DIY PRE ... Help please

[molitovv]

ok then, im going to run with that, see how we go, cheers for all your help

matt

[vanessa]

I would just add that a preamp that has an active eq will be more advantageous than one with a passive eq. When you get into the realms of tubes a tube pre with an opamp based active eq will yield far more tonal possibilities than a passive eq and (with the right chip) with low noise. I'm not sure what the real advantages of running a passive eq into a passive eq (guitar amp) would be other than introducing more signal noise to your signal chain and or muddying up your tone. I would just build the a simple tube pre section and leave off the passive eq in that case. I would design it with an active eq and have the ability to switch it in pre or post preamp.

[bancika]

passive eq will always make less noise than active one. Active has all parts like passive plus amplifiers.

[Rafa]

So the preamp is designed to work with 240VDC but with 190VDC it still works and is less dangerous, sounds interesting??
Ok, the hammond is good but its not availble in my country and shipping +transormer will be too expensive, do you know where can I find a power supply schematic that suits my needs (for the matchless DC30 preamp) if its 190VDC even Better??
Is the matcless DC30 an imitation of the VOX AC30??
Cheers
Rafa

Bancika I want to says thankssssssssss a lot to you since youve helped me a lot with the tube preamps and powersupplies
Sorry for my english

[vanessa]

passive eq will always make less noise than active one. Active has all parts like passive plus amplifiers.

Put that way you're right. I meant more in the form of mud. But given the right opamp it can actually clean up the signal a bit.

The question I pose is why would you want an extra passive eq if you already have one built into the amp? Active eq's can boost and cut frequencies giving you a much wider range to sculpt your tone. It's not something to be afraid of. You're not ripping that passive eq section out of your vintage Marshall Plexi. An active eq in a small external tube preamp can add a lot more function than the same with a passive eq and like I said you could have it able to switch off, or injected pre/post preamp for even more possibility's.

[bancika]

rafa, you don't understand man. You'll get 250VDC anyway at tube plates. I'm talking about AC voltage you get from transformer.
If you can't find suitable transformer thantwo transformers wired back-to back are excellent and cheap solution. What's mains voltage there? 110V, 220V..?
I don't know about vox/matchless, not much into that.

vanessa, don't get me wrong, I have nothing about active eq's, nor circuits that aren't tube based. I enjoy boosting my tube reactor with solid state overdrive. Having EQ (either passive or active) both in front and after preamp is great thing. But I don't agree that EQ is not needed here. What about putting that preamp in front of amp who has not tone control or has simple one knob? I'm building fender princeton with only one knob and plan to use it with reactor. Or what about direct recording guitar->preamp->speaker sim->PC?
And I don't agree about mud thing. I mean, why does evryone play on muddy amps fike marshall, fender, mesa boogie, bogner and virually all others who have passive eq?
Cheers

[vanessa]

And I don't agree about mud thing. I mean, why does evryone play on muddy amps fike marshall, fender, mesa boogie, bogner and virually all others who have passive eq?

We're talking about external preamps right? I would bet good money that if Leo Fender had the choice between an active eq or a passive eq to put into the first Fender amps or the Bassman for that matter, (and the cost of parts and labor were identical) I'm positive that he would have opted for an active eq over a passive. It would have been a better seller and he would have known that. Why? You can dial in way more tones than a passive eq. Get a little Marshall with your Fender, or Vox, etc... It's not amp modeling or speaker simulation by know means but it's the next best thing.

[Rafa]

Hi again:
Ok lets see if im right this is WHAT I THINK A POWER SUPPLY IS:

220VAC at the plug in the wall of my house ( enchufe in sapnish)------>transformer-------------------->310AC------------------------->Transforemer---------------->240VAC---------------->rectifier---------------------------->240V DC which is waht the preamp needs.

Im actually reading allaboutcircuits.com book but im still in the DC part so I dont understand much about Ac now but soon I hope I will.
The main voltage here is 220V
Furthermore do amps work with less voltage at the tube plates than the recommended one??
Thanksssssssssssssss
Rafa

[bancika]

Nope, here it goes:

220VAC (house) -> first transformer -> 9VAC -> second transformer (and tube heaters) -> 220VAC -> rectifier -> 310VDC -> PS filter -> 250VDC -> tube plates

when you take voltage down to 9VAC with first transformer you use that 9VAC to rectify them, regulate to 6VDC and power tube heaters but also you use that 9VAC to power second transformer (same as first, 220V->9V) which is turned back (secondary is used as primary) and it will increase voltage to 220V

Any clearer?

And tube amps usualy work at higher voltages than recommended. For 12AX7 MAXIMUM voltage is 250V at plates. Some amps (mesa dual rectifier) has over 400VDC at preamp tubes.

