Plate stop resistors?

[Steve Conner]

When I got the amp there was a 3 or 4 inch black whole inside the chassis where one of the screen resistors had blown up and taken everything around it with it. That tube had completely shorted, but all of the output tubes tested bad. Never could figure that out....

They are 3 watt resistors which seem kind of small to be in the plate circuit of a KT88. The first pair I put in got way too hot when I had the amp close to full power.

If I am going to remove the plate resistors and see what happens to the current, would it be better to remove them one at a time or all at once? I'm guessing all at once to maintain the balance.

And lets suppose one burned up? I'm assuming since there are two pairs of tubes in parallel that the other three tubes will keep going, albiet not properly. But will the tube who's plate resistor burnt up shut off, or will the screen begin to act like the plate and begin to draw excessive current?

Thanks for the help, as always. Still wrapping my head around all this, a little each day......

Yikes.

A single KT88 on full blast, as part of a push-pull pair, draws about 200mA of plate current. That's average. Because of the peak-to-average ratio of a sine wave, it is around 655mA peak, and because each tube conducts on alternate half-cycles, the RMS value works out to 328mA per tube.

(To get the RMS: divide peak by square root of two because it's a sine wave: and square root of two again because every other half-cycle missing)

328mA RMS through a 100 ohm resistor is 10.7 watts, so no wonder the 3 watt ones burnt up.

Whenever I've seen those plate stopper resistors used, they've been 10 or 22 ohms, not 100. A 22 ohm resistor would dissipate 2.4 watts, so if you want to use a 3 watt type, that would be a reasonable value.

Yes, if a plate resistor goes open circuit, the screen will probably burn out too. (Hopefully the screen resistor will die first.) So maybe using them as fuses isn't so wise.

[raiken]

a TATG or whatever.

On our side of the pond, we hams (or former hams) call that a TPTG. I had to think twice when I saw that.

(That's a tuned-plate, tuned-grid oscillator, for those not familiar with the acronym).

And yes, the 100 ohm plate resistor is indeed for parasitic-suppression, as others have pointed out.

RA

[alchemy]

Yikes.

A single KT88 on full blast, as part of a push-pull pair, draws about 200mA of plate current. That's average. Because of the peak-to-average ratio of a sine wave, it is around 655mA peak, and because each tube conducts on alternate half-cycles, the RMS value works out to 328mA per tube.

(To get the RMS: divide peak by square root of two because it's a sine wave: and square root of two again because every other half-cycle missing)

328mA RMS through a 100 ohm resistor is 10.7 watts, so no wonder the 3 watt ones burnt up.

Whenever I've seen those plate stopper resistors used, they've been 10 or 22 ohms, not 100. A 22 ohm resistor would dissipate 2.4 watts, so if you want to use a 3 watt type, that would be a reasonable value.

Yes, if a plate resistor goes open circuit, the screen will probably burn out too. (Hopefully the screen resistor will die first.) So maybe using them as fuses isn't so wise.

Yep they were pulling 200mA at full blast, and less than a minute of it was enough to make the resistor color codes unreadable...

I guess when it originally went out, the plate resistor probably burnt open and the sudden excessive current through the screen caused the screen resistor to spontaneously combust! I still don't get why all of the power tubes were bad though...

As for 100 ohms, I have no idea. Ampeg used 10 ohms, Sunn used 50 ohms. It doesn't seem like it would be to protect the OT because the screens are UL tied to OT and if the plate goes open then all the current is still flowing through the OT screen windings.

[redelephant]

...just want to add my pennies regarding ringing tubes. We can all agree the oscillation starts due to an electrically excited LC resonant circuit, but once the oscillations start, it can actually turn into mechanical motion, which in turn becomes an audible ringing. That's what microphonics is...

while at it...

I wouldn't regard plate stoppers as band aids. More like extra insurance just in case poor wire dress, layout, or something else causes oscillations. The standard tubes used in guitar amps normally don't need them, but as mentioned, radio tubes do. The 807 is a faster version of 5881/6L6 made for radio work, and needs them while 6L6s usually do not. Also as mentioned, 100ohms do seem a bit much since they will dissipate a lot of power. No way are they there to protect the OT.

Thanks Steve for the input regarding current sharing in pentodes, I didn't think about the fact that the screen has much more effect and makes the plate resistors useless in that way. I guess triodes are the only tubes where the plate resistors can help with current sharing.

[raiken]

...just want to add my pennies regarding ringing tubes. We can all agree the oscillation starts due to an electrically excited LC resonant circuit, but once the oscillations start, it can actually turn into mechanical motion, which in turn becomes an audible ringing. That's what microphonics is...

