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February 08, 2023, 09:20:50 PM

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Faint "fizziness" in Ampeg GT10

Started by yaryaryar, February 06, 2016, 02:14:06 PM

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galaxiex

Sorry, don't mean to hijack the thread, but would this type of crossover distortion be visible on a 'scope?
If it ain't broke I'll fix it until it is.

Seeker

Enzo...quick update
Fr: Seeker (yamaha g100)
First off, you guys are good and thanks for everything. Long story short a guy traded me an old bogner 75 watt head for my G100 412.  Major mouse nest in it and needs lots of TLC but I think this will be a good start on the SS pre-amp with tube power amp I wanted to build. The 412 cab was a bit unpractical for my apartment anyways (according to my wife anyways lol).  But thanks again, I really enjoy watching/reading you guys in action.
Seeker

Enzo

OK, a ten ohm trimmer.  Or a 10-12 ohm resistor with a 100 ohm pot too.  Or just a 10 ohm and be done with it like Juan said..  I didn't figure the whole 100 ohms would come into play, that was why I started with it at zero.  But Juan is also right in that if you turn the pot too far, the amp will be overbiased.

There is natural variation in parts.  And these diodes tend to have a junction drop that increases with current.  So maybe your 0.7v a=was a little lower than average, or your series resistances a trifle higher.   On th other hand for Ampeg to "correct" this "defect", they have to think it worth the trouble.   I suspect the louder you play, the less obtrusive this effect is.  is that the case?  I know a rubbing voice coil is much more noticeable at low volumes than high.   And in the world of guitar amps, "close enough" comes up often.  This is not a hifi.

Crossover distortion is one of the easier things to see on a scope.

Seeker, really, start a thread for your amp, we'd be happy to talk about it.

yaryaryar

A general question:   JM said to add another diode in series with the existing one, then a resistor/trimmer in parallel with those.  So the question is, what does adding an extra diode in series do?   

As I said in the first post, I'm way more familiar with older tube circuits.  As such, I'm not so familiar with diodes (or transistors for that matter).


Enzo

The existing diode has current through it causing the 0.7v drop you report.  Since the bases of the outputs are connected across that diode, they have that single voltage drop to share as bias. About 0.35v per each.  So by adding another series diode, we add a second voltage drop, so about 1.4v, for the outputs to share.  That is about 0.7v per transistor.  That amount is probably a little too much, meaning the outputs will be turned on a bit more than we want, causing excess idle current, and heat.  What we need at those transistor bases is more like 0.5 or 0.45 or thereabouts.

Measure the voltage drop across base to emitter with one of your outputs (out of circuit) to see what junction drop we need to overcome.

SO we put a resistor across one of the diodes.   The diode itself means we don't add any more than its drop.  The parallel resistor means that somewhat less than that drop will occur.  By making the resistor variable, we can adjust it.