New low voltage tube amp project. Runs on 12v walwart 62v inside. not to scary.

[solderburn]

Thanks again zambo!

In the spring I'll put finish on it. If i can remember to.
Now hears a gutshot (sorta). hard to see because of the multiplier caps. all of the parts are placed randomly as you can see. this is my third or fourth project. first in a long time actually.

WARING TERRIBLE SOLDERING SKILLS IN FOLLOWING PICTURES

[sajy_ho]

Hey,
why don't you use a Max1771 circuit instead? 240V and it's pretty loud;

I made one, and it's too loud to crank at night, but headroom is a little better.

take a look:

Based on Frequency Central Obsession amp

Nice, Unfortunately I can't find MAX1771 anywhere! In the LT1054's datasheet was mentioned that it can deliver 100mA too; so why not design a SMPS with that instead of MAX1771?

[zambo]

Solderburn- I love it! I am glad you got it working! I love the enclosure! Thanks for posting the pics  Glad to give back!

[zambo]

@ Thomasha, Thats a sweet sounding amp! Ive built them around 200 volts with 1363 a lot of times and various versions and spinoffs of the firefly and 4-4-0. all of them are to loud. great headroom etc. but i always end up turning them up to the its to loud point. I saw Frequency Centrals post about turning your valvecaster into an amp. he stated voltage equals volume. I have lived by this mantra ever since. Most of my bigger amps (18 watts usualy) I build with a vvr inside to drop the dc rail for a master volume. This one seems just loud enough to be usable at night or around people talking etc. about like an acoustic guitar.

How many ma does the 1771 put out? I would love to try and build with one! I may try a push pull 6v6 amp with the LT1054 just to be ridiculous  I have one I can wire it into just to see.

[zambo]

here is a gutshot of an incomplete one. I will try to post a done one pretty quick here. 

[zambo]

I still have to solder in a bright cap on the volume pot, but here it is right before testing it.

[sajy_ho]

I still have to solder in a bright cap on the volume pot, but here it is right before testing it.

Is it a new one?
I had a question; Why to use 22k OPT? two sides of 12AU7 are in parallel connection, so each of them will see twice of the load impedance. I have a 7k-5R OPT from an old Grundig radio, if I use it with 8 ohm speaker it'll give me about 11.2k load impedance; so can I use it instead?
Thanks
Sajad

[psychedelicfish]

I had a question; Why to use 22k OPT? two sides of 12AU7 are in parallel connection, so each of them will see twice of the load impedance. I have a 7k-5R OPT from an old Grundig radio, if I use it with 8 ohm speaker it'll give me about 11.2k load impedance; so can I use it instead?
Thanks
Sajad

22K:8r is a (relatively) common transformer, it comes in the form of a reverb transformer. I suspect this is why Greg used one. At this low of a voltage, you don't really need to worry much about what load you use, so your radio transformer should be fine.

[sajy_ho]

22K:8r is a (relatively) common transformer, it comes in the form of a reverb transformer. I suspect this is why Greg used one. At this low of a voltage, you don't really need to worry much about what load you use, so your radio transformer should be fine.

Oh, now I get it! so as long as my load line does not go far the tube limits I can use any transformer.
Thanks man.

[zambo]

Exactly! At this low of a voltage you could pretty much use a meatball and some duct tape and get it to work ( grass fed organic beef of course ) . As I said before I used a small audio trans from radioshack that is 100k to 8 ohm and it worked fine. 3 bucks well spent? Your radio trans will work great. Use it in push pull like on the firefly and it will sound as good as any of these little tiny amps. You can also get crazy and make it with an eq etc. it does work. another tube stage would help that too. This amp that i put gut shots of was finished last night. it sounds pretty good. I put a 250pf bright cap across lugs 1,2 of the volume pot. I also added 2 10uf reservoir caps to help the sag. it worked well. I will post some vid clips later if i can get to it.

