Speaker test clip / another clip added

[Bob-I]

Bob,

Please axe the UL. Convert to regular screen supply ASAP.

Hey... when can I come down to your place and do some tweaking?

[markusw]

Bob,

Please axe the UL. Convert to regular screen supply ASAP.

Sorry, whats UL??

Markus

[Bob-I]

Bob,

Please axe the UL. Convert to regular screen supply ASAP.

Sorry, whats UL??

Markus

Oops, sorry for sticking with TLA's.

UL is Ultra Linear. The output xformer has taps at 40% on each side that connect to the screens. The goal is to create a more linear output curve and get more power out of the amp as well as a more Hi-Fi sound.

IME this doesn't degrade the tone like Dogears says. Maybe his ears are simply more sensitive but I don't hear it. I hear more difference in other tweaks. That said there's something about this amp that just doesn't quite produce the harmonic content that Dumble's should.

Oh... and a TLA is 3 (or 2) letter acronym.

[markusw]

Edit: double post

[markusw]

Oops, sorry for sticking with TLA's.

UL is Ultra Linear. The output xformer has taps at 40% on each side that connect to the screens. The goal is to create a more linear output curve and get more power out of the amp as well as a more Hi-Fi sound.

IME this doesn't degrade the tone like Dogears says. Maybe his ears are simply more sensitive but I don't hear it. I hear more difference in other tweaks. That said there's something about this amp that just doesn't quite produce the harmonic content that Dumble's should.

Oh... and a TLA is 3 (or 2) letter acronym.

Ah, thanks for your explanation! Now I remember it from another thread. Will have to add it to my list of Dumble acronyms...
Wish you good luck for finding the missing harmonic content!

Markus

[dogears]

Give me a shout and we'll chat. Need my cell?

Bob,

Please axe the UL. Convert to regular screen supply ASAP.

Hey... when can I come down to your place and do some tweaking?

[Darkbluemurder]

One more vote for the EVM 15".

Maybe this is off topic but with all the discussion about NFB tweaking I wonder whether anybody tried a "soul control" (variable resistor in the FB circuit) with these amps? I am considering one to replace the presence control since I am dissatisfied with the presence controls in my amps.

[Fischerman]

Bob,
Just a Dumble lurker so far...it's next on my 'list' of amps to build...so pardon if I'm rehashing old news or anything. But UL is a form of NFB...if you have UL and NFB in an amp then it's likely overkill. Most folks know by now that the easiest/best/cheapest/quickest way to get a SF UL Twin Reverb to sound better is to completely remove the NFB. So you might want to consider nixing the UL or removing (or at least greatly reducing) the NFB...and since you'd probably like to keep the Presence control...you know what that means.

[Bob-I]

Bob,
Just a Dumble lurker so far...it's next on my 'list' of amps to build...so pardon if I'm rehashing old news or anything. But UL is a form of NFB...if you have UL and NFB in an amp then it's likely overkill. Most folks know by now that the easiest/best/cheapest/quickest way to get a SF UL Twin Reverb to sound better is to completely remove the NFB. So you might want to consider nixing the UL or removing (or at least greatly reducing) the NFB...and since you'd probably like to keep the Presence control...you know what that means.

Thx, but no UL is not a type of NFB, it's a way of supplying the screens from the OT so that the output is very linear. I have 2 of these chassis, one I used and one I used a standard screen supply. I'll compare and see if that's a difference.

I've also played with the NFB. This is critical to the Dumble tone so removing it isn't an option, but adjusting it may be. I'll re-visit that as well as the UL screens.

Thx for your input.

[Fischerman]

Thx, but no UL is not a type of NFB, it's a way of supplying the screens from the OT so that the output is very linear.

How does UL accomplish "making the output very linear"? I'm not an expert by any stretch...I'll just say that some amp guys that know way more than I do told me this when I was homebrewing a UL amp several years ago. Think about it...you have signal on both the screens and plates now...I think it is a form of NFB (although not usually refered to in that way).

[Bob-I]

Thx, but no UL is not a type of NFB, it's a way of supplying the screens from the OT so that the output is very linear.

How does UL accomplish "making the output very linear"? I'm not an expert by any stretch...I'll just say that some amp guys that know way more than I do told me this when I was homebrewing a UL amp several years ago. Think about it...you have signal on both the screens and plates now...I think it is a form of NFB (although not usually refered to in that way).

Here's an article on UL mode. I guess you could think of it as a type of feedback, but applied to the screens, not the grids.

http://www.webace.com.au/~electron/tubes/ul.html

[heisthl]

The way it was explained to me - One of the things the screen does is catch the electrons that via collision, bounce of the plate and rather than travel against the flow (causing more collisions) back to the grid they are caught by the positive screen potential. In a UL configuration this energy is transfered into the OT instead of being wasted. The end result is more clean power.

EDIT - I just read the linked article and I wish my above explanation was correct because it would be easier to understand. The science is confusing, - I understand the concept now, but I'm having trouble envisioning the part of the OT duty cycle where a screen would have MORE potential (attraction) than a plate when both are being fed by the same windings.

[dobbhill]

Would a lower impedance on the screen make it attract more electrons than the plate (at a higer impedance) with the same voltage applied?
Just thinking out loud....
D

[Bob-I]

The way it was explained to me - One of the things the screen does is catch the electrons that via collision, bounce of the plate and rather than travel against the flow (causing more collisions) back to the grid they are caught by the positive screen potential. In a UL configuration this energy is transfered into the OT instead of being wasted. The end result is more clean power.

EDIT - I just read the linked article and I wish my above explanation was correct because it would be easier to understand. The science is confusing, - I understand the concept now, but I'm having trouble envisioning the part of the OT duty cycle where a screen would have MORE potential (attraction) than a plate when both are being fed by the same windings.

My understanding is not that the screen has MORE potential, but LESS. It's to assist with acceleration of the electrons toward the plate. If it were to have more potrential it might prevent the flow of electrons to the plate. Just thinking out loud.

[heisthl]

My understanding is not that the screen has MORE potential, but LESS. It's to assist with acceleration of the electrons toward the plate. If it were to have more potrential it might prevent the flow of electrons to the plate. Just thinking out loud.

I agree, that is one of the benefits of a pentode. I was referring to the UL article where it says the Screen has more attraction during part of the cycle which is why UL works the way it does....