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Old 4th May 2006, 11:03 pm   #1
adibrook
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Default Miniamp prototype 1

I was bored so i started building an inverter, originally designed to power battery valves or somethgin. However, two days into the project, and now i have a prototype for a whole amp!

It's built into the wooden case from a ''suitecase'' type pye radio. I have a scrap one of these, so i pulled the shassis out and used it.

This amp is a total hybrid, and probably the wierdest thign i'v built yet.

The solid state circuitry is kept on two stripbords. One has the oscillator and power transisor, and one has the high voltage stuff (rectifier and smoothign caps/resistor). The actual valve circuitry is built in nails nailed into the wood.

Hers the schematic.
http://img.photobucket.com/albums/v4...brook/6at6.gif

The inverter actually works quiet well. What really got me is that the bdx53a is amazingly cool. It's cool to the touch and although riginally i was going to include a heatsink, now i dont think i'll bother. It never even gets higher than human body temp!

The transformer is run at 55.7Hz if i remember rightly. The 150ohm resistor was chosen pretty much on the spot, as it seemed to work the best. Lower values gave no improvement but overheated the 555.

I'm sure peopel woudl want to know what the output is. Well...the short answer is i dont have a clue! It's over 150v, sicne it fires a neon lamp, and it cant be over 400v since non of the caps blew up yet. I think maybe about 200 ish?

The preamp is a 6at6 because i accidently killed my only ef91, but dropping an o/p transformer on it! I ususally use a ef91 for jobs like this, but i just appened to have a 6at6 handy so i just used it.

Hers a coupel of photos of the amp.

http://img.photobucket.com/albums/v4...brook/amp1.jpg

http://img.photobucket.com/albums/v4...brook/amp2.jpg

It works quiet well, but distorts abit. I'm not sure if it's because the 6v6 is being starved of voltage/current, or because of the preamp. The cathode resistor looks abit high. I got it from some french hifi amp design or somethign. Maybe it's too high and it doesnt have much headroom.

Also, i think the amp would be better off with somethign like an el85. It would use less heater power and i think would work better under these low voltage conditions. A 6v6 is abit overkill.

Interastign project tho. And it works. I think if i take it further i'll put in alot more filtering and decoupling. maybe tying HT earth to battery earth would be a good idea too?
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Old 5th May 2006, 12:41 am   #2
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Default Re: Miniamp prototype 1

Nice job Adi! BTW, looking at the 6AT6 grid curves it looks like you're biasing that triode into class AB, even with an optomistic 300V HT! Try an 820 or 1k cathode resistor. Although, if HT is below 200V you can expect the 6V6 to flab out on you, why haven't you measured the voltages!
What transformers did you use?
I'm very interested in this switchmode thing you've got going- it'd make a great little battery practice amp using an ECC81/82 for a P-P power stage!- less current consumption too!
I get the impression you know your solid-state design straight off, but maybe your valve design isn't quite as hot? If that's the case then I want to swap knowledge with you! I can design valve amps back to front blindfold, but I could barely make a transistor work if you made me!
Oh yeah, try out an EF80 for a preamp stage for me- I haven't got time to experiment right now!
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Old 5th May 2006, 6:34 am   #3
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Default Re: Miniamp prototype 1

Thanks.

Yeah, i think the 6at6 is biased wrong. The 3.3k is just from a schematic i found. 1k would make more sence.

I havnt mesured the voltage because i dont have anythgin to do it! My multimeter has died on me again! I hate them. Even the not-so-cheap ones die after a few months. I need an avo.

Yeah, valves are pretty new to me. I'm too young to have seen them in use, and the only thing i know about them is what i lernt form audio stuff. Whereas solid state is abit more familiar.

The switch mode psu ismore sucessfull than i thought. It's pretty simple, yet effective. Not a normal combination.

And the heaters seem to run fine off only 6v. I'm using a big lantern cell rated at 11Ah. Not sure how long it'll last, but cirtenly worth 2 i reckon.

