Bass amp suggestions?

[firemedic]

My thought was this: the 6550 max gridleak resistor is a smaller value than 6L6GC or EL34, so I reduced the value to 68k (Bassman 100). Apparently I could use up to 150k, & still might.
So I think I need smaller value plate resistors on the PI.
Which the Bassman 100 PI already has.
So I used the Bassman 100 PI, which also has 330k gridleak resistors.
12AU7s seem to like smaller plate resistors; I would have tried a 12AT7 anyway.
I modified the tail resistor and NFB to suit (I guess) EL34s or 6550s.

The big input capacitance is for what I imagine to be large current reservoir needs on the output tubes. Maybe it is too much.

The resistors across the diodes I think I got from the Torres book. If it's a waste of time I'll delete them.

Thanks for looking it over Sluckey!

[SoundmasterG]

I think you will find that the 12AU7 won't have enough gain for you. You can always use the typical 12AX7 or 12AT7 LTP circuit from Fenders or Marshalls, and adjust the value of the tail resistor if you need to reduce gain. Fenders typically use a 10k, whereas Voxes with their sensitive EL84 tubes typically use a 47k for less gain. EL34's have more gain than 6L6's or 6550's, and less gain than EL84's, but a 10k is a good starting value. I think you'll find that the 330k grid leak resistors there don't sound as good as the more typical 1M's, and the 330k's will also lose more gain than the 1M. You may try a 12AY7 as the PI....they sound really good and give a good compromise between the 12AX7's extra gain and the 12AU7's extra current drive. You should be able to stick that right into a circuit intended for a 12AX7 or 12AT7 and get good performance and sound.

Greg

[firemedic]

All right SG, that is the plain talk I was looking for! I do not have the mathematical aptitude for designing circuits, yet for whatever reason can't build an existing circuit as is. There must be a sniglet for someone who has to get in over his head, again & again.
So, a stock Marshall output section. I always roll different tube types as a final tweak so I'll find the best one eventually.

[SoundmasterG]

All right SG, that is the plain talk I was looking for! I do not have the mathematical aptitude for designing circuits, yet for whatever reason can't build an existing circuit as is. There must be a sniglet for someone who has to get in over his head, again & again.
So, a stock Marshall output section. I always roll different tube types as a final tweak so I'll find the best one eventually.

Hey man, we've all been there. I'm currently in school for an EE but they don't teach tubes anymore so everything I know about them I'm self taught. Bass amps have different requirements from guitar amps and usually higher voltages both overall and in the preamp help, as do somewhat different tube choices and gain scheduling, since a bass string has more energy. But go too low on gain, and then then amp sounds and feels lifeless....so tweak away but stay within reason or you may find you are undoing a lot of stuff...haha.

greg

[firemedic]

I see kagliostro has gotten some good bites for bass amp ideas too.
Looks like the Weber 6S100 has already incorporated some of the ideas I'd had for my amp, including a paralleled cathodyne 12au7. I imagine a 5879 will work about the same as an EF86.
I think the B25 preamp will work better with it too, as a 2nd channel, since it is designed to feed a pentode-->cathodyne PI.
Alas, xmas is coming & money is tight, so I'm stuck for the moment with a 1/2 finished chassis and dreams. At least the heaters are wired!

[SoundmasterG]

I see kagliostro has gotten some good bites for bass amp ideas too.
Looks like the Weber 6S100 has already incorporated some of the ideas I'd had for my amp, including a paralleled cathodyne 12au7. I imagine a 5879 will work about the same as an EF86.
I think the B25 preamp will work better with it too, as a 2nd channel, since it is designed to feed a pentode-->cathodyne PI.
Alas, xmas is coming & money is tight, so I'm stuck for the moment with a 1/2 finished chassis and dreams. At least the heaters are wired!

The 5879 sounds really good, but has less gain (still plently) and a different sound character as compared to the EF86. The EF86 is brighter overall.

Greg

[firemedic]

How about this?
Some values will have to be tweaked in the real world, specifically, CS100 tone stack caps, PI grid stopper/voltage dropper and power rail resistors.
I like the paralleled concertina but the CS100 cathode and plate resistors would have made it too gainy IMO, and I may even drop them further. 

Again if I am very wrong about anything here please chime in.

