signal generator advice

[Marioverkill]

Hello all,

I have acquired a tek scope.I am looking for a signal(function) generator.I want to start using the scope when working on amps.I am new to this.I know for biasing and observing signals others use a 1Khz signal input to the amp.Will this one do the job?

http://www.rolls.com/productImage.php?pid=MO2020

The output is rated in dB.I am unsure if this output range is good for guitar tube amps.

What is the voltage range to simulate pickups?(1-5 Volts ?)

Thank you for any info and suggestions.

Mario

[Structo]

I think usually for checking an amp it's about 1v @ 1,000Hz

[Jana]

1 volt is way too hot if you are going into the guitar input jack. 100 millivolts is about the level of the average guitar pickup.

[Structo]

ooops

I'm sorry maybe I was thinking into the PI.

[Cliff Schecht]

Yeah I run the same, about 100mV P-P (about what light strumming on a Strat neck will get you).

The later Fender schematics (like 1990's and up) usually have something like a 3-5mV input labeled on the schematic. Ideally if you put their input signal (3.85mV @ 1kHz in the case of a '59 Bassman Reissue), you should be able to probe the other points of the amp and measure very similar amplitudes to what is labeled on their schematic. If you want to "up-convert" these numbers for your input signal, divide the test point value (say 213mV for the V1a plate AC component) by the input signal. This gives you the gain of the first stage which you can multiply by your input signal (say 100mV). Sticking with the 59BM example, 213mV/3.85mV=55.3 V/V. Multiply your input signal times this number to get your "expected" amplitude at the V1a plate (55.3*1mV=~5.5V of AC signal at the plate). Of course all of this is assuming you aren't clipping anything.

Also for those not familiar, V/V stands for Volts per Volt, a unitless way to express the gain of an amplifier. Sometimes we also express gain in dB by doing 20*log(V/V).

[Marioverkill]

Thanks everybody .

Can anyone check the link I posted and give me your opinions.If the Rolls unit is not suitable,I will get a function generator.Just trying to keep cost down without sacrificing reliability.

Mario

[Cliff Schecht]

Honestly you can build a simple function generator that will do 99% of the tasks that you would need as an amp builder. Everything you need can be bought at RadioShack or for very cheap from a place like Mouser.

Or kits for these things are very cheap. PAiA sells a little function generator kit for $14.00 which surely would work fine for our testing needs. Not trying to sound like an ad, I did work for PAiA for a few years, but these dinky little kits they sell can be quite useful.
http://paia.com/proddetail.asp?prod=K23&cat=50
http://paia.com/KRUKits/K23/K23.pdf

[John_P_WI]

I have an old tube HP generator that I use for a sine wave, but always thought some thing like this could be cool:

http://electronics-diy.com/product_details.php?pid=537

[Super_Reverb]

Another option is to purchase a used function generator online. I found a B&K for ~$40 on ebay. One this unit has is sweep frequency capability - useful for evaluating transient response over a frequency range (can help you flush out parasitic osc)

rob

[Cliff Schecht]

RG from Geofex and DIYSB posted this a few days ago. Pretty neat idea.
http://geofex.com/FX_images/Fake%20Guit ... llator.pdf

[Colossal]

Another option is to purchase a used function generator online. I found a B&K for ~$40 on ebay. One this unit has is sweep frequency capability - useful for evaluating transient response over a frequency range (can help you flush out parasitic osc)

rob

Yep, that's what I did. Got a pretty much new Goldstar FG-8002 for something like $50. It had been on a shelf doing nothing in an engineer's basement. Good score. It has quite a few useful functions.

[Firestorm]

The Rolls unit is probably too hot to be broadly useful. It says it has a 10dB pad which generates a -10dB "reference signal." By -10dB, they presumably mean -10dBV which is 316mV.

[Cliff Schecht]

Notice that the front panel has sine and square amplitude controls that range from -infinity (0) to +11dB. You can set the amplitude as low as you need methinks...

Also I think that -10dB pad means you can turn the level up all of the way, click this on and have the reference signal you mentioned (-10dBV). Just a guess there though..

Honestly I think $70 is a bit pricey for the Rolls function gen (that was the price I saw online anywho) that only does sine and square over a very limited frequency range (granted I use my function gens for more than testing amps). Triangle waves are incredibly useful for looking at where and how distortion is occurring and the ability to add a DC offset is nice if you work with any solid state stuff at all (pedals and such even).

This is a DIY forum right? Why not save a heap of cash and build one yourself? The circuits are typically very simple and not too fussy to boot (we don't need accuracy either!). I build stuff like this on perfboard all the time but if you are feeling especially lazy then surely you can find a pre-fabricated PCB from someone or buy a kit..

[Cliff Schecht]

http://www.amazon.com/Velleman-MK105-Au ... B000TA3FKS

This kit is $7 and generates sine, square, sawtooth and triangle waveforms in a pretty small package. Adding a frequency adjustment is as easy as changing one resistor to a pot. Or you could even make the pot switchable so you have a 1kHz OR variable function generator.

[Deric]

There are several free apps for iphone or Droid.