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I have noticed that a number of the low power amps don't have a standby switch. Does this make any difference?
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11-20-2009 10:12 PM
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I'm sure RandyC will chime in shortly. I've noticed the same: I think my Blues Jr NOS is the only tube amp I've owned that lacks a standby (I'd have to check my Pignose G40V, but that would mean getting up, walking down some stairs, and lifting the amp up, and it's too late in the evening).
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It's OK so long as the amplifier has a vacuum tube rectifier. (It's bad practise to apply plate voltage before the cathodes have heated to operating temperature.) In the smaller amplifiers, it takes longer for the rectifier tube to start passing high voltage than it does for the power tubes to commence operation. Therefore, there is a built-in time delay appropriate for the application.
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Neither of my non-standby-equipped amps have rectifier tubes.
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I don't know anything about that amplifier so I'm speculating when I suggest that a time delay circuit is included. If you know of a website that has a schematic of the amplifier, please let me know and I'll take a look - shouldn't take more than ten seconds to see what's up with the lack of a standby switch.
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Check this out:
I hope it's legible. I'll pull out the owner's manual to see if it works better.
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LPD:
It's tough to read the schematic at that reduction but if you look down in the lower right corner, you'll see a 12AX7 tube that "apparently" is not connected to anything .... that tube is in series with the high voltage supply at the output transformer and connects the high voltage to the output tubes AFTER the 12AX7 filaments are up to operating temperature.
In other words, the 12AX7 is acting as a relay that doesn't close until after the current starts flowing through the tube (which in turn, can't happen until the filament heat commences conduction through the tube).
Simple, effective !
Good job, Fender
Last edited by randyc; 11-21-2009 at 04:38 PM. Reason: correct bad typing
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Thanks, Randy. That gives me marginally more peace of mind.
Naturally, now that issue is settled, I've gone back to the Jazzmaster Ultralight.
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Originally Posted by randyc
LPD:
It's tough to read the schematic at that reduction but if you look down in the lower right corner, you'll see a 12AX7 tube that "apparently" is not connected to anything .... that tube is in series with the high voltage supply at the output transformer and connects the high voltage to the output tubes AFTER the 12AX7 filaments are up to operating temperature.
In other words, the 12AX7 is acting as a relay that doesn't close until after the current starts flowing through the tube (which in turn, can't happen until the filament heat commences conduction through the tube).
Simple, effective !
Good job, Fender
What!!!??? I don't know where you got that notion, but I can assure you that Fender did NOT put the spare section of the 12AX7 in series with the B+ to to output transformer. If you look again you'll see that the cathode, grid and plate of that spare section are all tied to ground. Randy, you really ought to know better than this. A 12AX7 will not pass anywhere near the current required by the output stage.
To millman: Guitar amps don't really need standby switches. There's a lot of folklore surrounding the function and proper use of a standby switch. If you do your research - and by that I mean going back to the source technical documents from the 1940s through 1960s; not reading what "everyone" on the internet ignorantly repeats as gospel truth - you'll learn that the tubes used in guitar amps are not harmed by the application of plate voltage before the filaments come up to temperature.
In short, don't worry about standby switches. The best use for a standby switch is to keep your amp from making noise while you're on break between sets. Otherwise it really doesn't matter how - or whether - you use an amp's standby switch.
If you're interested in my full treatment of this subject, here's an article I wrote:
David Lamkins - Guitarist - How to use a standby switch
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Yeah.. there are a lot of misconceptions about and not even the experts agree on this anymore. Google "cathode stripping" and you will see.
Ignorant question: Do tube hi-fi's have standby switches?
Ignorant question 2: On amps with tube rectifiers wouldnt it be cheaper to build them without standby switches if they only prolong tube life on amps with SS rectifiers?Last edited by dh82c; 11-22-2009 at 03:55 PM.
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EDIT: Ok.. disregard. I just read TDDs link lol.
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This is great. Every time I think that all the fight has gone out of forum members, an issue like this comes up. Now shake hands, and come out fighting.
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GUILTY ! I can't see the connections and the terminal identifications on the reference schematic, so I inferred the purpose of the spare tube, sorry guys. I can't read that schematic very well on this little laptop (surgically inserted plastic lenses in both eyes don't help - they're only good for long distance) ! Originally Posted by TieDyedDevil
Thanks for pointing out my error, of course that 12AX7 can't pass the 30 mA (or whatever) that the power tubes require. Definitely my bad !
Insofar as standby switches being necessary, dunno' that I've ever seen a high power ampifier without some kind of high voltage switching/time delay to allow the cathodes to come up to temperature.
Source material: RCA manual 30, which on page 81 states:
"When the (filament) voltage is low, the temperature of the cathode is below normal, with the result that electron emission is limited. The limited emission may cause unsatisfactory operation and reduced tube life."
