Two tube preamp for bass design

[ElectronAvalanche]

The bass player in our band asked me to build him an all-tube preamp to use with his power amp.

I was thinking along the lines of a Dumble Clean section using one 12Ax7, maybe a FET for boost.

He asked for a 4 way EQ section for high mids. I searched the web a bit but can not find schemos for a simple 4 way EQ. Maybe one could put a pot before the mid cap?

He asked for a boost for solos (yeah, he does play solos once in a while).

Maybe I could make the Mid boost ala Dumble switchable via relais.

Anyone out there who has done a Bass preamp?

thankful for any tips and hints/ideas!

Cheers,

Electron

[Cliff Schecht]

Why dick around with the Dumble route? Those are guitar amps and maybe too complex for what your bass player wants.

For the tonestack, have you looked at LC-based tonestacks? You can even build the L's out of op-amp (or tube!) gyrator circuits and this way you aren't dealing with unwieldy large inductors. The Mesa Boogie Mk4 has a good example of an LC-based EQ and IME (and I think many others) this is an extremely effective EQ and very linear and predictable as well. The typical guitar EQ is too interactive between the knobs for bass IMO, I want relatively independent control over the frequencies I'm boosting/cutting with bass amps.

Is a parametric EQ out of the equation? That's the ultimate titties for control with as few knobs as possible.

[ElectronAvalanche]

Hi Cliff,

thanks for the input.

Dumble style: this is the circuit I have the most experience with. The famed Alembic F-2B is based on the Fender preamp. Dumble is based on the Fender as well, with the LNFB and a bit more elaborate TS.

I am open for all TS variants, as long as they are passiv. Best would be to only use a single tube.

Would love to hear what you would suggest or maybe you can point me to some existing designs. Will look at the Mesa TS. Inductors might be out, since this should be a "budget" project.

Thanks!

Electron

[wicker]

Look at Traynor YBA3 schematic, it has 2 preamp tubes, but cool Twin T filters, or you can kick out this 2 stages (the've got very small amplification) and use 1 Twin T with bass and presence control + typical 3 pot eq.

[jon]

I would do a blackface fender preamp and use a pot in series with a resistor for the slope. like a 25K resistor and a 50K linear pot.

But to be honest I still like my B-15N stack with the ultra high and ultra low switches. I just works and Jelle will tell you it is capable of some seriously big signal.

[billyz]

Too bad you cannot do a two channel preamp. One Blackface fender and the other Ampeg B15n. But if I could only do one for Bass it would be the Ampeg with the Baxendale tone circuit.

[marcoloco961]

+1 on using a Bandaxall TS. The FMV is not a great TS for bass as I am finding out. It is way too sensitive and the treble pot acts as a mid scoop as you adjust it. You can get a good tone out of it, but it is a challenge.

You might check out the TS set-up on an old SVT pre-amp it has a neat little mid set-up that looks very versitile. I am guessing it is a winner since the SVT is the industry standard for the pros.

Here is a link from Duncan amps about Tone Stacks: http://www.duncanamps.com/technical/tonestack.html

[Reeltarded]

Oh! 2-3 stages of gain.. like 12at7s or mixed with au. I like more tubes, less gain.

EQ that shelves from like 800hz down, and from like 4k up, with a mid sweep between them, and a cut that dumps off like 12db/oct from around 10k. That kind of tone stack would cover many different bases.. err.. basses.

When you get that all worked out, I'll take one too.

[renshen1957]

The bass player in our band asked me to build him an all-tube preamp to use with his power amp.

I was thinking along the lines of a Dumble Clean section using one 12Ax7, maybe a FET for boost.

He asked for a 4 way EQ section for high mids. I searched the web a bit but can not find schemos for a simple 4 way EQ. Maybe one could put a pot before the mid cap?

He asked for a boost for solos (yeah, he does play solos once in a while).

Maybe I could make the Mid boost ala Dumble switchable via relais.

Anyone out there who has done a Bass preamp?....

Hi Electron,

I had very good results with the Bass preamp as illustrated in the Tonnes of Tone or in the The Ultimate Tone Vol. 5. if you have access to either book, aka London Power Bass Preamp. The design uses one 12A_7, it is an adjustible gain input (feedback) rather than a free running gain stage to accomdate high-output active basses as well as low-output passives - and even piezo pickups. In essence it is the adjustible gains preamp found in solid state amps without the solid state. A second 12A_7 to accomodate whatever his solos needs were, etc. can be added.

I'd use a James/Baxandall Tone Control as starting point myself, less interaction. The Fender Stack's greatest virture is that it uses the least amount of components, i.e. it's cheap.

As to Cliff's post, Gar Gillies book has an interesting EQ systems using tubes on page 137 although not used in his production amps, is an interesting modification to the James/Baxandall Tone Control to prevent interaction between the tone controls. It would increase the tube count.

Electronic Projects for Musicians has a LC project that if followed by a gain stage (to compensate for the losses) and shielded well is worth looking into.

