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Mini Block class-A PP EL84 Amp project
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Author:  anto_manna83 [ 18 Oct 2012, 15:21 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Bruce,
my idea was to use two identical transformers (see the link on the previous post) in my project. Every transformer has a secondary with 200V@0.1A and another with 6.3V@1.5A. Connecting them in series I can get 200V@0.2A (CT) and 12.6V@1.5A. Accordingly to your advices this should work properly.
The primaries are connected in parallel of course.
What do you think?

Thank you again,
Antonio

Author:  gofar99 [ 21 Oct 2012, 19:51 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Antonio, I plan on using a similar arrangement with a stereo version... actually a dual mono version on a common chassis. The difference is that it will be two completely independent power supplies. Two transformers, two filter chains. Only a common power switch, volume and input selector controls. I generally don't recommend hooking transformers like you indicate. It is done on occasion, but my concerns are related to how identical they really are. If the tolerance is say 20%, then one transformer will try to put power into the windings of the other. Worse if there is some type of phase difference. So with two supposed to be the same trannies I would go for dual supplies. There are benefits to that type of arrangement in terms of channel separation if nothing else.

Good listening
Bruce

Author:  bernard [ 11 Jan 2013, 10:41 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Bruce,
Did you try to run the EL84's in triode-mode?
rgrds.,
Bernard

Author:  gofar99 [ 11 Jan 2013, 22:55 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Bernard, Yes I did try triode mode. The power output was cut about in half and the sound really wasn't any better. The distortion measurements were also slightly worse in the range below 100HZ. IMHO the U/L mode does a better job in all areas. BTW I also tried the triode mode on the bigger amps (KT77 and KT88) and the results were the same, no improvement, less power.

Good listening
Bruce

Author:  bernard [ 05 Feb 2013, 13:10 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Bruce,
What preamp do you advise to use with the miniblocks: passive or active?
Rgds.,
Bernard

Author:  gofar99 [ 05 Feb 2013, 15:52 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Bernard, I use a passive one about 95% of the time. The amps are sensitive enough for most sources to drive them easily to full output. If you do vinyl though you will need a phono preamp to get it up to the line level needed for the amps. You can build a passive preamp in many ways. From just a simple 100K pot in a box to a full digital control one (I use that).

Good listening
Bruce

Author:  faenor [ 11 Apr 2013, 07:00 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi,

To make simple i'm nearly a newby in diy tube amp (and i'm not used to express myself in "technical" english). So, please ... be indulgent.

Studying the diagram, I do not understand what is tp1, tp2 ... tp4 "dot" near the lm317 regulator.

Thanks for reading.

best regards.

Author:  bernard [ 11 Apr 2013, 08:21 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

TP = Test Point. See: "Mini Block Amplifier Operation and Final Checks".

Author:  dlucy67 [ 23 May 2013, 21:43 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Bruce,

Thanks for all your generous ideas and time.

I'm assembling the gear to try my fix DIY tube build and your Mini Blocks sound like a good candidate. I don't need a lot of power at the moment as I live in a duplex and the neighbors don't care for my mix of 70's rock (Rush, Thin Lizzy, Pink Floyd), 70-80's Pop (Steely Dan, Thomas Dolby) and Jazz (Chet Baker, Ella Fitzgerald) on vinyl.

The one concern I'd like your opinion on is "will the Mini Blocks drive my speakers?" I've got a pair of JBL L46 for the heavy stuff and am looking for a pair of Boston Acoustics A70 Series II for the jazzier and mostly-vocals stuff. Both pairs claim they want at least 10 watts of input. Will 5.8 watts from the tube amps be enough? Or should I suffice with a gainclone on the power end for now and focus on a Groovewatt first?

Author:  gofar99 [ 24 May 2013, 14:46 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi, The JBLs are rated at 88 db/watt and the Bostons at 90 db/watt. The Bostons would be OK since you say they neighbors won't like your choice of music the JBLs are close to OK and for what you indicate probably OK as well. In large rooms I recommend that the speakers be about 92 db/w or higher. But for smaller rooms 86 and up is normally OK. The amps are over achievers and sound a lot louder than the power output would indicate. So I would expect them to be fine. A nice and pretty easy diy project as well.

Good listening
Bruce

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