Agreed. To further clarify, some elements can indeed vibrate mechanically as a result of the electrical oscillation (if the frequency is low enough), but that isn't necessarily a resonant condition, i.e. a mechanical "oscillation", rather, it is a sympathetic vibration at the frequency of electrical oscillation, which is a different thing. Ceramic capacitors and transformers are notorious for "singing" whenever an AC current is passed through them. Tubes can do the same and produce an audible noise. Oscillations don't have to be present for this effect, just a large enough signal to cause an audible excitation of the elements of the component. This is why you can hear your guitar playing through the output transformer when you accidentally leave the speaker disconnected. It is called "magnetostriction", in the case of a transformer, and "piezoelectric noise" in the case of a capacitor.

Where the microphonics do cause a mechanical oscillation is when the feedback mechanism is through the air - for example, a coupling of audio-frequency waves from a speaker back to the mechanical envelope of the tube, causing a feedback path that results in sustained oscillation, or if there isn't enough "gain" around the loop, a ringing that dies out, such as when you tap the tube to create noise. You can tell whether it is a mechanical oscillation or an electrical oscillation if some form of mechanical damping will kill or change the frequency of oscillation. If damping does affect the oscillation, it is mechanical in nature. If electrical shielding or component changes affect it, it is electrical in nature.

One more thing about parasitics - they don't have to produce sustained oscillations to be troublesome. It is very common to see what is called "boxcars", or small patches of oscillations riding on the sine wave at either the peak or at the crossover point. This is caused typically when there is a small disturbance, or discontinuity, that injects a signal into the circuit, causing it to "ring" at a particular frequency for a short period of time and die out. These are common in output stages where the leakage inductance of the output transformer resonates with stray and other circuit capacitances, and is excited by the zero-crossing switchover of the output stage, or the current reversal at the peak of the sine wave.

RA

[Steve Conner]

Parasitic oscillations of the kinds that are stopped by stopper resistors are in the 10s to 100s of MHz. No part of a tube can oscillate mechanically at that frequency, the effect is purely electronic.

[R.G.]

S'what I get for never having been a ham and built radio transmitters.

[raiken]

S'what I get for never having been a ham and built radio transmitters.

Every geek kid wanting a future in electronics should be a ham. I was, until I figured out the girls just didn't go for it, so I switched to guitar.

I used to love buiding QRP (low power) CW transmitters. I also learned an awful lot of tube theory from all the old ARRL handbooks and other electronic books and magazines.

The funny thing is that some of that antenna theory is now becoming useful to me as I'm dealing with ways to test my equipment for CE compliance. I'm looking into building a 10-meter OATS in the back yard, complete with a 30MHz to 2GHz combilog antenna on a 1-4 meter mast, and a 16'x48' aluminum ground plane (odd-looking, but it will mean less grass to cut!).

RA

[Steve Conner]

Me too! Except then I couldn't get any gigs, so I switched again to bass.

Building your own OATS is a pretty hardcore solution to the lawn cutting problem. We lately got some of our stuff tested at York EMC's facility in Scotland. York EMC Services Ltd :: Donibristle Laboratory

It costs several thousand bucks a shot, but that couldn't even buy an EMC test receiver, let alone build a lab. They do a precompliance test where they test EMC and ESD sensitivity, and tell you if you're going to fail the real test, and by how much.

I was responsible for the PCB layout and all of the noisy circuitry on this design, so I was very happy that it passed first time.

[raiken]

Me too! Except then I couldn't get any gigs, so I switched again to bass.

Building your own OATS is a pretty hardcore solution to the lawn cutting problem. We lately got some of our stuff tested at York EMC's facility in Scotland. York EMC Services Ltd :: Donibristle Laboratory

It costs several thousand bucks a shot, but that couldn't even buy an EMC test receiver, let alone build a lab. They do a precompliance test where they test EMC and ESD sensitivity, and tell you if you're going to fail the real test, and by how much.

I was responsible for the PCB layout and all of the noisy circuitry on this design, so I was very happy that it passed first time.

I have dealt with more EMC labs than I care to count. They are so expensive! Just to test my little Wombat bias meter, which sells for around $100 or so, I got quotes ranging from $4000 to $16,000! And, as you know, if it fails, you go back and pay it again.

Multiply that by just a few products, and you've paid for your own OATS, antenna, and a used receiver (although, as I understand it, they are now supposedly allowing a lower-cost spectrum analyzer with quasi-peak detector to be used for certification). I'm also building an ESD/static test bench and conducted emissions test LISN setup. I figure the only thing I can't test is EMI susceptibility, which requires bombarding the unit with up to 100W (depending on the antenna factor) from 30MHz to 1GHz, which requires a sealed, semi-anechoic room.

At the very least, I can do pre-compliance and save a lot of money. Also, it is much better to test your ESD compliance on your own nickel, in your own lab, than on their clock where you have no equipment to make modifications on the fly - if it dies when they zap it, all you can do is go home with your tail between your legs and try again next time after you've repaired the unit.

RA

[Steve Conner]

Well, this is wandering right off topic, except that an amp with parasitics would fail EMC.

