Bass guitar amplifier project

[kreso]

Hello!

I wanted to make a bass amplifier, mostly from parts that I have around. It’s my first bass amplifier so far and to be honest I have no clear picture what I’m aiming for. That’s why I’d like to ask you guys for some advice and guidelines.

I checked some schematics around and considering parts that I have, decided to base it on Sunn amplifier with two KT88 in UL connection. Haven’t had any 6AN8 around so I used 6BL8 instead. Therefore split load PI and preceding driver stage was modified, if I remember topology for that part was mainly taken from Playmaster amp.
 
At first I assembled it, but it had some oscillations due to bad layout. I didn’t mentioned that I wanted to build it as a birthday present for my friend and  it was only like 15 days to his birthday when I came to this idea. So I was in rush and didn’t plan it well enough. He only get a bottle of French Cognac

Second attempt, layout was improved, although it’s still on test chassis. Amplifier is silent, no oscillations, no hiss…until you hit the strings…than it’s very loud, very bold and clean
Amplifier is playing very nice with guitar and taking pedals extremely well, but it’s way too much for my guitar box.

Amp still hasn’t been tested with proper bass speaker cabinet. There is limiting factor here as OT has only 16 ohms tap that is usable, so probably we’ll go with 2x10” or 2x12” speakers.

I’d like to ask you to check the attached schematic, voltages and advise me is there any usable modification which would make it more bass friendly? Probably, first thing I would do is rewire tone stack as Sunn 200s, with lo-boost option, which is more suited than the current one.

I saw that some of you guys have a lot experience with this type of amplifiers. Any help is more than welcome!

Kreso

[sluckey]

Just copy the 200S preamp ***EXACTLY***, everything from input to grid of pentode. First amp I ever built was a Sorado (same as 200S) back in the late '60s. Simple amp that works very well for bass, especially with a 2x15 bass cab.

http://sluckeyamps.com/misc/1st_build.jpg
https://el34world.com/charts/Schematics/files/Sunn/Sunn_sorado.pdf
 

[kreso]

OK, thanks!
Mid pot goes out than, this 0.0075uF could be little hard to find, but I'll fix something.
So you think 6BL8 values are acceptable, AC coupling instead DC as well?

http://sluckeyamps.com/misc/1st_build.jpg
 

Nice build, I like this Star trek tolex/grill style

[sluckey]

I'm not familiar with 6BL8. It's probably OK but I would have used 7199 if no 6AN8s available. DC coupling is simpler but AC coupling works too.

[DummyLoad]

OK, thanks!
Mid pot goes out than, this 0.0075uF could be little hard to find, but I'll fix something.
So you think 6BL8 values are acceptable, AC coupling instead DC as well?

http://sluckeyamps.com/misc/1st_build.jpg
 

Nice build, I like this Star trek tolex/grill style

7500pF cap here

--pete

[kreso]

Thanks Pete!

One more question... Output transformer that I'm using has two secondary windings. One is the one which has three taps (4.15k/16R is used for speaker), and second one with only one tap (the one that is marked 0-37R). When was tested on the bench I calculated that it is reflecting 4.15k/37R. Measured approximately 8 ohm DC resistance.

Could it be used for some kind of line out maybe?

[PRR]

I'm not familiar with 6BL8. It's probably OK but I would have used 7199 if no 6AN8s available....

7199 is super scarce, exotic hi-fi. 6AN8 being the usual alternate, that too is scarce. There's a bazillion triode-pentodes from TV tuners and most will work fine for a stage-amp.

[thetragichero]

Could it be used for some kind of line out maybe?

for line out i just use a resistive voltage divider off one of the regular ot secondary taps like I've seen in many amps (bassmen, acoustic control corp, etc). figure if it's good enough for a production amp with warranty it's good enough for me

[kreso]

Hello again,

Amp has been rewired as per Sorado/200S schematic. Everything went fine, no surprises, no squeal, no hum... It's very loud and clean.
But it's also very bright, too bright I'd say. There is so much treble that by switching  treble boost on, there is no any difference.
With initial (Spectrum) setup, bass was more present as well. That doesn't surprise me as first coupling cap was 100nF and slightly different tone stack, but I liked that sound more.
I did replace V1 Phillips miniwatt with some new EH, but I doubt it could effect that much.

So is this usual with this type of amp or it's just this one? Any suggestions?

[Ethan]

Have you tried it through a proper bass cabinet yet?  If not, I would give it a try before passing judgement.
If you have and still find it too bright, and you liked the previous circuit better, well...  better is better, right?  You could either start subbing cap values from the other circuit or just go back to the other circuit entirely. 

[sluckey]

But it's also very bright, too bright I'd say. There is so much treble that by switching  treble boost on, there is no any difference.

May be a wiring error. I never used the hi boost on my Sorado but I always used the Lo boost. With proper bass speakers my amp was never too bright. In fact, it sounded rather dull with a guitar. Good for bass guitar though.

[kreso]

Have you tried it through a proper bass cabinet yet? 

I tried it trough 2x15 cab but box design was rather poor. Maybe you are right...

May be a wiring error. I never used the hi boost on my Sorado but I always used the Lo boost.

I double checked the wiring, I'll do it again. There is no need here for hi boost as well, I was trying to emphasize how bright it sounds.
With guitar and proper 2x12 cabinet it's the same thing.
Thanks for your help guys, I'll see what I can do. Although, for a better bass cab I'll need some time...

[sluckey]

Will you be using this amp mainly for guitar? If so, consider replacing the bass tone circuit with a guitar friendly tone circuit, ie, Spectrum II.

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_spectrum_ii.pdf

[thetragichero]

do you have an as-built schematic? i find that can be helpful when trying to troubleshoot bad sounds. sometimes it comes out to a resistor that was put back into the wrong bin or moving a decimal place in cap value

[kreso]

Will you be using this amp mainly for guitar? If so, consider replacing the bass tone circuit with a guitar friendly tone circuit, ie, Spectrum II.

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_spectrum_ii.pdf

No, it was built for a friend who is bass player. I'm not and it's just more convenient for me to plug it into guitar equipment as I have it at home. I have never dealt with bass amp so far.

[kreso]

do you have an as-built schematic? i find that can be helpful when trying to troubleshoot bad sounds. sometimes it comes out to a resistor that was put back into the wrong bin or moving a decimal place in cap value

It is not a problem. I'll make it tomorrow and post it.
Thank you guys.

[kreso]

Here is as built schematic. Hopefully, I didn't miss anything.