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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Suggestions please??
PostPosted: 12 Jan 2016, 08:41 
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Joined: 26 Sep 2015, 13:59
Posts: 60
Hi all,

I have completed my first project, a preamp using 6AK5 valves in triode mode It is virtually silent in operation and makes a pretty good sound. I am very grateful indeed for all the invaluable advice I have received from this forum and related blogs. This was a learning experience so that my next project, a preamp based on the Forewatt design here, will go smoothly. I do like to understand a circuit before building it.

So, I now need to prepare to build a steree power amp (or two monoblocks). I just need to decide what to build. Ideally I think I want to build single ended class A, although p-p is still a possibility but output transformers are more expensive. Cost is a big consideration so 300Bs are definitely out. KT66/88 are also pretty expensive but not totally out of the question. I have a preamp (the one I have built) so input sensitivity need not be that high. I guess I need output of around 8-10W, I am driving fairly inefficient speakers from the mid 80s (although I plan to bild my own the future).

My thinking is something like ECC82 input valve, half for each channel, with an EL34 per channel on the output?

Any ideas, thoughts, advice greatly appreciated and eagerly sought. One of the projects/circuits on here would be good.

Thanks.


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 Post subject: Re: Suggestions please??
PostPosted: 12 Jan 2016, 08:59 
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Joined: 02 Mar 2009, 12:41
Posts: 1110
Location: Vänersborg, Sweden
I'd say go for EL34-SE. If you're not reinventing the wheels (drawing a new set of loadlines) there are hundreds of proposals for a single ended EL34. Besides the tubes are available anywhere. BUT I'd say that 6L6 is also a reasonable choice, then you're into the KT66 family of tubes.
A KT88 or 6550 will give you more power but compared to EL34, it's not that significant.
Else use EL84 in pairs (two each amp).

With just three or four tubes it's no big point going for monoblocks = need for two mains transformers.

Generally SE-transformers use to be more expensive than PP-transformers. Depending where you live (USA?), check Edcor and pick their XPP or XSE transformers. Cheap but with a price/performance rate that is GOOD! (Their CXPP and CXSE range of transformers are not that expensive either).

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 Post subject: Re: Suggestions please??
PostPosted: 12 Jan 2016, 11:48 
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Joined: 04 Jun 2008, 20:59
Posts: 4097
Location: Arizona, USA
Hi, Very true that SE trannies are more costly than PP ones. The EL34 is a really nice tube, if you don't need that much power the EL84 is great as well. The GXPP transformers from Edcor are quite good for the price. I routinely get response down to 30 and sometimes below that depending on the circuit and they reach out easily to 18KHZ. The single 6 ohm output impedance is suitable for both 4 and 8 ohm speakers. I have not found it to be a problem with either.

Good listening
Bruce

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 Post subject: Re: Suggestions please??
PostPosted: 12 Jan 2016, 12:56 
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Location: Vänersborg, Sweden
I also would like to add - octal tubes are FUN! EL34 with 6SL7 or 6SN7 is a perfect choice.

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“If you don't know where you are going, any road will get you there.”
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 Post subject: Re: Suggestions please??
PostPosted: 12 Jan 2016, 16:11 
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Joined: 26 Sep 2015, 13:59
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Thanks for all the helpful advice so far. I am warming to having a go at this http://diyaudioprojects.com/Tubes/5751-KT88-Tube-Amp-Kit/

It seems a fairly simple circuit design and offers huge flexibility of valve choice. Certainly it seems I could use ECC82 and EL34. Bruce's genius strikes again!

I do have one or two issues before finalising my choiuce though. The main issue is transformers. Edcor seem very good indeed but I am UK based and they have no UK distributer. They will ship to the UK but the cost is pretty high. There are some UK transformer makers that I can find but specs don't match exactly, and they mostly seem to deal with guitar amps. I think I can sort the mains transformer so long as I can get the B+ voltage right and get provide enough current. I will probably use a combinatin of resistors and LR8 regulator. I am much less familiar with output transofrmers and, my guess is, they make the most difference to the sound. So:

1. How much current do I need to provide to drive a pair of these amps? I envisage making a stereo pair rather than two monobloicks.
2. What is the impedance requirement of the primary side of the output transformer?
3. What maximum power output do I need to specify for the output transformer?

