Bass Pre-amp Project

Karel-Mars

Hi all

My brother asked me to organise him a bass amplifier for his weekend jams. A few options was discussed, and we settled on a preamp+poweramp+speaker format.

So: on to the pre-amp. A CAD drawing was made, and our local engineers made us a nice steel enclosure - laser cut/bent and welded.

I knew a design from a web site that I liked

The output master volume control driven by a cathode follower looked like just the thing the doctor ordered for a ass kicking bass amp 8)

The circuit was modified to Fender bassman specs - I used o.1 uF capacitors in the tone stack section for better low frequency response.

I also prefer the tone stack to be driven by a cathode follower such as in a Fender Bassman 59 or a Marshall. So this pre-amp actually has two cathode followers in series.

All was mounted and wired up

I decided to keep the gain low - so none of the cathodes were bypassed.

A amp rack cabinet was assembled

and the power amp and the pre-amp was mounted. I used the power amp in bridge mode for extra power.

All is done! The faceplate design has been referred to the graphics department; I'll update you when that becomes available.

This amp sounds very retro to me... tube roundness and tons of harmonics. The pre-amp has enough gain to get the power amp into the red, so the basic gain characteristics turned out on target.

If you have a power amp that is under powered for PA use, try using it as an instrument amplifer. Pre-amps are easy to design to and fun to build ?

FatBoy

Nice one! I still need to get a bass amp as well.

matta

Karel,

Very impressive as always... was wondering if you were going the "Bassman" route ?

Thanks for the schematic, will dissect it later.

The PSU has me wondering... did you go with S/S rectifier or valve? If you went S/S would a standby switch not help? Silicon power diodes are almost immediate in their response time and it has me wondering if the constant jolt on the cathodes damage/strip them over time?

Another 'taboo' question is on the ac lines. Typically valve amps use UNREGULATED voltages and it seems you have done the same here for both HV and Heaters... have you ever tried regulating them?

I don't work with HV enough to have an opinion on the mater and just wondering in your own experience if you find that regulated lines sound/react better? I know it might seem sacrilegious given that they were built this way for decades!

I'm fairly ignorant when it comes to valves/tubes, so always want to learn more. Good to have you around these parts!

Cheers

Matt

Karel-Mars

Matt

Next in line is the Ampeg B15 baxandahl circuit with 6SL7's ?

Actually, I think a mute switch might be a nice addition rather than a stand by.

I went for SS rectifier. The "jolt/cathode stripping" issue is more relevant for output tubes in power amps with fixed bias where the output tubes are directly (via center tap on output transformer) connected to power supply (esp in high power amps with high HT >500VDC).

In this case the power supply is rectifier bridge>cap>tuning resistor>cap>10K resistor>cap>pre-amp.

The 10K chills out the surge significantly.

The 12AX7's run at only 1mA each triode, so the tube do not really need to be warm to give good tone.

On the regulated vs. unregulated topic: I like a bit of resonance in the power supply. Guitar amplifiers typically have very little regulation, giving them the warm vintage tone of small power supply capacitors (look at the fender twin power supply for example - 35uF on the first section regulating a 100W power amplifier). I am actually trying to make a pre-amp that adds a bit of second order harmonics.

I actually rectified the filaments to DC.

The pre-amp is very hum free.

For a studio pre-amp I would certainly consider regulated HT so that there is no hum in the signal.

Manfred-Klose

Awesome!!

The first 2 pics dont want to load up? do you have any pics of it inside?

So you just run it through a PA Power amp and it kicks ass compared to other bass amps?

.....mmmh.......i hate it when i get ideas ?

Karel-Mars

Mo' pics....

Try this link:
http://picasaweb.google.com/marskarel/CustomProjects?feat=directlink

matta

Karel Mars wrote:Next in line is the Ampeg B15 baxandahl circuit with 6SL7's ?

Nice! I used a B15 fliptop on a couple records I worked on Stateside, they sound fan-flipping-tastic!

Karel Mars wrote:Actually, I think a mute switch might be a nice addition rather than a stand by.

Hmm, indeed, a nice addition, gear for 'silent tuning' if your brother has an in-line tuner with out a 'mute' option.

Karel Mars wrote:I went for SS rectifier. The "jolt/cathode stripping" issue is more relevant for output tubes in power amps with fixed bias where the output tubes are directly (via center tap on output transformer) connected to power supply (esp in high power amps with high HT >500VDC).

Thanks for the explanation, as I shared me and HV are like long lost cousins... you only see each other once ina blue moon and not quite sure what to say/do when you do... LOL And the HV I do work on tend to use simple interstage transformers and no output valves ?

Karel Mars wrote:In this case the power supply is rectifier bridge>cap>tuning resistor>cap>10K resistor>cap>pre-amp.

The 10K chills out the surge significantly.

I see

Karel Mars wrote:The 12AX7's run at only 1mA each triode, so the tube do not really need to be warm to give good tone.

Off topic somewha... I've always wondered about 'valve' stuff that uses 'starved plate voltages' like valve stompboxes for eg. These units are normally powered by 12-18 Volt wallwarts... clearly they cannot get the HV plate voltages off those!

Since I'm not that clued up about valves, how does this effect things tonally? I assume it would reduce headroom somewhat, but what are the other effects/byproducts of using 'starved plate voltates' be?

Karel Mars wrote:On the regulated vs. unregulated topic: I like a bit of resonance in the power supply. Guitar amplifiers typically have very little regulation, giving them the warm vintage tone of small power supply capacitors (look at the fender twin power supply for example - 35uF on the first section regulating a 100W power amplifier). I am actually trying to make a pre-amp that adds a bit of second order harmonics.

I actually rectified the filaments to DC.

The pre-amp is very hum free.

For a studio pre-amp I would certainly consider regulated HT so that there is no hum in the signal.

I'm with you on the limited regulation on the guitar amps, I've seen it by looking over old valve schematics. I'm just wondering how this plays into the 'tone' factor? I'd also hazard a guess that by suppressing the physical size of the filter caps you may incur increased ripple? The cause of much hum... which for 'studio designs' is annoying at best, LOL!!!

Karel Mars wrote:Mo' pics....

Don't see em but would like to! Pop that hood... I want to see the guts shots!

Cheers

Matt

Karel-Mars

??? Can you not see the pics? I can see them on my browser....

I have contacted Norio to find out what's amiss

nick

Karel Mars wrote: ??? Can you not see the pics? I can see them on my browser....

I have contacted Norio to find out what's amiss

the only one showing for me is

they did work earlier though.

Karel-Mars

Here is a link to the photo album:

http://picasaweb.google.com/marskarel/CustomProjects?feat=directlink

NorioDS

No idea what could be wrong here. The pics load fine if I load them individually in separate tabs but they don't show up for me in your post.

Sorry - no idea what could be wrong. Never experienced anything like this before.

FatBoy

Maybe google has some tricksey code in place to prevent more than one image from an album being hot linked, only thing I can think of...