#1
Hi guys,
I just purchased a Peavey Duel for pretty cheap of a local guy.
Once I meant the guy, I found it was playing nicely and bought it after trying it out for about 20 minutes.

When I got home, before powering it up, I check the impedance of the speakers: 8 + 8 (series) --> 16 ohms

Wow the switch was at 4!?!
Now I play with the switch at 16 and find it not too be SO loud. No biggie, I check the tubes in the back, can't see anything wrong with them:

Let me know what you guys think, I'd hate to find my good deal turn into a bad one!

The wiring just to make sure I'm not stupid (or am...) 8 + 8 =16 ohm, no?




Shot of the tubes




shot of the small tubes:



Buddy Guy strat just for fun:

Last edited by vince_badaz at Mar 7, 2007,
#2
hard to really tell, but looking at your pic, it looks like the (2) negative terminals are conected together which would be parallel wiring. If that's the case, and the speakers are 8 ohm each, it's 8ohm + 8ohm = 4ohm. Using 16ohm is an unsafe mismatch, you can easily fry the transformer that way. mismatching in either direction causes a power drop, but mismatching with a lower impedance cab than the head is expecting will draw more current than the transformer was designed for.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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Last edited by Erock503 at Mar 7, 2007,
#3
Pic updated, sorry about that!

I mentionned that it's not so loud... I should've explained that the issue really lies in the master volume knob, it's nowhere near being linear: 3 sounds as loud as 9...
#4
ic, yeah, as long as you know the pot is good, lower volume output "can" be a sympton of old power tubes. Did the previous owner give you any idea how old the tubes are?
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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#5
He said "the tubes are new" but didn't mention anything about pre-amp ones or power ones.
I planned on replacing them with some JJ from eurotubes anyhow, I just want to make sure it isn't something else first...
#6
how many tubes are there btw, and what kind of tubes? That's a 120W amp right?
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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#7
4 6l6
4 12ax7

I emailed eurotubes for some advice
I think i'll just go with a replacement right of the bat
The sound is decent, maybe it wasn't used to be played at reasonably high volumes like I like to...

Mind you, this is my first tube amp so I'm still new to this thing
#8
The lower the impedance, the more current the amp puts out.

If the switch is set to 4, the amp will be putting out more current because its expecting a 4 ohm load. So, when you set the switch to 16, the amp is expecting a bigger load and thus will put out less current. And less current = less dB.
The more you know, the less you understand.
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#9
actually, that's not quite true. It's a matching output transformer, so if you use the switch set to 4 ohm, with a 4ohm cab, it should be the same as having the switch on 16ohm and using a 16ohm cab. Less current isn't the complete reason for lower volume when mismatching either. The amp will put out less "power" mismatching in either direction. So even mismatching with more current, your still putting out less watts, which is directly related to SPL(dB).

example of a 100W amp rated at 8ohms:

PLoad = current^2 x RLoad
100W = I^2 x 8ohm
I = 3.5 amps. This is what the output transformer is designed for in this example.

Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (Vs^2 x 8) / (8 + 8)^2 = 100W
Vsource = 56.6V

I = Vsource/(Rsource + Rload) = 56.6V/(8ohm + 8ohm) = 3.5 amps using a matching impedance cab of 8ohm

PL = load power = 100W with an 8 ohm cab
Vs = voltage source (fixed in the circuit) = 56.6V
Rs = impedance source (fixed in the circuit - impedance matching output transformer) = 8 ohm

__________________________________________________ ______________________

mismatching using 4 ohm cab*
I = 56.6V/(8ohm + 4ohm*)= 4.7 amps which is more current than the output transformer was designed for.

The power formula is:
Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (56.6^2 x 4ohm) / (8 + 4)^2 = 89W

There is less power, but there is also "more" current than the transformer was designed for, therefore more strain on the transformer.
__________________________________________________ _____________________

mismatching using 16 ohm cab*
I = 56.6V/(8ohm + 16ohm*)= 2.35 amps which is less current than the transformer was designed for and therefore will run cooler.

Pload = (Vsource^2 x Rload) / (Rsource + Rload)^2 = (56.6^2 x 16ohm) / (8 + 16)^2 = 89W
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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#10
^^^ math makes the world go round! (<-- eng student)

Ok let's settle this, it IS a 16 ohm load. 8ohms in series --> 16 ohms
the manuals also states it...

I'll get the tubes and post pics when everything is in to let you guys know whats up.
Hell, I'll even throw in some sound clips for the not-so faint of heart...

yes yes, with pics of course... threads without pics are just plain boring