That's the way that I've always understood it, standby switch enhances tube life. I suppose the amplifier manufacturers made the same assumption. Given no contrary evidence, I'll continue the practise of with-holding high voltage until cathodes are up to temperature. (It's worked pretty well for the three of my amplifiers that are over 50 years old.)
As an aside, I'm also thinking that perhaps it's more important to include standby switches in guitar amplifiers universally operated in push-pull configuration. (I mention this because I've never seen a standby switch on Class A amplifiers or on Class B amplifiers operated at less than ... say 275 - 300 volts.) The peak voltages can be as high as FOUR TIMES the high voltage provided by the power supply. Frankly I don't know that this is the reason for the practise, I'm just tossing the idea out.
It's been pretty hectic today (you know, all the work that's required to WATCH FOOTBALL) but I'll try to look into this a little further..
cheers,
randyc
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That's a different issue: running the heater too cold. Originally Posted by randyc
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My wife used to do that. Originally Posted by TieDyedDevil
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It is the same issue: passing (or attempting to pass) current through a tube whose cathode has not reached operating temperature while high voltage is applied. Originally Posted by TieDyedDevil
One might make the argument that there is a long-term consideration and a short-term consideration. I couldn't argue either side of that, not being an tube reliability expert. But the scenario is identical.
cheers
randyc
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And my wife wonders why I get angry when she fires up the electric blanket with with window open. Originally Posted by lpdeluxe
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This is SO simple to explain: she does it because she's a woman, and you do it because you're a guy. Originally Posted by dh82c
I had an interesting conversation at lunch today. A lady made the statement that men don't talk enough. I said, that's not true: here's a deep and meaningful conversation between two men: I'm selling my Les Paul. Response: cool.
Here's the same conversation between two women: Bob's selling his guitar, whatever it is that he calls it, it has a name, anyway, he's selling it and expects to get more than he paid for it, god knows why he has so many guitars, I tell him, you can only play one at a time, and he just gives me this look, you know what I mean....Response: Frank is exactly the same way, I don't know where they get it, if they used the money for someone you really wanted, like a Caribbean cruise, that'd be one thing, but no, they have to go buy some other, pardon my french, damn guit-TAR, I'll never understand it as long as I live....
You'll note that the male conversation, in addition to a feature known as "laconic"(see below), is composed of actual diagrammable sentences that end in full stops, commonly known as periods; the female equivalent never ends, but is interrupted.
The lady I was speaking with said, I don't understand how men can pretend to bond, when they don't say anything.
Necessary footnote, although not in the form of a footnote: "laconic" derives from "Laconia," an alternate name for Sparta, a city-state on the Northern (Mediterranean) coast of Africa. It was a male-dominated society, with valor of arms and machismo the primary virtues. At one point, a rival of Sparta sent an inflammatory missive, which, in part, read "when we defeat you, there will not be a single brick left to place atop another brick!"
The laconic reply: "When."
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OK, while watching a boring sci-fi movie, I thought some more about standby switches and why they are still used. I don't "own" any of the conventional theories regarding applying high voltage to a tube that is not at operating temperature, my slight knowledge comes from reading old literature, like the RCA manual I referenced previously. The problem seems to me not well-related with either of the two theories that usually justify the use of a standby switch. I think that a more likely failure mechanism in beam power tubes would occur in the fragile screen grid structures of these tubes.
Bear with me for a moment while I consider the possible screen grid problem:
Amplifiers producing 15 watts or more use beam power pentodes in the power section. The screen grid of these tubes is always operated at lower voltage than the plate (screen grids frequently CAN'T be operated at plate potential without catastrophic failure).
The universal means of obtaining the bias voltage for the screen grid is by using a voltage dropping resistor from plate to screen grid. The correct screen bias voltage cannot be obtained without a voltage drop through this bias resistor.
The voltage drop can't occur unless current is flowing through the bias resistor. Current cannot flow through the bias resistor until it flows through the cathode - plate path of the tube.
That can't happen until the filament/cathode are at operational temperatures.
Without current flow, the screen grid is at the plate potential.
AND without current flow through the output tubes, the power supply voltage is much higher than under normal loaded conditions (because guitar amplifiers have primitive, non-regulated power supplies).
Consider an amplifier using EL-84 tubes as an example, where the plates are operated around 375 volts and the screen grids around 300 volts. At switch-on, there is no current flow through ANY of the tubes (assume that the rectifier is solid state).
There is no voltage drop across the secondary winding of the power transformer, so the voltage appearing at the plate AND the screen of the output tubes is at nearly 420 volts.
The maximum screen voltage specified for quality American-made EL-84 tubes is 300 volts.
