Bass amp crash course

[xtian]

Client has asked for a custom bass amp. Seems serious!

His idea is an SE design (one 6550) with a line out available from the OT secondary. Would be a good amp for playing at home or for small gigs. Then, he plans to build a cab with a class-D amp in it, so he can use the head as a preamp and boost the volume for large gigs with the solid state power amp.

He hopes the SE design and line-out will give him the benefits of the harmonic distortion from the 6550.

1. Is this idea, getting good sounding, SE power amp distortion from a line-out, an idea worth chasing?

2. Need some good preamp building blocks for bass, esp. tone stack.

[LeftyStrat]

That could work. You may want to consider a reactive load on the output. This is a lengthy thread but worth the read:

http://www.thegearpage.net/board/showth ... ?t=1072793

There are sound clips a few pages in that show a very stark different between a resistive dummy load and a reactive one.

As far as preamps. The Alembic FB2 is well thought of by many bassists. It's basically listed directly from dual showman.

Older Ampegs used the James tone stack (sometimes erroneously called the Baxandal).

The other to look at would be the Sunn 200s preamp.

[Leo_Gnardo]

Another choice is the early 60's "white" Fender Bassman. For something a little bit different. Used it to good effect in a 4x6550/KT88 120W custom bass amp

[xtian]

Ultra linear OT or not for bass amp?

Looking at Hammond 1627SEA (30W UL) versus 125GSE (25W non-UL).

Remember that client WANTS output tube distortion, though of the gentle, even-order kind.

[LeftyStrat]

Ultra linear OT or not for bass amp?

Looking at Hammond 1627SEA (30W UL) versus 125GSE (25W non-UL).

Remember that client WANTS output tube distortion, though of the gentle, even-order kind.

Might be nice to have the amp switchable between UL, pentode, and tetrode operation. Different characters of distortion.

However, pentode and tetrode are probably good enough. Not sure I'd spend an extra $60 to get UL.

[LeftyStrat]

I was googling for something else, and happened upon an old thread about tone stacks for bass amps. Thought you might find it interesting:

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

[cpollack]

First off, I’ll have to agree with the Alembic F2B and Ampeg tone stack suggestions. Merlin has some nice ideas in his book, too. You may want to consider using a lower gain tube in the preamp such as a 12AU7. The goal here seems to be to make it clip in the output stage and a class A1 SE 6550 doesn't need much drive. A 12AT7 in the V1 position can sound a but too "glassy" to me but works fine later. A 12AY7, 6CG7 or 6SN7 might be good to try, too.

Regarding the OT connection, I’d say that UL has a lot going for it in a bass amp, even one you want to give up a little dirt. The main reason is that UL does a great job of reducing IM distortion, which, IMO, can sound particularly nasty on bass. The GEC KT66/KT77/KT88 manual covers IM levels for various connections. A triode connection would be even better on reducing IM at some power levels, but might be overall too down on power. I’d also couple the screen to the UL connection with a resistor to limit screen current. If you use one large enough, it would allow some compression as it nears clipping. A 470 Ohm / 5W is where I’d start.

Also for gently giving up the goods, I suggest using cathode bias. You may also want to try leaving out the cathode bypass cap. It will clip more gently without it, but again power will be down. I also wouldn't be shy about using generously-sized grid stoppers.

The last item to cover is whether or not to use negative feedback from the speaker connection on the OT back to the driver. No feedback would again allow it to go into clipping more gently, but I always like some feedback on a bass amp to help control the cone and reduce IM. It just sounds more like a bass to me.

I have used the Hammond SE UL iron in a HiFi amp (1640SE with an 826 triode using Shishido’s design), and that stuff is BUILT. It should hold up fine in an SE bass amp. The 1640 would allow you to do a parallel SE (2 x 6550) version. The 125 Series is not in the same league.

YMMV, but some things to consider anyway.

[xtian]

Yesgreatthankyou!

[selloutrr]

This has been done by alembic. Slightly modified fender showman preamp with an output transformer. Multiple outs pre or post OT. Then feed a solid state amp. It's a great pre (pre) OT. You will have to becareful to not overdrive the solid state amps input when the pre is pushed to limiting. Other than that sure have at it.

[xtian]

Observe the FMV tone stack, as used in Alembic F-B2:

[img:351:428]http://pickroar.com/wp-content/uploads/ ... lified.gif[/img]

Do you know how to add a Deep switch, as in Bassman?

[LeftyStrat]

In Merlin's preamp book, he shows an ultra-low switch, which adds a 22n cap between the junction of the slope resistor and mid cap to ground. The switch is paralleled with a 1M resistor to eliminate any pops.


So, from the bend in your diagram where the slope resistor and the middle cap connect, you have a 22n connected, the other end of the 22n connects to a switch and a 1M resistor, both of which are connected to ground.

[Firestorm]

Do you know how to add a Deep switch, as in Bassman?

The Deep switch is kind a misnomer: it's really an "undeep" switch. It's a 0.1 cap switched to ground. When the switch is closed, it shunts some of the bass; when the switch is open ("Deep" position), it's just the stock tone stack.

[xtian]

Next question, client now wants Presence control. My power section is going to look like a 5f2 with NFB. Will my Presence circuit (shown in red) work?

[Firestorm]

I don't think you meant to take the new control to B+; you meant to draw it to ground I'm sure. It might work better if you ground only the cap and leave one end of the pot open so you don't parallel the cathode resistor.

[xtian]

Yes, to ground. Duh. Revised drawing attached. Can it really be this easy? I came up empty with a search of "presence control for single ended amp".