DIY Tube Preamp question

Bass4lyf · #1 03-30-2010, 04:10 PM

I want to build a really cheap but half descent tube preamp that I can use for mainly bass guitar, but still be open for other applications if I have to. Im considering doing this as part of a school project. I came across some cheap tube preamp kits on ebay and was wondering if they would be adequate enough for what I want to do.

http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

I plan to build it as a rack.

Also, if I have an equaliser on it, I was thinking just for a budget type thing, I could buy a cheap behringer bass eq pedal ( http://cgi.ebay.com.au/FX-PEDAL-BEQ7...item35a6de4d1f ) , take the insides out of it, and put it inside the rack before the tube preamp.

Would this work? Would that tube preamp kit work well?
Thanks for the help

Bootzilla · #2 03-30-2010, 04:17 PM

I wouldn't put in a graphic eq but put in a nice passive tonestack intead (for the mojo and all). You could go with a fender or a baxandall (used in the B15 I believe). For more info you could check out some of the DIY effect forums (there are some people building small amps on there and overdrive pedals are not that different from preamps in many ways). Freestompbox.org is a good forum with a lot of knowledge on circuits.

Edit: I looked at the kit and it doesn't provide any info at all so I wouldn't know.

T-Bird · #3 03-30-2010, 10:49 PM

Hi.

Not enough information in the ad to draw any conclusions. Other than it's Chinese origin

.

If everything exept the transformer is included, the price is great IMHO.
If You buy the transformer, make sure it'll fit the 1U rack case.

The quality of the parts will determine the s/n ratio, noise floor level etc, so building one from scratch with the zillions of istructions in the net will in theory yield better results. It will cost considerably more though.

For a 1U rack chassis, I've collected quite a few obsolete router boxes, a definite + is the miniature fans. You don't want to buy one, it'll break the bank.

I wouldn't try to mate a Behringer EQ into anything, if You want something passable from them into Your project, try the DI.

Regards
Sam

Bass4lyf · #4 03-31-2010, 12:28 AM

ok cool, im kinda confused about the transformers, why cant I just use a 6v power adaptor/wall cube thing intead of a big, heavy, kinda expensive transformer? Also, Ive got an old toroidal (pretty sure) transformer from an old bass amp. It says its out put is like white 0v, black 30v, yellow 0v, orange 30v. Im guessing this means its output is 2x 30v, is that right? But those tube preamp kits say they need a single 6.3v to power it. Will this transformer overpower the preamp by putting in 30v, or is the transformer actually rated at 0-30v per side?

Sorry for being a massive noob but id rather not buy a new transformer as im trying to be as budget as possible and im happy with recycling old unused equipment if itl work well enough. Another thing is, this transformer was made in 1996, is that bad? The bass amp that its in does work, has cleanish sound, but does distort when the volume is on about 5.

I appreciate the help!

johnk_10 · #5 03-31-2010, 12:45 AM

you need a transformer to supply the 6.3 AC volts and the high voltage for the tubes. the listing states: "voltage AC 180V-0-180V 2AC 6V 30W", so your transformer will not work.

looking a those pics, there's no way that the circuits will fit in a 1U chassis, since the tubes would need to be mounted horizontally, in which case the pcb's that they're mounted on are too large.

his torodial tranny looks to me like its to large for a 1U case too.

Bass4lyf · #6 03-31-2010, 12:50 AM

it doesnt have to fit in a 1u case, il go bigger if I have to. Why wont the transformer work?

johnk_10 · #7 03-31-2010, 01:11 AM

i thought that i explained it in my last post.
does your transformer have 180-0-180 and 6 volt taps?

Bass4lyf · #8 03-31-2010, 01:23 AM

sorry but I dont know what that is. All I know is, the input voltage is 240 volts. And on the label it has a list of different coloured wires and voltages next to these wires, theres a few with 0v, some with 20v, and some with 100v, then theres another part on the label where it has four different couloured wires, 2 wires are 0v, and another 2 are 30v. I really dont know much about how a transformer works, I only learnt what it kinda does like yesterday.

johnk_10 · #9 03-31-2010, 01:34 AM

then you'd better study up on it a bit more before you attempt building a tube preamp. tube circuits require high voltages, and unless you know what you're doing, you could electrocuted.

Bass4lyf · #10 03-31-2010, 01:39 AM

ok fair enough

seamonkey · #11 03-31-2010, 08:53 AM

Quote:

Originally Posted by Bass4lyf

I want to build a really cheap but half descent tube preamp that I can use for mainly bass guitar, but still be open for other applications if I have to. Im considering doing this as part of a school project.

Take a look at
http://paia.com/tubestuff.asp

For complete kits, schematics, and circuit explanations.

As a school project I'd think you'd want to do something more modern and get some experience to carry you into the future. IMHO - The popularity of this stuff is a lot of marketing hype. White/clear box marketing.

Bass4lyf · #12 04-01-2010, 07:09 PM

i was checking out this site a few days ago, seems pretty good. So these kits come with absolutely everything needed to run it?

Bass4lyf · #13 04-02-2010, 08:45 AM

I came across this stereo preamp, marantz design, think its designed for hifi applications. http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

How well will that work for bass and guitar as a 2 channel preamp?

Also, for eq, could I just use a 3 band active eq that is meant to be used in the actual guitar itself? something like this. http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

or would it be better to buy a bass graphic eq pedal and rip the guts out of that and use that inside the preamp?

I also want to have a knob that adjusts tube overdrive to get some clunky overdriven bass sounds. How can I do this, can i just put a volume knob before the preamp, or does it have to go somewhere in amongst the preamp? Surely one of you guys that can understand preamp circuitry will know this by looking at the preamp or will you need a wiring diagram or something?

