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Old 25th Sep 2006, 10:55 pm   #1
adibrook
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Default The Junk Amp

Damn. I seem to be living in the homebrew section right now .

Anyway. Another day, another creation.

I had a radiogram replacement shassis. It was made in the 50's or 60's to replace a 1930's shassis i think, and as far as i'm aware is a HORRIBLE HORRIBLE device. Because it was meant to "upgrade" beutifull 30's radios with PX4 output valves etc, into ''better'' modern radios by removign the original shassis and replacing it with this one.

Anyway. The shassis was pretty trashed, and i have no 30's radiograms. I didnt have much desire to restore it really. It had a horrible self-splitting 6BW6 push pull output, run straight from the detector/1st af valve. I'v allredy ripped bits out of the shassis and stuff, so i decided that i should recycle it.

I have had a really annoying problem for a while. I'm a total tube head, yet i listen to music through a pair of TDA2050's on a stripbord! (which incidently blew up on me when i was trying to modefy the amp yestoday).

So, i thought, why dont i build an amp out of this scrap shassis i have.

It allredy had an octal socket for a 5Z4 rectifier, and a power transformer. It had two 9 pin socket holes, and the rest were 7 pin. However, one of the 7 pin holes got converted to fit a 9 pin socket with a needle file.

So now it looks like this.

http://img.photobucket.com/albums/v4...ok/junkamp.jpg

The idea of this amp is that the shassis, and the power transformer, are both actually quiet big. Also the shassis has lots of holes in it from fittings which were on the radio. This is good i think, because it will keep everythgin nice and cool. I dont think thers much of a safety issue with the holes. The amp will just sit on the shelf all the time, and i will never even touch it. Thers no volume control because i use my laptop for that. This is just a stereo power amp to drive my acoustic solutions speakers, and hopefully sound good.

I'v put in an ECC83, each half being a preamp for one channel. And then either EL84 or 6BW6 outputs. Shoudl be enough to give reasonable power. I dont usually have it up too loud, because most of the time i use the speakers in my room at night (keeps my brain conditioned )

Now i just need to find a couple of se output transformers with simular charectaristics.
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Old 26th Sep 2006, 8:11 pm   #2
Ed_Dinning
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Default Re: The Junk Amp

The spirit of home construction/ experimentation/ lashups lives on!
Well done Adi, I suspect many of us have made up bits of kit exactly like that.

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Old 26th Sep 2006, 9:44 pm   #3
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Talking Re: The Junk Amp

I'll echo Ed's comment!

Reminds me of when I was just "starting out" as a youngster. Someone gave me a similar amp - obviously home-brew - but all the bits seemed to be there. Knowing next to nothing about electronics, and armed with a battered version of "The Practical Wireless Encyclopaedia" - but feeling a trifle nervous - I ran a long mains lead to it, placed it in the middle of the back lawn, mains lead through my bedroom window, and plugged it in. Nowt happened - at first. Then BANG! I've been very cautious about ali can electrolytics ever since! ~ But you should have seen that cap fly!

Good luck, Adi.

Al / G8DLH
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Old 26th Sep 2006, 11:25 pm   #4
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Default Re: The Junk Amp

As usual for me, the amp has changed function half way through the project.

I decided that i have a p/p output transformer from that shassis, and therefore i think i will just make a push pull mono amp. However, the o/p valves are two QQV03-10's!

I'v allways been a big fan of the QQV03-10, as some of you may know, and now i think i found a use for them. I will use them in paralell push pull, with the two tetrodes in each bottle paralelled up.

Should be actually reasonably powerfull. The phase splitter is an ECC83/82 and the preamp is an EF91.

Everythgin is fitted, but i ahvnt tested it under power yet.

I'm hoping that if this amp is sucessfull, it will prove my belief that the QQV03-10 is a reasonable power o/p valve.

I will use the original mains switch and volume pot from the shassis too.

