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Old 13th January 2006, 03:39 PM   #1
stoo is offline stoo  Canada
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Default Screen grid resistors

Hello everyone.
What considerations are taken when chosing the screen resistors on a EL34 p-p output section. I see Fenders use 470 ohm to B+ and Marshall uses 1K to B+. I'ave also see the 2 screens tied together then going to B+ through a 1K.

What's the science here??
Thanx all
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Old 13th January 2006, 03:49 PM   #2
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It limits peak screen current, a good thing for reliability. The higher the B+, the more they're needed. May help prevent high-frequency oscillations. And they limit the damage if a tube shorts... resistors are cheap! They may cause a very small loss of maximum power, a little more distortion, but it's worth it in an instrument amp.
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Old 13th January 2006, 04:14 PM   #3
stoo is offline stoo  Canada
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Tom how are those values chosen. Does the voltage on the screen have to be a certain percentage of the plate voltage or am I out to lunch here?
stew
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Old 13th January 2006, 05:04 PM   #4
Joel is offline Joel  United States
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Hi Stew,

I have no idea what companies like Fender used them for (very little science to guitar amps), but in Hi-Fi amps they are there solely to prevent parasitic oscillations.

Mullard, the original maker of the EL34, used 1/2 watt 47 ohms consistantly. Make sure they are carbon composition, or other non-inductive type.

Joel
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Old 13th January 2006, 05:42 PM   #5
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There probably was very little science in guitar amp designs. And a lot of emperical design. Anyone who has spent some time probing around inside them, or watching the tubes, while driving the amp to extreme clipping knows what the resistor is for. It keeps the screen grid from melting.

A guitar amp is often subjected to conditions that your typical stereo would never see, like continuous clipping for extended periods of time. In a hard clip the output tube can see the plate voltage way below the screen voltage for 50% of the time (the bottom half of a square wave). During this time the screen current must be limited, or it will glow red.

I can't say for sure, since I wasn't there, but the resistor value (and connection) was probably a compromize between power, tube life, and sound quality. It is my experience that some (not all) 6BQ5/EL-84's are particularly prone to glowing grid wires, followed soon by sparking out, and tube death.
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Old 14th January 2006, 01:07 AM   #6
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I can echo the observation of Tubelab regarding red screens, in my case with EL34s. Not to discourage their use, but after some experience there I went to beam tubes.

Regarding the value of these resistors (and apart from guitar amps where certain desired effects come in), I fear they are often piously put there .... but, as resistors are cheap.... Only, especially with beam tubes, the screen current can vary quite substantially during operation and the value must be kept low, say not much above 220 ohm. As voltage reducers, i.e. where the screen voltage must be substantially lower than anode voltage for whatever reason, a voltage divider (wasteful of energy) or better, a voltage regulator should rather be used.

The manufacturer's recommendation for EL34 for lowest distortion is a 1K.ohm screen resistor. I have never seen why so high, but they confirm with graphs, so I do not argue (typical load 6.6 K, anode-anode in PP).

Regards.
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Old 14th January 2006, 02:56 AM   #7
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There is an obvious difference between a guitar amp and a HiFi amp. A HiFi amp is designed for low distortion. The screen grid supply needs to be stiff, and is often regulated.

A guitar amp is often designed to have a distortion "signature", the amps "sound". The manufacturers use several tricks to get this "sound". Poor power supply regulation "sag", screen circuit resistance, and "special" (undersized) output transformer design.
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Old 14th January 2006, 06:26 AM   #8
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Screen grid tubes are usually unreliable at high power without screen resistors. Small wirewound resistors do just fine as screen resistors, as long as they are not excessive in physical size. Around 500 ohms is a good general purpose value for screen resistors, especially in an EL34 based design. I've frequently seen 100 watts from a pair of EL34 tubes, so power loss with screen resistors is not a very real concern.
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Old 14th January 2006, 11:19 AM   #9
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tubelab.com hit the nail on the head. Not being able to do anything other than carry a musical instrument I was awe struck when I had the chance to listen to different brands of guitar amplifiers. The Marshall and Mesa had vast differences even with similar circuit designs. Not to mention Fender and other amplifiers each with their sonic signature. Vastly different than any HiFi amplifier. Each seemed to have a special sound for a special purpose? I liked the different output stages, the 6L6 or EL34 choice each had a different sound.
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Old 1st February 2006, 03:44 PM   #10
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Each manufacturer do financial calulation when designing an amplifier. The most expensive marshal is costing no more as 100$ to manufacturate, inclusive the manual and the tape on the box. So they have to compromize.

Peavey have very cheap main transformer, so half of the saturation you can hear at high volume is the saturation inside the main transformer and not inside the amplifier. It is why peavy is a good amp for jazz, but not for blues or rock. It is also why, in most case, the main transformer in a such amp, is the first component that will smell when playing at high volume.

Fender and mashall have better main transformer. I don't know for marshal, but with fender, if the output transformer have a good design, they are small. And they will smell quickly if you are playing ofen with high volume.

At the other hand, what I like with Fender is at they have a very good design of all parts of the amplifier. They compromize on the cost of the components, but not much with the design. As exemple, their preamps have a lot of gain and it is very easy to sature the whole preamp from the first stage to the last stage.

All the commercial guitar amps I know have a very poor design in regard to the driver stage. No one is using a driver transformer when it is the best solution to obtain a low output impedance with a good symmetry. The raison is at a good driver transformer cost too much money.

Guitar amp are using class B for the output stage, and in class B, the grid of the tube can be positive. It mean at it will use current, and we need a driver that can drive that current. The RMS value of that current is certainly below the watt for an EL34, but can be a few watts in peak. In consequence, the driver of a guitar amp determine most of the sounding of the final stage.

I have try to stabilize the g2 in one of my amp using 2x807 at the output. The result was 2 time more dynamic when clipping, but the whole tubes was becoming orange, so I went back to a more traditional and less expensive alimentation. I would not recommand even to try it if you don't know exactly what you are doing.

A last word about the EL34. Its a tube that is manufactured both as a classic pentode and as a screen grid tube. So I beleive at variant manufactured as screen grid tube would be better for a guitar amp. It is what a good friend to me tell me. I don't know more and can not ask him more on that subject, because it just desapear and leave the country with his wife it was 2 yeras ago.

Wikipedia on http://en.wikipedia.org/wiki/Beam_tetrode tell this:
"Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34 (6CA7) although manufactured by Mullard as a pentode was also produced by many manufacturers around the world, either as a beam tetrode or as a true pentode.

Even Philips/Mullard themselves were not immune. Several examples of Mullard marked ECL82s (a signal triode and low power pentode intended for single ended operation) have turned out to be triode/beam tetrodes."
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