Revisiting Bass Amps, Final Critique

[bluesbear]

I need some info on bass amps. "Search" didn't bring up anything useful. My bass player has an old Traynor. It's wonderful but since he's a guy that prefers the sound of a cranked amp (even for bass), we're finding it too powerful for the gigs we play. I have an AB165 Bassman I gutted years ago; I figure the chassis, cabinet, and trannies, and a pair of 6L6's will make a great bed for a nice lower power (compared to the Traynor) bass amp. Having never built one, I'd love any help with design. My criteria are:
I only need one channel
Stupid simple (a concept I use with every amp)
I'd prefer a standard Fender preamp style. I can deal with adding midrange, master, etc. It's the basic concept I need help with.
Thanks,
Dave

[PRR]

Bass is a lot about the speaker. Instead of "tone" and "voice" like guitar speakers, you want some reasonable compromise between balls and size/weight, with an overall low-high balance you can live with.

Fender 5F6-A would have been a fine bass-amp if Fender had put the right speaker with it. You won't go far wrong just copying the 5F6-A plan. Omit a channel and the mix resistors. The AB165 Bassman tin and iron will be fine with 5F6-A operation (the final stages are essentialy the same). Use the solid-state rectifiers ala AB165 (bottle-rectifier "sag" may be useful on 5F6-A with guitar, but for bass you *probably* want big and un-saggy). You may use a good 50-100uFd 500V main cap instead of that 70u+70u (35u) string used in AB165.

[Tone Junkie]

BluesBear please keep us posted on how you come along . i would like to build one for myself in that size range in the near future so your project intrigues me greatly. Thanks Bill

PRR Ive been doing some reading on building bass cabs and the consensus is guitar speakers dont work just like you said, woofers that only go up to 1 to 2k in there frequency range seem best for bass, and some of the woofers in the car stereo industry work very well cheap. but I would have to pull my notes out to see which ones they were. I think they were dayton speakers off the top of my head , i was thinking of a 2/12 inch speaker cab, if i needed more I could build two my backs just not the same as it used to be.

[PRR]

> my backs just not the same as it used to be.

Then there is no alternative. Use solid-state for bass-amp.

Bass waves are BIG. You have two choices:

1) Big speakers

2) Big amp

Little speakers can slosh a fairly big wave if you have excess power to cover the slosh-slop.

In tube amps this gets silly: what you gain in smaller speaker is equal to what you lose in a heavier amplifier.

SS amps have no OT and half the PT power/weight of a tube amp with the same measured power. Since a bass-amp OT may be as heavy as the PT, going SS gives 1/4 the weight or 4 times the power in the same weight. With 4 times the power you can use 1/4th the speaker. That's significant. In fact modern SS stage amps may do better than this, a 400W head can be astonishingly light (to an old phart who lugged an SVT and Crown DC300s).

If your ear complains, tell it to talk to your back. If your back spasms, you won't be gigging that week; if you go back to the same abuse your back may not stop hurting. Your ear can hear SS amplification, or it can hear your back moaning in restless pain.

And the audience will never know the difference after the 3rd beer, probably not before the first beer.

I kinda know what bluesbear does. He's not blasting stadiums, nor 400-seat disco halls. And he says his bud's Y3(?) is too much power (rare for bass). Then a 40W-50W Bassman is probably a better fit. And he knows how heavy the AB165 is, how that weight suits his bassist. But if you are lugging a BIG tube amp, and hurting, admit your age, leverage your musical experience to make entertaining booms with a brick-amp and compact speaker.

Yeah, you don't want zingy highs on a bass cab; that trespasses on the guitarist's turf. If the bass-amp clips, the speaker should muffle the sharp edges and deliver a midbass phattness which won't fight the guitar's soprano voice.

[Tone Junkie]

What about a good pair of kt88 for good clean power.

[bluesbear]

I populated the circuit board yesterday. I'm trying a single channel 5F6a with .1mf coupling caps. I'll use 6L6's. I'm not overly concerned with as much clean power as I can squeeze out since the whole reason for this exercise is to lower the power as opposed to the Traynor. Should be a few days till done. I'll post results.
Thanks,
Dave

[Tone Junkie]

So Mr. knewhimwell are you saying you put that grill cloth on sideways. 

[bluesbear]

Well, I tried the amp out through a 1 - 12" and 1 - 10" bass cabinet Sunday. It's way not as loud as it ought to be (quieter than 2 - 6V6's) and it sounds a lot like a Fuzz Face. I'm betting I've got a resistor screw-up in there somewhere (1m instead of 1k, etc). I'll check it out tomorrow. If anyone has any ideas, I'd sure love to hear them!
Thanks,
Dave

[tubeswell]

Well, I tried the amp out through a 1 - 12" and 1 - 10" bass cabinet Sunday. It's way not as loud as it ought to be (quieter than 2 - 6V6's) and it sounds a lot like a Fuzz Face. I'm betting I've got a resistor screw-up in there somewhere (1m instead of 1k, etc). I'll check it out tomorrow. If anyone has any ideas, I'd sure love to hear them!
Thanks,
Dave

Voltages please

[bluesbear]

Well, here are the voltages. It would seem I have a problem somewhere in the B+. A big one!
This is a single channel 5F6a. Half of tube one is unused. Tube two is a cathode follower, of course.

http://www.el34world.com/forms/valvedata/dave5e3.htm

Any thoughts? I haven't even opened the filter cap cover since I replaced them 3 years ago so I can't imagine what could have gone wrong. They were fine a few weeks ago.
Thanks,
Dave

[sluckey]

Cathode follower voltages indicate that it may not be wired correctly. Pin 6 should be connected directly to a B+ node and will be 300v or more. You gotta fix this.