Is there anything else I need to think about?

Thanks,

Mark


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 Post subject: Re: Suggestions please??
PostPosted: 12 Jan 2016, 17:15 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
UK?! Haven't you checked Mikes LivingInThePast-site? His transfmers are definitely OK and well worth the price. I have used numerous of the smaller SE-trannies rated 5-10W. Mike can surely advice what transformers are best suited to what tubes.
Also check German guy Jan Würsten: http://fragjanzuerst.de/ Also very good stuff and check his tube lists for Russian equivalents!

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 Post subject: Re: Suggestions please??
PostPosted: 13 Jan 2016, 05:22 
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Location: Vänersborg, Sweden
Mikes UK address: http://livinginthepast-audioweb.co.uk/index.php?p=messages&sw=none

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 Post subject: Re: Suggestions please??
PostPosted: 13 Jan 2016, 08:57 
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Joined: 04 Jun 2008, 20:59
Posts: 4097
Location: Arizona, USA
Hi, yes the design is adjustable to nearly any size output tubes. There are some adjustments that improve the lower power versions though. A slight improvement in efficiency for EL84 and smaller tubes. You can use triode/pentode tubes like the 6GH8, 6MB8 and 6MS8 or their odd voltage equals as well as the ECL85. I attached a circuit doing that. The Russian 6F5P works well too. The output transformers are GXPP10-6-10K from Edcor. They are early ones without the pretty blue metal cases other wise they are the same. It is a low cost design and provides really great sound, far better than the components would seem to indicate. If you sub the better tranies (CXPP10-MS-10K) it will rival the quality of much more pedigreed gear in sound quality. The impedance of the output transformers as used is 10K however 8K is fine. The lower impedance will give a tiny bit more output but it trades off with a bit higher distortion.

Current demands on the EL84 stereo version are at least 200 ma B+ at 225-250 volts with approximately 3-4 amps (preferably more) of 6 volts for the heaters or about 2 amps at 12 volts.

Good listening
Bruce
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PP 6GV8 Amp Front.jpg
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 Post subject: Re: Suggestions please??
PostPosted: 13 Jan 2016, 18:30 
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Joined: 06 Jun 2008, 18:23
Posts: 5188
Location: Australia
One of Bruce's kits would be a good starting point. I'm a big EL34 SE UL fan. There are some very simple schematics using this tube on this site. Beauty is you only need a few components. Remember with any SE amp a dead quiet PS is required. So a 10H choke and good size caps on a two stage PS is an essential. I like to keep it very simple and two stage SE power amps are just that, simple.

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 Post subject: Re: Suggestions please??
PostPosted: 14 Jan 2016, 05:18 
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Joined: 26 Sep 2015, 13:59
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Thanks All,

Magnus: I have discovered the 'livinginthepast' site and he has some really good looking stuff at good prices. The problem is he doesn't seem to have anything to match the transformers Bruce specifies from Edcor. How close a match does an output transformer need to be? I do have family in the US so I might be able to get the Edcor transformers through them.

Bruce: thanks for your advice and tips, very helpful.

Having read up a little on SE circuits I am drawn to a P-P output stage. As I understand it SE amplifiers are subject to noise if everything is not perfect. I'm sure they sound the best when everything is spot on but P-P has a degree of noise rejection by virtue of its design. Is this right?

I think I will use Bruce's OddBlock design modded slightly to make a stereo pair with a single shared PSU. I will Use EL34 power valves unless I can find some cheap KT77/88/90 valves. I will use ECC82 on the input as I already have some of these.

I really appreciate Bruce's work in producing the kits but shipping a kit from the US is uneconomic I think. Also I'd rather build from scratch for that sense of pride and acheivement (so long as I can get it to work!)

Thanks again,

Mark


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