Ampeg Portaflex Bright Channel?

[Volume4]

Calling all Portaflex owners

For some reason I've never plugged into the bright channel of my SB-12 before this morning and I'm pretty surprised at how good it sounds....actually sounds a little SVTish... as in more mids.

Pretty cool, so I'm thinking of trying my BBM with the dry out into the normal channel and the other output into the bright channel.

Silly question maybe.....but this can't hurt anything can it? I'm pretty sure the answer is no. Will this even work? or will one bypass the other?

Also, how many of you B-15 or other Portaflex variant owners find uses for the bright channel?

[JimmyM]

nope, won't hurt a thing. and i have occasionally used the bright channel. i occasionally record some weird songs using only drum machine and bass, and i split my signal, use the first channel clean, and use the bright channel with a rat pedal, then blend the two to taste and mic it.

[Volume4]

As usual....Jimmy comes through!!

Thanks man, much appreciated as always.

I also took your advice and went to home depot and bought the self stick rubber seal for a new cab gasket. Worked great.....getting the original gasket out was a chore though but it's nice and tight now

[JimmyM]

nice! ya, it's not regulation but it's cheap and works very well.

[fu22ba55]

I've jumped the channels on my Magnavox fliptop since the day I got it. It gives you a more "gainy" sound, and you can use the two channels to kind of adjust bass/treble to your needs.

And if you plug a tele into this setup... instant Keith Richards.

You could probably use a y-cable on the SB-12 for the same effect?

[Volume4]

I'm thinking about trying a Y-Cable.....but what I'm about to try in a few minutes is the Bass Big Muff as a splitter.

Which will work two ways, with the pedal off it will still split into both channels and I kick in the pedal on the bright channel if I want or put a pedal after in the bright side also.

I've been playing this song by OM called At Giza and it's played cleanly and then erupts at a certain point dripping with gain.....so I want to try playing it with this setup.

The bright channel by itself is pretty cool too but loses some low end.

[MR PC]

Originally Posted by fu22ba55

I've jumped the channels on my Magnavox fliptop since the day I got it. It gives you a more "gainy" sound, and you can use the two channels to kind of adjust bass/treble to your needs.

And if you plug a tele into this setup... instant Keith Richards.

You could probably use a y-cable on the SB-12 for the same effect?

Isn't the bright input on those pretty much the same as just flipping the bright switch on when using the normal input?

[JimmyM]

Originally Posted by MR PC

Isn't the bright input on those pretty much the same as just flipping the bright switch on when using the normal input?

nope. the bright switch is more like adding instant mid scoop. the bright channel rolls off low end and adds highs and upper mids. besides, most sb-12's don't have a bright switch.

[Volume4]

Originally Posted by MR PC

Isn't the bright input on those pretty much the same as just flipping the bright switch on when using the normal input?

Hmm, maybe on a B-15? The B-15 owners can maybe answer that. (Jimmy just did above )

I've got an SB-12, no bright switch just 2 inputs+ vol-Bass-Treble.....nice and simple, the way I like it

[MAMMOTHvolume]

Originally Posted by JimmyM

nope. the bright switch is more like adding instant mid scoop. the bright channel rolls off low end and adds highs and upper mids. besides, most sb-12's don't have a bright switch.

bright channel doesn't remove any low end it just boosts high frequencys

[JimmyM]

Originally Posted by MAMMOTHvolume

bright channel doesn't remove any low end it just boosts high frequencys

if you noon the controls on both channels, it does actually attenuate low freqs by comparison. however, you can easily get them back by boosting the bass knob so they're not gone, just attenuated.

[beans-on-toast]

I have an analysis of the SB-12 input circuit.

If you look at the frequency response of the bright and normal channel you find the following:

On the normal channel, there is very little attenuation over audio frequencies. There is no attenuation until 10K. The response is -3db down at 64 kHz.

In the bright channel, there is high pass filter made up of a parallel resistor (100K) and capacitor (0.005 uF) in line with the input. This cuts the low end so the channel appears to be brighter than the normal channel. The input is -3 db down at 1K, -6 db down at 375 Hz, -10 db at 318 Hz.

This is just looking at the input circuit and doesn't take into account the tone control settings.

[JimmyM]

Originally Posted by beans-on-toast

In the bright channel, there is high pass filter made up of a parallel resistor (100K) and capacitor (0.005 uF) in line with the input. This cuts the low end so the channel appears to be brighter than the normal channel. The input is -3 db down at 1K, -6 db down at 375 Hz, -10 db at 318 Hz.

man, i've never seen anyone so prepared with all the fliptop answers! i was going by what i heard with my b-15n but it looks like it sort of applies to the sb-12 as well.

i'll resist the urge to gloat

[beans-on-toast]

I'd love to have ears like your's Jimmy!

