Would this work? Tube Amp Question

[doitle]

I was looking at FrequencyCentral's Murder One amp for some time now thinking about how a simple tube amp could be made without a fancy power transformer. I came up with this:



Could this work? I realize I removed the 1:1 Isolation Transformer that should probably be in there but in this configuration I could get 170ish volts for the B+ and 12V to run two series'd filaments and then just drop 6ish volts for the single filament on the El84. I saw that the Fender Reverb Transformer on the Murder One will also work with an EL84 at least according to the site that was selling them. Obviously there are no values for most of the amp or the biasing. I hope to learn more and eventually be able to do some more tube amp design calculations but just as an overall idea could this work?

PS: Ooops looks like I drew the volume pot wrong... it's obviously not just a resistor to ground but it should be a pot with the wiper going to the grid of the EL84... Regardless that's not really the important part of this.

[sean k]

Well you can as they used to wire up some radios with 50V filaments in series to get the eventual 120V and run the B+ as you have but for the price of a couple of 120/12 transformers back to back it's hardly worth the risk. you've already got one 12V transformer there anyways to run your filaments so why not go and get another and the whole thing would be tidier anyways.

I'm not saying not to do it... it'd work but the risk involved is in the being silly to save a few bucks territory and you'll have possible ground loop problems because you're ground on your earth return and not the third prong which is earth and this may see some current on the chassis which isn't a good idea. I think 

Wait for the heated No's!

[doitle]

This is by no means a final or even rough draft. This is a napkin sketch of something amp looking with the filaments on a 12V adapter and the Plates on rectified AC. For sure with an isolation transformer and maybe with a different power transformer but I'm trying to avoid having to buy a specialized "Guitar Amp" power transformer which drives the cost of the amp up a ton. If I wanted to throw a 12ax7 into an el84 run into an old cabinet I have just to play around with this stuff I could probably do it fairly cheaply. Throw one of these guitar power transformers I see online in and the price doubles. I'd like this to be a relatively safe project and will keep trying to research the way to do this without adding any dangerous or dubious engineering. That's what I was looking for was just some suggestions to lead me in the right direction as far as research and planning goes.

[sean k]

Well you can do doublers and triplers from 50 to 80V transformers but even they are quite expensive new but they are easier to find second hand or surplus than Tube stuff. Old radios are always good for SE in low watts but I guess you already know about those.

Whats a EL84 draw in current. I can't remember 35-45 mA maybe at 300V. Thats about 12-14 watts so a 60V on a tripler would give you about 250V and that 60V would want to supply at least 200mA for the B+ and two 12AX7's and an EL84 in series equals 30V so if your 60V had a centre tap you could run the filaments off that but it's a bit dicey just using one half for a high current draw. Maybe a thirty volt trannie at two amps on a, well, six little capacitor diode pumps.

[doitle]

I made a revision to show how I think the transformer section should work.



I was also doing some research and thinking I might replace the wall wart with just a 120V/12.6V transformer with a coil tap. That way I could feed the two filaments of the 12AX7 in series and get the single 6.3V required for the EL84 more easily. This configuration would be using AC for the filaments but they can certainly be run that way it seems. AS a very first foray into tube amp building I wouldn't be suprised if this thing was a bit noisy so I certainly wouldn't be disapointed by some hum. If the thing makes any sound at all other than an explosion I'll be so proud.

I may just redraw this since it started out looking messy and now looks REALLY messy.

Also I was researching some slightly more exotic tubes: 6D10 or the 6AV11...

They are triple triode tubes. They would allow me to get a little more gain out without adding a a second preamp tube. I'm not sure how good they sound but it might be a fun experiment too! They may have been used in guitar amps before but I couldn't' find any sites detailing anything about them other than for TV use.

[Jered]

  The 6AN8A is commonly used in amps and is a pentode/triode. Might be worth looking into.

[doitle]

A triode/pentode? Interesting. Almost like you could make a simple clean tube amp with just one tube. Sort of like those pocket amps only with tubes. Well tube that is. I will be su re to look into those when I'm researching some more. Any other interesting tube suggestions would also be welcome! I'm not looking for any one particular sound so it doesn't have to be any style of tube amp. I was just looking for a very simple design to learn off of.

[GibsonGM]

I like the 2nd drawing a lot more, having the tranny for isolation.  Any other way and you risk being dead from an electrician's miswiring of an outlet, regardless of how well YOU build the thing!
The back-to-back dual tranny thing, a la McTube, works very well.  You can connect 2 RS transformers that way, cost about $25-$30, and get your 120 (or more, if you get 25V transformers) - and it can be 'pumped up' with diode/cap doublers.   Feeds the filaments AND provides isolation.  I have one that's giving me about 100V right now, going to do the doubler soon to see if I can heat things up a little, ha ha. 

[Boogdish]

If you haven't looked at it yet, I'd suggest reading through the Valve Wizard site.  It's a great resource for tube amp design:

http://www.valvewizard.co.uk/

[brett]

Hi
I think you should take your time with this.

Firstly, you can't use a power transformer in this way.  You need a "gapped", single ended transformer for a Class A amp.  You CAN use a power transformer in a "push-pull" Class AB amp.