[sluckey]

The CS100 bass pot does nothing. Maybe connect the wiper to the top side of the pot?

V4 cannot work. The cathode is tied to node C just like the plate. No current can flow thru the tube. Maybe ground the cathode resistor?

[firemedic]

Duh, that was a dumb mistake. Let me work on the stack, stand by.

All right, let's try that again....

[moody]

The transformers arrive next week and I've procured 2 15" MDF cabs. Anybody have opinions regarding the use of folded horns (w/ crossovers) in bass cabs? I could either keep the horns or cover the openings & install ports. I'm leaning towards porting.

Something isn't right in this paragraph.  If you cover the horn in a folded horn enclosure then you won't get any sound.  A folded horn is a physical construction built around the woofer in order to create more sound and low end extension with less power.  As to using them, I did for a couple of years - big folded horn 15" cab below a regular front loaded 15" cab with two channels of a solid state amp and an active crossover.  Thunderous low end.

As to normal (seperate driver) horns in bass cabs, opinions vary, it won't hurt to try them and if you don't like the sound then disconnect them and leave them there for the look.  Porting will depend on the given speaker in the given enclosure and if done incorrectly could easily cause damage to the speaker.

[firemedic]

It's a regular horn, not a folded one. I'm never afraid to use a term whether I know what it means or not!

[firemedic]

IT'S ALIVE!!!!!!
For anyone taking notes, 4 EL34s will push a Peavey Black Widow 15" 400W speaker. It is so loud.
As soon as I get a chance I will post the semifinal schematic. I think my preamp needs just a little tweaking, as the low E string sounds pretty loose & undefined. To be absolutely sure it's not the speaker, I will have to go to Guitar Center and hook it up to some commercial cabs. Probably I'll have to do 4-10s or something.

[firemedic]

Here is the schem to the best of my memory. I will fill in voltages when time permits. I was focused on the pentode feeding the cathodyne, as the stock 270k plate resistor left 40 or so volts on the plate. So a little tweaking of power supply & plate resistors seem to have fixed that, now its more like 150v. IIRC the cathodyne grid has @-8v bias to work with which is good, right?

Anyhow, the low E string sounds pretty undefined and loose at any kind of mid-to-high volume, I can't be sure if it's the amp (with a 100w Marshall OT, possibly too large tone caps, and just-kind-of-guessed PI output impedance) or the homemade sealed 15" speaker cab, or even the speaker itself. Peavey Black Widow 1508 SPS. The B25b channel is a little tighter, but lows are still loose when turned up.

So let the tweaking begin..... 

 

[firemedic]

Well I tried it again, after rolling some tubes thru the preamp I settled on a 5751 in the B25b and a 12AV7 in the 6S100. I also tried turning down the bass knobs and that really helped a lot. It sounds pretty nice.

Methinks the 100w Marshall OT just isn't going to give me a tight sound on really low frequencies, no matter how much I tweak.  
I'll tighten up this speaker cabinet here over the next couple of days and call it a day for now.

I ought to keep a diary & track the times when I really want to mess around with my amps, vs the times I can't be bothered. In this 2-month episode, I built a bass amp & a Vibrochamp, and put gridstoppers on all my Vibroverb grids. I'm tired of this stuff. Tube fatigue. There's a pattern here....

[firemedic]

Here are the voltages.
I disconnected the parallel cathodyne triode, and changed load resistors accordingly.
Increased the main capacitance.
The 6S100 channel was too flabby so I went to partial bypass on the V2a cathode.
Stand by for results...

[firemedic]

Could be. There were quite a few possible problems, so I'm gradually changing a little at a time. If the little tweaks I just described fix the muddiness, it's all to the good.
That paralleled cathodyne was a big question mark for me so I figured I'd simplify a little.
I'll report back tomorrow night after I work it out properly.

[firemedic]

Well the tweaks were beneficial. The mud is mostly washed off. The Ampeg channel, which is stock, of course sounds the best. I think I will eliminate the bypass cap on the 6S100 channel entirely as it is still too boomy & gainy, even with a 12AY7.

[firemedic]

Excellent link drgonzonm.
I wonder, however, if we are comparing apples & oranges, since the Marshall & 5F6a tone stacks are both driven by cathode followers, whereas my stack is not.
OTOH, I have a free triode now, maybe I could work a CF in.