Questions for all the portaflex gurus out there...

[Pomaikai]

I would love a vintage B15 for my recording studio, but I'm just a lowly guitar player and don't have the money to spend on something like that. So I've been working on putting together a poor man's substitute that will get me close to that kind of sound for recording from stuff I already have hanging around.

I started with a Bogen CHB35a amplifier, it's 35 watts, pretty darn close to a SB12 with the 7868 power section and the James tone stack but with a bit more power. It sounds really good so far (and I mean really good!) as it is into my sealed 2x12 with a pair of Gauss 12s. Messing around on a tone stack simulator I was able to get the 10khz treble control center frequency down to 5khz like an Ampeg tone stack by changing a single cap and resistor. The Bass control is close enough as it is centered on 50hz instead of Ampeg's 40hz, will also experiment with adding the 270k bass pot dropping resistor to the tone stack. For speakers I have a pair of Eminence Square magnet 15s from the 45 week of 1982 to work with and enough plywood around to build either a TL606 or double baffle Portaflex type cab, but unfortunately not both (though I may do that later since I already have 2 speakers).

So these are the questions I have;
1) How close does an SB12 sound to a B15 tonaly when played through a 15" cab instead of the Ampeg 12" cab?

2) With the later ('82) Squareback speakers, should I just build a TL606, or go for the double baffle? Which will be close enough to a vintage B15 sound and/or work best?

3) For folks that record with an old B15 what is the best sounding DI option for going to the board to mix with a mic'ed signal, clean DI split before the amp, pre-amp out from the amp, or line out voltage divider tapped off the speaker output? I would like to incorporate something inside the amp so I can just add an XLR out on the back panel.

Thanks! Pretty sure the collective braintrust can point me in the right direction on this project.

 

[Stylee Mousse]

I sometimes run my 66 SB-12 head through a shelf ported cab with a JBL D-140 in it.

Sounds really good, definitely cleaner than through the ampeg labeled alnico 12 in the SB cab.

the 15 cab is essentially half of a sunn 2000s.

 

[Aloe]

I'd just grab a VST from Plugin Alliance

 

[Short4since64]

Just a personal opinion: The double baffle cab (with the Jensen) is the defining B15N sound. Have a clone with a Jensen Vibranto-series alnico 15 in it, don't know how the Eminence speaker will sound, likely very close. Bear in mind that the double baffle box acts as an acoustic bandpass filter rather than a classical tuned bass reflex.
The amp itself is just a generic design straight from the RCA Receiving Tube Manual. Nothing special.
Very familiar with those Bogens. Rated at 5% THD. About 30W at clipping and indestructable. Might want to consider getting rid of the grid leak biasing and the remote mute circuit in the input stage and going to a conventional cathode resistor bias to get more preamp headroom, originally designed to work with piezo ceramic squawk box microphones.

 

[direktor]

Honestly, having spent last few years playing with many types of modeling plugins, and hardware, a model is probably closer than any “kinda like” hardware one is likely to gather together.

Captures other people have made on my Quad Cortex with appropriate IRs applied is the most accurate and believable I’ve heard.

I understand a suggestion like this is not what we come to the Amps & Cabs section for, but you have to ask is the sound more important or is how you got there more important? (Sometimes how wins, I know)

 

[Pomaikai]

Just a personal opinion: The double baffle cab (with the Jensen) is the defining B15N sound.

I think that's the direction I'm going to go. The TL606 is kind of a know quantity, so I think the double baffle will get me further down the road of discovery on looking for the sound I want. If I want to change it up from there I can use my sealed Gauss 2x12 or TL506 with JBL E155-4.

That's a great idea about the cathode bias, I think I'll leave Mic 1 stock and convert Mic 2 to cathode bias and get rid of the Aux/Mag/Remote stuff.

Thanks for the suggestions on the modeling stuff, I have a pair of Neve clone preamps with DIs that I use in my bigger rack rig, and use the plug in alliance STV plug-in when I want to add punch and grit to the DI signal, but there's no substitute for moving air in the room, getting a little bleed in the drum mics, changing up bass cab mics, twisting amp knobs and the compression and drive that come from real tubes and speaker cabs with zero latency.

 

[Short4since64]

I think that's the direction I'm going to go. The TL606 is kind of a know quantity, so I think the double baffle will get me further down the road of discovery on looking for the sound I want. If I want to change it up from there I can use my sealed Gauss 2x12 or TL506 with JBL E155-4.

That's a great idea about the cathode bias, I think I'll leave Mic 1 stock and convert Mic 2 to cathode bias and get rid of the Aux/Mag/Remote stuff.

Thanks for the suggestions on the modeling stuff, I have a pair of Neve clone preamps with DIs that I use in my bigger rack rig, and use the plug in alliance STV plug-in when I want to add punch and grit to the DI signal, but there's no substitute for moving air in the room, getting a little bleed in the drum mics, changing up bass cab mics, twisting amp knobs and the compression and drive that come from real tubes and speaker cabs with zero latency.

Try 220K plate load, 2.7K cathode resistor unbypassed for lower gain, bypassed with 25uF for higher gain, feed directly off the 240V node. The more common 100K/1.5K will likely load the 240V node down too much for the remaining stock input to function properly. Ampeg used 270K/3.3K but that was with a 6SL7 which has lower mu. That 6EU7 is just a alternate-pinout 12AX7

 

[Pomaikai]

Thanks,great tips!
I was thinking I might as well wire it for 12AX7/7025 pinout at the same time.
Would it be best to just change them both to 100k/1.5k cathode bias then with 120k grid like the SB12? The 240v tap on the CHB is lower than the 280v node on the SB12, but theres's an additional 68k resistor going to the plate, it looks like just having 100k on the 240v puts the plate right around 150v like in the SB12. There's no cathode bypass cap on the SB12, maybe add one on the second channel as a way to do hi and lo channels?

 

[Short4since64]

Looked at the schematics. Think the 220K/2.7K, slightly colder biasing than the 2.2K used on the second stage of the SB12 would be about the right choice although there's no reason why the 100K/1.5K or 1.8K wouldn't work, although those values are more common with a 300V supply rail. The series resistors on the SB12 are there simply to get rid of gain. The 1M grid leak is fine, replace the .1 input cap with 68K resistor (typical grid stopper). The input stage is running into a variable impedance load with the reverse-pot mixing arrangement so leave the series resistor and HF compensating capacitor as-is. As the 6EU7 is made of pure unobtanium, going to the 12AX7 pinout is a good idea, opens possibility of using a 12AY7 for lower gain. If you wanted to experiment with B15N values, a 5751 is a 6SL7 with 12AX7 pinout.
BTW been designing tube amps since high school, then engineering school, could come up with a circuit while skydiving, sex or having a root canal. Glad to help.