying the car is unsafe to drive in the city.

[CB Amp]

There is a direct relationship between gain and noise(hiss). As gain increases noise also increases. No way around that fact.

tube amps hiss on high volume/gain.
if you ever stood in front of Brian May's 6 AC30's you wouldn't believe the hiss that comes from them just sat there. Once everyone starts playing it's not an issue

Sorry, perhaps "hiss" is not the good term (excuse my french, I'm french... ). I should rather say a big oscillation at very high frequencies. You can't do or play anything with that. This is absolutely not normal.
I certainly do something wrong, but I can't figure it out.

[CB Amp]

I would think that the correct outside foil orientation of the caps might help?

Yes, I tried it too. Not a big influence...

[erwin_ve]

Big oscillations are often caused by a bad lead dress. Find the #124 pics onhttp://thesubjectmatter.com/dumblearchive.html and compare your wiring to what Mr Dumble did on his.

[CB Amp]

Play guitar through the amp. First stage aroung 4. Tone controls moderated, OD controls OD gain around 6-7 and OD volume to match clean volume (arond 3-4) . Master on 5. That is how these amps are supposed to be used. Cranking every control is like flooring the gas in yiur car and sayint the car is unsafe to drive in the city.

Yes, it's ok this way. But again, I guess I under-qualified the problem. This is an oscillation.

If I compare the discontinued Fender Prosonic amp (no offense, it' not the same amp nor manufacture of course), it had such a 2 triodes preamp OD channel with 2 gain controls an not too much filters, and you could turn the pots all way up without this oscillation...

[CB Amp]

Big oscillations are often caused by a bad lead dress. Find the #124 pics onhttp://thesubjectmatter.com/dumblearchive.html and compare your wiring to what Mr Dumble did on his.

Yes you're right. I also did some changings in the wirings. In fact I use this OD schematic for years, and now I use very few wires. I don't use turret boards anymore, and I solder the components directly to tubes/pots as much as I can (big work), Matchless like, but cleaner !... But this way or with boards, I still get this hiss/oscillation at some point.

[erwin_ve]

Big oscillations are often caused by a bad lead dress. Find the #124 pics onhttp://thesubjectmatter.com/dumblearchive.html and compare your wiring to what Mr Dumble did on his.

Yes you're right. I also did some changings in the wirings. In fact I use this OD schematic for years, and now I use very few wires. I don't use turret boards anymore, and I solder the components directly to tubes/pots as much as I can (big work), Matchless like, but cleaner !... But this way or with boards, I still get this hiss/oscillation at some point.

It's not about the schematic or lay-out. It's about how you wire and were wires are in relation to each other and other components.
You sure must know some basic design rules when drifting away from the standard Ods lay-out.
Maybe pics help to pinpoint?

[pompeiisneaks]

The other random thing you may not have checked, is the OT may be out of phase and causing oscillations. Sometimes the problem is subtle and only shows up at pretty high levels of output. You could test quickly by either swapping the OT leads to the output tubes or by swapping the PI output wires.

Other ideas sound more likely but this is one that's bitten me more than once. It wasn't the usual "SCREAMING NOISE" you get if it's reversed in some cases.

~Phil

[erwin_ve]

The other random thing you may not have checked, is the OT may be out of phase and causing oscillations. Sometimes the problem is subtle and only shows up at pretty high levels of output. You could test quickly by either swapping the OT leads to the output tubes or by swapping the PI output wires.

Other ideas sound more likely but this is one that's bitten me more than once. It wasn't the usual "SCREAMING NOISE" you get if it's reversed in some cases.

~Phil

Good one!
Or disconnect the nfb wire