So far everythgin lights up, but i will connect the anodes up tomorrow, and see what it actually sounds like.
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Old 30th Sep 2006, 5:49 am   #5
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Default Re: The Junk Amp

Update:

The junk amp has been finished (well..the 1st prototype anyway) and it changed function again!

A member of the forum has kindly donated a pair of identicle o/p transformers, and i finally have a stereo amp!

Hers a pic.

http://img.photobucket.com/albums/v4...k/P1010061.jpg

The circuit is pretty simple. It has an ECC81 preamp triode, with 330K anode resistor, 1K/47uf on the cathode and 1M/0.47uf on the grid. Then an EL84 power stage with 220R/100uf on the cathode, 470K/0.1uf on the grid, and the screen grid beign connected straight to HT.

There are two identicle circuits like this, one for each channel.

I used an ECC83, but then decided to use an 81 isntead because not much gain is needed.

So far there are several problems.

It sounds horribly trebely. Even with windows media players eq on full bass and lowest trebel, its still too trebly.

It clips bass way too early. Other amps i'v built were alot meatier.

For some reason it also picks up noise from my laptop. The only way to get rid of this noise is to either unplug the laptop, or to disconnect the aerth wire on the amp. This lets the shassis float at the laptops own earth i guess, and the noise problem is cured. Maybe i shoudl put a switch on the back to connect/disconnect the earth to chassis.

The components wrere mostly recycled. The values arnt idea, but theyre allright.

I also found that a Brimar EL84 and a Mullard EL84 are totally different. But i put in a pair of matched Marshall EL84's, and now the channels are surprisengly simular.

Do any of these component values suggest any problems? Maybe it just needs a high value resistor on the input to help eq it?

But i'm just happy i have a stereo!
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Old 30th Sep 2006, 6:13 pm   #6
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Default Re: The Junk Amp

Hi Adi,
Excellent work! Makes me want to go out to the shed and knock one up!
I remember when I messed around with EL84 single ended circuits I had the same problem with it being tebly and lacking bass, I found it was cos I had connected the screen like you straight to HT and I guess lots of cathode current was being absorbed by the screen and not getting as far as the anode and throught the primary of the output tranny, Try fitting a 2.2k resistor in series with the screen to HT Adi, This worked for me and the amp was a lot more meatier!

Hope this helps.......... enjoy and keep up the good work!

Best regards
Lee
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Old 1st Oct 2006, 6:15 pm   #7
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Default Re: The Junk Amp

I connected a couple of 2.2k 1/2W resistors between the screens and ht, and they burnt up straight away. Maybe i need bigger resistors?

I allway. Screen grid resistors shoudlnt dissipate that much heat...should they?
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Old 1st Oct 2006, 10:31 pm   #8
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Default Re: The Junk Amp

Crikey!

What is your smoothed HT voltage Adi, you can work out the current by measuring the voltage across the resistor (before it burns up) and then divide this voltage by the 2.2k, this will give the current flowing through the resistor. Then all you need to do is multiply the measured voltage by the calculated current and hey presto! thats the wattage dissapated in the resistor, now employ a resistor that has a suitable power dissapation

Good luck and keep us posted!

Cheers Lee
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Old 2nd Oct 2006, 9:57 am   #9
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Default Re: The Junk Amp

An ECC81 in that circuit will provide more gain than an ECC83, if you do the maths you will see. I suspect the bass is clipping too soon due to the very low bias voltage on the ECC81, try reducing its anode load to 33k or something.

The screens must have been pulling more than 15mA through those resistors. If you connected them directly to HT it sounds like you're running the EL84 in triode mode? Or is there a filter between HT and the screen/preamp supply?

The only way to reduce hum is to shield everything! Can you post a schem, with voltages?
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Old 2nd Oct 2006, 5:39 pm   #10
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Default Re: The Junk Amp

http://www.mif.pg.gda.pl/homepages/f...10/e/ECC83.pdf has design values as an AF amp. 100K should be good for anode load assuming you have 250V or so, much lower and you lose gain due to the high plate resistance.
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