B-15N preamp design

[txstrat]

Hi folks,

long time no hear.

I'm wondering if somebody could help me with this question.
The B-15N copy I built a couple years ago sounds really great to my ears.
My bass player loves it too BUT it distorts very early when pushed. That's why I added two more output tubes and a 120 watts ultralinear OT.
It's a little better now (because you don't have to turn it up as far as before), but still there.
Actually it's not a distortion but more of a compression, which is IMHO part of the signature sound.
Does anybody know if this is a matter of the preamp design or the preamp tubes (6SL7) or anything else?
Would it be different, if I used 12AX7 instead of the 6SL7? Both tubes have roughly the same values.
What could I do to get the bass more tight with less compression? Would it be better to change the whole preamp design?

Thanks in advance

Matt

[Leo_Gnardo]

The B-15N copy I built a couple years ago sounds really great to my ears.
My bass player loves it too BUT it distorts very early when pushed. That's why I added two more output tubes and a 120 watts ultralinear OT.
It's a little better now (because you don't have to turn it up as far as before), but still there.
Actually it's not a distortion but more of a compression, which is IMHO part of the signature sound.

Good moves, doubling up output tubes, bigger better OT. But all the B15's I've run across do much the same. They're 30W amps give or take 10%, just run out of steam especially on low notes.

OTOH the similar but *bigger & better* B18 has always satisfied me - 55 watts at clip, 18" speaker. Low notes project without effort. Your next project?

[DRH1958]

I've never played one but I would think the paraphase PI would add its own brand of distortion. Maybe an LTP PI?
Also the schematic I found didn't have voltages shown. I'm wondering if using one of those SS replacements for the rectifier will help? I'm wondering if it will increase the 6L6 plate and screen voltages too much. I'm not familiar with this amp, just a couple of suggestions to maybe try. It might also give the preamp a little more headroom as well. Higher plate voltages. These theoretically would help, just not sure about the real world result. A quick and easy mod to try.

[txstrat]

I've never played one but I would think the paraphase PI would add its own brand of distortion. Maybe an LTP PI?

Yeah, that might help.

I'm wondering if using one of those SS replacements for the rectifier will help?

I've used a SS rectifier from the beginning. Voltages run at aprox. 450 volts at the power tube plates. Schematic is the B-15n 1968.

[loudthud]

Negative feedback tends to counteract compression. In the preamp that means use unbypassed cathode resistors. In the power amp, increase the feedback but with an ultralinear connection, compression should be low already. A stiff power supply helps both preamp and power amp, raising the preamp B+ also helps.

[Mike Sulzer]

Negative feedback tends to counteract compression. In the preamp that means use unbypassed cathode resistors. In the power amp, increase the feedback but with an ultralinear connection, compression should be low already. A stiff power supply helps both preamp and power amp, raising the preamp B+ also helps.

But why would this preamp have any more compression than any other nearly identical circuit? I think that this is just a matter of producing low fre

[octal]

Replace the dropping resistor in the screen supply with a choke to reduce sag. Did you beef up the power transformer when you doubled up on the output tubes? If not, the voltage sag may be limiting your power output. Measure it to verify! If the power supply is solid, maybe running class AB2 by adding some mosfet source followers to drive the output tubes would get you where you need to go. You can get a fair bit more power that way. Check out RG Keen's Mosfet Follies.

[Mike Sulzer]

Negative feedback tends to counteract compression. In the preamp that means use unbypassed cathode resistors. In the power amp, increase the feedback but with an ultralinear connection, compression should be low already. A stiff power supply helps both preamp and power amp, raising the preamp B+ also helps.

But why would this preamp have any more compression than any other nearly identical circuit? I think that this is just a matter of producing low frequencies at high volume takes a lot of power.

[J M Fahey]

Now going to the real problem: you doubled power tubes and installed the proper OT BUT left the original PT.

Enzo once said (and I fully agree) something like "the power amp is that thingie between the power supply and the speaker"

A 4 gallon beer barrel with a much larger spout will still serve you 4 gallons , not a drop more.

In my version of B15N , I use the Ampeg preamp(s) but a 100W 4 x 6L6 "Fender" type power amp, fixed bias and all, plus an efficient speaker (suggest an EVM15L).

Guess what? Still sounds like an Ampeg but pushes like a M*th*rfuck*'

Or to be more precise: at least can be heard besides a drummer.

You might do the same with little effort (besides adding a larger PT).

There's nothing Magic about B15N , it's a *killer* recording amp (or Jazz trio duty), but for anything approaching Rock live, even in a small Club, you must add what it takes.

[txstrat]

Hey that's some quite good replies. Thank you all so far.
Well, since I built this one from scratch I used a PT which is rated at 550 milliamps (when I planned the amp I thought 'just in case').
I would say that's enough for the 4 6L6 GCs and a couple preamp tubes.
Actually what you say J M is what I intended to do. The power tubes are already wired like a Silverface Twin, including grid resistors.

Did you use a long tail PI?
Which tubes. 6SL7?
What if I just use the given PI-tube to rewire it for a long tail PI?

Regarding the headroom I can just say it is (even now) loud enough to keep the drummer going: 'please turn it down a little' :-)
The sound is just, well, a little too compressed.

[loudthud]

But why would this preamp have any more compression than any other nearly identical circuit? I think that this is just a matter of producing low frequencies at high volume takes a lot of power.

I suspect 6SL7s compress a little more than 12AX7s. Much depends on the details of the circuit. Solid state designs using opamps do not compress at all unless a compressor circuit is designed in. Some bass players have come to expect the no compression (and big headroom) solid state sound. For this reason (and the expense of high power tube amps) they have migrated away from tube bass amps. A tube in front of a solid state power amp can warm up the sound with a little even order distortion but it won't compress much with a relatively fixed B+.

It occurred to me that the problem might be in the power amp. The ultralinear connection reduces the overall gain so there isn't much gain to counteract the compression in the phase inverter.

[txstrat]

In my version of B15N , I use the Ampeg preamp(s) but a 100W 4 x 6L6 "Fender" type power amp, fixed bias and all, plus an efficient speaker (suggest an EVM15L).

Guess what? Still sounds like an Ampeg but pushes like a M*th*rfuck*'

Could you post a schematic?

[Enzo]

OTOH the similar but *bigger & better* B18 has always satisfied me - 55 watts at clip, 18" speaker. Low notes project without effort. Your next project?

I have one of those B18 amps, it does make lows nice.

[DRH1958]

It occurred to me that the problem might be in the power amp. The ultralinear connection reduces the overall gain so there isn't much gain to counteract the compression in the phase inverter.

Thinking of the power amp and PI, what did you do with the NFB resistor? Increasing the power will change the NFB voltage. The schematic says it's a 10k from the 16 ohm tap to the cathode of the first triode of the paraphase PI. You could play around with the value of the NFB resistor if you left it in.

[txstrat]

Thinking of the power amp and PI, what did you do with the NFB resistor? Increasing the power will change the NFB voltage. The schematic says it's a 10k from the 16 ohm tap to the cathode of the first triode of the paraphase PI. You could play around with the value of the NFB resistor if you left it in.

That's right.
At the moment I think about changing the paraphase to a long tail PI. In that case I'd have to change the NFB resistor anyway. Think I'll try different values.
I just don't know if it makes sense to change for a 12AX7 in the PI position.
My gut feeling says keep the 6SL7 as a first try.
That should change the compression to the tighter side.