1 watt tube amp

[Cliff Schecht]

What formula did you use to obtain figure?

Take your 120V RMS, double it and multiply by 1.414. You get about 340V so I assume Martin is accounting for some losses both in the PT HV windings series resistance and in the circuit itself. 310-320V seems about right.

[9VOLT1967]

I am trying a simplicity on this project. Hope to get a decent sound with some distortion for a micro amp type layout. Small with a nice punch running thru a 6-8" speaker. Thanks for all the great feedback!

[selloutrr]

The firefly is a great little amp. I added a dummy load to the output Jack using a shunt Jack. As well as a line out Jack with a volume control. It makes a great distortion box as well as a rocking little studio amp. My daughters is pushing two jensen 8" MOD speakers.

[camerongrieve]

Anybody here played with using 70V line transformers as output transformers?

Getting output transformers here in Australia can be quite expensive, so this is a great option.
I've successfully used PA line transformers for push-pull designs - a 2 x EL84 build, and a 50W 2 x 6L6GC Fender style build (not my designs, http://www.ozvalveamps.org/ava100/ava101lamington.htm)

I've just finished building an Ace Pepper HeavyWatter using this 10 Watt line transformer wired for 20K primary Single Ended. I'm still chasing oscillations out of the amp (my own layout ), but the transformer works, sounds good, and doesn't get hot.
http://jaycar.com.au/productView.asp?ID ... rm=KEYWORD

More info on PA line transformers on this excellent site http://www.ozvalveamps.org/optrans.htm

Regards,

Cameron

[9VOLT1967]

I have made a few changes to schem. How can I apply feedback to powerstage?

[martin manning]

Take the feedback signal from the speaker jack tip through a resistor to the cathode of the stage driving the power tube. This resistor should be 10-20x the size of the cathode resistor on the driver stage. The cathode bypass cap on that stage will now be controlling the range of frequencies in the feedback, so you will have to reduce or eliminate it. I'd try 0.05uF, maybe less. You'll also have to pay attention to the phase relationship of the OT windings.

Edit: Another way is to do it like the Fender Vibro Champ, where a small resistor is placed between the cathode resistor of the driver stage and ground. The feedback is applied at the top of the small resistor, which takes the cathode bypass cap out of the picture. The values used for the divider were 2k7 for the feedback resistor and 47 ohms for the tail.

[LeftyStrat]

Has anyone built one of these:

Discussion:
http://g.landry.myweb.absamail.co.za/uTube.htm

Schematic:
http://g.landry.myweb.absamail.co.za/uTube2.htm

[renshen1957]

Anybody here played with using 70V line transformers as output transformers? For a 1W application there should be no problem finding a CHEAP 70V distributed line system transformer. I have a few but have never played with this myself. I know that the right ratio for a tube output stage can be achieved with one but they aren't meant to handle static DC (SE amp). But at the same time, a 10W line transformer will have no problem handling the static DC from a 1W output stage..

Hi Cliff,

I have a pair of 70V line xformers, one I will be using to build a mini herzog. As to the static DC, I will be using a capacitor to couple the signal to the OT.

Best Regards,

Steve

[El_Martin]

@Cliff

All of my ECC82 output amps. (SE and PP) use cheap line transformers. No problems yet. I do not run them with maximum B+. They are around 250-280 V.

I wouldnt hesitate to use Hammonds 125 Series, though. Theyre cheap enough.

I just wanted to use the line thingys.

The little screamers give lots of fun.

Ciao
Martin

[Dr-Joned]

[quote="LeftyStrat"]Has anyone built one of these:

Discussion:
http://g.landry.myweb.absamail.co.za/uTube.htm

Schematic:
http://g.landry.myweb.absamail.co.za/uTube2.htm[/quote]

You won't get anything out of that schematic with that input jack wired like that! It does need to be inverted so the center pin doesn't go to ground.

[9VOLT1967]

Received new output transformer. I would like to clarify color codes. 4 wires to include-- . black / green - red / blue. The input side of transformer is blue to the plate connection 1 & 6. Red is the other half of input that connects directly to A1. Then the secondary is black ground with green feeding 8 ohm output. Please advise if incorrect. I will be posting pics of my project as it progresses. Also I will be using NFB and it will be fed from the green output leg thru a 22K resistor to pin 8 of pre-amp second stage. Thanks

[LeftyStrat]

You won't get anything out of that schematic with that input jack wired like that! It does need to be inverted so the center pin doesn't go to ground.

I hadn't noticed that, LOL.

[martin manning]

Received new output transformer. I would like to clarify color codes. 4 wires to include-- . black / green - red / blue. The input side of transformer is blue to the plate connection 1 & 6. Red is the other half of input that connects directly to A1. Then the secondary is black ground with green feeding 8 ohm output. Please advise if incorrect. I will be posting pics of my project as it progresses. Also I will be using NFB and it will be fed from the green output leg thru a 22K resistor to pin 8 of pre-amp second stage. Thanks

I believe the blue primary lead is in phase with the black secondary lead, so I think you'll want to use green for the secondary ground to get negative feedback.

[9VOLT1967]

Progress pics. Just about ready to wire in board!