ESR Quest

[Alan0354]

Yes, ESL is more important if the circuit can reach that frequency range as I posted the article in post #9. That's where the small cap can take over.

I really did not think much about this for guitar amps until this post. really the question is whether this is even important for tube circuit where the circuit is low current ( say no more than 400mA for 100W amp). Even the +B winding has quite a few ohms resistance. Say peak current of 400mA and ESR of 10 ohm, that's only 4V droop out of 400V. A few ohms or lower ESR in the reservoir cap is not going to affect a whole lot. The RC in preamp always have R in kohm range, the ESR is not going to affect the performance. For SS amp where you are talking about 10A of peak current and filter caps of 33,000uF range, ESR and ESL can sure become important and I can understand the reason of using parallel caps. Even ESR of 1ohm is 10V at peak current of 10A. If supply rail is only 60V, 10V droop is big deal!!!

Like Chuck said about people tried putting a resistor in series. People here should really try a resistor below 10 ohm in series with the filter caps is if the ESR increase to see how does that affect the sound. I don't worry about this, I don't use small caps in parallel and I use cheap caps. I am happy so far. Until something happens in the future, I am going to keep doing this way.

[R.G.]

The biggest issue with ESR is not DC voltage loss, but the fact that ESR in the filter caps will support a voltage proportional to, and in phase with, the current drawn by some later stage. If this gets into the signal path in an earlier stage by modulating the plate voltage on a preamp tube, you can get intractable oscillation - either motorboating or high frequency. The details all matter in that case.

1V of signal injected in the plate supply of a preamp tube can cause problems.

And you're right - it's easy to experiment with ESR by deliberately "dirtying up" a cap by inserting a resistor in front of it. This is the standard argument to us if someone doesn't want to change out old electros because it will change the tone. You can artificially put at least that artifact back.

[The Dude]

Aside from the "working" ESR discussion going on here, it's important to note that an ESR meter is a good tool for finding caps that have begun to dry out or leak when leakage is not visible. Like the average tube tester can't tell you for certain a tube is good, an ESR meter can't tell you for certain a cap is good. It's still a useful tool for finding caps that are definitely drying out without having to remove them.

[trem]

What meter do you have/like.?
Thanks

[Alan0354]

The biggest issue with ESR is not DC voltage loss, but the fact that ESR in the filter caps will support a voltage proportional to, and in phase with, the current drawn by some later stage. If this gets into the signal path in an earlier stage by modulating the plate voltage on a preamp tube, you can get intractable oscillation - either motorboating or high frequency. The details all matter in that case.

1V of signal injected in the plate supply of a preamp tube can cause problems.
.

JMHO

The preamp tube will not see close to even a few mV due to ESR because they are not drawing 100mA+. It's only when you draw big current at the reservoir cap that you see that kind of current. At preamp, you are talking about a few mA and it is not varying much as they mostly work in class A. If the +B at the preamp passes even a few mV of signal, that's oscillation big time!!! Something is wrong.

RC in +B on the preamp tubes usually have R over 2Kohm, that makes the ESR not that critical. I don't know enough, I don't deal with old caps. But I use all different cheap caps like China caps and Illinois cap etc. I don't have problem at all as long as I have 20uF. I did have problem when I used 10uF for high gain, that cause oscillation. But I chuck it to not enough capacitance rather than ESR.

[g1]

it's important to note that an ESR meter is a good tool for finding caps that have begun to dry out or leak when leakage is not visible.

Yes, another tool for the arsenal can never hurt.
The DER EE DE-5000 is supposed to be a good unit, and reasonable price.
Der EE de 5000 Handheld LCR Meter | eBay

[The Dude]

I have the Sencore LC75. It's a full capacitor-inductor analyzer. It has an ESR test and I use it often for finding dried out caps. Probably more than most want to spend. A simple ESR tester is a useful tool and much less expensive. The only one I've ever used is the blue faced one pictured on this page.

ESR Meters for Measurement | Info Service TV

It worked quite well and I would gladly buy another if I needed one. That said, it's the only "ESR test only" device I have any experience with, so I don't have any basis for comparison.

[R.G.]

The preamp tube will not see close to even a few mV due to ESR because they are not drawing 100mA+. It's only when you draw big current at the reservoir cap that you see that kind of current. At preamp, you are talking about a few mA and it is not varying much as they mostly work in class A. If the +B at the preamp passes even a few mV of signal, that's oscillation big time!!! Something is wrong.

It's not the current the preamp tube pulls. It's the current pulled by larger current stages later in the power supply. If there is any shared impedance - like the ESR of the first filter cap that feeds the other stages - the high current stages can modulate the voltage getting into later (from the power supply's point of view) power users which run at lower signal levels and higher gain. These stages are supposed to be decoupled by their own R-C sections, but if those RC sections are not working right, the signal impressed on the higher voltages by the high current stages can be seen in the lower voltage preamp tube supplies. This is, by the way, the reason for the existence of those series-R, shunt-C sections in the supply chain. And the less current the preamp stage uses, the less loaded the series R is, and if the shunt C is not good, you can get signal fed through.

