Additional questions raised by the answer

>> Bonjour.

Hi !

>> If I understand correctly, you'll be feeding this beast with another
>> preamp.
>> If the preamp doesn't have enough gain, you may have to add another gain
>> stage in front of this. A 12AX7 with the first stage a cathode follower
>> (buffer) followed by a common cathode gain stage would do well.

Actually, i just meant the first stage of the power amp, the dephasing
stage. There are 2 halves of a 12AX7 (or AT7) tube used. Can it be the same
tube or is there some interaction between the left half-12AX7 and the right
half-12AX7 that could be bad for the sound ? I think not but i'd rather ask.

>> The 1609 is 10K P-P, a good choice for triode mode. For pentode mode the
>> 1608
>> (8K P-P) is a better choice.

Ok but let's say that a dual EL84 in pentode mode have around 8K of
impedance and going through the 1608 is fine to get 4 output ohms, if i go
through the 1609, the output impedance will be (10000/4 = 2500 ->
8000/2500=) 3.2 ohms. I thought that as long as the speaker has a higher or
equal impedance compared to the output, it's fine. Ain't it the case here ?
Because sticking with the 1609 would allow me (i thought about this reading
your answer) to put a DPDT switch to go from triode to pentode mode. If i'd
do this with the 1608, i'd get 10000/2000 = 5 ohm on the output side which
would be a bit higher than my speakers -> bad.
Is this a correct reasoning ?
Also, i see that the 1609 and 1608 are rated at 10W. I read that dual EL84
in triode dissipate 10W while they burn 17W in pentode mode. looks like i'm
over the ratings here, am i not ? At the same time, it seems strange to me
that Hammond would produce dual EL84 PP transformers that would never fit
any circuit so there's probably something wrong in my head.

>> The 150K are in parallel with the caps not series. They're there to
>> evenly
>> split the voltage between the caps.

Right, right, should have listened to my physics teacher more carefully...

>>> - Any idea why there's a dissimmetry between what goes to the top tube
>>> and the bottom tube (100k + 120pF cap vs. 82k)
>>
>> The top triode is common cathode fed via the grid. The bottom is common
>> grid
>> fed via cathode. If one did the math, one would find that if using the
>> same
>> plate load, common cathode has slightly higher gain than common grid. The
>> 82K
>> versus the 100K balances the two.

I saw similar circuits who didn't bother that subtlety. Boogie is up to its
reputation.

>> Makes no difference to the transformer. SPST is fine.

And a few cents cheaper, great !

>>> - Should i make the aluminum casing bound to the ground
>>
>> Yes

Safety first, i'll bind ground and casing

>> Absolutely no ground switch. That would be a serious safety hazard.

I have a Kittyhawk tube preamp too and when i bought it for 50$
(yesssssssssssssssss, long life eBay and suckers !) i could only get sound
through headphones on the lineout.
I opened that baby and saw that there was a wire obviously added by a
non-factory technician, going from somewhere on the power circuit to the
rack box. I asked about it to some guy at Kittyhawk (in Germany) and the guy
told me to cut that wire, that it was there because some rigs require a
"ground lift". Instead of cutting it, i put a switch in case one day my rig
would require that too. The Mesa Boogie Quad also has a ground lift that i
used once to check the effect : It dramatically reduced the hum.
So my questions are :
- Is a ground lift switch the same thing than that switch i considered
putting to hook and unhook ground and chassis ?
- If it's unsafe, why does my Quad has one ?
- Is it correct to say that in case of high tension reaching the casing,
having it grounded would make blow my home's fuse immediatly while not
having it grounded would blow my head if i touch the case ?

>> Bienvenu

Actually, i've been reading topics on this NG for a few months now, trying
to get familiar with the subject but thanks anyway.

Thank you for your quick and precise answers, when (if) i get my power amp,
i'll try to record a little something for this NG.
In the meantime, i'm pretty sure i'll come up with other questions !


.

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