I assume that you understand that the fuse doesn't stop mains spikes or protect you from electrocution.
Running direct from the mains is a complete mistake.  Besides being illegal in most states and countries, the mains contains spikes of hundreds of volts.  Without a large inductor (ie mains transformer) in the way, your amp will be short-lived.  Maybe you will be, too.

Running transformers back-to-back (a la McTube) can work, but is best for low power applications.  You really should buy a power transformer (maybe give up the booze for a week, dance at a friend's hen's party.....do whatever for the $$$).  And an SE output transformer.

In fact, you can get a second-hand Epi Junior (or similar Chinese valve amp) with a similar circuit to yours for about half the cost of the parts needed for this amp.  Then you could have all of the fun of adding a tone section, cascode boost, power soak, whatever.  All in comparitive safety and learnign a lot about valve amps.

god luck

[doitle]

Did you not see the second schematic? I'm just wondering, I didn't think I was running "Direct" from the mains anymore. I certainly will be taking my time with this. I really want to get everything planned out beyond a doubt before I purchase anything and make sure I totally understand the amp inside and out. Also I was hoping to keep this very low power. 1W would be more than enough. Right now I've got a Roland Micro Cube and it is more than loud enough for me.

As for the money the problem is mostly the lump sum nature of it. Every paycheck I can buy something little, maybe that Fender Reverb transformer for output one week at around 20$. Then a EL84 tube at 15$ next paycheck. Buy some big caps the next paycheck, get the sockets the next. The transformer looks to be upwards of 50$ though before shipping. That's what has been stopping me from planning on a real "amp" power transformer. Same problem with the valve junior. I see the heads come up for 150$ on Craigslist but that's too much for me to just throw down on an amp in one lump.

I really wish we had gone more in depth in my electronics courses. They were pretty barebones: This is a PNP BJT. This is the Common Emitter configuration. This is the Common Base Configuration. Here's the frequency response of this one. Here's the frequency response of that one. This is a NPN BJT. Repeat. This is a NMOS Mosfet. This is CS amp. This is a CE amp. This is a CG amp. Here's the frequency response. This is a transformer... (ooh!) You can find the voltages by taking the turns in a proportion. Onto IC cmos amps... I guess there's just too much information to cover. Although I need to bust out that Sedra and Smith Electronics book and see what I can learn extra come to think of it!

[Boogdish]

If you're looking for deals on transformers, I've been able to get good deals from ebay:

http://electronics.shop.ebay.com/items/Transformers__W0QQQ5ftrkparmsZ66Q253A2Q257C65Q253A10Q257C39Q253A1QQ_dmptZVintageQ5fElectronicsQ5fR2QQ_sacatZ67815QQ_scZ1QQ_trksidZp3286Q2ec0Q2em14QQ_sopZ1QQ_scZ1

You just have to know what you're looking for, be willing to sift through a lot of crap and have some tolerance for risk with your money (which isn't always a great quality to have).  I'm finishing up an amp this week that I think I paid $50 for my power transformer and output transformer together.  Another tip is to keep an eye out for "broken" amps.  You might be able to get a chassis/cabinet/transformers/sockets at once for $50.

Also, you're correct, with the isolation transformer, you're no longer going to be running direct.

[brett]

Hi
sorry for sounding paternalistic.  I was trying to help, and was concerned about your safety.  Although I looked at the second schematic, I figured that there was a key piece of information in the first schematic: that you were considering running an amp from the mains.  That's why I suggested something less dangerous to start.  I was lucky when I started - I got lots of advice from people here in the forum.  When I asked about using AC from the wall (here in Oz, the wall gives 240 V, which I figured was perfect for a valve amp) I think it was R.G. who said "stop right now and do a lot more research". 

I ended up buiding a Firefly.  A Firefly would be a great amp project for you.  Unlike the EL84, there's a good chance that you can use a PT as an OT, because it uses the two triodes in a 12AU7 in push-pull.  As I said above, you simply cannot use a PT as a class A OT.  This is because it has significant DC flowing in it, which saturates the core (SE OTs have a "gap" of paper or air that block DC.  A power transformer has an endless core that forms a saturated electromagnet under DC conditions). 

I built my Firefly with small back-to-back transformers (240:12:240), and if you are only after 1 W you could also use such a system. 120:12 and 6.3:120 would give you about 220V AC and 300V DC (1.4 x DC.  Because 1.4 is the ratio of the peak to the average absolute voltage of a sine wave).  You can also use the first transformer for the heaters (12V DC from an LM7812 regulator works well.  Or search for getting 12.6 V from a 7812 with just one diode extra).  You'll probably want about 10VA for the step-down transformer and 3VA for the step-up transformer.

good luck

[doitle]

Ah I will definitely look into the Firefly then. I didn't know that it was a push pull power section. I've already worked with the 12AU7 a little on the valve caster. Thank you for the extra information on the transformer and the difference between a gapped and non gapped one. As I said we really didn't cover transformers further than this is what a schematic symbol for a TF looks like. They transform voltages.

Off to do more reading!