Designing my first tube guitar preamp: a few questions

[tubeswell]

Will the the heaters only take what they need or will all 3a flow through them, killing them.

Current is like running water. Your heaters are like a waterwheel that powers an old-fashioned mill - they use the bit of current that they need to make them go and the rest flows by...

[jaywalker512]

Tubswell: Alright, thank you for clearing up my misconception there. I also got your post about how to arrange my transformers in my email, but not here on the forum (?) Anyway, if Im following you correctly, this would be a better arrangement:

[JoeM]

Why not use a PT for Fender Reverb unit? You wouldn't have to deal with the 2 transformer arrangement.

[jaywalker512]

Why not use a PT for Fender Reverb unit? You wouldn't have to deal with the 2 transformer arrangement.

Im not sure specifically what transformer you're talking about, a google search for "Fender Reverb unit transformer" gives me lots of options

So can someone verify if the power supply arrangement I posted above will (finally) work?

http://img190.imageshack.us/img190/21/powersupply.png

Also, as far as the actual preamp circuit goes, I found a schematic for an amplifier I really like:

http://www.metaltronix.net/manuals/m...eamp_schem.pdf

If I wanted to simply copy the preamp from this amplifier, would it be as easy as changing some of the values to compensate for the difference in tubes gain and voltage, but otherwise everything remain the same?

[JoeM]

Im not sure specifically what transformer you're talking about, a google search for "Fender Reverb unit transformer" gives me lots of options...

Not a transformer for driving a reverb pan. It's a PT for a Fender standalone reverb unit.

Here's one from Weber:


https://taweber.powweb.com/store/wrvbptsch.jpg

[jaywalker512]

Alright, that would surely simplify things and just let me rectify and then divide the B+ voltage to whatever I need. I was hoping to order all the parts from mouser to cut down on shipping costs but I'll surely keep that suggestion in mind, thanks

[tedmich]

don't be afraid to buy components off surplus sites,
Electronic Goldmine - Electronics, Circuits, Robots, LED, Solar, Kits & Surplus Electronic Parts
All Electronics | Electronic and Electro-Mechanical Parts and Supplies at Discount Prices
Apex Jr.Home Page
West Florida Components: Your Source for Electronic Parts and Supplies

to save money for decent iron.

Edcor has the XPWR024 120V primary, 350V(175-0-175)@60mA, & 6.3V(3.15-0-3.15)@2A
for $20 (use zener trick for lower B+)

and don't be scared; these guys are quite nice once they're convinced you're not about to burn your house down. We were all beginners once (I am still!)

[jaywalker512]

Alright, well that would certainly be sufficient. What is the zener trick you're talking about to lower the voltage? After rectification that transformer would give me about 500v, double what I need for these tubes!

and don't be scared; these guys are quite nice once they're convinced you're not about to burn your house down. We were all beginners once (I am still!)

Well I am certainly still a beginner, but you dont need to worry about me burning my house down Im definitely not about to stick some strange transformer arrangement in the wall and start messing with it without asking questions (annoying questions by now, im sure ) about it and trying to understand it first.

As far as the actual preamp, since these tubes arent completely comparable with 12ax7s, would it be a sensible idea to start off with 5 center biased, nearly identical gainstages in this arrangement:

[gstage-gainpot-gstage-gstage-gstage-tonestack-gstage-mstrvolume]

and afterwards tweak them towards the tonal result I want to achieve? A question I have about gain stages/grid input that I havent been able to find on the web yet: Is there a limit to the signal level you can apply to the grid of a tube (aside from the current limit on the datasheet, im talking about p-p voltage ie. a guitar signal or signal amplified by another stage)? Ive seen some schematics where one tubes output goes straight into anothers grid, and others where there are some pretty hefty voltage dividers in between. I suppose the amount of signal reduction between stages you employ all depends on how much clipping you want in the end?

[tubeswell]

Alright, well that would certainly be sufficient. What is the zener trick you're talking about to lower the voltage?

You put a reverse-biased zener diode (banded end/cathode pointing to ground) in series with the HT secondary centre tap and the ground return path. The zener should have a power rating about 2 x the maximum current that it is likely to see for the voltage it is dropping. (i.e. a 12V zener on a 200mA winding will need: 2 x (0.2A x 12V) = 4.8W)

[Steve Conner]

The recommended transformer will only give 500V if you use a 4-diode bridge. Use a two-diode fullwave circuit and you'll get 250V.

Yes, there is a limit to how much signal you can hit a tube's grid with. Exceed it and the distortion just starts to sound bad, like a crappy old transistor radio with a flat battery.

