Kofi Annan in: "Push and Pull with Me"

[ghpicard]

You will need to buy probes for it too...
Cheap ones (guess where they come from...) are about 20-30 USD each though...

[valveitude]

Hi Kofi,

You should have little trouble finding a good scope for around a hundred clams..just make sure you get a working one

I got my Tek 2236A for $150.00 on ebay, there are a few listed now..this one for instance.

Not only do you get a smokin' scope, but a freq. counter and a 6 digit DVM!!

Best test equipment purchase I ever made.

-Casey

[cerrem]

Make sure to check the voltage ratting of the probe...
Most standard probes are CAT III 300V probes...... Make sure to use the Higher Voltage probes for tube amp circuits...
The next step up is the CAT III 600V probes....which is good for pre-amps...but risky if you like the measure the AC voltage across the OPT primary in high voltage power amps..
You can buy used high voltage probes on Ebay such as a TEKTRONIX P6007 X100 HIGH VOLTAGE OSCILLOSCOPE PROBE, which is good to 1200V.....
You should be able to get one used for under $40....

I also use the Tektronix P5100, is good up to 2500V..

Chris

[valveitude]

You want 50MHz minimum, 100MHz would be better, at least two channels

Sy's point is well taken. 50-100mhz is needed to sleuth out rf oscillations...but...I would submit a 20mhz scope is better than NO 'scope

Make sure to check the voltage ratting of the probe...

Likewise cerrem's points are valid..but..I have gotten away with my cheapo 300V probes by lashing together voltage divider's when needed. Alternatively, I have used a high voltage blocking cap, with a high value resistor to ground after the cap to ground. The proper probe is the first choice, but there are work arounds.

I only point this out so as to not scare the soon to be "former" Seceratary General off with the higher price of admission of doing it "right".

I use the HP-9100 probe found here for 20 bucks.

-Casey

[wa4htz]

Kofi,

If you read the fine print (I assume you are used to doing this since we know there are countries which try to slip all kinds of stuff in those UN resolutions), the Tek 2236 'scope Casey linked to runs on 220 VAC, 50 Hz. In the US, you'd have to hook it to the dryer or stove outlet! The 465 is a good 'scope and was the workhorse of Tek's line for years. I have its faster brother, the 467, and see no reason to want another 'scope. You can get standalone pieces of test equipment to fullfill the functions that newer 'scopes might include and, if something breaks, you aren't completely without any test equipment. Just a little food for thought.

Ken

P.S. Glad you like your P-P 6BQ5 amp and hope you get it tuned to your happiness. I have an opt suitable for 6BQ5s (wish I hadn't given away its twin years ago) and am going to build a P-P with it and the used GE 6BQ5s in the junk box.

[Johan Potgieter]

Kofi,

Me jumping in and out of communication. Sure you will understand what a busy schedule is.

We must be careful here with that 910pF - I would most certainly encourage the use of a scope here. That capacitor (as has been said by Sheldon) is not there for frequency control in the first place, but phase compensation.

That means that phase has already been shifted as a result of output transformer leakage reactance and other little capacitances in the circuit, which will give a peak somewhere around 70 KHz for this circuit. (This is most conveniently evident from a square wave display as an overshoot.) While it is not audible, noise there might intermodulate to give audible components etc.; better not to have peaks anywhere. The 910pF shifts feedback voltage phase in the opposite direction, thus affording a correction. (There are also other secondary effects, which need not concern us now for simplicity's sake.)

If the feedback resistor is now altered (8,4K?), such phase shift needs to remain at approximately the same frequency (again ignoring other factors), so the "best" capacitor value needs to come down, say to about 270pF. If you keep the 910pF over a 8,4K, you will probably not see a good square wave (too much roll-off); also other squiggles might come in depending on circuit parameters. This is only sorted out optimally by looking with a scope and proper load there.

A long description which may be superfluous, but to get matters clear. I would echo the advice that if you are at all interested in further audio DIY (and you definitely appear to be!), you must get at least a scope and a signal generator. No use working in the dark with hunches and (possibly) wrong deductions leading nowhere or worse.

To conclude also, one needs not to worry about inductance in resistors and suchlike in audio. This only starts to be of consequence in the r.f. region, if at all. Where a circuit is so heroically misdesigned that this plays a role in audio amplifiers, other things need to be examined or better still, throw the circuit in the fire.

Regards

[ghpicard]

Originally posted by Johan Potgieter
To conclude also, one needs not to worry about inductance in resistors and suchlike in audio. This only starts to be of consequence in the r.f. region, if at all. Where a circuit is so heroically misdesigned that this plays a role in audio amplifiers, other things need to be examined or better still, throw the circuit in the fire.

Johan, if we put gridstoppers, then we are worrying about RF. Thus, we have to use the right component for the task. In the audio frequency range, a gridstopper is just an ornament.
Using your phrasing, the worst case of heroical misdesign ( )would lead a circuit to plainly oscillate in HF.
The normal usage in which everyone puts the ECC88, an RF double triode designed for VHF cascode operation to do audio amp tasks, is to prevent oscillation due to stray capacitances, inductance of the wires and all such normal stray components of a circuit. Layouts that look neat and work OK in the audio range, are usually a very bad practice in the RF one.
The mild consequence could be HF ringing in fast transient signals (square waves anyone ?)

So IMO putting gridstoppers will add some tiny cost to the bottom line, and save several headaches in the case of a design that looks neat but is a bad RF layout. And of course won't help if the circuit itself is badly misdesigned...

Gastón

[Sheldon]

Originally posted by ghpicard
So IMO putting gridstoppers will add some tiny cost to the bottom line, and save several headaches in the case of a design that looks neat but is a bad RF layout. And of course won't help if the circuit itself is badly misdesigned...

Gastón

Taken in the context of previous posts, I didn't read Johan's comments as recommending against using grid stopper resistors - only that the small inductance difference between common small resistor types is not important in this application.

Sheldon

[ghpicard]

Originally posted by Sheldon


Taken in the context of previous posts, I didn't read Johan's comments as recommending against using grid stopper resistors - only that the small inductance difference between common small resistor types is not important in this application.

Sheldon

Yes, I read the same, and perhaps I wasn't as clear as I wanted to be. I wanted to remark that for gridstopping inductance is important. Not so for other usages that are not related to HF parasitics supression.

Gastón

[Sheldon]

Originally posted by ghpicard


Yes, I read the same, and perhaps I wasn't as clear as I wanted to be. I wanted to remark that for gridstopping inductance is important. Not so for other usages that are not related to HF parasitics supression.

Gastón

My question, and it's an honest question, is; is this a practical issue with common metal film resistors, or is it a matter of "it's a remote possibility, but why chance it"?

Sheldon