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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 26th Sep 2014, 8:14 pm   #1
G6Tanuki
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Default Stereo single-ended amp from junk.

OK, so I've collected together a slew of bits and want to use these to build a 5+5W stereo amp.

I truly don't have 'golden ears' and am rather more interested in said amp being able to survive hours of 'being turned up to eleven' - so freakishly-complex feedback arrangements are off the agenda - I'm more focussed on power-supplies that don't sag.

Power-supply is a 250-0-250V 120mA, 6.3V 3.5A transformer. I've got some nice Sprague 47uF 450V-rated can-electrolytics for smoothing.

I've also got a couple of EZ40 valves, or a couple of EZ81s as rectifiers - I'm thinking using these with the anodes strapped in parallel will give better HT regulation - but which to go for?

Similarly, for the single-ended output-stages I've got a choice:

Edicron EL84s, Brimar 6BW6 or Mullard EL41s.

Which is likely to survive best when driven hard? I'm minded that the 6BW6 has the lowest heater-demand but from past experience of a pair of these running in push-pull to deliver 30-odd Watts of audio they don't seem to 'go off' as a result of being driven hard.

Decisions, Decisions!
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Old 26th Sep 2014, 9:00 pm   #2
bikerhifinut
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Default Re: Stereo single-ended amp from junk.

Funnily enough I have just built a rather nice stereo se el84 job.
based it on the "RH84" design from Alex kitic. Its dead simple and works lovely. I'm listening to it now. I had to use a slightly lower HT than the circuit but it measures fine and wonder of wonders it's got a silent background, no hum. I was very careful with the heater wiring and also used a 10H choke on the HT.

heres a link You don't have to use the LM317 based CCS or the zener feed to the screen grid but I did and it does make life easier.

http://rh-amps.blogspot.co.uk/2013_02_01_archive.html

I hope its ok to paste that link, none of his stuff is copyright and there for all to enjoy.

Andy.
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Old 26th Sep 2014, 9:22 pm   #3
G6Tanuki
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Default Re: Stereo single-ended amp from junk.

An interesting design - though I'm inclined to skip the Zener and the LM317; I like to keep things simple!

Given that I don't see overall gain as an issue [I have a couple of 6BR7s to use as drivers] I'm inclined to include a bit of negative-feedback by way of undecoupled cathode-resistors for the output valves.

My approach to HT won't include a choke - I intend to feed the 'hot' ends of the output transformers direct from the 47uF-smoothed HT rail [or maybe wire a pair of valve rectifiers in parallel so can I then use 100uF of first-smoothing? I've got enough LT in hand from the transformer to use two parallel-strapped-anodes EZ81s to get a 'stiffer' HT rail] and then feed the screens from the second 47uF cap with a 1KOhm resistor feeding it from the main HT.

Hum's not likely to be noticed - I have pretty much zero functional hearing below 150Hz after being 'closely involved' in a major explosion back in the 1990s.
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Old 26th Sep 2014, 9:36 pm   #4
bikerhifinut
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Default Re: Stereo single-ended amp from junk.

fair comments...............

I found the circuit works well and I also tried the original version which used a normal cathode resistor and resistor to the screen grid. I just happened to have a few LM317's and thought it was worth a try.
It worked fine without the choke too, I merely refined the PSU along the way. I'm also using a transformer gleaned from an old Armstrong Radiogram which doesn't give as high an HT as the original design but it seems to work pretty well anyway.
I used a pair of those edicron EL84's too, strangely the same batch number as yours. Russian 6P14PEV as I remember from deciphering the etched marks on the envelope.

Be interested to see how you fare with your project.

Andy.
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Old 27th Sep 2014, 3:25 pm   #5
bikerhifinut
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Default Re: Stereo single-ended amp from junk.

Just to add something, that circuit I used isn't that sensitive, it needs around 600mV for full output so anyone using a low-ish output source like a lot of older fm tuners and some RIAA disc stages (my REGA FONO springs to mind) may need to use a simple gain stage in front if not feeding it from a separate preamp. Or maybe reduce the feedback a bit.
Theres a few variations on that circuit from a parallel SE EL84 design which would be an inexpensive way to get 10watts, not counting the cost of the output transformer, to a "Universal" design that you can tailor to use bigger valves such as EL34/KT66/6L6/6550 etc etc. I have to say that apart from not having the outright "ooomph" of my P-P KT88 power amp (kit built), the SE EL84 does not disgrace itself at all in my Hi Fi system which I guess is borderline "Audiophool". I just use good quality standard bits in my builds, I feel that modern components have the quality and reliability that the older stuff didnt and most "Traditional" designs work really well because we have affordable parts that are in tolerance and stay that way.
Oh I think i have a 6BW6 circuit from a "radio Constructor" article that a friend passed on. I shall pass it on if I can find it. I think the 6BW6 was the same as a 6V6 in a 9 pin envelope?

Andy.

Last edited by Brian R Pateman; 27th Sep 2014 at 4:11 pm. Reason: Propriety.
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Old 27th Sep 2014, 6:15 pm   #6
G6Tanuki
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Default Re: Stereo single-ended amp from junk.

The output transformers I have don't look up to handling the resultant HT current of paralleled EL84s - and like I say, this is a 'made from junk I have lying about' project!

The 6BW6 is essentially a 6V6 in a 9-pin glass bottle - they run very hot but seem to survive surprisingly well (unlike the 6AQ5 which is the same electronics squeezed even more tightly into a B7 bottle).

Sensitivity shouldn't be an issue - this amp's going to take input from the likes of laptop sound-cards, MP3-players, and tablets: I'll be fitting relatively low-value (100 Ohms or so) input-load resistors so the audio source has something to work against. Similarly, I won't be doing any in-amp tone-adjustment. Keep it simple...!

One area I *will* be paying attention to is RFI-proofing: stereo speaker leads make a fine HF-band dipole.
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Old 27th Sep 2014, 6:46 pm   #7
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Default Re: Stereo single-ended amp from junk.

I like the 6BW6 i've used them in a number of small harmonica map projects, like you say they dont mind being driven hard.

Jay
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