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Old 14th Apr 2020, 2:44 pm   #1
Beardyman
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Default Hybrid valve amplifier

With plenty of time on my hands lately I was scanning round the workshop taking stock of the various bits & pieces that tend to accumulate. I found a pair of AC2/PEN's, a couple of the RS 3.5W output transformers & there was the basis for a hare brained idea! What could I build with that lot? I know the AC2/PEN or the RS transformers aren't what could be called audiophile preferred devices but then again, my hearing ain't what it used to be either! The next obstacle was something to put it all in, my other half came to the rescue with an oak box from an old canteen of cutlery, the Lord alone knows why she keeps such things but she probably knows me better than I do. Next was the power supplies. With 4 volt heater @ 1.75A this wasn't going to be cheap or so I thought. Digging around I found a 160VA toroidal transformer with two 15V outputs. Further digging revealed a few variable voltage 5A switching regulators which had been bought for something else but it was cheaper to purchase five than the two I needed at the time so these were surplus to requirements. That's the filament supplies sorted! The next hurdle was the HT. During my rooting around I'd found another smaller (30VA) toroid, split primary & two 15V outputs. After a few experiments on the bench I managed to get about 190VDC @ 100mA by back driving the smaller transformer. This, for me, goes right against the grain. I did a fair bit of research but could find no good reason why this would not work in this particular application nor would it be detrimental to either transformer. Next was the detailed bits like capacitors etc. Looking around all I could find were some devices I'd rescued from an HV PSU at work, a few 5600UF 350V "bean cans" & a handful of 470UF 400VDC. A couple of heavy duty bridge rectifiers surfaced, both over 30A & 1000V rated, they'll do! So far so good. Looking at the AC2/PEN spec it needs around 3.2V RMS drive to get full power at optimum HT volts etc. This is where I kind of hit a brick wall. Then I remembered I had some LM386 modules from yet another project, running them at 12V (right on the edge for these as it happens) they would provide just about the right drive level with a 400mV input without clipping into a 470R load. Again this was found by much bench experimentation. Whilst these modules were working ok they would sometimes break into oscillation at the higher end of their output, this (after looking at the datasheet) was due to the module manufacturer not including a 47nF in series with a 10R tied to ground on the output (Zobel network), cheap skating in the extreme, once this was added all was stable. I hooked out a passive tone/volume control from the junk box as well. The speakers were from a defunct radio/CD player my neighbour had binned, tiny 3" full range drivers & reflex enclosure but for this they were just what was needed. Please bear in mind that this happened over about 3-4 weeks, a little here & a little there. I was determined that I would use whatever was to hand rather than buying anything specifically for it. Although it looks like a "junk box" amplifier it performs quite well, I added a BT/MP3/FM module just to make it more usable, again, this was lurking in the "surplus to requirements" box, one up from the junk box! I get the feeling that it will be one of those never quite finished projects, it certainly won't be sold.
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Old 14th Apr 2020, 5:09 pm   #2
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Default Re: Hybrid valve amplifier

Brilliant way to use a variety of technologies in a single project. Low cost coupled with a raid on the parts box to create a very usable piece of equipment. The red "are you sure you want to do this" switch guard is a nice touch!
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Old 14th Apr 2020, 5:23 pm   #3
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Default Re: Hybrid valve amplifier

Yes its great use of bits.

Nice result

I thought the switch was to prevent premature switching off the Archers no matter how painful it got!

Cheers

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Old 14th Apr 2020, 5:53 pm   #4
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What next in the many weeks of lockdown left?
 
Old 18th Apr 2020, 11:13 pm   #5
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Default Re: Hybrid valve amplifier

I'm impressed by the amount of knowledge that underlies this project--and by the fact that you had the right sockets for the AC2/PENs. Thanks for sharing!
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Old 20th Apr 2020, 8:06 am   #6
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Default Re: Hybrid valve amplifier

I think it was the sockets that triggered off this project in the first place, you know how sometimes one single thought can just roll on & on? Well, this was one of those moments! I got them for a Defiant MSH902 as it had a couple of the paxolin ones which looked decidedly worn/tarnished, turned out they weren't. An occasion where it looked far worse than it really was! The hybrid amplifier gave me something to focus on for a while, quite important in these trying times I feel. The AC2/PEN's are actually quite an easy valve to use it turns out. I could go into all the guff about it sounding warm & well staged (whatever that means?!?) but it's certainly loud enough! Many thanks for all your comments, much appreciated.
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Old 22nd Apr 2020, 4:55 pm   #7
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Default Re: Hybrid valve amplifier

That looks fantastic! Now I wish I hadn't let my sister have that early 20th C canteen of cutlery that belonged to my grandmother! The knowledge you have to put something like that together is really impressive!
David
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Old 30th Apr 2020, 1:42 pm   #8
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Default Re: Hybrid valve amplifier

Addendum:

Despite being absolutely delighted with the fact it actually worked I, nonetheless, began taking some readings with the scope. After a bit of poking around I found there was about 6V of ripple (100Hz) on the HT rail. This was, of course, reflected on the output. My hearing isn't perfect but its not that bad either! Part of the reason it couldn't be heard was the teensy speakers couldn't go down that far! They're only about 3.75" in diameter but the cones could be felt to be vibrating. A little research took me into the realms of PSU design, what a wonderful place that is. Not having a high value choke on hand I went down the C-R-C route. A little experimentation found that inserting a 100R 5W resistor reduced the ripple to about 200mV, very promising. I went for the lowest value I could as the HT has nothing to spare being only 190V. 100R drops about 3V so not too bad. I'm learning a lot from this junk box amplifier! More updates as & when I find other problems! Take care one & all.
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Old 24th Jul 2020, 2:17 pm   #9
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Default Re: Hybrid valve amplifier

Well, it has metamorphosed into what is (hopefully) its final iteration. It now has a valve pre-amp & magic eyes. I admit to cheating with the valve pre-amp & bought a kit! Same with the eyes but, to mitigate myself, buying the kits was significantly cheaper than buying separate parts. The whole point of this was just to see if it could be done, looks like it can! I had to re-vamp the HT side of things to get it up around the 250VDC region. Found some SMPS type boost converters which take 10-32VDC in & give (I think) 150 to 390VDC out @ up to 200mA. I had to install an RF/EMI shield around it to prevent pickup elsewhere but it works well. My better half likes purple, hence the under lit valves, a small price to pay in my opinion. Probably not to everyone's taste but she loves it. So far it has proved to be very reliable, she uses it when she is working from home which, at the moment, is all day every day! I may well write it up with diagrams etc at some point, we shall see.
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Old 25th Jul 2020, 8:15 am   #10
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Default Re: Hybrid valve amplifier

"I'm learning a lot from this junk box amplifier!" Designing and building your own amps rather than building a kit of copying an existing design teaches you a hell of a lot, I'd recommend any beginner to give it a go, it's good fun too. Beware of a lot of the modules out there that use valves, a lot just use an opamp with the valve doing nothing.

Your amp has got a bit blingy but still, good job. Andy.
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Old 25th Jul 2020, 11:03 am   #11
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Default Re: Hybrid valve amplifier

I agree, I looked at a lot of the so-called valve pre-amps & you can see, quite easily, that the valves are there purely for decoration. Not so with the one I bought, it actually works quite well, not audiophile stuff obviously but good enough. The output was an adventure, I spent a lot of time looking at SE output stages & the many & varied implimentations, particularly those that used the AC2/PEN, it was a very popular valve apparently so plenty of examples to study. It is quite capable of driving a pair of Mordaunt-Short bookshelf speakers to uncomfortable levels despite its 3W RMS per channel capability. As a learning exercise it fulfilled its purpose, as a usable piece of kit it far exceeds my expectations. It certainly caused a lot of interest from the teeange grandchildren! One in particular was asking all kinds of questions, possibly a new enthusiast in the making? Who knows?
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Old 25th Jul 2020, 1:40 pm   #12
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Silly, daft and wonderful, give that man 11/10!!
 
Old 25th Jul 2020, 1:47 pm   #13
Edward Huggins
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Default Re: Hybrid valve amplifier

I am thinking of buying those RS OPTXs, which I notice you have taken the trouble to screen was there a reason for this? How well do they handle the 3 watts or so per channel?
BTW this is beginning to look very Audiophoolish - you might even sell this as a piece of unique exotica for some outrageous price!
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Old 25th Jul 2020, 3:17 pm   #14
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Default Re: Hybrid valve amplifier

The only reason for screening them was simply to protect them from prying fingers seeing as they have around 250VDC on them, nothing more than that. The screens were salvaged from the scrap bin at work! The RS TX's work very well, I believe they are made by OEP Transformers of Oxford. Link here:
http://www.oep.co.uk/

(I have no connections with either company other than using their products)

I hadn't considered the audiphoolish look as such but yes, I'm sure you are right. Somebody may well be willing to pay a kings ransom for it, more fool them!
My other (better) half loves the silly, daft, wonderful look of it, the fact it works as well is very much a bonus.
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Old 25th Jul 2020, 5:40 pm   #15
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Default Re: Hybrid valve amplifier

I like it!

The distressed look of the box would rule it out of the high-end maket, but there are other opportunities.

In some markets it would have to have a little pressure gauge, a bit of small bore polished brass piping and a few gears.

Brass labels for the controls engraved in an Edwardian script font?

Not a king's ransom, but ideal for the sort of client quite prepared to set out in his dirigible airship to rescue any king held hostage. Aided only by his trusty blunderbuss and a gramophone record of Kipling's 'If'.

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Old 25th Jul 2020, 8:17 pm   #16
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Nice work! If you advertised it as "Steampunk" it might double the value Cheers, Jerry
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