Yorkville/Traynor SS Bass Amp Bias

[bobloblaws]

Happy Canadian Thanksgiving!

I have a Yorkville (same company as Traynor) Bloc 80B solid state bass amp here. I'm curious about a note on the schematic, which I am attaching. It says " Bias adjust resistors R29 and R30 should be cut out until 4.5 to 5.5 mV is measured across R33 or R34." I don't know what is meant by the term "cut out". Does that mean dropped in value? If that were the case it would seem odd to me that they used fixed value resistors in those spots as opposed to pots. And what if the bias level needed to be adjusted the other way? In this discussion https://www.indiabix.com/electrical-.../discussion-50 the user named Latha sort of uses that same terminology when talking about a pot/rheostat. But in this context it doesn't make sense to me. Can someone please enlighten me? Incidentally, I'm measuring 6 mV at R33 and R34.

- Bob

[Enzo]

Instead of putting in a trimmer control, they put in a bunch of parallel resistors. The instruction means to actually physically cut the resistors off the board until it is right. Those parallel resistors effectively lower the 680 ohm R27. SO by clipping one or both out, the 680 ohms is closer to the truth.

You have 6mv, which is probably close enough, but by removing one of those resistors, R27 effectively raises. Turns on Q7 a little harder which reduces idle current.

They did it this way because the snip method is close enough, and far more reliable than a trimmer pot.

[bobloblaws]

Instead of putting in a trimmer control, they put in a bunch of parallel resistors. The instruction means to actually physically cut the resistors off the board until it is right. Those parallel resistors effectively lower the 680 ohm R27. SO by clipping one or both out, the 680 ohms is closer to the truth.

You have 6mv, which is probably close enough, but by removing one of those resistors, R27 effectively raises. Turns on Q7 a little harder which reduces idle current.

They did it this way because the snip method is close enough, and far more reliable than a trimmer pot.

Interesting, thanks!

[bobloblaws]

Enzo, since you have familiarized yourself somewhat with the circuit, do you mind giving me a quick overview of what is going on with the rest of it? I know even less about solid state amps than I do about tube amps (I'm doing what I can do get a better handle on the subject of SS), but a quick glance at this schematic kind of looks like it might actually be a push/pull type of arrangement similar to the beloved class AB tube amps. Is it more or less loosely based on the same amp class principle or is it in fact directly analogous, just with transistors instead of vacuum tubes?

[J M Fahey]

far more reliable than a trimmer pot.

+1
Add to that : users with itchy fingers can not "turn the tiny wheel and see what happens"

[Enzo]

since you have familiarized yourself somewhat with the circuit

Actually all I did was look at the bias circuit, which I knew was going to be where those resistors were.

The power amp is very basic, out of a text book. Darlington outputs, common differential pair at the input. Diffy pair drives Q6, the voltage amp. Q7 establishes bias. Preamp is also pretty simple, the two JFETs acting roughly like triodes.

[bobloblaws]

Actually all I did was look at the bias circuit, which I knew was going to be where those resistors were.

The power amp is very basic, out of a text book. Darlington outputs, common differential pair at the input. Diffy pair drives Q6, the voltage amp. Q7 establishes bias. Preamp is also pretty simple, the two JFETs acting roughly like triodes.

Fair enough, sorry to be presumptous. Thanks for the overview, I will do some home schooling :-)

[g1]

As far as your question about push-pull class AB, it can be done with tube or SS. So yes, the Traynor in question is class AB push-pull.

[Enzo]

Class AB, but the design really has no relation to tube circuits.