Sound City B150 Bass Amp

[W5FH]

Hello-
Studying bass amp circuits here for a good preamp section to put ahead of KT88 power section. Came across the Sound City B150 schematic (attached). I don't recall seeing a couple of features this amp has in any other amp. One feature is the configuration of the input stage V-1. The input triode of V-1 (pins 1,2,3) has some signal taken off of cathode to drive the grid of the other V-1 triode (pins 6,7,8). The signals from the plates of these two triodes are mixed at the Overtone pot. This is an interesting circuit to me and I am asking for commentary on how this amp sounds when using this control. The other feature is there are two signal paths leaving the tone controls, one from the junction of Bass pot wipers and one from the Treble pot wiper.
Can anyone comment on how this amp sounds and might this circuit be worth the time to build and try out?
Thank you!

b150_schemo_left.jpg

[Stevem]

Sound city preamps / tone controls are active so they can take a bit of getting use to.

I played one in my early bass playing days when we were starting to play bigger gigs and 5 nights a week.

After 3 years I got tired of its big weight I switched to a SS amp.

I like there tonal range for bass and for clean guitar sounds, but they don’t feel right for dirty lead guitar, nor sound quite right either.

Please note that these impressions of mind are 50 years old now, so perhaps with what I know now I might think a bit differently.

Hopefully others will chime in who still use them.

[W5FH]

Thank you Stevem-
I appreciate the reply and comments. I am going into a head cab build that will have a very clean (near-HiFi) KT88 back end. I plan to have a bass voiced channel and a clean channel for pedals. I have two 12AX7 type tubes (4 triodes) for each channel's preamp. Have been looking for a decent bass-oriented preamp.
Byron W5FH

[sluckey]

Have been looking for a decent bass-oriented preamp.

Take a look at the Sunn 200S bass amp. You may even like the power amp which is a Dynaco Mark III monoblock.

The Sunn 100S is a very clean guitar amp.

Both amps use the same KT88 power amp. The only difference is preamps and speakers. Worth a look.

[W5FH]

Thanks Steve-
Yes I have studied some of the Sunn amps, including the 200S. Since I have no playing experience, I sift through the comments I read on all the various forums and study the schematics to see if I see any hint of reasons for these comments. It seems I read too many unfavorable comments on some of the bass forums about the Sunn 200S tone. You know the story as we all have different tastes so I try to sort it all out. Of note is that the 200S has a pentode, plus with with 2 gain stages ahead of it. That introduces a lot of possibilities for tonal characters. I will not have the pentode so maybe I should revisit the 200S circuit, less the pentode.
Thank you for the help,
Byron W5FH

[Fischerman]

Those two stages at the input should be out of phase, just like a phase inverter. But the output bandwidth of the cathode-input triode is limited by the cathode cap, coupling cap, and even the 1000pF cap. I can't really tell how much gain it has relative to the 'main' input; seems like it would be way less. And the 1nF cathode cap should prevent higher frequencies from being input to that triode (those frequencies go to ground). At least that's what it looks like to me.

I'd be interested in how that sounds. At first glance it looks like a 'tone killer' circuit, but it's there for a reason. Very strange to my untrained eyes.

[martin manning]

Those two stages at the input should be out of phase, just like a phase inverter.

In the Sound City B150 schematic, the signals coming off the "Volume" and "Overtone" pots are in phase.

[Fischerman]

Those two stages at the input should be out of phase, just like a phase inverter.

In the Sound City B150 schematic, the signals coming off the "Volume" and "Overtone" pots are in phase.

Yes I don't know why I saw it as input to Overtone thru the cathode, the input is the grid. My mistake. Interesting circuit.

[sluckey]

Of note is that the 200S has a pentode, plus with with 2 gain stages ahead of it.

The pentode is considered a part of the power amp. The preamp is just the single 12AX7 and the output of the preamp is taken from the treble pot wiper.

[chief mushroom cloud]

This circuits intrigues me....I got bored so I drew it out so it easily make sense. It kinda looks like another way to make a 'jumpered' preamp, but fine tuning the second channel.
I think this could be modified for a guitar circuit w/ just a few component changes....could sound unique...but I know how you guys are....you just want to sound like someone else's record or tone....

[didit]

This circuits intrigues me....I got bored so I drew it out so it easily make sense. It kinda looks like another way to make a 'jumpered' preamp, but fine tuning the second channel.
I think this could be modified for a guitar circuit w/ just a few component changes....could sound unique...but I know how you guys are....you just want to sound like someone else's record or tone....

build it and they will come ..