The 200 Watt Fender (tube) Bassman had a Semi Active tube driven LC EQ. If the inductors were replaced with IC's and tweaked, this might make the circuit useful, however the EQ in this edition Bassman is not that well regarded by owners of the amp. On the other hand the Marshall Major's tone controls (also semi-active) weren't appreciated that much either.

Depends on the level of sophistication your Bass player is interest in, however. He might want an effects loop (to EQs, whatever) depending on his level of GAS.

Best Regards,

Steve

[selloutrr]

It's also hard to go wrong with the Showman preamp (alembic used the showman as thier rack mount Preamps.)

Ampeg B15 or V9 is also a studio favorite.


I don't really see the point in going the route of Dumble. Even HAD used Ampeg in the Winterland.

[selloutrr]

Look at a Neve 1081/1073 or API EQ

A simple 4 band parametric eq with hi pass low pass filter would be useful in taming pickup pop.

Direct Out with volume control that is uneffected by the master input volume. Pad and Ground lift. 1/4" balanced/unbalanced and XLR out.

compression is a nice feature if the player does slap or plays a bit on the sloppy side.

Gain to drive the tube

Overall master volume

Bypassable (EQ, FX Loop Buffer)

Relay switching

Fx loop pre and / or post EQ (send and return level controls serial / parallel, Mix) - Foot switchable

Boost? though It would be best used as a pedal in the FX loop, or as feature built into the wiring of the bass.

Mute Switch or Standby

isolated Tuner out

Cooling Fan

IN and Thru (2 inputs) for multi setups.

Rackmountable

Take a look at Trace Elliot, Walter Wood, Ampeg B15, Ampeg SVT (pre '80), Alembic, Fender Showman, Pultec EQP-1A3, Neve 1073/1081, Avalon DI, Demeter DI, Countryman Type 85, Urei 1176, LA-2a LA-3a, DBX 160...

most of the bass tracks at taken directly into the console. Look at studio gear schematics and build a signal chain to make a bass amp, It's basically a channel strip in a rackmount.

[jaysg]

Too bad you cannot do a two channel preamp. One Blackface fender and the other Ampeg B15n. But if I could only do one for Bass it would be the Ampeg with the Baxandal tone circuit.

Imo, these are the two options and it's good to know that the popular on-board active circuits from Sadowsky and Aguilar are simply the Fender tone stack adjusted for jfets....100K slope becomes 10K and so on. I think I've posted my F-2B schematics here somewhere. hth

[billyz]

Too bad you cannot do a two channel preamp. One Blackface fender and the other Ampeg B15n. But if I could only do one for Bass it would be the Ampeg with the Baxandal tone circuit.

Imo, these are the two options and it's good to know that the popular on-board active circuits from Sadowsky and Aguilar are simply the Fender tone stack adjusted for jfets....100K slope becomes 10K and so on. I think I've posted my F-2B schematics here somewhere. hth

Use a pot for the slope resistor. might need to decouple it though.

[ElectronAvalanche]

Hi gang,

thanks for all the replies!

I have decided to use two 12AX7A. I find the B15N preamp most interesting, albeit the Baxandall only has (of course) Bass and Treble.

This preamp needs to stay rather simple, but the Bass player requests a Mid control. Hmmm...

Although I have TUT5 I could not find a schemo for a bass preamp.

@renshen1957: could you point me to the page where this preamp is described in TUT5? Thanks

Concerning the Neve, I was unable to find a schemo on the net showing the parametric EQ. Remember that this thing needs to be 100% tube (maybe except for the FET boost).

XLR out using a small transformer would be a nice feature, but the xformers are rather costly (out of budget of bass player).

FX-Loop: This might be where the Dumbleator could come in.

I will try to come up with a schemo and post. Open for all critique...

Thanks again for the help!

Electron

[selloutrr]

if you are limited by the number of tubes and $ you might want to look at the Orange AB2000 it's stripped down straight forward.

The DI transformers are under $25 if your pinching pennies that hard. A quality FX loop is also probably out of the question.

Solid state Opamp circuits can be adapted to tube.

Look at the Pultec EQP-1a3 it's a studio standard for bass EQ and tube driven.

I've assembled several DIY best of bass heads for clients. To do one right you will most likely be $1000+ in parts. Plus your labor, and design time. The more you can take from a working time tested design the easier it will be to cut cost.

The bass and treble tone pots could be enough. if not you should be able to swap out the tone circuit or modify it with minimal effort / effect to the tone. If the player uses active electronics on his bass you will want to include a PAD on the input of the amplifier to prevent the input from clipping when the tone controls are dimed.

the Altec 436C is a great tube compressor. Around $450 to clone from scratch being that you would already have the chassis and PT it would be a great addition for around $150-$200 in parts.

IMHO before I started this project I would make sure the client has enough $$$ to cover the time and effort this is going to take. Even before you start to build it or order parts you will be 20+ hrs just working out the details. If you at least know what your $ limits are and what the absolute requirements are you can find out if it's going to work fast so you don't waste your time.

One of the other most used bass preamps in the studio is the Fender Twin Reverb. That would give you a Bass Mid Treb EQ. Parts are cheap.