Your path of discovery sounds almost identical to the first experiments I did with tubes: I was trying to get the Metallica chug out of two 12AX7s given to me by a friend. It was quite a learning curve, far too long to reproduce here. If this forum had existed (heck, if the WWW had existed!) I'd have just been able to download a SLO100 schematic.

Short answer: I suggest you wire it up one gainstage at a time and listen to the results as you add stages and pile on the gain. Be prepared to remove cathode bypass caps, add oversized cathode resistors, use undersized coupling capacitors to reduce bass, throw signal away between stages and so on. Or maybe not: as you say, the tubes you're using aren't 12AX7s.

PS: It's a relief to hear that you never did try sticking any of those voltage divider arrangements into the wall.

[tedmich]

the Seymour Duncan Twin tube pedals (Mayhem, Classic, Blue) may be good designs to study, if you could get a schematic...

Same for the Siegmund Guitars Micro Tube Double Drive
which uses a 6021W tube

[R.G.]

A great deal of the commentary you're getting is motivated by wanting you to not kill yourself or burn down your house. Those things can happen in a terrifying small amount of time when you play with AC power line voltages. As a quick aside - the DC power supply for your tube plates can kill you too. We're all kind of afraid that you don't know the danger lurking inside those little bottles and cans.

A question I have about gain stages/grid input that I havent been able to find on the web yet: Is there a limit to the signal level you can apply to the grid of a tube (aside from the current limit on the datasheet, im talking about p-p voltage ie. a guitar signal or signal amplified by another stage)? Ive seen some schematics where one tubes output goes straight into anothers grid, and others where there are some pretty hefty voltage dividers in between. I suppose the amount of signal reduction between stages you employ all depends on how much clipping you want in the end?

It's a little more involved than that. A vacuum tube grid goes from near-infinite impedance in normal operation to looking like a few-K resistor as you go from the grid being slightly below the cathode to being slightly above the cathode. The clipping at that point can be as razor-edged as any transistor circuit; it's softest when driven from a low impedance that can pull the grid up even if it has some loading on the source, and sharpest/hardest when it's driven from a high impedance - like a tube plate.

It is also true that gain builds up incredibly quickly. With five gain stages, you will be hard pressed to keep it from oscillating uncontrollably from stray coupling. My dabbling with two 12AX7s was a week worth of tinkering to keep it from oscillating, and I'm not a beginner. The practicalities of wiring something up to keep it from screaming at radio frequencies when there's lots of gain and high impedance grids close together can be daunting.

The dividers and those mysterious grid stopper resistors have a lot to do with keeping things from singing uncontrollably.

[unparalleled]

Amp project with easily available parts

This amplifier uses all "off the shelf" components which you could get from Mouser or similar supplier.
You will find the site is a wealth of information, there is an article about using 100V audio line transformers as tube OP.


AVA101 The Lamington

[jaywalker512]

We're all kind of afraid that you don't know the danger lurking inside those little bottles and cans.

If it makes you feel any better, a short while ago a friend of mine lent me his Fender Super Reverb in hopes that I could fix it (crackling inputs, connections were just loose). Anyway, when I got it open, I spent at least 20 minutes draining each and every capacitor I could see, measuring to make sure they were empty and then draining them again just to be sure. Did all of that with one hand behind my back so that no charge could cross my heart as many things I have read suggest.

Im sure as hell not about to start poking around in a live amplifier or do anything I am unsure of. Hence why I have been posting so much here on every little thing I question, and you know where I'll be coming if I have any more questions (provided a google search doesnt yield me a detailed answer first). I definitely would like to survive to see the end result of my efforts

I also have the local library ordering in 2 books on tube amps for me: "Valve Amplifiers" by Morgan Jones and "Ultimate Bench Warrior" by Lee Jackson. I asked them if they could get Merlins book for me but they couldnt because it is too new, and not in any libraries yet.

[jaywalker512]

A 36V transformer could provide about 50Vdc,, which is enough to make some cool sounds with submini tubes without being quite so dangerous, and a separate 6V transformer could supply the heater.

Ive decided this would be the best approach for now, to experiment with what I have available and not be so dangerous as 250+ volts. Im just going to play with a single tube for now to learn some things. I dont have a 6v transformer though, would I be able to power the tubes heater by using 4x AA batteries for 6vDC? If so is there anything special I need to know about connecting one in this fashion, any resistors in parallel with it or will a heater in series with 4x AA batteries work just fine alone?