Suggestions for a 6550C bass amp build?

[mirage_indigo]

I have a pair of Svetlana 6550Cs lying around and also need a bass amp, as I just switched over to the other team in my band. I play a fretted Jazz and will be building a fretless at some point so high end is desired.

Any ideas? I've built a couple amps before so I kinda know what I'm doing.

I don't need anything particularly fancy. I was thinking something along the lines of a single channel two tube pre into a standard longtail PI into a cathode follower driven 6550 pair. Possibly with a post PI master. Fixed Bias. The logical choice would be an SVT but this is about 3x more powerful than I need. Maybe an SVT with just a single output pair?

So 3 or 4 preamp, 2 power tubes. SS rectified, quite possibly with a toroid PT. No reverb, but possibly a passive split effects loop. Should come in about 75W, which is as much as my homebrew 1x15 cab can handle. I might also replace the cathode follower with a mosfet source follower as it's a bass amp and I'l lalreay have a neg supply anyway.

I have couple hi power Hammonds, so already have an OT that'll handle a pair of 6550s.

[sluckey]

I built several of these many years ago. Works very well in a medium size country music club. Not a LTP PI but just as good IMO.

https://el34world.com/charts/Schematics ... sorado.pdf

[mirage_indigo]

Thanks. That's an odd design with a concertina splitter. I used on of those in my Deluxe with reverb and I liked it. I'd have expected a LTP.

[didit]

Hello -

Perhaps this will prove somewhat familiar --

Untitled.jpg

Best .. Ian

[Stevem]

I would build a Sunn 100 / Sonaro, but ditch the tube recto and run 500 volts on the plates over the stock 480, then talor the preamp V+ dropping resistors to keep those voltages in line with stock.

[didit]

Endorsing Steve's endorsement of Sunn/Dynaco. That 6an8 is still available for sensible prices for NOS, but best understanding only as NOS.

Alternatively perhaps look through this thread for other inspirations -- Bass amp ideas, Odyssey, Winterland etc.

Best .. Ian

[mirage_indigo]

Those sound great! Although why would Sunn name one Sorado and one sonaro? Marketing Fail. That's a really interesting design, particularly the NFB headed to the cathode of the pentode stage. I think I even have a NOS 6an8 lying around someplace.

I think I could redo the PI section to increase the clean headroom with triode from the 6AN8 as a contertina then into a LTP them into a pair of CF drivers to a KT88 or 6550c pair. I guess I'd insert the passive loop before the small signal pentode stage.

I also found someone's thread where they suggested building a smaller kt88 amp and then DI-ing that into a 300W SS Class D head. Which seems brilliant.

[sluckey]

why would Sunn name one Sorado and one sonaro?

Same amp but... Sorado = head, Sonaro = combo cab.

[M Fowler]

You already stated what you want for power section, long tail pair PI and power section for which you have your Hammond transformers.

I am uploading this schematic and layout simply for ideas on simple bass preamp section.

Mark

Mojotone_50W_Bass_Amp_Kit_-_WD.pdf

Mojotone_50W_Bass_Amp_Kit_-_SCH.pdf

[mirage_indigo]

Thanks Mark!

[Kagliostro]

Full detailed project with schematic, layout, chassis measures and all you need to build your own

Tribute to the classic Ampeg B-15N

http://ctgelectronics.weebly.com/custom ... n-51b.html (full project here - on the bottom of the link there is BOM)

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http://ctgelectronics.weebly.com/upload ... 081810.pdf

http://ctgelectronics.weebly.com/upload ... 0_r0.2.pdf

http://ctgelectronics.weebly.com/upload ... 081810.pdf

http://ctgelectronics.weebly.com/upload ... 081810.pdf

http://ctgelectronics.weebly.com/upload ... 081810.pdf

http://ctgelectronics.weebly.com/upload ... 2_orig.jpg

http://ctgelectronics.weebly.com/upload ... 7_orig.jpg

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Franco

[roberto]

Great job Franco!

[Kagliostro]

Ciao Roberto

Franco

[mirage_indigo]

Thanks everyone for the great suggestions. I stumbled across KoC's Stentorian in my TUT library too and have been looking at that for ideas. I may need to build a pair of bass amps, One with a more conventional PI and one Sunn.

I've been studying the Sunn Sorado circuit more and the Sunn bass amps in general. The pentode into concertina is really interesting, it looks like the high input impedance of the concertina is a big part of juicing the pentode stage for more gain. I thought I had kt88s shoved in my tube parts drawer, it was another pair of 6550s. I could not, alas, find a 6an8 in the tube pile, but at $11 a pop NOS, I'm good.

I think I've talked myself into building that with a stiffer power supply, a post-pi dual pot master and cathode followers into a 500V+ 6550 pair. I'll post the schematic from KiCad once I work out the post PI bits. I used a dual pot master after a concertina splitter in a Tweed Deluxe + MV build and it works well but can fart out at high drive + low volume. I'd always assumed this was grid blocking from the lack of drive current. I think the cathode follower 12AT7 would help with that problem. There's no point building a 75W amp without master volume for bedroom use (IMHO).

Questions:
It seems like all the Sunn amps pull the V+ PA feed from the far side of the choke. This seems an odd choice to me. Any problems tapping off the normal place after the diode bridge / first cap?
At least a couple Orange amps use a dual pot MV post a concertina splitter, but the output coupling caps look kind of undersized. Last time I did this I had concertina into .1 uF coupling caps out to top of pot, then 1uF caps coupling into the grid stops. KOC has this arrangement with a LTP and recommended at least 10x output coupling cap but maybe this is way overkill. Any experiences with a dual gang MV into a cathode follower? I'm going to go study the Marshall preamp topology for ideas.

[martin manning]

I think I'd do a pre-PI master so as not to upset the NFB.