Why does nearly every tube circuit connect the screen bypass cap to ground not C - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 22, 2022, 11:13:21 pm
content: ...not the cathode. I've read in a few articles that say you should always connect the screen bypass cap to the cathode instead of ground. but every radio or audio amp circuit I've seen connects it to ground instead.and radio books even show it connected to ground.so are there circuits where you don't want to connect it to the cathode?

author: CaptDon
author_rank: Super Contributor
timestamp: September 22, 2022, 11:26:59 pm
content: And what specific type of circuit did they not want the bypass connected to the cathode? At RF frequencies the bypass is always grounded to avoid the tube going into parasitic oscillations. In fact the screens are painstakingly capacitor bypassed to ground. What reason was given for capacitor coupling the screen to the cathode?? In some tubes the screen is internally attached to the cathode and generally in that type of tube the cathode has multiple pinouts that are grounded!! Who is the source of your articles??? You can find articles supporting flat earth also but I wouldn't try to navigate based on flat earth theories.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 23, 2022, 01:02:00 am
content: Quote from: ELS122 on September 22, 2022, 11:41:39 pmI think the fact he describes himself as "The Valve Wizard" & the sub-heading "how to designguitaramplifiers" is the vital clue--- guitar amplifier sites are full of "woo-woo" nonsense, which was never used by the "hard-nosed" designers of real HiFi amplifiers of the valve era.Just looking at the example amplifier stage further down the page, what the screen is actually seeing is Cg2 & Ck in series to ground.As Ck is so much larger in value than Cg2 the series combination is "near as dammit" to that of the latter----44.45nF as compared to 47 nF, so why bother?As even CK is unusually small, the value of Rk may be of concern here----- as the whole idea of bypassing the screen grid is to have it as a "hard" DC voltage, unvarying with signal level, Ck would be much greater than 820nF, in a normal case.As to returning bypass caps to HT-----in theory, DC anode/screen supplies (& , in fact, any DC supply to an active device) look like ground to ac, but in practice, nasty things like stray inductance & capacitance come into play at higher frequencies, so the most common connection of bypass capacitors is to ground.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 23, 2022, 01:42:08 am
content: I'd suggest that the typical vintage amp has the screen bypass cap going to gnd/chassis due to practicality - that cap was typically by far the largest below-chassis part in that vicinity, and a chassis connection was the most convenient means for manufacture to secure and locate such a large part. And some vintage first stage circuits connect cathode directly to ground, so by default the screen bypass is going to cathode. Nowadays such caps are physically much smaller so can be terminated wherever with ease, and grounded cathode circuits are rarely used.I wouldn't dis people just because of their moniker - his first name is Merlin, hence the play on words. He certainly does focus on the guitar community as that is bread and butter for him as he is not want to making a snake oil living from hi-fi or guitar hoop-la like many others - he also has a very detailed knowledge and practical experience of what does and doesn't matter with respect to tube circuitry. As noted, some guitar focussed amps purposefully use low values of cathode bypass for specific tone shaping, so the connection point of the screen bypass can be of concern and well worth writing about.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 23, 2022, 02:57:03 am
content: Quote from: ELS122 on September 23, 2022, 02:15:42 amOut of interest, I Googled for "Mullard Amplifiers", & found this:-https://archive.org/details/19400164MullardCircuitsForAudioAmplifiers/mode/1up?view=theaterLooking through it, I found theydoreturn the screen bypass to the cathode, but it seems to be only in EF86 input stages where the tube is operated with very high value anode & screen resistors in what they term the "starvation" mode, combined with a split cathode resistor with a bypassed part around 1K& an unbypassed section of around 100, with negative feedback from the secondary of the amplifier's output transformer applied across the latter.Ck in all instances is around 50uF---much larger than shown in the "valve wizard's" example.In each instance where this is done, DC coupling of the EF86 anode to the grid of the following phase splitter is also used.In other, more conventional EF86 stages, the screen bypass is directly returned to ground.Unfortunately, Mullard don't go further into why they sometimes do this, & sometimes not.As to a "sturdy screen voltage"--in high power Tetrode RF stages, the screen supply is regulated, so it has no adverse effects, seeing many such stages are driven at very high levels.The only time the screen will be damaged is if the anode voltage is lost entirely, & the screen voltage is still present.Variations of anode voltage with signal levels will not cause excessive screen current.I was brought up in the valve era, I even built the 3 watt amplifier shown in the Mullard book.Much of my work life was involved with "hollow state" devices---in Radio & TV Broadcasting, the "valve era" hung on a long time!

