Preamp sag

Harald.Nowak · Post by **Harald.Nowak** » Thu Oct 06, 2011 8:46 pm

People, help me pls out on this:

A while ago (1 years or so ) there was a discussion here

http://ampgarage.com/forum/viewtopic.ph ... preamp+sag

about note flipping; the discussion spun also partially about the preamp sagging under full clipping conditions even if MV was full down (so, not as a side effect of power amp sag, but all on its own).
Who of you - if any - observes the same effect (I'm talking about the supply voltages B4 and B5, not the anode voltages) and if so, by how much do they sag? Back then there was talk about 20-30V; what do you observe?

thanks,
Harry

Harald.Nowak · Post by **Harald.Nowak** » Mon Oct 10, 2011 2:48 pm

Seems like the effect is mostly non existent - I do now have data from another Dumble clone, that mostly confirms my own measurements: preamp sag of B4+/B5+, if there at all, is at max around 3-5V, typically more in the 0-2V, even with the high dropping resistor (22k) that most Dumble schematics have. So i guess, "preamp sag" as an important part of the "Dumble recipe" can finally be ruled out.
(if nobody turns up that observes otherwise); the 22k does not seem to "fullfill" this function (as was assumed in the thread mentioned above).

Roe · Post by **Roe** » Mon Oct 10, 2011 5:45 pm

seems plausible to me. but on some amps, notably plexis, the b+ will sag 90+v when running flat out and the screens will sag even more, suggesting that you have lots of preamp sag as well

Harald.Nowak · Post by **Harald.Nowak** » Mon Oct 10, 2011 6:56 pm

Roe wrote:seems plausible to me. but on some amps, notably plexis, the b+ will sag 90+v when running flat out and the screens will sag even more, suggesting that you have lots of preamp sag as well

Yes, sure - when the Powertubes draw lots of current everything sags - including the preamp; however the thread mentioned above was partially about preamp sag without power tube participation, i.e. when MV is low or even 0. For some time I had the feeling that this preamp sag without powertube participation might really exist - however it does not, or so it seems right now. As I mentioned elsewhere, my Bassman/Dumble crossover also sags pretty heftily 50V or more, when driven hard and set to tube rectifier - so I can imagine your number of 90V+ sag for a Plexi is indeed possible.
cheers,
Harry

LeftyStrat · Post by **LeftyStrat** » Tue Oct 11, 2011 1:53 am

Which begs the question, why hasn't anyone produced just a Dumble-style preamp? I've read several posts on here that just the preamp alone didn't have the mojo of a full Dumble build. Could it be slamming the PI?

Can't wait to get a build under the scope.

groovtubin · Post by **groovtubin** » Tue Oct 11, 2011 1:58 am

LeftyStrat wrote:Which begs the question, why hasn't anyone produced just a Dumble-style preamp? I've read several posts on here that just the preamp alone didn't have the mojo of a full Dumble build. Could it be slamming the PI?

Can't wait to get a build under the scope.

Quinn amp has built them.., boldaslove ran a preamp into a BM power section....there IS ALOT mojo in the output section!! youtube it..

respectfully, jim@Omegaamps

boldaslove6789 · Post by **boldaslove6789** » Tue Oct 11, 2011 3:01 am

groovtubin wrote:
LeftyStrat wrote:Which begs the question, why hasn't anyone produced just a Dumble-style preamp? I've read several posts on here that just the preamp alone didn't have the mojo of a full Dumble build. Could it be slamming the PI?

Can't wait to get a build under the scope.
Quinn amp has built them.., boldaslove ran a preamp into a BM power section....there IS ALOT mojo in the output section!! youtube it..

respectfully, jim@Omegaamps

yuppers uber mojo

http://www.youtube.com/watch?v=CXZdn-8I ... er&list=UL

http://www.youtube.com/watch?v=Y4FDjBuI ... er&list=UL

http://www.youtube.com/watch?v=LFCpTFsr ... er&list=UL

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