I need to get hold of a really small audio o/p valve like an el85. I think it's charectaristics would suit this better.
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Old 5th May 2006, 8:41 am   #4
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Default Re: Miniamp prototype 1

What's going on here?
I can't see any photos!
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Old 5th May 2006, 1:10 pm   #5
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Default Re: Miniamp prototype 1

Adi, it would be worthwhile trying an ECL80 as the output valve in a design like this. These valves are rattling around in almost any valve junk box - they were a mainstay of 50s 405 line TVs. You don't need to worry about the triode section, just use the output pentode. The heater is only 300ma so would help your battery life. I'm sure lots of fourum members would be happy to send you a couple without charge.

http://www.r-type.org/exhib/aaa0227.htm

Another low powered output valve is the EL42, which is electrically the same as an EL85 but with a B8A base.

http://www.mif.pg.gda.pl/homepages/f...030/e/EL42.pdf

Good luck, Paul
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Old 5th May 2006, 2:12 pm   #6
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Default Re: Miniamp prototype 1

Regarding the distortion again, you're probably getting blocking distortion from that giant coupling cap and grid ref resitor (the resistor is twice the max rated value for a 6V6!). Try a 47nF and 220k combination.
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Old 5th May 2006, 6:18 pm   #7
adibrook
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Default Re: Miniamp prototype 1

Wow. I didnt know that.

I thought the bigger the cap the better, since the bigger it is the lower impeadence it has at lower frequancies.

And the grid resistor is to stop the grid floating. It's 1M because at that resistance current through it will be very small, but it will stop it floating.

Damn...thers so many thigns i dont know yet.
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Old 5th May 2006, 8:17 pm   #8
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Default Re: Miniamp prototype 1

How do you mean floating?
The problem with big caps is that they can lead to blocking distortion because they, um, have big capacities! Also, passing very low frequencies can lead to oscillation in the following stage- a problem in vintage long-tailed-pair designs. -Ignore the data sheet at your peril!
A 47n & 220k rolls off at 15Hz anyway, plenty low enough. 0.47u & 1M is practically letting DC through!
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Old 6th May 2006, 4:20 pm   #9
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Default Re: Miniamp prototype 1

Hello,

to improve the power supply, you could

1) use a transformer with two identical windings for primary and take opposite polarity as common, the other two each to one transistor switch, where both transistors are driven by complementary rectangle. That would result in better symmetry and thus avoid core saturation.

2) Use Power-MOSFET instead of a Darlington (even a normal transistor would be better, as it doesn't have that high saturation voltage).

Regards

Dieter

Last edited by Station X; 15th May 2006 at 4:42 pm. Reason: Error pointed out by poster corrected.
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Old 6th May 2006, 5:03 pm   #10
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Default Re: Miniamp prototype 1

Thanks. I dont think i have any mosfets at the moment. I'll see if i can scavange one off somethign. The trouble is they arnt really used in tv's or computer monitors, which is where i usually scvange transistors from. I'll get some from maplin next time i'm there.

The psu section seems to be managing allright. It even light a 20w lightbulb (but dimly). I think the problem is in the biasing around the triode preamp, and also the 6v6's grid circuitry. I'll try to fix that when i next have time.

I need to get a multimeter and see how much current the while thgin is drawing from the battery.
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Old 6th May 2006, 6:57 pm   #11
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Default Re: Miniamp prototype 1

For the sort of thing you are doing Adi, a multimeter is essential. One of those cheapo ones from Maplin would do, but check it has a current range up to 10A AC and DC (some cheap ones just do voltage and resistance).
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Old 6th May 2006, 7:54 pm   #12
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Default Re: Miniamp prototype 1

For a cheapo DMM look for a Mastech MAS830.
You can read about them here: http://www.tubelab.com/MeterUse.htm

Look on your favourite auction site, you can buy one from the USA delivered for a tenner (yes that's meter+postage=a tenner)
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Old 8th May 2006, 9:28 am   #13
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Default Re: Miniamp prototype 1

I have a spare AVO Multiminor, Adi, if it's any use to you (free). PM me your address and I'll send it on to you,
Jim
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