I hope im not sounding like an idiot, I really am a noob, but would still like to learn about and succeed in building a half descent low cost tube preamp.

ColinB · #14 04-02-2010, 09:12 AM

I've built and heavily modified my own tube amp for guitar, and repaired quite a few tube amps. Can I suggest buying "Inside Tube Amps" by Dan Torres and reading through that first.

From your posts it seems like you know very little indeed about the workings of a tube amp and the chances of killing yourself would be unacceptably high. I don't mean to be patronising, but I'm really not kidding on this!

You might want to open up some of the lay-out diagrams and schematics on this Weber kits site to give you a flavour of what might be involved. (The Weber original Kit AB200 is a bass amp.) They all include the power amp section as well, but it may get you started on reading scematics and what's involved in tone stacks etc, and if you can't figure out what's pre-amp and what's power amp, then you're not ready to build anything yet.

BassmanPaul · #15 04-02-2010, 09:41 AM

much as I applaud the OPs desire to build something tubed he should read up on tube amps and electronics before he makes the attempt. The schematic for the Normal channel of a Fender Showman is very simple and has been used as a model for several highly regarded pre-amps. however he makes no mention of what he wants to use the pre-amp for. Recording? Driving a power amp? The final build has to reflect the input impedance of the following stage for good results with a bass. Typically the output impedance should be a decade or so lower than the impedance of the following stage. I would aim for 1KΩ or less.

The power supply for such a project will always be problematic especially if the aim is to build into a 1U rack case. It's hard to find iron that fits into about 1.5" of vertical chassis space. That means special toroid PT or a couple of small low voltage transformers one wired in reverse. Local electrical codes will have to be followed for the mains wiring.

I feel the OP is nowhere near the knowledge level to complete such a project safely.

Paul

"Q" · #16 04-02-2010, 02:42 PM

Quote:

Originally Posted by ColinB

From your posts it seems like you know very little indeed about the workings of a tube amp and the chances of killing yourself would be unacceptably high.

This is a bit dramatic isn't it?

And instead of trying to scare the poor guy why don't we tell him how to NOT kill himself?

In order to preserve your mortal soul on this plane.... whenever you work on a piece of tube gear make sure (or any gear really) you short out the terminals of any of your power supply capacitors with a screwdriver or a piece of insulated wire to discharge them. A resistor in the megaohm range is better and you wont get a bang when you do it.

That said, I dont think you'll be using caps big enough in a preamp to kill you. But you might as well develop good habits in case you ever decide to work on a power amp.

Definitely get some books about building tube amp projects. There are quite a few out there.

johnk_10 · #17 04-02-2010, 03:15 PM

Quote:

Originally Posted by "Q"

This is a bit dramatic isn't it?

judging by the questions that he asked, i don't think so at all.

Quote:

Originally Posted by "Q"

And instead of trying to scare the poor guy why don't we tell him how to NOT kill himself?

IMO, this forum isn't the place to 'teach' people how to build amplifiers. offer advice? yes. but its obvious that he is in over his head thus far, and IMO, 300 plate volts in a preamp is enough for me to advise him learn more about, it since he didn't seem to know anything about power supplies and power transformers.

Hookus · #18 04-02-2010, 03:26 PM

Quote:

Originally Posted by "Q"

This is a bit dramatic isn't it?

And instead of trying to scare the poor guy why don't we tell him how to NOT kill himself?

Well, I think he was trying just that. Honestly, I doubt many folks could solder up a properly working overdrive pedal, a tube preamp is pretty advanced stuff. Not only do you need to understand a schematic thoroughly, you need to be able to calculate voltages and signal strengths at various points, so really, if you barely understand a transformer, a full on tube amp or preamp is waaaay beyond the OP's knowledge base.

Not harping, just sayin'

MAMMOTHvolume · #19 04-02-2010, 04:05 PM

Quote:

Originally Posted by Bass4lyf

I want to build a really cheap but half descent tube preamp that I can use for mainly bass guitar, but still be open for other applications if I have to. Im considering doing this as part of a school project. I came across some cheap tube preamp kits on ebay and was wondering if they would be adequate enough for what I want to do.

http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT

I plan to build it as a rack.

Also, if I have an equaliser on it, I was thinking just for a budget type thing, I could buy a cheap behringer bass eq pedal ( http://cgi.ebay.com.au/FX-PEDAL-BEQ7...item35a6de4d1f ) , take the insides out of it, and put it inside the rack before the tube preamp.

Would this work? Would that tube preamp kit work well?
Thanks for the help

from what you've said in your other posts please don't try and make this go to someone a bit more proffesional.

The kit you linked to i'm assuming is a buffer and is meant to sit in front of a hifi not really ideal for a bass preamp. It also doesn't include the power section which is the most dangerous part of any build.

If you really want to undertake this project do some reading there are plenty of decent sites on the net such as the valve wizard etc.

BassmanPaul · #20 04-02-2010, 04:08 PM

Quote:

Originally Posted by "Q"

you short out the terminals of any of your power supply capacitors with a screwdriver or a piece of insulated wire to discharge them. A resistor in the megaohm range is better and you wont get a bang when you do it.

You should NEVER EVER short out an electrolytic capacitor. Doing so could well deform the plates and render the capacitor useless. While using a resistor is the correct way to drain a capacitor, resisters in the megohms are way too high a value and will take too much time to drain the cap. A better value would be 10KΩ 10w. Mine is clipped to the wall above my bench and is connected to insulated leads with clips on the end. Safe and easy to use. The leads are red and black to remind me to always connect to chassis before B+. Doing it the other way round would make the other clip "hot" posing a shock hazard.

Paul