[Rafa]

OK THANKS A LOT BANCIKA NOW EVERYTHING IS MUCH MORE CLEARER.
Another question where can I get a schematic for that, The hotbox has only one trasnformer. Then if addpat that power suply for two transformers, when matching both transformers I should put some rectifier in the middle like in the real mc tube (for the heaters), and then the another rectifier and the PS so Its to complicated for me to design all that. Or maybe using the same power supply as the real mctube and changing the caps voltages, the  resisitros wattages and the transformers ??

Do you think the matchless would be a good idea?? The soundclips at the page sound amazing but I think the schematic is not verified, maybe to start the real mactube would be a better idea,even though the clips Ive heard are horrible (sounds like Creendence on ¨HEY TONIGHT) and it does not have a tonestack its chepaer and easier, and I think its verified.
Another question in the  hotbox there are two outputs from the power supply one is  242V and the other 250V so whats the difference?? 242 is mor less the same as 250
Thankssss alot
Rafa

[bancika]

Hi,
nice to hear that ou're getting into it.
I haven't built real mctube but as I've heard it's very much fuzzy which can be done with few transistors instead. Power supply is similar so you can use it as starting point. It uses two 110V-9V to get 110VAC into PS, but you can use the same with two 220V->9V to get 220VAC. The rest is same. I suggest using voltage regulator and large caps for heaters.
If you don't need tone stack you can just use one 0.022uF or 0.047F cap instead of whole tone stack.
To be honest, stock version of hotbox is little fuzzy at high gain settings. Mods I've done  are there to make sound smoother. Depending on what you need go with stock or modified one.
As for 250-242, it's just two voltage stages, usually called B+1, B+2, etc. With amps highest voltage goes to power amp and they reduce a bit for each tube going to preamp. Here we have the same just without power amp. Second tube gets B+1 which is 250V at plates (both triodes) and first gets B+2 = 242V. Get it?
Cheers

[molitovv]

hello again,

one question, im just laying out the design of my front panel and i was wondering, does the gain knob only control the dirty channel?

also for vanessa, my amp doesnt have tone controls so im kindof excited about the ones on the tube reactor

cheers

[vanessa]

my amp doesnt have tone controls so im kindof excited about the ones on the tube reactor

I understand. I'm just looking at the parts count and for a little (I mean a little) extra work you would have a more versatile preamp at your disposal (and with amps that you may get in the future that have built in tone controls).

[bancika]

yes, gain only affects dirty channel, as well as EQ. So clean channel only has master volume knob (that's why I put different knob for that one one my build). It's very neat that way because you need separate EQ's for clean and dirty playing. For instance, when I tune my EQ to scooped mids, pronounced bass it would sound bad with clean. Anyway, I don't think clean needs EQ anyway, it's very nice stock
Cheers

[molitovv]

Hi again, i thought i might throw this in the mix as an after thought, i have a heap if balancing transformers available,

how would i go about making a balanced output for this pedal? can i use a balancing transformer straight out, or is than not possible due to impedance or other such issues?

Matt

[molitovv]

Bumpity Bump Bump

[Rafa]

Hi:
I wacthed te real mctube power supply and the caps are rated 160V, so I should change them to 300 or more(also change some resistors) , furtherome I kept on looking for other preamp designs at scheamtic hevaen, and the Matchless is still the best, all others were too complicated or I couldnt separate the preamp form the poweramp, so I should find out If that scheamtic is accurate. The tubes apparently here in a shop the 12ax7 are at US$12 and at Buenos Aires at US$13 which I think is ok. Do you think the matchleess should work properly with 250V isnt that risky, I mean maybe its too much and everytihng burns up? The heaters work with 6VDC or 12VDC?
Cheers
Rafa

[bancika]

13$ is little too expensive. In europe 12AX7 costs about 5euros=6$.
Don't worry, 250V isn't much for 12AX7 at all. Many amps run them at much higher voltages. As for filter caps, I recommend using at least 350V. It's better to be safe than sorry. Mine uses 450V for all except last one which is 350V.
Hotbox is great because it can easily be modified. It's cuircuit which looks very like marshall JCM800 preamp. You can get pretty much any sound you like (except for death metal stuff) from it with a little fine tunning.
As for heaters, you can use either 12V or 6V but with 220V->9VAC transformer you won't be able to get stable 12VDC to power heaters. It will be better to use 6V. Connect 6V to pins 4 and 5 and ground pin 9 to star ground point. Take a look at how I did my grounding here

My power supply is in different enclosure, so I take 4 cables from power supply to preamp. One is 250V, one is 240V, one 6V and ground. I use speakon jack (4 pole) to connect it. See on the left side where I connected all grounds to one point (those blue and white wires). I connected one from ground buss (that wire on board), heater grounds, LED grounds and it's screwed to make contact with enclosure. It's important to have enclosure grounded but only at one place.
This photo shows it little better

Ask if you need any help
Cheers

[molitovv]

hi again, bancika, do you think it would be possibel to add ballancing transformers to the output of this preamp?

[bancika]

why not, you could add speaker simulator after tone stack and then make ballanced output. I don't know much about it cause I don't have any gear with XLR so I don't know anything about ballancing