Low voltage tube amp experiment

[frank_p]

PS.: Change your name: nobody will remember what it is...  Even if it is not your "real" name... We don't care. ( Well, I don't, as long as I (and others) can remember you )

I think his name is Bradley, only some letters are transposed to numbers. I'll sure remember him now that I cracked the code! 

Greetings

Miguel

Miguel, I feel stupid to write long messages like this, just for someone asking for a vero.  I should have kept my hands off the keyboard... This was not my business, but hey, Ian is still perfecting his creation...

Anyway, sorry for the OT, Miguel "Cracker-Jack" !

[frequencycentral]

Ian, did you ever look at this? http://www.jjs.at/electronic/class_a_subminiature.html

The designer states that his NE555 based power supply isn't exactly safe, but I would love to know if using a LT1054 would be sufficient to power a small tube amp from a 12 volt wall wart.

[slacker]

Yeah I had seen that before, I didn't realise or didn't remember that he'd also gone with a Fender reverb type arrangement though, just goes to show that nothing's ever really a new idea I guess.
I like the idea of getting proper tube voltages out of low voltage wallwarts but I for now I'm happier sticking to low voltages. 

I'm pretty sure the LT1054 would work fine. I've taken some measurements and the amp its self (discounting the heaters) only draws about 10ma at about 33 volts. It also works at lower voltages, I've tested it at 20 volts and it draws about 6ma, it sounds Ok at this voltage but is a bit quieter than at 33 volts, any less and it distorts too much and in my opinion is really too quiet to be of any use.

I've only had a quick look at the datasheet so I might have missed something but the LT1054 is rated for input voltages up to 15volts and the basic positive doubler circuit can provide 50ma. Taking into account the losses mentioned in the datasheet you should be able to get about 22volts at 50ma from a 12 volt wallwart, this is more than enough for the amp. Next time I need some parts I'll order a couple and have a play with them.

Cheers to everyone for the tips on getting rid of the hum. I've solved it by adding an additional RC filter between the rectified AC and the amp.

[Minion]

Maybe try a power mosfet at the output as opposed to a NPN Transistor...The mosfet would probably give you more output current ,Probably breakup more symetricly and with an output cap you could drop the output transformer...With a strong power supply you might be able to get 10w with 36v Rails....Just a thought....


Cheers

[comfortably_numb]

I think if you set up the second triode as a cathode follower you could forgo the transistor all together.  Although, to do that the two have to be DC coupled (no decoupling cap) so that would change your tone stack placement, and putting it after the follower may make doing so useless, not providing enough current to drive the power amp.  If you added a second tube you could do this immediately following and coupled to the second triode, or you could borrow a triode from the power amp, taking them out of parallel. 

Maybe none of this meets your design goals, but a bit of information for others perhaps.  To make a cathode follower after stage two, output from the anode of triode 2 directly into the grid of triode three.  Triode three's anode should be connected directly to your B+ and the cathode should be connected to ground through a resistor between 47k and 100k (lower for more compression on the negative side of the waveform (I think).  Take the output from the cathode this time (between the cathode and your resistor) through a decoupling capacitor into your power amp.

I guess maybe I should draw that up.

[Br4d13y]

Ian, did you ever look at this? http://www.jjs.at/electronic/class_a_subminiature.html

The designer states that his NE555 based power supply isn't exactly safe, but I would love to know if using a LT1054 would be sufficient to power a small tube amp from a 12 volt wall wart.

oh my gosh! did anybody not notice this looks exactly the same as a zvex nano head? hmmmmmmmm

[ambulancevoice]

Ian, did you ever look at this? http://www.jjs.at/electronic/class_a_subminiature.html

The designer states that his NE555 based power supply isn't exactly safe, but I would love to know if using a LT1054 would be sufficient to power a small tube amp from a 12 volt wall wart.

oh my gosh! did anybody not notice this looks exactly the same as a zvex nano head? hmmmmmmmm

oh course, there was a thread about it
its not the same circuit though

[Br4d13y]

oh.

[slacker]

Here's a much better clip of the amp http://www.aronnelson.com/gallery/main.php/v/slackers-stuff/rascal/rascal2.mp3.html

This was recorded using a real mic and 1x10 closed back cab. The clip starts off clean, then with volume at about 75% and finally on full. Then at the end I'm boosting the already dirty amp with a Colorsound Overdriver.

[John Lyons]

I think that I like the Clean to slightly broken up sounds best.
The clean sound has a nice sparkle to it.
The overdriven sounds are a little harsh sounding to my ear.

Thanks for the new clip.

john

[slacker]

Yeah I'd agree with that John the lower gain settings work best. The higher gain settings are a bit rough and the bottom end gets a bit flabby, I don't know if it can be tweaked to sound better or whether I've reached the limits of what I can do with it.