York have the susceptibility test rig, powered by a stack of RF power amps the size of a dishwasher. Ironically their own CCTV camera that they use to view the unit under test is susceptible.

In my previous job we dealt with the CE/EMC issue by "self-certifying", and hand-waving about how our products were subassemblies for OEMs, etc. I managed to work in electronic R&D for a good 6-7 years before anything I designed ever made its way to an EMC test house.

It may be different for you, Randall, but there is no way we would ever have our own full in-house EMC facility. For a start we're based in the city and couldn't afford the land for an OATS.

But, we have a spectrum analyzer with quasi-peak and some near-field probes, and even the guy at York EMC told us to save money and hassle by getting our own ESD gun.

[raiken]

It may be different for you, Randall, but there is no way we would ever have our own full in-house EMC facility. For a start we're based in the city and couldn't afford the land for an OATS.

But, we have a spectrum analyzer with quasi-peak and some near-field probes, and even the guy at York EMC told us to save money and hassle by getting our own ESD gun.

Yes, this is off-topic, but still interesting (well, to us geeks, anyway...). And, in addition to the parasitics you mentioned, you can't sell an amp in an EU country without testing for conducted and radiated emiisions (which can also occur due to rectifier switching), so it is topical, sort of...

A 10m OATS doesn't require much land at all - all you need is a metal and obstacle-free area within the CISPR "ellipse" of 20m x 17.32m, with a ground plane centered in that area. That will easily fit in the upper right corner of my back yard, where the garden used to be. The ground plane doesn't need to be anything special, just a continuous area of aluminum or steel, or even metal mesh, that meets the minimum area standards of EN55022. I figure I can make a frame of 24 4'x8' sections of OSB (at $6/sheet) and cover it with 24"wide x 50' strips of aluminum flashing (9 at around $35 each) and seal the seams with metal tape. I should be able to build the whole thing for around $500, not counting the adjustable antenna mast and rotating turntable I'll have to build.

As for the antenna, yes, they are rather expensive, around $3000 - $3500 for a new biconilog, but you can rent them for around $250/week. I have never seen a used one for sale. You do have to make sure the background noise in your area isn't too high to render the area useless.

I can rent or buy a comb generator calibrated for a 10m site to get the correction factors for the site attenuation, or I can rent a couple antennas and do my own normalized site attenuation measurements.

I even plan to design my own ESD gun for the hell of it, using a switchmode current source fed Royer oscillator driving a CCFL transformer into a Cockcroft-Walton voltage multiplier into the HBM RC network, under control of a microcontroller to set the voltage.

Crazy? Probably. Expensive? Yes. Fun and educational? Definitely!

RA

[R.G.]

I did always enjoy a trip to the EMC lab back in my last career. We had an RF anechoic chamber 10x10m which was fully suspended so it could be un-linked from earth ground as needed. It was coated walls and ceiling with 1.5m high tetrahedral carbon composite radio absorbers. I think there was a layer of these under the raised floor as well.

Funny thing is, it was actually more acoustically quiet than the audio anechoic chamber we used for audio testing.

[Steve Conner]

They make it all quiet like that so they can sneak up on you and seize your wallet.

Randall, please let us know how the ESD gun works out. I sure hope the microcontroller in it isn't ESD sensitive. I think you could save hassle by using a flyback from a CRT computer monitor, these have an EHT rectifier built in and can kick out up to 25kV. You might get help with it over at the 4hv.org forum for high voltage hobbyists, where I'm also a moderator.

[raiken]

Randall, please let us know how the ESD gun works out. I sure hope the microcontroller in it isn't ESD sensitive. I think you could save hassle by using a flyback from a CRT computer monitor, these have an EHT rectifier built in and can kick out up to 25kV. You might get help with it over at the 4hv.org forum for high voltage hobbyists, where I'm also a moderator.

Thanks for the link, I'll have to check that site out. Nothing is more fun than HV!

I don't think a standard high-voltage flyback transformer will fit in my handheld "trigger" enclosure, unless I get one custom made. It doesn't need to be anywhere near the power a monitor flyback tranny produces, as it is only charging up a 150pF cap to 12kV max. A variable high-voltage flyback controller would certainly be easier to design, with less circuitry. Do you know of any physically small high-ratio flyback transformers?

CCFL transformers, like the CTX210659-R, are very small, efficient, and good for up to around 1.3kV RMS (before the CCW multiplier) and I have designed plenty of switchmode current-fed Royer circuits for CCFL backlight controllers in LCD displays, so I am quite familiar with those. I have already spice'd up my royer/multiplier, so it has a good chance of working. In addition, the CCW multiplier allows use of multiple low-voltage rectifiers and caps, so I can keep them relatively small, except for the final 150pF cap required by the HBM, which will be required to handle the full 4KV - 12KV range.

(apologies to the OP for this continued sidetrack...)


RA