[sajy_ho]

Hi, it's me again; with another issue! I've built the PSU and added some more stages to get about 100 volts. My problem is finding a SE OT, I'd just found a PP transformer around,(I'm living in the middle of nowhere!). So what do you think? Can I use this PP transformer (7k-5R rated at 40W and weights like a ton!) in this parallel SE design? or the transformer's core will sature and make noise?
And I don't know how to bias a firefly type self-split output stage, so I caould go for a PP stage.
Please share your knowledge.
Thanks
Sajad

[Will.mendil]

Might have been asked but what's the OPT input?

[Mustachio]

I believe they are referring to the Out Put Transformer. Some of the other terms are PP -push pull transformer and SE- single ended transformer.

lil while back I found this guy on ebay selling what looked like decent transformers for a decent price

http://stores.ebay.com/musicalpowersupplies/
http://www.musicalpowersupplies.com/ordering/

I haven't tried any of them yet , so far I've only built a superfly with a hammond 125b.

[Will.mendil]

So what's the Doreen e between OUT And OPT?

[zambo]

Hi, it's me again; with another issue! I've built the PSU and added some more stages to get about 100 volts. My problem is finding a SE OT, I'd just found a PP transformer around,(I'm living in the middle of nowhere!). So what do you think? Can I use this PP transformer (7k-5R rated at 40W and weights like a ton!) in this parallel SE design? or the transformer's core will sature and make noise?
And I don't know how to bias a firefly type self-split output stage, so I caould go for a PP stage.
Please share your knowledge.
Thanks
Sajad

Hey Sajy, use it. if its there just do it and see. i am sure it will be fine. big, but fine. use a 390 to 500 ohm cathode reistor. if you have a 1 ohm resistor put it in series with the power tube cathode resistor ( 1 ohm first then 390-500 ) . you can measure across the 1R in millivolts and that number is the milliamps your drawing. you can figure the bias from that. just use the firefly self split design and you should be fine. just follow the schematic. I believe it uses a 440 ohm resistor for the power tube cathode.that should be great.

[zambo]

@ Mustachio- I have heard those are decent transformers but never used them. I ccaros has and he makes good stuff.

@ Will- I dont know what Doreen e between input and output means. Sorry, Im may be dumb on this one.

[sajy_ho]

Thank you Greg; I'll go with the Firefly and when finished will report the results...
Thanks again
Sajad

[jazbo8]

Thank you Greg; I'll go with the Firefly and when finished will report the results...
Thanks again
Sajad

I think the 7k:5R OPT you found would be an overkill for this project, see if you can find/salvage a small mains transformer say 120-0-120V (for 240V mains) to 5V or something close it, this will give you a good match for the 12AU7s and be much smaller than the 40W OPT.

[sajy_ho]

I think the 7k:5R OPT you found would be an overkill for this project, see if you can find/salvage a small mains transformer say 120-0-120V (for 240V mains) to 5V or something close it, this will give you a good match for the 12AU7s and be much smaller than the 40W OPT.

Hey jazbo8, thanks buddy; That OPT is so old and the writings on it's secondary taps has been removed; the only writing has remained on the primary side says 6k.
The other day I measured  turn ratio of that OPT using 3.5V ac on secondaries and measured 350V p-p, and 175V from center tap to each plate!!
The numbers are so extraordinary; so using 8R speaker, it gives me 80k P-P and 20k P-CT. So what do you think? That iron is faulty or it was designed for an exotit speaker!?
And also looking at its huge size I think it's core will not even get close to saturation with SE operation, so can I use half of the primary side(20k P-CT) for a SE amp?
Thanks
Sajad

[jazbo8]

100:1 turns ratio! That's an odd one, wonder what it was used for? There was an interesting project using a PP OPT in an poor-man's UL amp (a la Dumble SSS) over at TAG that might be worth a look, with 40W you are not going to saturate it easily... another project for you!