I feel like I'm not explaining this very well for some reason.

[Alan0354]

I feel like I'm not explaining this very well for some reason.

I think you explained very clear, It's just only my opinion that it's not as critical in the tube amp......Well at least I have not seen it with my limited experience.

Anyway, I think we got into too deep on this, the ultimate thing is for people to judge the sound. If people think it makes a difference then they can make their choice. As I said, I am born cheap, I tend to justify buying cheap caps and not worry about the ESR. Ha ha, I think one of the FT cap is like two to three times the cost of the Weber special AND the Weber special is 600V instead of 500V of the FT.

[The Dude]

I did a little digging. Apparently the company that built the ESR tester I referenced above is no longer in business. I did find out through some web searching that it was a highly regarded machine amongst experienced techs. The link below is supposed to be the successor of the analog version. You can get it assembled or buy it as a kit. I'm not endorsing it because I haven't used one, but if it's as good as its predecessor, it would be worth the $. It will save you hours of unsoldering caps to test them, especially if you do any SMD work where unsoldering a cap means you are going to replace it. Putting them back on the board after unsoldering is not a good idea.

AnaTek Corporation - Blue ESR Kit

[Jazz P Bass]

It's not the current the preamp tube pulls. It's the current pulled by larger current stages later in the power supply. If there is any shared impedance - like the ESR of the first filter cap that feeds the other stages - the high current stages can modulate the voltage getting into later (from the power supply's point of view) power users which run at lower signal levels and higher gain. These stages are supposed to be decoupled by their own R-C sections, but if those RC sections are not working right, the signal impressed on the higher voltages by the high current stages can be seen in the lower voltage preamp tube supplies. This is, by the way, the reason for the existence of those series-R, shunt-C sections in the supply chain. And the less current the preamp stage uses, the less loaded the series R is, and if the shunt C is not good, you can get signal fed through.

I feel like I'm not explaining this very well for some reason.

Oh yeah.
Think Fender Hot Rod series amps.
That first stage decoupling cap goes bad & you can see the signal riding on the B+.
(especially true for the 'Dirty' channel.
With a good cap, the signal presents itself very briefly, but then gets decoupled.

[nevetslab]

I did a little digging in the area of ESR Testing methods, found a number of interesting approaches on the internet using a moderately high freq narrow pulse (1uS, 10V high with 1mS rep rate) and a scope, using the 50 ohm source of the pulse or function generator and a 49.9 ohm series resistor (100 ohm source). it yielded effective results for a limited ESR range of 0.1 ohm to 10 ohms. Another method used a 100kHz square wave (50 ohm source) with a BNC T at the generator output, a 50 ohm BNC cable with a 50 ohm feed-thru terminator at your scope from one side of the T, and a BNC cable with test probes at the other end from the other side of the T. Sig level 100mV P-P. Short the probes together, and you get a very narrow leading edge pulse, with little or no level past the narrow pulse. Apply the signal (coming down the test probes) across a good cap, it shows about the same as shorting the probes, but a bad cap or one with high ESR, there's a lot of signal past that leading edge pulse. This method wasn't calibrated, but was more of a good/bad approach. I gotta try this.

Tektronix has an App Note to measure ESR and ESL using a generator, a known series resistor, and a digital scope that yields numeric readout of the probe measurements on both sides of the series resistor, feeding the cap under test. Record the values, use the formulas for calculating phase and one can derive the impedance and ESR of the part. Not a rapid test approach, but old-school methodology using current digital instruments.

Tektronix Cap & Ind Measurements using scope-gen.pdf

There were a number of other variations that all seemed to be using a HF signal or pulse, a known series resistor, a scope and you're looking at the magnitude across that series resistor while feeding the test signal to a grounded power supply cap, or across a coupling cap (grounding one end).

General Radio gives you formulas for calculating the ESR of a cap (out of circuit, of course) based on the dissipation factor (tan of theta of the cap) in the operators manual of their impedance or capacitance bridges. And on their Digi-Bridges, like the 1657 or 1658, you can measure ESR directly, though only at the test frequency of 1kHz or 120Hz.

I'm guessing the Blue ESR box is a calibrated variation of injecting a HF signal or pulse thru the two probes which are placed across the cap IN CIRCUIT. I haven't stopped to try any of the methods, but I did like the simple, non-calibrated method yielding a Go/No Go approach, once you got used to seeing what looks good and not good.

Knowing there's always some residual voltage left over after discharging power supply electrolytic caps (from inherent dielectric absorption), using a high quality 600V film cap in series with the pulse generator signal, you protect your generator. Using AC coupling on the scope, you protect it's input from any voltage that you forgot to discharge, or what charges back up from the DA of the cap.

Here's a couple interesting links that I found using simple instrumentation (shown with late generation digital scope with waveform measurement capability):

Measuring capacitance :: Electronic Measurements

All about ESR measurement | PA4TIM's opvangtehuis voor buizenbakken

[dai h.]

