Re: Why no headphone jack on tube amps?

Charmed Snark wrote:

On 29 Jun 2007, Jim <askme@xxxxxxxxxxxxxxxxx> wrote in
alt.guitar.beginner:


Charmed Snark wrote:


On 28 Jun 2007, "David Martel" <marte005@xxxxxxxxxxxxx> wrote in
alt.guitar.beginner:



Snark,

Using a power soak, I call it a dummy load,


Ya, I think "Power Soak" was a brand name (still?).

Wasn't that the Scholtz model that was notorious for bad contacts
on the switch and using only power resistors (unfriendly to many
higher powered vintage amps)?


Possibly-- I gave it up a long time ago (early 80's), before I got back to guitars. Mine never gave me trouble, though I do recall it was basically a resistor box. ;-)


Well, it's not just freq response (which is what EQ can adjust
for), but that the speaker's physical characteriscs and motion
colours the sound (both in the air, and to some effect this gets
reflected back into the final stage of the amp). Obviously any
reflected "electronic influence" also to some extent flavours
your power amp distortion and sound qualities.

It's the fact that there is an impedance curve, and the fact that
an output transformer reflects that impedance back to the output
tube plates as different loads at different frequencies.


Absolutely.


But at the end of the day, what you have to determine is whether
it is "good enough". At this level you'll have many people in disagreement over what is good enough. But your opinion is the
only one that counts here. ;-)

First and foremost is the safety of the finals in the amp (a suitable load). Second is the sound level in your earphone jack.

The best way to settle it for yourself is to simply borrow a
dummy load, and wire up an external jack arrangement (get 3 jacks
and just wire it up without a box). This will give you the power
level you want in your earphones by suitable choices of resistor.
Just watch out for the power level of your resistors (5 to 10W
should be plenty good for earphones, assuming a good choice of
resistance values used).

Then if you like it, you can house that in a little box. Have 1 speaker input jack, a dummy load jack and a earphone jack. If you
want to get fancy you can add a monitor jack for recording
purposes (again, some suggest this ain't good enough for
recording, but I beg to differ, until I try it for myself ;-)

Snark.

I'd be wary of using a non reactive dummy load on high powered
amps,


I don't believe that the reactive nature makes a bit of difference to the operating safety of the finals.

I do. Marshall did: http://www.seattle-attorney.com/guitar/2204_hangtag.jpg

I've been told that it can push an amp into parasitic oscillation. And additional heat to the OPT.


Don't forget that inductive or capacitive reactance is still resistance-- it just has a non-zero phase angle (a resistor's phase angle is zero).

What _is_ important to the tube finals is that the power gets absorbed by the load. If you don't have a load, it gets disappated in the finals itself, because that is where the power remains (on the primary side this leads to high voltages, eventually causing the tube to conduct more (disappating some of the power) -- but the high voltages also cause arcing in the OPT, disapating the rest of the power). This exact same problem applies to RF finals in RF transmitters with an unmatched antenna attached (the antenna is the load and must radiate the power out to space).

Resistor dummy loads are used all the time in the RF world. They take care of absorbing the output power, and prevent it from being refected back to the final stage (SS or tube). So ignoring faulty switching contacts, there is really nothing wrong with a resistive dummy load for tube amps.

Now of course, I am not saying that a non-reactive load won't influence the sound-- but sound and amp safety are different things of course. And the real issue is whether it makes enough of a difference (blind hearing tests???)


and I'd use a dropping resistor then maybe a wire wound or
L-pad for volume adjustment on the headphones.


Pure resistance is fine, and certainly simpler.


That way, you
adjust for amp tone, then set your headphones volume to the tone
(instead of possibly having to BLAST your ears before you hit
power tube crunch).


Well, of course if you want to adjust earphone volume, you'll need a variable resistor. But you can easily do this the same way that many consumer earphones do it-- a little pot in the wire.

Snark.

.

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