Make your own decision about using the standby switch, I know what MINE will be J
PS: LPD, any possibility of providing a higher-resolution schematic ? The one in photobucket seems to be right at this laptop monitors limits and I'd like to look a little bit closer at the power supply ... thanks.Last edited by randyc; 11-22-2009 at 10:55 PM. Reason: add PS
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I'll dig out the BJ manual manana. I think (or, suspect) it may be there. If not, I'll do whatever's necessary to expand the existing jpeg: I wasn't (and am not now) on my Photoshop-equipped computer.
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Thanks, John. Did a bit more poking around on the internet and I'm even more convinced that the standby switch is still common because of screen grid problems that I mentioned above, and not the reasons frequently mentioned .... I may even do a separate thread about this later, thus keeping the standard of boredom here in the forum reasonably low
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TDD: As a rule, I prefer to discuss things like this privately, rather than complicating possible misunderstanding with public exposure (which might consequently encourage a participant to adopt "attitude"). However, attempting to send you a PM, a forum response states that you have chosen not to receive private messages from the forum, so ... Originally Posted by TieDyedDevil
I have several problems with what you posted (above). I don't mind when my errors are noted and I'm told of them - how else is one to learn to be more careful ? But NOT in the disrespectful, patronizing manner that you used. There's a long-established standard of professional courtesy between colleagues - even when major disagreements are experienced -that you chose to disregard.
I hope that this doesn't occur again. I'm going to assume that impulsiveness generated your "gotcha" post rather than deliberate rudeness. I wouldn't feel comfortable exchanging "in kind" comments so please give me no reason to do so.
Thanks,
randyc
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Quite a few fenders run their screens at VERY close to the plate potential.
It is common mod to put 470ohm resistors on the grids (why 470? Betty Crocker recommends it
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Specific example: Blonde Bassman +432V on the plates.. +430V on the grids.
As you have no doubt pointed out before they also do silly things like run 6V6s with higher plate values than a sane person would. Their cathode bypass caps are rated 25V (an running 23-26V in a healthy 12ax7).
Funny thing is fender amps are (from the 60s anyways) generally considered reliable (at least compared to Vox and Marshall). I think they knew a little more than they sometimes let on
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Fair enough .. this is the best source of collective information regarding screen grids that I located tonight: Originally Posted by dh82c
Quite a few fenders run their screens at VERY close to the plate potential. ..... I think they knew a little more than they sometimes let on
http://www.webace.com.au/~electron/tubes/screens.htm
I strongly recommend reading the entire article to confirm that I'm not "cherry picking" this stuff to support some bogus argument. Here are some bits and pieces from the article:
BIT 1
So a review of commercial circuits shows that for the whole of that 60 year period between 1940 and 2001, only a few basic types of tubes were used in all the audio amplifiers ever produced in the whole world.
The result is that:
1. there is very little literature about Screen Grids
2. there are are few examples of innovative design variants
3. audio amplifier design standards reflected the need for simple tubes that could be overloaded and abused by users
4. audio amateurs - ie hobbyists and project builders - have had to remain within a very rigid published design framework
5. published manufacturer's tube data invariably fails to provide information about the effect of Screen Grid voltage upon
Plate Current
6. there is little published manufacturer's data available for non-popular tube types
7. there is little practical knowledge available to facilitate experimentation with non-popular tube types
8. a self-destructing commercial approach manifested that inhibited innovation in the tube based audio equipment
industry, paving the way for their displacement by semi-conductors
This page attempts to quantify some of the major principles and possibilities regarding improving vacuum tube technologies in the area of Screen Grids.
BIT 2
RCA Receiving Tube Handbook RC-19 explains at Page 7:
"The Screen Grid is operated at a positive voltage and, therefore, attracts electrons from the cathode. However, because of the comparatively large space between wires of the Screen Grid, most of the electrons drawn to the Screen Grid pass through it. Hence the Screen Grid supplies an electrostatic force pulling electrons from the Cathode to the Plate. At the same time, the Screen Grid shields the electrons between Cathode and Screen Grid from the Plate so that the Plate exerts very little electrostatic force on electrons near the Cathode.
So long as the Plate voltage is higher than the Screen Grid voltage, Plate current in a Screen Grid tube depends to a great degree on the Screen Grid voltage and very little on the Plate voltage" (end quote)
BIT 3
Notwithstanding the above JETEC design specifications - determined from extensive practical and theoretical research, design type tested performance criteria and endorsed by leading manufacturers'- numerous examples of commercial Guitar amplifiers and Public Address (PA) amplifiers demonstrate typical design with a common Plate and Screen supply (as a cost saving measure) having B+ supply voltages well above the above specified maxima.