I don't see any voltages for a PI tube (V3)?

Looks like you are using cathode bias instead of the normal fixed bias on the output tubes?

Got any pics?

[Dave]

The concensus is, as far as I have ever heard, that this circuit should be used for everytihng from 20 watts to 300 watts, as it is simple, straight to the tonal point and works flawlessly.

I thought this was an interesting statement.

Wouldn't you need to make some changes to the phase inverter if you were going to try to drive a bigger power section?
For example, say I wanted to build a bassman at 150 watts..... would the standard 5F6A PI drive over 3 times the power section that it was designed to?

Dave

[bluesbear]

I figured out the weird voltages. A testament to why one shouldn't keep working when tired!
The 10K resistor on the front side of the phase inverter is supposed to tie in with the 27k resistor in the feedback loop and the presence control. On my layout, that meant running a jumper one space to the left. I went one space to the right.... right to the B+! No wonder the voltages were high. All seems well now.
I'll know better about how it actually sounds after practice this afternoon. Tell you later about that.
Thanks for all the help.
Dave

[bluesbear]

I recommend the 5F6a preamp for a bass amp. We used mine all afternoon and it does exactly what I hoped. Cranked up, it gets plenty loud but when back way off, it still sounds warm and punchy. Also, it's about 1/4 the weight of the Traynor my bassman was using. The Traynor is a great amp but it doesn't like being turned down much.
Thanks for all the advice. The great people here have helped me pull out another one.
Dave

[PRR]

> Wouldn't you need to make some changes to the phase inverter if you were going to try to drive a bigger power section?

Not much.

The peak grid swing is some fraction of the G2 voltage times how far you go toward class B. 50L6 in straight A at 100V on G2, about 7V, 5F6A in deep AB with 400V on G2, about 40V.

Most driver circuits, the peak output voltage is a fraction of their supply voltage. 12AX7 simple gain stage: 7V with 90V supply, 40V with 300V supply.

Simply feeding your driver with something "near" your output G2 voltage will probably scale correctly.

Anyway "sane" high-power receiving-tube amps don't use a wide range of G2 voltage. 280V on a little 6V6, 450V on a big hot 6L6GC beast. (100V amps and 560V G2 amps are freaks.)

Now, current. This is mostly the power tube(s) grid resistor. For most large tubes, 500K max in self-bias, 100K max in fix bias. In self-bias we may wave-off 500K, or even 250K (two grids per side), because most driver triodes won't strain. 100K in fix-bias is real work; yet many Fenders get-away with 220K because the idle dissipation has some headroom for grid leakage. And while two tubes per side should use half the value, in fact you probably won't get two leakers in the same side. And it's only rock-n-roll.

For two tubes per side you can copy Twin 5F8A which still uses one 220K per two tubes.

So the stock 5F6-A driver can work fine up to 100 Watts.

Or ponder the QuadReverb Twin or SF-135W-UL which goes to 68K per pair and a 12AT7 driver with 47K loads; it isn't much of a difference.

At the 180W level and three tubes per side, this gets strained. Super Twin 180W uses a couple cathode-followers to buffer the 33K grid resistors from the voltage-amp.

[Dave]

Thank you for that outstanding information.

Dave

[PRR]

4CX300A is a stiff tetrode. Maximum grid resistor is 100K, higher than 6550 or 8417 (50K). Peak voltage per grid is 50V. Odds are that a 5F6A-type driver will do the job.

A pair of 4CX300A costs $200, not out of line with a quad of 6550 or a sextet of 6L6GC.

Two 4CX300A in push-pull AB1 can give 240Watts-800Watts.

[bluesbear]

My bass player used the amp last night in a small club. It worked just as I hoped. Unlike the Traynor, this bass amp doesn't get thin and weak sounding when the volume is backed off. He was able to play much quieter and still sound good, which brought the drummer back to a moderate sense of volume reality... not an easy thing to do! Not a single person left with their hands over their ears.
Thanks to all for the help and advice!!!
Dave

[Boots Deville]

Thanks for reporting back - I'm interested in building something like this. 

Did you stick with the 2x6L6 power amp? 

For my bass amp build, I'm considering mating the 5F6A preamp to the AB763 twin reverb poweramp, hoping that 100W is enough.

[bluesbear]

It's a single channel 5F6a with extra filtering. I was planning on 6L6's but I had a pair of 5881's on hand and they turned out to be exactly right. I went with volume, treble, mid, bass, master, and presence. For what I call a "standard" size bar, it's just right. Next step (as soon as I get a few bucks ahead), I'll try it in a small combo cabinet with a 15" neo bass speaker.  If that works out, it'll be a full bass rig for most of our jobs that'll weigh less than half the Traynor head. My bass player will love that! He's almost as old as me!
Dave

[Tone Junkie]

My friend i thought you were making a suttle joke on yourself as in JD knewhimwell being you  my mistake sorry . Bill