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Old 4th August 2009, 07:10 PM   #1
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Join Date: May 2006
Default RH84 based amp

Here is my just completed RH84 based amp. I built it this weekend. I forgot to take a picture of the underside, but will if anyone is interested. The changes include replacing the 12at7 with a 5687 at 10 ma and 3 hlmp-6000s in the cathode loaded with a Gary Pimm self bias ccs. Edcor Xse-5k-8 outputs.

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Old 5th August 2009, 02:54 AM   #2
tim614 is offline tim614  United States
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Old 5th August 2009, 03:17 AM   #3
cubby01 is offline cubby01  United States
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I am interested in the underside
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Old 5th August 2009, 01:36 PM   #4
Join Date: Dec 2004
Location: Zemun
It's nice to see the family growing

Still, I must say one more time, if that means anything, that the 5687 will not make you happy in that position, i.e. the RH84 will not be at it's best.

That is simply because the mu=20 family has not got enough gain to drive the output tube in the configuration required.

You need either a very high mu tube (like the ECC83 and ilk), or preferably a tube showing an optimum compromise between mu and transconductance: a few examples would be ECC81 and family, E180CC, etc.

The 5687 in this case has a lot of transconductance, but very low mu and very low Rp. The result is not good.

Anyway, now that you made it in a nice box, there is no reason whatsoever not to change the components to the original values and install a NICE ECC81!

I must say that I do not understand what is it that people have against these ECC81/12AT7 tubes? To me what matters most in the designing and calculations stage is whether a tube has the right specs to be able to perform its task on a given position. Only after being satisfied with the design, and having built and tested the sonics of the amp -- only than I consider "tube rolling".

Keep up the good work and get yourself a few adequate tubes for this nice amp!

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Old 6th August 2009, 01:05 AM   #5
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Hi Alex,

First I would like to say thanks for making the RH84 design available.

Regarding gain, in my system, the 5687 has way more gain than I will ever use. I have a preamp, and gain needs to be considered on a system whole, not just one particular stage. Seriously, with a 12at7 I would only get about 4 or 5 clicks of my 24 step volume control. Now, when it moves into my bedroom system without a preamp, I may need to change to have more gain. And my deciding on the 5687 isn't anything against your choice, it just fits my system better. You should try it, it sounds pretty good. And isn't that what DIY is about, trying your own thing.

The only thing I don't know about is if the lower RP will affect the feedback resistor. Any help would definitely be appreciated.

Thanks again for releasing the design, and here is the under bits.

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Old 7th August 2009, 02:19 PM   #6
Join Date: Dec 2004
Location: Zemun
Default gain

Regarding gain, in my system, the 5687 has way more gain than I will ever use. I have a preamp, and gain needs to be considered on a system whole, not just one particular stage.
Hi, Chris.

Point is, we are not talking about gain in the sense that you need to ampify some signal more or less... but in the sense that the input tube, due to its low mu, low (thus inadequate) Rp, etc. cannot DRIVE the output tube properly. The result, sonics aside, is NOT HAVING ALL THE POWER which it was designed for.

Not having a sim at hand, and to simplify things, let us assume that instead of the 4W at 1% distortion, you are getting some 1.5W (never mind the distortion). Maybe you listen at low levels, or have very efficient speakers (like having 104dB/W/m speakers means 2.5W or even 1.5W is more than you will ever need, in comparison with the standard 89dB/W/m speakers which will frankly not satisfy you without a certain power (i.e. 10W, which is the reason why the 2nd generation of RH amps was created...) -- so you do not need the extra power the RH gives you in comparison with the triode connected EL84, or other similarly (1.5W) powered designs.

I hope it is now more understandable what I meant by GAIN...

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