Yes, another tool for the arsenal can never hurt.
The DER EE DE-5000 is supposed to be a good unit, and reasonable price.
Der EE de 5000 Handheld LCR Meter | eBay

I posted about the apparent source for these on ebay before FWIW:

http://music-electronics-forum.com/t36853/

(about 41 bucks incl 8% consumption tax(from the store--not the ebay sellers) for the meter only at today's USD-Yen exchange rate--I assume you would want some of the other accessories though--so (if you buy from one of the ebay sellers,) you might pay whatever you think is fair in addition to the ebayer)

[Mick Bailey]

I use my home-made ESR meter all the time. It's my most-used piece of test gear after my DMM and cost me nothing to build. I can quickly run right through an amp (TV, SMPS or whatever) and get a good idea of the state of the electrolytic caps and their prospective remaining lifespan (unless the cap is shorted). Caps with increasing ESR still generally measure up at their rated value on a capacitance meter. I find ESR particularly useful in troubleshooting older SMD acoustic preamps suffering from signal loss. Those small caps rise to tens of ohms and eventually will go open and kill the output.

The absolute measurement of ESR isn't the important thing, it's whether the value has risen compared to similar caps in the equipment, or compared to a new one of the same type. So lets say you look at those Illinois caps in a Fender. You measure one and it's 0.8 Ohms, the next one the same. But then the third is 7 Ohms. You then know that cap is on its way out.

In less time than it's taken me to type this up, I can test pretty much every cap in an old amp and spot where there's a problem. Particularly useful in 70s amps where everything works but the output is low or it doesn't sound right.

Lots of people have good things to say about the Anatek unit, but there are plenty of schematics that can easily be built up and calibrated. Make sure you get a meter that can test charged caps, or circuits under power.

[nevetslab]

Simple ESR Test set-up using Square Wave source & scope

After reading thru and trying the basic ESR Measurement Test Setup as outlined in the first of several approaches in this article I found yesterday

Measuring capacitance :: Electronic Measurements

Following the information presented in the paper's Measuring with a Square Wave & Figures 1 & 2, I set up my Tektronix FG501A Function Generator for a 10V P-P Square Wave, used a BNC-T to feed both my Tek 7623 100MHz scope and a short BNC cable with a Pomona female BNC to EZ-Hook lead adapters to connect to a variety of Electrolytic caps, ranging from 10uF to 10,000uF to see how this would work for a rapid visual inspection tool to spot caps that showing problems, as well as being able to select thru on-hand parts, including NOS parts that have been aging.

I didn't stop to use the measurements to determine the capacitance and the ESR, though those can be derived from the scope measurements. I suppose if I wanted to do that, I'd use my LeCroy Digital scope to yield accurate data.

What I did find was, following the comments in the Meettechniek article, adjusting the frequency to show a significant step-ramp waveform that results when applying the 10V P-P Square Wave from my 50 ohm source (which yields a 200mA constant current source), at a glance you can tell a part that has higher or lower capacitance as well as higher or lower ESR, comparing more than one part of the same value.



In these scope images, the vertical rise portion of the waveform contains the ESR magnitude, while that of the ramp portion contains the capacitance magnitude, These 10,000uF/80V electrolytic caps were pulls from an SWR SM900 bass amp. Of the four seen in the photo, one shows much higher ESR than the other three. During the evaluation process of this approach, that one cap changed numerous times under test, and it wasn't test lead contact related. It was a bad part getting excited by the applied 200mA constant current. It had tamed down a bit at the time I took these scope images.

Next, I looked at two different 22uF axial Electrolytic caps, one was a 500V part, pulled form a Fender Twin's supply board, the other 450V part was new.



Looking at the vertical rise in the scope photo, the used part shows higher ESR than the new one, while the magnitude of the ramp looks about the same, indicating the capacitance is nearly the same.

I then looked at some NOS 10uF/350V Mallory Axial Electrolytics, having a date code from 1985. Each varied in capacitance, as well as ESR. I showed the two extremes of the 3 I measured.



I haven't yet tried this method on parts IN CIRCUIT. One issue you have with them in circuit is if there's more than one cap in parallel with another to increase the overall value at that circuit. Placing a part across one connected shows increase in capacitance and decrease in ESR, as seen in the magnitude of the ramp and the vertical rise portion of the step-ramp waveform.

So, this method does lend itself to rapid testing, can yield accurate results if care is taken in recording the data, and can be done with basic bench instruments, while the test set-up isn't difficult at all.

I tried a couple other methods that didn't live up to their claims. I'll try this one out on some amps coming onto the bench shortly to see how it behaves IN CIRCUIT.

After reading the notes at the bottom of the article linked in this post (Measuring Electrolytic Capacitors), I failed to offset my 10V P-P Square Wave, so that applied voltage in this series of measurements DID apply reverse polarity to the caps being tested. I'll check the results after making tht adjustment.