Pulling two output tubes in a Tweed Twin

[Leo_Gnardo]

Perhaps a bass cab would show some significant differences.

The point's been made before (credit Enzo IIRC) having a "dead" speaker in an open back cab is much less of a problem than in a sealed or vented/ported cab.

For passive radiator hi fi speakers, there's typically some fancy trick internal baffles & ports to delay the back pressure so the PR won't be working against the woofer. And the PR won't work wide band that way, it typically reinforces a fairly narrow band of bass. I remember some 70's Genesis speakers that used passive radiators, they sounded pretty good in the showroom, had some disco "sock" appropriate to the era.

[Chuck H]

I had a pair of old Pioneer stereo speakers that sounded pretty damn good. They had a 12" woofer and a 12" reflex with no port. I couldn't begin to decipher the alchemy involved in sorting out a sealed cab reflex "pad", "thing". My sensibilities are naturally skeptical. In a sealed cab the woofer moves one way and the reflex moves the other... Period! My logical brain can only interpret this as a loss due to phase opposition. But then my real world brain has heard actual speakers with these features that sounded good.?. Certainly there's some science involved in determining resonant peaks relative to speaker and cabinet particulars as it relates to port or reflex size. Probably has more to do with flattening the response than boosting LF.?. Since that's out of my wheel house by a mile I leave it to Bose and Pioneer to figure it out. A little of this tech has come into guitar amp cabs with the infamous "detuned 1x12 cabinets". Usually hosting an EV speaker. And they sound very impressive on the *outube clips. Makes me want to learn more about it. Oh the time...

[galaxiex]

..................... A little of this tech has come into guitar amp cabs with the infamous "detuned 1x12 cabinets". Usually hosting an EV speaker. And they sound very impressive on the *outube clips. Makes me want to learn more about it. Oh the time...

Are you talking about the Kevin O'Connor detuned cabs?

I've built 3 of them.

1X10
2X10

and a huge 1X15

They sound massive. esp the 1X15, as you would expect.

[olddawg]

Honestly I use a speaker attenuator a lot. I like the Weber ones.

[nsubulysses]

I think it likely that removing 2 6L6 will screw up the amp's dynamic response (because the HT becomes relatively much stiffer)[/url]

What would you recommend to remedy this? Make the B+ and screen nodes capacitors lesser value? Or would it be more like lowering value of V1 power supply node so it is less stiff, since V1 is the main part that dictates the sound of the amp? Or something else?

I looked at a few different schematics for 50W and 100W amps and it seems like the power supply cap values usually remain the same. Maybe this doesn't matter though since the PT and OT are different for the 50W versions???

[nsubulysses]

Sound_City_LB_120_MK_IV_(11-73)_Schematic.pdf

is the 10 ohm resistor in series with rectifier and B+ node in the attached schematic installed to make the power supply sag and fluctuate more under load?

Seems like you don't see that resistor in most amps but once in a while you do. I never really understood its purpose but that was my guess

Edit: actually, interesting. I have two of these schematics on my computer. the one I linked above has no 10 ohm resistor.

This one does --

[pdf64]

What would you recommend to remedy this? Make the B+ and screen nodes capacitors lesser value? Or would it be more like lowering value of V1 power supply node so it is less stiff, since V1 is the main part that dictates the sound of the amp? Or something else?

I looked at a few different schematics for 50W and 100W amps and it seems like the power supply cap values usually remain the same. Maybe this doesn't matter though since the PT and OT are different for the 50W versions???

Equivalent PTs for a 2 x 6L6 amp would probably have higher equivalent winding resistances, so yes, I guess you could just add external resistors 5F8A PT to get the voltages right with 2 6l6 pulled.
Bear in mind that as it only has a 400V HT (and maybe a 3k primary?), the 5F8A won't be anywhere near 100 watts output.

Regarding that Sound City amp, note that there look to be series resistors on each output of the rectifier, 10 ohms on the +ve, 1.8 ohm on the -ve.
Yes they will definitely add some sag to a 200W amp, but also they will act to limit peak / switch on surge currents through the rectifiers and reservoir caps.

These schematics have the 1.8 ohm but not the 10 ohm http://soundcitysite.com/sc_120_2.jpg and also note the revision for mixed bias on later MkIV amps http://soundcitysite.com/Sound_City_..._Schematic.pdf

[Zozobra]

You could try increasing the 470r bias resistor in the LTPI too? That reduces the LTPI headroom before breaking up and also reduces the swing to the output tubes. Some mojave amps use this as a ghetto "power scaling" with a pot in series with the resistor.