B15 build

[Smokebreak]

I'm preparing for a B15 build. My bass playing buddy has stated that's the amp he wants, as he wants "old school" and "vintage" bass tones, and doesn't need it to be super loud. Big Jamerson fan. Mostly for home and recording purposes.
Easy enough!...Until I realized there is a lot of variation to this circuit over the 60's/70's : http://www.vintage-blue.com/schematics.htm .

Cathode biased vs fixed, different supply voltages, location of James stack and volume control, SS vs Tube rectification. Not as subtle as say a 1987 circuit over the years.

While I'm planning an all-octal build, I think I ran across a couple that used 6SN7 as PI instaed of the more common 6SL7GT. Then there's also some with a ultra high and ultralow switch.

Of course I can kinda pick and choose what elements I want, but I'm looking for some guidance from those experienced with these amps. Also, when I asked, my buddy stated that he'd prefer the amp more on the muddy/mushy side than the tight side.

Any input is appreciated.

[stevlech]

I'm no expert on Jamerson, but I believe he was known to favor the earlier circuit with the tonestack after first stage. The later preamp is brighter with more growl. I think the reissues have both preamps in them.

[Smokebreak]

Now that's a great idea! Thanks a bunch . That's definitely an option, though I may need to compromise one of the channel's pre-voltages.

Here's what I've narrowed the options down to, the B15N, 1968, and the B15NC, 1964 . This probably shows the biggest differences in design over the years too, I think

[Stevem]

Build it so the output stage can be switched from the cathode biased version and the fixed biased latter one.
I have both types and the sweet sustane of the cathode biased model is the one I use for recording 75% of the time!

Recording thru one of these driving through a Altec 421A speaker is Jaw dropping in regards to tone!

[Structo]

Does a 50w bass amp get loud enough for gigs?

500pf bright caps?

[Smokebreak]

Thanks Steve, I found the scheme for the Heritage B15, which does just that, as well as have the two differnt preamps. Looks like they split the difference on the preamp voltages.

Also found a thread elsewhere with TUT3 scans of the B15 section and he favors 370VDC on the plates, as opposed to the ~420 that Ampeg did.
I'm gonna use the 25W 1650F that is 7K6 , and the MC 40-18094, which has 710VAC and 610VAC CT windings . I know that the 710 taps with a 5AR4 gives about 480 plates in PP AB1, so cathode biased 610 taps should be around 400V.

So yea, Tom, it's not even a 50W amp, and I imagine would only be loud enough with quiet bands in a small room But this is a bedroom/recording amp. I'm gonna put some switches on there too....cathode bypass (sparkle?) and bright, and deep on one of the channels.

I would like to put a line out jack from the OT sec, as well as the preamp out jack that was original. I haven't started looking yet but I seem to remember Mr. Fowler posting some nifty circuits in the past. Do you have those handy Mark?

[jon]

I built a B-18 a few years ago. I Used a Hammond 272 power Transformer and a Dynaclone A470 output. The amp was perfectly suited for a normal jam session and great for recording. If I were to build another I would mod the amp to include a post PI cathode follower ie: V4B. otherwise the amp is darn near perfect.

[IMG:800:600]http://i82.photobucket.com/albums/j250/Jhtjon/Amps%20and%20Stuff/IMG_0521.jpg[/img]

[IMG:800:600]http://i82.photobucket.com/albums/j250/Jhtjon/Amps%20and%20Stuff/IMG_0508.jpg[/img]

[IMG:600:800]http://i82.photobucket.com/albums/j250/Jhtjon/Amps%20and%20Stuff/IMG_0301.jpg[/img]

[drew]

Nicely documented DIY build: http://ctgelectronics.weebly.com/custom ... n-51b.html

[Smokebreak]

Thanks Drew, I hadn't seen that one. It's got the two different eras as channels, too, and the switchable bias.

Great looking build Jon, I've got a bunch of those CD Greenies too!
Which B-18 did you build?

Well I called the guy and apparantly I have overdesigned. He just wants a single channel early version. End of story ;(

Of course I finished up an eyelet layout this morning for the dual era channel, so here it is, so it doesn't go to waste. The PDF should be to scale . 16.9 x 3.125. The .22s were switchable 2nd stage K bypass.

Now I shall commence to shrinking it!

[jon]

Mine was the B-18V\B-15ND version with tube recto and post gain stages tone stack.

[M Fowler]

Started to build 5B6 but changed to B15N with Fender Bassman replacement transformers for more power, plenty loud.

Mark

[Firestorm]

Interestingly, when the SVT was designed, there weren't any speakers that could reproduce 40Hz at power, so the focus was on the harmonics. We likely can't hear low frequencies anyway, but only FEEL them. So it's ultimately about air movement. Tone shaping is only about the part we can hear. Bass is complex.

[Stevem]

I am mainly a Bass player and anytime a have another Bass player over the place who has only played thru a 4-10" set up all there life and then plays thru any one of my 2-15" cabinets is hooked with a big smile on there face!

Even with 8-10" drivers such a cabinet can not carry the B string on a 5 string bass as there are many 15" drivers that hardly can get down there without a lot of roll off!

[Smokebreak]

I'm getting ready to fire up this amp. I've been referencing different schematics, and just noticed that in the KOC "improved B15", the signal off the James tone stack comes off the Bass wiper, instead of the Treble, per usual. What gives?
I also noticed that in his verbatim redraw of the original, he has it drawn this way as well. Typo or design?

[Smokebreak]

Fired right up and sounds great but I've got a lengthy question.

I basically used KOC's Improved B15 power supply, as far as 370V plates(340V plate>cathode) and a 270ohm cathode resistor shared between the 6L6GCs, as opposed to stock 430V plates, and 250ohm resistor.
However, with this setup, it's about 50mA through each tube, and 18W dissipation , which is around 55%.
Why would one want this setup, at least generally speaking, in a cathode biased amp? Seems awfully cold to me, though it sounds great, There's just not a ton of headroom, or possibly volume as I'm imagining it might have. I'm not quite sure why one would want less plate voltage and subsequent power, as well as less of everything from increasing the cathode resistor.