The later B15's with the bright channel and the SVT's have the same input circuit. The results would be similar.

The numbers are based on the 12AX7 that is found in the SB-12. The B15's 6SL7 and SVT's 12DW7 will change the curves a little. As will the rest of the circuits, including the tone controls, that follow the first stage. At very high frequencies the instrument electronics and cable come into play as well. Not much of a problem for bass unless you are interested in high end sparkle.

The bright vs normal channel difference seems to cause a lot of confusion.

[Volume4]

Great info!!

I love this place

[JimmyM]

Originally Posted by beans-on-toast

I'd love to have ears like your's Jimmy!

The later B15's with the bright channel and the SVT's have the same input circuit. The results would be similar.

The numbers are based on the 12AX7 that is found in the SB-12. The B15's 6SL7 and SVT's 12DW7 will change the curves a little. As will the rest of the circuits, including the tone controls, that follow the first stage. At very high frequencies the instrument electronics and cable come into play as well. Not much of a problem for bass unless you are interested in high end sparkle.

The bright vs normal channel difference seems to cause a lot of confusion.

ya, and apparently i'm still confused. after rereading what you said, i missed the part where you said there was an hpf on channel 2. maybe i'm wrong then. maybe you can't match the tones on channel 2 with channel 1. been a very long time since i used channel 2, but i always thought i could match a nooned ch 1 with the ch 2 knobs. i'll tell you, hearing those lows are very tricky for me, and it's taken me forever to develop my ears to where i can sort of recognize what's going on with the lows. and just when i think i hear something right, i find out it's something else altogether. so don't envy my ears THAT much. and for that matter, i used to think ch 1 and 2 were identical. of course, that was before i got mine fixed right

otoh, i have perfect pitch well, i used to...it's still very good but it's deterioriating with age.

[beans-on-toast]

Originally Posted by JimmyM

ya, and apparently i'm still confused. after rereading what you said, i missed the part where you said there was an hpf on channel 2. maybe i'm wrong then. maybe you can't match the tones on channel 2 with channel 1. been a very long time since i used channel 2, but i always thought i could match a nooned ch 1 with the ch 2 knobs. i'll tell you, hearing those lows are very tricky for me, and it's taken me forever to develop my ears to where i can sort of recognize what's going on with the lows. and just when i think i hear something right, i find out it's something else altogether. so don't envy my ears THAT much. and for that matter, i used to think ch 1 and 2 were identical. of course, that was before i got mine fixed right

otoh, i have perfect pitch well, i used to...it's still very good but it's deterioriating with age.

I wonder which is better, perfect pitch or good ears. I see perfect pitch as more of a technical thing. It is real asset but at the same time it can drive some people nuts. Good ears is more of a musical thing. It helps one to know what sounds good. Maybe this is what's better to have more of. Having both is a blessing.

If your ears say it's so, then trust them. I don't think that you're wrong.

Remember an amp is composed of several stages that run from the input to the speaker. They all come together, complimenting each other, by boosting, cutting, summing and subtracting.

If the input has a high pass filter on it you can undo this to some extent by boosting the low end and the gain and keeping the treble flat. The curves of the input high pass filter and the tone section will not match but are close. You are not recreating low end that was taken away at the input, but by boosting the lows and keeping the highs flat you are essentially passing the signal through a low pass filter. Gain will boost that level back up again. This is a complex way of approximating the normal channel. This is not as pure as plugging into the normal channel and keeping the full sound. But the purpose of the bright channel is to take away the lows to make the sound bright.

_______________________

Just for completeness, I want to add a comment about channel inputs. I was talking about channels that have a bright and normal input like the SB-12, SVT's and later model B-15's. The bright inputs have the high pass filters on them.

Some amps like the early B-15's have two channels but do not have the bright input. These amps do not have the high pass filter. In the case of the early B-15's for example, there are two channels, channel 1 has two inputs, channel 2 has one input. There are voltage dividers on each input that present a slightly different input level to the first tube stage. In the case of the B15NC, channel 1, the first input has -1db of attenuation, the second input has no attenuation. This applies to low and mid audio frequencies. There is more attenuation at high audio frequencies. For bass, this normally isn't a concern.

On some amps the inputs are labelled high and low. This is input attenuation for instruments with different gains. The inputs labelled bright and normal affect frequencies. People often confuse the two.

I hope that I haven't opened a can of worms with this.

[JimmyM]

yep, you done it now, david! now i'm going to have to set up my b-15 and play with it when i should be getting real work done around here today