[John Lyons]

The coupling caps are pretty small as is so there should not be too much bass making it sloppy down there.
Mainly I think it sounds a little grating for the higher gain settings.
Something to smooth out and gain some clarity and note definition when the volume is up would make it better.

I'll have to look at it some more.

john

[Fernando]

Hi. I'm going to try your amp and was thinking in using 4 x 9V batteries for the 36V plate and a lead acid battery for the heaters. That would make a lovely battery powered amp! (...and eliminate AC induced hum) A 2.2 Ah battery would last for more than 4 hours easy and the 9V should last about 12 hours.

I like the crunch sound but I really love the cleans of it. Making the amp cleaner would be great so one could always overload it externally (with a small BS170 booster (*) for example or your OCD, etc.)

No idea of how to make this amp cleaner without lowering the overall power...
Some ignorant intuitions :
- A more sucking tone-stack (Ampeg baxandall? Fender AA764?) may make it cleaner but lower the power too...
- Using 12AU7 for the pre maybe...
- Adding a master vol control before the 12AT7 "amp" to keep it at max power before saturation when you like? (don't know how to do it)
- supress the 33uF bypass cap?
- add negative feedback?

Any clue?

Fernando



(*) http://www.kilback.net/homebrewtweaks/pedals/cleanboost.htm

[slacker]

I think your idea for running it on batteries will be fine, it's only the heaters that need a lot of current.

I tried adding negative feedback and that does clean it up a bit at lower volumes, it's still dirty at maximum volume though. Using a 12AU7 for the power tube makes it cleaner and adds a bit of sparkle, it sounds good but I prefer the grit of the 12AT7.
A 12AU7 in the preamp sounds brighter and has bit of compression in the midrange, I'm not sure if it's any cleaner though.
I haven't tried a master volume so I'm not sure how well it would work.

[Fernando]

hi, thanks a lot for the help

Don' you think the transistor may be too easely overloaded? Maybe a vol control just before the transistor would permit to avoid that whenever you like... so you get only tube saturation.

Wich would be the correct way of controling the amplitude of the signal going to the transistor?

[Fernando]

duplicate, sorry

[Fernando]

any solution found to eliminate the transistor?

comfortably_numb did you draw your suggestion?

>>>
I think if you set up the second triode as a cathode follower you could forgo the transistor all together.  Although, to do that the two have to be DC coupled
(...)
I guess maybe I should draw that up.
>>>

[brett]

Hi
a couple of suggestions, which may not be what you're looking for:

1.  Power supply.  What about a printer's supply?  My $49 injet printer (now busted) came with a supply that gives 32V AC 400mA and 16V AC 600mA. You could probably pick up such a thing for $5 at a charity shop in the electronics section (mine sells wall-warts for 50c).

2.

Yes it's single ended class A with 2 triodes in parallel to get a reasonable volume. Push pull would probably be better but I think I'd need a proper output transformer for that and I wanted to do this on the cheap. Now I know the idea works I might try a push pull version.

You don't need a push-pull transformer at all.  Just use two taps that are equally spaced (in either DC or AC ohms) from the centre tap.  I'm a little surprised that the low impedance primary works so well for you.  That puts a low load on the output valve (which the AU7 should handle better than the AT7).  If you do decide to go to push-pull, I think you'll find that you can get around 0.2W from such a system. (ie almost as much as a Gem/Ruby/Noisey cricket)

nice work!

[DougH]

Just listened to your "better" soundclip. Sounds great!

[slacker]

1.  Power supply.  What about a printer's supply?  My $49 injet printer (now busted) came with a supply that gives 32V AC 400mA and 16V AC 600mA. You could probably pick up such a thing for $5 at a charity shop in the electronics section (mine sells wall-warts for 50c).

That's a great idea Brett, I'll have to see what I can find locally.

You don't need a push-pull transformer at all.  Just use two taps that are equally spaced (in either DC or AC ohms) from the centre tap. 

Thanks for the info, I'll try a push pull version and see what happens. I found this site that has some info about using these transformers with real valve amps. I'm trying to digest it all at the minute, but it looks like useful stuff.

I'm a little surprised that the low impedance primary works so well for you.  That puts a low load on the output valve (which the AU7 should handle better than the AT7).  If you do decide to go to push-pull, I think you'll find that you can get around 0.2W from such a system. (ie almost as much as a Gem/Ruby/Noisey cricket)

I'll be honest I was surprised it worked at all! For the output section I basically just built something that looked like an amp with out any real technical knowledge and it worked, guess I just got lucky.

Just listened to your "better" soundclip. Sounds great!

Thanks Doug.