Combo cab response - Suggestions?

[Gainzilla]

Hi all,

I'm building a pair of combo amps for a guy who is looking to use them as a clean stereo pedal platform. The amps will be based around a 68 SuperBass circuit, but with closer to 35 watts each. No master volume. Just Volume, Treb, Mid, Bass, Presence. Pretty straightforward. Since I mainly work in the head format, I'm not as well versed with combo cabs as I probably should be. Any suggestions for a top-mounted cab with good/tight bass response? Bonus points for actual plans.

Thanks in advance!

- Bryce

[echuta13]

Tight/defined bass to me usually indicates that you should be going with a closed cab for starters.

[nworbetan]

Connecting the output tubes as triodes gives them a bit more clean headroom and a lower output impedance which flattens the speakers' natural eq effect and tightens the bass response. It's best to put the triode opttion on a switch on the back panel as high voltages are involved. The triode setting will also make the amp sound rather "dull" and "lifeless" compared to pentode connection setting until you adjust the preamp. It works well for a pedal platform, but not as useful otherwise.

[pdf64]

Connecting the output tubes as triodes gives them a bit more clean headroom...

I think that the 'H' word is somewhat ambiguous / problematical in our application; there seems no proper definition for it in regard of guitar amps, or at least folk use it to mean various, often conflicting, things.
It seems preferable to use unambiguous terminology, eg for an amp designed for pentode operation, connecting the output tubes as triodes will reduce gain, output impedance and power output, of the power amp. In addition to the likely consequences of that already noted, it may reduce the effectiveness of the presence control.

[nworbetan]

Good points. I actually meant something specific the way I used the H word but just now when I started writing it out I also started second guessing whether that's even true. I was thinking the triode connected output tubes would need a bigger input swing before they started clipping, but I can't defend that idea in a fixed bias amp.

I actually consider the reduced gain to be a benefit in a pedal platform. But I can understand how a lot of people would disagree with that.

[nworbetan]

I figured out where I got the half baked idea that triode connected pentodes have more headroom:

https://www.jj-electronic.com/en/kt66

The part I mis-remembered was that the extra headroom before clipping isn't a given, you have to bias the tubes colder to make it happen.

[bmx]

I can't give you much help besides to point out that there is a "cabinet" section on this forum with some really interesting reading. On the topic of open back cabs, one member says to keep the depth around 11", which I thought was interesting.

https://ampgarage.com/forum/viewforum.php?f=11

[Gainzilla]

Hi all,

Thanks for the responses. Interesting about the 11”. I started out looking for a “go with a 5E3 cab” or similar level of advice, but there’s nothing better than learning something.

Also, I feel silly that I completely forgot about the cabs section. Doh!

Thanks again!

- Bryce

[thetragichero]

what if you make the cabinet like a bad combo: sealed bottom portion for the speaker (i would use winisd for proper dimensions based on the driver's thiele/small parameters) and an open top portion for the chassis?

[TUBEDUDE]

You might consider ultralinear operation. A little more control than pentode operation with greater power than triode mode and avoids the triode tonal shift.

[nworbetan]

I lucked into a Marshall Major clone when I was a teenager and I also approve of ultralinear guitar amps. I've never seen an open back cab I'd plug it into though.

I have no idea why I didn't remember this earlier, but I actually played bass through my '59 Bassman reissue for a gig a couple years back. (It was more of a recording session with cameras than an actual show really.) In that amp I put an adjustable negative feedback pot and a trimpot on the board to set the minimum nfb resistance. I also heavily modified the cabinet so it's structurally more like a typical Marshall 4x12 but with the top half open. The floating baffle Fender used in '98 was shit imo.




I can post pics of that cab, but I can't say I prefer it to a sealed cab like thetragichero suggested.

(edited to a different video with a more flattering mix)

[Gainzilla]

(edited to a different video with a more flattering mix)

Sounds great, thanks for sharing. It looks like there are 2 schools of thought. The first being, you need to carefully account for the parameters of the speaker and build a box specifically taking that Into account. The next being just build whatever you think will work because for guitar, the accuracy of reproduction is less important. I know I've certainly heard the stories about the Marshall 4x12 cab and how the basically just slapped it together. At this point I'm on the fence about just trying something tried and true, and maybe modifying it to have a little more depth... Or I may just try and convince the customer to go with a small head and 1x12 cab route. lol

Regardless, thanks again for the insight. I'm the Jon Snow of combo cabs.

Cheers,

- Bryce