True!
Though an actual audio transformer would surely be better? I'd planned to simply use a lead acid battery for the b+ and filament.
Hammond have a 119da which looks ok?
Hammond Mfg. - "Classic" 600 Ohm - Speaker Matching Transformer - Hammond P/N 119DA
Though an actual audio transformer would surely be better? I'd planned to simply use a lead acid battery for the b+ and filament.
Hammond have a 119da which looks ok?
Hammond Mfg. - "Classic" 600 Ohm - Speaker Matching Transformer - Hammond P/N 119DA
Yes, but one can re-use decent iron in other amps/projects.
This is a small experiment to show my son what a circuit is, how tubes work & more importantly, operates at low voltages....
Drive a small pair of sensitive speakers in a child's bedroom.
These are space-charge tubes, more of historical significance than anything else.
See here for a good outline of a successful project (use google translate if necessary as it's in Spanish).
Mini Amplificador valvular | Inventable
Edcor
I have a pair of 600:8 I got from Edcor (off the shelf design). For an additional $25 they'll make it 680:8 if you really need that.
I have a pair of 600:8 I got from Edcor (off the shelf design). For an additional $25 they'll make it 680:8 if you really need that.
Headphones. Or make it a push-pull amp. like thie one in this radio:
An externally hosted image should be here but it was not working when we last tested it.
You might get a whopping 100mW.
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Don't headphones typically want to see more like 300 or 600 ohms - we'd need a different output transformer, surely?
I might try the spanish circuit first - can anyone convert these files to gerber files?
Tom,
There is a .BRD file in the zip file.
That is an EAGLE PCB layout File.
I think that the freeware Eagle program should be able to create the gerbers from the .BRD file.
Might be worth downloading it for a play.
Freeware | CadSoft EAGLE
Cheers,
Ian
There is a .BRD file in the zip file.
That is an EAGLE PCB layout File.
I think that the freeware Eagle program should be able to create the gerbers from the .BRD file.
Might be worth downloading it for a play.
Freeware | CadSoft EAGLE
Cheers,
Ian
I built this amp, after trying and failing to get much interest in it from others. I was, to put it bluntly, somewhat underwhelmed with the results.
Like you, I faced a quandry on the OPT, so I ended up buying a set of these tiny things (Radio Shack Model #: EI-19):
Radioshack - Products
It's a 1K center-tapped primary to 8 ohm output transformer, super-small, and very inexpensive. Clearly not optimal, but acceptable for the intended use. I believe they are rated at 100 mW, but that's fine for a 35 mW output tube, I think.
You might also take a look at cheap 70 volt line transformers. They can be used as a poor man's output transformer. No air gap, so not good for single-ended use, but with 35 mW output, I don't think you'll have to worry much about saturation issues. And did I mention cheap?
Anyway, output volume with mine was VERY low. Perhaps I did something wrong, but I checked the schematic carefully and I've built a few tube amps, so I have an idea of what I'm doing (I am still a newbie, though).
I have since moved on. I still like the idea of low-voltage tube amps for fun (not for hi-fi as such, just for fun). But I don't talk about it much, because people get sniffy and tell you to build a 300B amp instead. Lots of attitude, so I keep it to myself now.
FWIW, there are a couple ways to go with low-voltage tubes if you care to explore. From line-voltage on down, there are many stops you can make, with varying degrees of success, difficulty, and output power. I've built with 117P7GT tubes (a half-wave rectifier and a beam power pentode in one tube, powered by a simple isolation transformer 1:1), and I've experimented with lower plate and grid voltages on other non-space tube types. I'm still playing around. I've found that tubes which have a high heater power draw often work pretty well with lower plate voltages. How linear they are, I have no idea; I don't put them on the scope, I just build them on a piece of wood, listen to them, and take them apart again.
That would echo the sentiment from most of the reports on the web of this circuit!
I think, given, the difficulty in getting a decent output transformer and the fact the circuit would require some exceedingly sensitive speakers, perhaps it is best to re-design this circuit as a headphone amplifier?
I have the tubes to build it, and it's a rather simple point to point exercises, so what output transformers would be suited to driving some headphones? If the output impedance of the circuit is 690R, then potentially we could drive some 600R headphones (eg Beyerdynamic DT880) without any transformers?
Well that is a better circuit!
Increasing the B+ gets more current going at least, and the filaments can be run in series. Plus it is a more standard output transformer spec.
Probably need some >100dB speakers to get a decent sound level.
Where did you source this circuit? Anyone actually built it?
Increasing the B+ gets more current going at least, and the filaments can be run in series. Plus it is a more standard output transformer spec.
Probably need some >100dB speakers to get a decent sound level.
Where did you source this circuit? Anyone actually built it?
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