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 23, 2022, 10:23:18 am
content: In a typical common cathode voltage amplifier the screen grid should be bypasses to the signal ‘common’. For Vin and Vout, this is 0 V (ground). For Cathode bias using a resistor (and optional parallel cap - to increase the gain), connecting the screen bypass cap to the cathode creates feed back. AC Modulation of the screen current caused by amplifier operation will feed back to the cathode. This would have a small frequency dependent measurable effect on the Vout/Vin transfer function but probably not significant for most amplifiers.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 23, 2022, 02:29:54 pm
content: Perhaps some separation is needed for applications (and hence valves). My previous comments related to input and low signal level stages.ELS122, it seems like your focus is on amplifier output stages, where the screen can conduct relatively substantial current (especially when the anode is driven towards saturation). Datasheets for some output stage valves show representative distortion levels for a given circuit configuration, and they are normally based on a stiff screen voltage supply and hence decoupling to 0V.Some output stage configurations that use cathode and anode coupled output transformers (eg. unity coupled style) by necessity use a decoupling cap from screen to cathode as the cathode is not fixed to 0V (as per fixed bias) or typically cathode biased but with a strongly decoupled cathode.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2022, 04:13:48 pm
content: Note that when a pentode is used as the input tube in a feedback amplifier, the cathode is usually not bypassed to ground and the feedback resistor connects to the cathode.In that circuit, the screen bypass capacitor should be returned to the cathode, not to ground.The classic Dynaco amplifiers used a 6AN8-type pentode-triode at the input, with the pentode grid as the input, an unbypassed cathode resistor, and the feedback resistor to the cathode.The pentode plate connected directly to the grid of the triode, used as a phase inverter to drive the output tube grids from its plate and cathode.

author: eugene
author_rank: Frequent Contributor
timestamp: September 23, 2022, 04:33:27 pm
content: Good grief. So much guessing. Most of us could learn more than a little from John Broskie (https://tubecad.com)The bypass cap on the screen is intended to bypass AC currents. So look at the circuit and decide where theAC signalcurrent loop is. In terms of electrons (as opposed to conventional current) it's up through the tube, out the screen, and back to the cathode. In the case of a pentode used for gain, grid bias is usually achieved with a cathode resistor. That cathode resistor also has an AC bypass cap. So if the screen is bypassed to ground, then the AC screen current goes through its own bypass cap to ground and then also through the cathode bypass cap. The fact is, it really does make more sense to bypass the screen directly to the cathode, thus skipping the detour through the cathode bypass cap.My guess is that it's usually connected to ground because (1) few people take the time to think about the current loop, and (2) because most circuits are just copies of some other circuit and that's what everyone else does.

author: b_force
author_rank: Super Contributor
timestamp: September 23, 2022, 08:16:38 pm
content: Quote from: vk6zgo on September 23, 2022, 01:02:00 amI understand where you're coming from, because there is a lot of woo-woo, not only with guitar amplifiers, but also with tube/valve amplifiers.But I also know for sure you never really have read anything from Merlin Blencowe or talked to him as well.Because he is most definitely not a "woo-woo" person.In fact, he's pretty down the earth and open minded about stuff.fyi, if you read a little further he also has a book about hifi amplifiers as well.I have talked to him quite a bit, and I know that he get his books, explanations and resources checked by quite some people in the field.With that I don't mean "woo woo" people.And to quote Merlin about exactly this question/problem; (on a different forum)QuoteWhich would be also my answer.So yes, to cathode is the technical correct answer, but since the impedance is so small, it doesn't matter an awful lot.That being said, why most people connect it to ground; because the very vast majority of people just copy-paste circuits.Or, depending on the physical connections, is just a little easier to mount.

author: TimFox
author_rank: Super Contributor
timestamp: September 24, 2022, 02:09:05 pm
content: I just remembered an example of proper usage of screen bypass capacitor to ground instead of cathode.I have an ancient E H Scott short-wave receiver designed for merchant-marine service in WW II.Since many merchant vessels had DC power, it is an AC/DC set with series heater strings and off-the-line rectification.Since "DC ground" in the circuit is not connected to the metal chassis directly, the RF bypass capacitors are "bathtub" hermetically-sealed multiple paper capacitors with common connection to the case.While the DC connections to grid returns, screens, cathodes, and plates come from the rectified voltage, their bypasses are all done to the chassis through these bathtubs.(This particular receiver was optimized for low local-oscillator leakage to the antenna, for security reasons, and used a 6K7 input tube with grid cap to reduce the leakage.)

author: eugene
author_rank: Frequent Contributor
timestamp: September 25, 2022, 07:55:24 pm
content: Quote from: TimFox on September 24, 2022, 02:09:05 pmOK, but... why?

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2022, 08:11:24 pm
content: Quote from: eugene on September 25, 2022, 07:55:24 pmThe "DC ground" is just a wire net connecting the negative side of the off-line rectifier.To properly RF-ground the cathodes, screens, plate returns (tuned transformer primaries), and grid returns (tuned transformer secondaries) to a real shield, the RF bypass capacitors must be connected to the chassis.