You will need to buy probes for it too...
Cheap ones (guess where they come from...) are about 20-30 USD each though...
Cheap ones (guess where they come from...) are about 20-30 USD each though...
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
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
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
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
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 😉
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
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.
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.
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
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
Johan Potgieter said:
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
ghpicard said:
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
Sheldon said:
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
ghpicard said:
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
I will throw in my 2 cents....
The inductance is good becuase it allows a greater filter slope..at -40dB instead of -20dB ...
The R with the tube input cap...is -20dB...
The LC is a double Pole -40dB slope...
Many times, in the old days, you will see a small inductor wound around the grid-stop resistor making a small LCR network...
The LC make -40dB DOUBLE POLE slope then as you go higher in frequency the resistor in parallel with the inductor makes a ZERO, which cancels one of the DOUBLE POLES and your left with a single pole and back at -20dB per decade slope the rest of the way.... there can be many reason why to start out with such a sharper slope....
Chris
The inductance is good becuase it allows a greater filter slope..at -40dB instead of -20dB ...
The R with the tube input cap...is -20dB...
The LC is a double Pole -40dB slope...
Many times, in the old days, you will see a small inductor wound around the grid-stop resistor making a small LCR network...
The LC make -40dB DOUBLE POLE slope then as you go higher in frequency the resistor in parallel with the inductor makes a ZERO, which cancels one of the DOUBLE POLES and your left with a single pole and back at -20dB per decade slope the rest of the way.... there can be many reason why to start out with such a sharper slope....
Chris
ghpicard said:
Totally conceded, Ghpcard. I should have been clearer and added values or at least order of magnitude. I meant that the inductance of metal film resistors that I measured would only start to act in the several Mhz region where amplifier response is so low that it would not play any significant role. Another way of measuring this will be to plot h.f. response with metal film as well as carbon resistors. I have not done it, but will be surprised if there is any significant difference. Cerrem's basics are also correct, etc.
Grid stoppers themselves are of course a different matter, though some amplifiers do not have them. I would suspect that especially in triode output stages their presence would be noticable in a frequency plot or square wave display.
Thanks for pointing this detail out.
But after that, Sheldon, I would not use carbon resistors because one worries about the inductive component of metal films, as per above experience. Then again at the levels prevalent in power amplifiers I confess that there would not be much of an extra noise problem. One simply uses metal film because they are generally better, if not practically so - a little vanity, I guess?
Regards.
Johan Potgieter said:
Not faulting you here Johan, as your posts are very informative. But as a someone limited in experience in these matters I sometimes find it hard to make sound (as in properly engineered) decisions based on some of the discussions on the forum. The word "magnitude" is the one I should have used when posing the question as to practical significance of grid stop resistor type.
Facts, such as "less than" or "more than", are often taken to judgements such as "better for" or "worse than", without the supporting numbers or even, as you say, at least the order of magnitude. The practical outcome is that I think in this case I won't worry about changing some grid stoppers of the metal film variety.
Sheldon
Sheldon said:
I used metal film grid-stoppers for years because they were what I had. A while ago, I was putting in an order for bits and needed to make it up to the amount that would qualify for free postage so I ordered some carbon resistors. I now use carbon for grid-stoppers because I know that they are theoretically better and I have them. One day, I might just make something where the carbon resistor will make all the difference and I'll be saved the nuisance value of having to diagnose a problem.
To summarise; it's a very small difference, but if you have the choice, carbon is the one to use.
So, this falls under the why-I-did-this-I-don't-know category:
I was just about to push the button on a Tektronix 465B 100mHz scope and then I called Mrs. Annan to make sure she was OK with it. You know the rest.
So, it looks like I'll have to wait a few months before getting my new toy. In the meantime, I have 910pF, 470pF and 330pF micas in stock and I think I should try and put them in series to get close to the recommended 270pF value for now.
So, that means I can wind up with 310pf (910 & 470) or 242pf (910 & 330) given the available values. Any particular combination preferrable over the other or are all equally satisfctory / unsatisfactory? I'd assume the 242pF combo is better as it's a bit closer to the 270pF we want, but what do I know.
Nothing. That's what.
Don't get married.
Kofi
I was just about to push the button on a Tektronix 465B 100mHz scope and then I called Mrs. Annan to make sure she was OK with it. You know the rest.
So, it looks like I'll have to wait a few months before getting my new toy. In the meantime, I have 910pF, 470pF and 330pF micas in stock and I think I should try and put them in series to get close to the recommended 270pF value for now.
So, that means I can wind up with 310pf (910 & 470) or 242pf (910 & 330) given the available values. Any particular combination preferrable over the other or are all equally satisfctory / unsatisfactory? I'd assume the 242pF combo is better as it's a bit closer to the 270pF we want, but what do I know.
Nothing. That's what.
Don't get married.
Kofi
Kofi Annan said:
Not altogether, Kofi. It is often a matter of things not making a mentionable difference. Similarly, capacitance variation of a few percent often does not make a practical difference. At least wait until you can look at a square wave before deciding on fine differences like 240, 270pF or such. One cannot predict or calculate that; practical circuit parameters mainly including the output transformer have too much to do with it.
Success!
Kofi-
Don't look at the final cost of that B&K scope I pointed you to, unless you have a strong masochistic bent
Mrs. Annan probably would have given that a green light.
-Casey
Don't look at the final cost of that B&K scope I pointed you to, unless you have a strong masochistic bent
Mrs. Annan probably would have given that a green light.
-Casey
So, I ran home early today and soldered in the mica to get the 240p and WALLA WALLA WASHINGTON! The high end is back!
I don't hear any warble or washout as yet, but I need to spend some time with it and see how it goes.
More to follow.
Yeah!
Kofi!
I don't hear any warble or washout as yet, but I need to spend some time with it and see how it goes.
More to follow.
Yeah!
Kofi!
After a listen.... it needs more NFB. Sounds a bit grainy and rough. That's a good sign that NFB is warranted, right?
You've been updated.
KofiifoK
You've been updated.
KofiifoK
More updates...
I changed the makeup of the NFB circuit today. The cathode resistor is now around 700R and the feedback resistor is down from 8.4K to about 4.6K with a 470pF mica bypass cap. I think this one's the winner.
It sounds natural and not as stifled as the original configuration and it really needed more correction than I had going with the 8.4K NFB resistor.
I'll play it for a week and see how it sounds but I had a little... uh... accident while I was soldering.
I guess I was manhandling the third pin (cathode) on one of the E88CCs and it snapped off. I managed go get it back in and I soldered it back into the socket, but I'm guessing a responsible person would remove the entire socket and engage in a do over.
Right?
Kofi?
I changed the makeup of the NFB circuit today. The cathode resistor is now around 700R and the feedback resistor is down from 8.4K to about 4.6K with a 470pF mica bypass cap. I think this one's the winner.
It sounds natural and not as stifled as the original configuration and it really needed more correction than I had going with the 8.4K NFB resistor.
I'll play it for a week and see how it sounds but I had a little... uh... accident while I was soldering.
I guess I was manhandling the third pin (cathode) on one of the E88CCs and it snapped off. I managed go get it back in and I soldered it back into the socket, but I'm guessing a responsible person would remove the entire socket and engage in a do over.
Right?
Kofi?
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