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Old 14th November 2002, 04:40 AM   #1
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Talking Newbie's 1st Amp, comments please.



I am looking at making a Hi-Fi valve amp. Though I have a
backround in electronics, valves are completely new to me.

Some general comments please.

I would like to use something like this -
preamp - 12AX7
PA driver - 12AU7 (is this enough?)
Power Amp - KT88 or 6550C x2 (push-pull)
all x2 for stereo.

Is this feasable?
What do you think of the valve selection?
I have not seen any ciruits that use this combination.
I have seen some circuits, but I would like some ideas.

I can run the filaments for all these tubes from 6.3V,
AC or DC? some think AC, others DC - what reasons?

Layout seems to be very important, why - hum?
Earthing too, but I have read that some people have hum
no-matter how carefully they wire it.

I was thinking of a solid state power supply but someone
said they are noisy. A 5AR4 will not deliver enough
current, can they be paralleled?

I have seen some photos of nice wooden chassis, do they
need to be shielded?

I need to do bulk research.
Any comments would be helpful.
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Old 14th November 2002, 04:48 AM   #2
halojoy is offline halojoy  Sweden
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you will certainly get at lot of alternative suggestions
on your choice of tubes (everybody have their favos)
but I think you have made right decision on that

they are "mainstream" tubes, easy to find and replace
and a lot of knowledge is around of how they behave
and how they should be designed for

Good Luck
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Old 14th November 2002, 05:06 AM   #3
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Default VALVE CHOICES

Hi,

Just to give some short answers:



Quote:
preamp - 12AX7
Too much gain and one of the least linear triodes so that's a no go from me.

Quote:
PA driver - 12AU7 (is this enough?)
Probably not.You are going to do the phase splitting with the other half of the twin triode?

Quote:
Power Amp - KT88 or 6550C x2 (push-pull)
Why not.

Quote:
I can run the filaments for all these tubes from 6.3V,
AC.

See:DC Filament power supply

Quote:
Earthing too, but I have read that some people have hum
Shouldn't be a problem with PP amps and IHT.

Quote:
I was thinking of a solid state power supply but someone
You could use a pair of GZ34 in a Graetz bridge configuration.

Quote:
I have seen some photos of nice wooden chassis, do they
If you don't suffer from severe RFI then there is no need for shielding.

Ciao,
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Old 14th November 2002, 05:08 AM   #4
Colt45 is offline Colt45  Serbia
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I would try something a little different for the 1st two stages. AX7 and AU7 are so over used.. try something intresting.

and besides, they look kinda goofy next to a big 6550

AC on the heaters (output tubes at least)


you can put them on the gain tubes if you want.. i dont think its needed.

I often tell people to try AC first, DC if AC hums.


maybe try a 5U4.. they got what.. 250mA DC out?

or, if you want to try something neat, try a pair of damper diodes..
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Old 14th November 2002, 06:30 AM   #5
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Some very quick replies, Thank you very much.


I have ordered this book -

Valve Amplifiers by Morgan Jones
http://www.amazon.com/exec/obidos/tg...glance&s=books

and judjing by the reviews, it is a good book to have.


I value your opinions, but let me defend my valve choices a bit -

- I would like to keep this simple and easy to build (i.e.- use parts that are easy to get hold of). The 'interesting' stuff can come later.

- These valves are current and common, and readily available. there are perhaps only 20 or so types i can get my hands on locally. perhaps suggest a common alternative?

- I have seen the first 2 stages in a number of circuits. Is this not a proven design?

some silly questions -

Colt45
If I play it safe and design with max values in mind, 4 x KT88's would be 900+ mA yes?

fdegrove
What is -
IHT
Graetz bridge (a standard bridge rectifier?)
Aren't the GZ34 and 5AR4 equivalents?

Thank you all and more comments please!
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Old 14th November 2002, 06:49 AM   #6
halojoy is offline halojoy  Sweden
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Quote:
Originally posted by Robert-V
I have ordered this book -
Valve Amplifiers by Morgan Jones
and judjing by the reviews, it is a good book to have.
I know the name Morgan Jones.
He has designed a little, interesting Headphones-Amp.
It uses E88CC/6DJ8 tubes. (is that 12AU or 12AX ?)
I wouldn't mind try to build that one.

Morgan Jones Tube Headphones Amplifier
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Old 14th November 2002, 07:32 AM   #7
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Quote:
Originally posted by halojoy
Morgan Jones.
I enjoyed the Morgan Jones book. Have to read it again -- i'm just a tube rookie too, a little ahead of you.

IHT -- indirectly heated triode.

dave
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Old 14th November 2002, 07:40 AM   #8
Duo is offline Duo  Canada
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Hey, nice to see another person interested in tubes. They're great for sure. About that ax7 and au7 config, well, those tubes, as was mentioned before are a little overused and not that linear. But for newbie purposes and to keep the work simplistic, an ax7 is fine and the sound will still be impressive. The KT88 is my personal favorite tube. British, high power, high fidelity, nice looking, inefficient, just perfect for audio. I don't think you totally need to have two gainstages though. Unless you were planning to use the au7 as a phase splitter or something. I find that the KT88 with one other medium gain tube is more than plenty gain, unless you use crazy amounts of negative feedback.

Just keep asking around and look at lot's of diagrams and notes. It's quite simple really to make a tube amp.

Hope you have good luck.

/Duo
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Old 14th November 2002, 09:44 AM   #9
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I've just been going through the same considerations for a KT88 based amp and as I davour simplicity I ended up thinking I would build one of Andrea Ciuffoli's designs

http://www.audiodesignguide.com/my/sekt88.html

Of course, life being what it is, my son, whose 14, decided he would like to build the amplifier.... so I've changed direction 'cos he needs a proven kit for his first project...

However the above design impressed me as very capable...what do the panel think?

James D.
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Old 14th November 2002, 10:58 AM   #10
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Default GRAETZ

Hi,

Quote:
Graetz bridge (a standard bridge rectifier?)
Indeed that's it.
One can use such rectification with tube diodes as well and since you wondered about current capability I suggested a pair of
GZ34/5AR4 in such a setup.
It will give you full wave rectification with higher current capability.

I do not hold the book you refer to so it's a bit hard to judge for me.
If however I find a copy of the circuit on the web I'll comment further.

Ciao,
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