DIY PRE ... Help please

[molitovv]

Hello All

i have been recently working on a guitar cab,and it runs off a standard power amp, id like to build a tube pre, so my setup sounds alot more colourful than all the solid state electronics i have at the moment, anyone done anything like this or know of any plans?

cheers
matt

[brett]

Hi
preamps give quite a lot of tonal influence, so choose something that you like in an amp.
I used to like Fenderish tones, but these days I play a Marshall 18 watter and a JTM60 almost all of the time.
They have similar pre-amps -  a single 12AX7.
The DIY 18 watt site has schematics for an 18 watt "lite" which has an excellent sound.  It is kinda low-gain and almost clean, especially with low-output single coil pickups, but humbuckers or even a modest amount of boost pedal makes it come alive with distortion.  A big boost or high volume distortion pedal can be magic into such a pre-amp.

You'll need skills in handling potentially fatal 250V DC when constructing such a pre-amp, so be safe and have fun.
cheers

[molitovv]

I just want something simple and high quality, and don't worry about the HV, im an Electrician

cheers for the comment, anyone out there have any plans?

[fikri]

Try www.ax84.com they do have LOTS of things to share.

[bancika]

I have similar setup at the moment:
solid state overdrive as boost -> tube preamp -> solid state power amp and it sounds pretty cool.
my preamp is based on hotbox but I modifed it with different tone stack, has smoother sound and more compression with less distortion (that's how I like it). There is schematic and more info here as well as few sound clips
http://storm-software.co.yu/diy/index.php?project=tube_reactor
Cheers

[fikri]

Awesome bancika !
I built the hotbox too : http://fikri_ilyas.blogs.friendster.com/its_a_musical_techthing_/2006/09/the_matchless_h.html
But i like the tone control just the way it is now. So i keep it just like that. Actually i have a sound sample too. I recorded it using a marshall 9100 tube poweramp with 2x2x12 Celestion cabs. It sounds amazing. If you want to hear that, i will send one for you.

[molitovv]

a question about your tube reactor schematic, someone else might be able to answer too. is it important to use a single transformer with 2 secondaries. because i have a separate 220 volt and a 9 volt transformer available.

cheers
matt

[bancika]

nope, you can either use two transformers for separate heaters and HV or you can use two transformers wired back to back. Forst lowers voltage to 9V (or so) and second rises it to 220V so you need either two 220->9V transformers (common, at least in Europe) or 110V>9V and 220V-18V if you're in US.
Cheers

[Rafa]

Im interested too, can I just take a tube amp and build just the preamp, I guess the answer is no, so what mods would be needed maybe something like a bogner shiva would be great??
Thanksss
Rafa

[Rafa]

This on looks nice, but the poer supply    ??
Cheers
Rafa

[bancika]

yes you can take only a preamp of any amp but it doesn't mean you'll capture tone of the amp 100%. Tube power amp also gives character and in many cases little more distortion.
As for this DC30 I don't know but something similar to hotbox (or my tube reactor) will give you B+ of 250V which should be fine

[molitovv]

why is that bancica, from what i can tell from the schematic, the 6v is only used for indicator leds. and by the way we are on a 240v system here. so there are lots of those transformers available.

also... would 1 power supply be capable of powering 2 tube reactors?

cheers
matt

[bancika]

6V is used for heaters also, that is not drawn on schematic. You can use same supply for powering more than one but:
- 7806 regulator is rated at max 1A, two tubes and two ultra bright LED-s can draw up to 650mA. If you double that you get about 1.3A which is too much for 7806. You need either two regulators or one with higher current rating, if it exists
- HV secondary I use is 20mA which is more than enough. One triode draws about 1mA, so two tubes will draw about 4mA. You're totaly safe with two preamps. But, more current means less voltage. So if you use same power supply for two reactors you'll get lower anode voltages. I can't tell exactly how much, but you can expect 20-30V. So you need a bit smaller supply resistors. I can run simulation in Duncan PSUD if you want to determine resistor size. Also, I'd suggest increasing PS filter caps for at least 50%.
By the way, I'm very pleased with my reactor. It's the most quiet pedal I've ever built (noise-wise), thanks to DC heaters, separate power supply unit, shielded cable and taking care about component placement.
Cheers

[molitovv]

Hi Again Bancika

Cheers for all your help. i would quite like to use the same transformers for both units and have separate rectifying circuits. i have 2 x 240 ->  9-0-9 transformers available, do you think you could please work it out for those transformers, it would be muchly appreciated. i have already ordered the valves and they are on their way. and have dug out a 2u rack case for it all.

looking forward to hearing your great circuit

matt

[Rafa]

Hi:
Well guys you are scaring me, I wanted to built one with less transofrmers as possible, One would be perfect 
Is that possible, my country is 220/240V AC directly from the plug in the wall I dont remeber how you call it english.
Thankssssss
Rafa

[bancika]

Hi,
rafa, you can use just one transformer, like hammond 369EX, but two transformers back to back are way cheaper. Using directly mains voltage for powering tubes is dangerous and I wouldn't recommend it.

molitovv, I don't have PSUD here, but tonight I'll be home and try to work out resistor values. I guess that using two preamps with 240VAC at the start would give just about right voltage with existing resistor values. You'll have more current consumption but you'll also have 20VAC more than I have so you'll stay in a ballpark.
I'll get back with exact values later. I assume that your mains voltage is 240VAC?
What's power rating of those transformers? I'd suggest using 1A or more.
Hotbox is great circuit to start with. There's lot's of place for experimenting. I used perf board with enough distance between components to let me swap them, add ones in parallel/series etc to find my sound.
Cheers

[molitovv]

Hi again, i would really appreciate it if you did do the maths for me. we do have 240v at the wall in most places. and those transformers are 2.2amps so they should be fine.

thanks again
matt

[bancika]

PSUD cannot work with multiple rectifiers at one transformer, so I couldn't make any calculations regarding that.
But, if you use single rectifier both resistors at 10K would give voltages close to mine according to simulation. Anyway, I think it's better to have only one power supply: takes less time, money and space. Maybe you shouldn't increase filter caps at all. I already increased them compared with stock (stock version has only one 22uF for first tube and two 22uF caps for second). My version has 47uF+47uF for second tube and 47+47+22 for second. Nice idea is to add 0.22uF poly film cap (not electrolyte) rated at 400V or more in parallel with last filter cap. It should improve the sound, at least I heard so.
Cheers

[Rafa]

So any power supply that gives me 240VDC would be fine??
I dont understand why a transfromer if the plugs in my house are 220V so if I turn AC into DC with a much smaller and cheaper circuit like a rectifer, should work or not??
Cheers
Rafa

[bancika]

not really 240VDC. My 220VAC input gets rectified to 220*1.41~310VDC which then gets filtered by RC (resistor-capacitor) network and gets to ~250V at second preamp tube. One more RC filter and it's 240V. All my voltages are under load. You can get away for 190VAC at the start but you'll need smaller filter resistors (which will filter a bit less, but it's ok). Hammond 369EX is universal transformer that has 190-0-190 secondary. It can work here.

I don't know enough to thorougly answer your second question but everybody says it's a *NO-NO for safety reasons..
Cheers