ForumVibe resistor values

[JBK1]

I'm currently building the forumvibe and came across a couple of resistor value conflicts. On the parts matrix R47 is listed as 100 ohms, on the layouts its 150 ohms. Also the parts matrix has R34 and R35 as 76K5 on the layout its 76K8, I don't believe resistors come in either value, should this be a 75K ohms?


thanks
Jerry

[Hides-His-Eyes]

In E24 standard, both 100 and 150R would be possible, but you'd only find a 75K.

Does the schematic specify that a higher level of tolerance than 5% is needed? 76k8 does exist, but only at a tolerance level of .5% or higher, which is implausible in this case.

[slacker]

75k will be fine instead of the weird 76k8 resistors, not sure why he used those because there's a trimmer between them that will have a much lower tolerance than any resistors people are likely to use so there's no point using super accurate resistors.
100 or 150 ohm should work for R47 I don't think it matters which.

Schematic http://classicamplification.net/forumvibe/

[R.G.]

75k will be fine instead of the weird 76k8 resistors, not sure why he used those because there's a trimmer between them that will have a much lower tolerance than any resistors people are likely to use so there's no point using super accurate resistors.

The accuracy isn't needed, but maybe it's picked for using metal film for low noise. That's fine, probably gilding the lily on a circuit like this, but it's a defensible reason maybe. Some people always use metal film out of habit now that they're cheap.

[PRR]

> maybe it's picked for using metal film for low noise

Perhaps, but... the 22K input resistor is the most noise-critical; that and its 47K neighbor are the lilies you could guild, though neither matters in e-guitar context.

Precision is moot, since each "76.5K" has another 0-23K-47K trim in series.

I think he had a bucket of 76.5K to use-up.

47K or 100K will be fine, just make both the same color-code and better than 20% precision.

Page 24 wants you to rig so you can find the "50:50 mix" quickly. If that's not very-near center, try 68K, 75K, 82K etc for one of the two "76.5K" resistors.


The 100/150 resistor is really about the lamp you use, and lamps have huge tolerances. Low-current lamps will want larger emitter resistor. That's why there is 500 ohm trim. If you use 150 ohm fixed and have to dead-short it to get any action, try 100 ohms fixed.

[R.G.]

> maybe it's picked for using metal film for low noise
Perhaps, but... the 22K input resistor is the most noise-critical; that and its 47K neighbor are the lilies you could guild, though neither matters in e-guitar context.

Much like thinking that rubbing germanium transistors on a fuzz pedal will make you play and sound like [insert guitar god here], there's a belief amongst some folks that just putting in a metal film resistor eats some of the noise out of the circuit.

I agree with you, that front end is atrocious. Presumably it was designed to mix two line level inputs, or that was carried through from the predecessors of the circuit. Not only are the 22k/47k tone sucking loads, they add noise AND cut signal at the same time. Ugh. My simulation and mutilation of a proto shows me that changing the 22K to perhaps 0 ohms to 1K and the 47K to 2.2M will improve things a lot, as well as probably obviating the "unity gain mod".

[JBK1]

Thanks for all of the input,

R.G. I will take your advise on R1 & R2..thanks and I'll try rubbing germanium transistors on the fuzz pedal thing, crossing my fingers.


Jerry