However this operating configuration does not promote either long tube life or high-fi standard performance - in fact some tube guitar amp designers deliberately configure the output stage to ensure desired distortion characteristics under sustained overload conditions. But it can also be a recipe for overheating, unreliability, short tube life, instability, parasitic oscillations and/or dynatron action in the output stage because the output tubes are running with the Plate Voltage less than the Screen Grid Voltage (because of DC voltage drop in the primary of the output transformer).
This is particularly true of low-cost output transformers having high DC resistance windings - not to mention low primary inductance and high leakage inductance which also facilitate parasitics.
BIT 4
It follows that the critical design element for a Tetrode, Pentode or Beam Power Tube is the Screen-Grid voltage, because this is the effective Anode voltage.
As a rule of thumb, the screen grid supply voltage should NEVER be more than the manufacturer's rating. Higher applied Screen-Grid voltage is likely to cause self-oscillation, parasitic oscillation, dynatron action or thermal runaway - any of which can easily destroy a tube and associated components. MINIMAL Screen-Grid voltage will provide better performance including cleaner, crisper sound with less distortion.
Tube Data handbooks typically recommend Screen Grid operating voltages at only half, or even less than half, the rated maximum for a given tube type, warning us of the great control the Screen Grid has in determining tube performance.
BIT 5
Radiotronics Magazine No. 80 of October 1937 says:
"The power dissipated in the Screen circuit is added to the power in the Plate to obtain the total B supply input power. With full signal input, the power delivered to the Plate circuit is the product of the full signal Plate supply voltage and the full-signal DC Plate current. The power dissipated by the Plate in heat is the difference between the power supplied to the Plate circuit and the power supplied to the load.
Screen dissipation increases with load resistance. In order to visualise this relation, assume that the sum of the Screen and Plate current is independent of Plate voltage for zero Control Grid bias, or for a negative value of it. A decrease in Plate voltage causes a certain decrease in Plate current; it is assumed that the Screen Current rises by an equal amount. Hence, when the Screen Grid valve operates with a load which intersects the zero-bias characteristics below the knee, the Screen current rises to high values during low-Plate voltage excursions of the output voltage. This action produces a rise in the DC value of Screen current with signal. Therefore, the Screen dissipation with full signal input may be several times the zero-signal value. To reduce Screen dissipation, the load should always be chosen so that it passes through the knee of the zero-bias characteristic.
Increasing the applied signal voltage to a value higher than that for which the load is designed also increases Screen dissipation. For this reason, it may be advisable to use a value of load which is slightly less than the optimum value. This precaution has another advantage, which is especially important at high audio frequencies. The impedance of a loudspeaker increases with frequency. When the load is adjusted for the proper value at 400 Hz, the load is usually too high at 2000 Hz; thus a Screen dissipation limit may be exceeded at 2000 Hz even though operation is normal at 400 Hz. The use of a load which passes through the zero bias characteristic somewhat above the knee is desirable for these reasons." (end quote)
Note: The conditions described above are very likely in lead guitar amplifiers where the signal is of a single frequency nature.
BIT 6
It is interesting to note also that although RCA state in Transmitting Tube Manual TT-4 at page 9: " Beam Power Tubes may also employ Suppressor Grids rather than space-charge effects to prevent the reversal of electron flow when the Plate swings negative with respect to the Screen Grid." - a study of tube specifications reveals that RF Beam Power Tubes always have a rated Screen Grid voltage substantially lower than the rated Plate voltage, thereby rendering the foregoing statement by RCA as somewhat theoretical for both Pentodes and Beam Power Tubes.Last edited by randyc; 11-23-2009 at 01:25 AM. Reason: add underlining
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BIT 7
As a rule of thumb, the screen grid supply voltage should NEVER be more than the manufacturer's rating. Higher applied Screen-Grid voltage is likely to cause self-oscillation, parasitic oscillation, dynatron action or thermal runaway - any of which can easily destroy a tube and associated components. MINIMAL Screen-Grid voltage will provide better performance including cleaner, crisper sound with less distortion.
BIT 8
RCA Transmitting Tube Handbook TT-4 also states at p62:
"The danger of excessive screen-grid voltages is present principally when screen-grid voltage is obtained from the plate supply through a series dropping resistor. In this type of supply circuit, sufficient resistance is connected between the screen-grid and the plate supply to assure that the screen-grid voltage and dissipation at the values of screen-grid current, bias and driving voltage required for full output are within the maximum ratings for the tube. Any condition which reduces the current through the screen-grid dropping resistor to a very low value, therefore, may cause the screen-grid voltage to rise to an excessive value."
(end quote)
BIT 9
Voltage drop from DC Screen Current is a particular challenge with parallel-push-pull operation. Care is also needed with conventional Class AB or Class B operation of single paired tubes
Soooooo ... make up your own mind, follow your preconceived opinions, whatever you like
cheers,
randycLast edited by randyc; 11-23-2009 at 03:03 PM. Reason: delete a sentence repeated in previous post
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