Fender '64 Vibroverb - A cosa servono quei due diodi ?

[Kagliostro]

Rispolvero una vecchia domanda che mi son fatto tempo fa e che sottoposi all'attenzione di un altro forum, senza, peraltro, riuscire ad arrivare ad una risposta

In pratica si tratta di due diodi (D4 - D5) collegati in parallelo alle res da 1R che congiungono i catodi delle finali a massa

in pratica un diodo per resistenza ed i diodi hanno i catodi collegati a massa, vedere schema ed estratto dello schema qui sotto

Per quanto riguarda D2 e D3, si vedono in diversi ampli, solo che di solito sono collegati con gli anodi direttamente a massa

http://www.prowessamplifiers.com/schema ... ematic.pdf



Qualcuno ha idea di che caspita ci facciano quei diodi (D4 - D5) collegati tra massa ed i catodi delle finali, in parallelo alle resistenze per la misura del bias ?

Grazie

Franco

[robi]

Ciao, i due diodi sugli anodi proteggono il trasformatore d’uscita da sovratensioni negative.

Quelli sui catodi limitano la massima tensione sulle resistenze di catodo a 0,6 V.

[Kagliostro]

Ciao Robi, Grazie

OK, ma secondo te ha un senso logico ? Può servire perché montano resistenze flameProof che se saltano si aprono ?

Secondo te c'è un motivo per il quale gli altri due diodi (collegati alle placche) sono collegati ai catodi anziché a massa (è lì che li vedo collegati in tutti gli altri casi che ho visto)

Che i diodi in parallelo alle resistenze abbiano a che fare con il fatto che han collegato gli altri sui catodi invece che a massa ??

Mah, tante domande, scusa

Se sai qualcosa, grazie, altrimenti grazie ugualmente

Franco

[robi]

Ci penso meglio e ti faccio sapere, Franco.
Io non avrei messo D4 e D5, ma dubito abbiano speso migliaia di euro in diodi per nulla.

[Kagliostro]

Si, un qualche motivo deve pur esserci

Certo che sembra abbastanza criptico

Franco

[robi]

Glitch Protection
During a major problem, the anode (plate) current meter and other amplifier components can be subjected to a large current surge as the HV filter capacitors discharge. The peak discharge current can exceed 1000a if a series resistor is not used to limit the short circuit current that can be delivered by the HV filter capacitors. The current limiting resistor is placed in series with the positive output lead from the filter capacitors. A wire wound resistor with a high length to diameter ratio works best. A 10 ohm, 10W wire wound resistor is adequate for up to about 3kV & 1A. For higher voltages, additional 10 ohm, 10W resistors can be added in series to share the voltage drop during a glitch. Wire wound resistors with a high length-to-diameter ratio are best for this type of service. Since about 1985, Eimac® has recommended the use of a glitch protection resistor in the anode supply circuit. Svetlana® typically recommends using a 10 to 25 ohm glitch resistor.

A HV current limiting/glitch resistor may disintegrate during a major glitch--so it should be given a wide berth with plenty of chassis clearance. If the chassis clearance is minimal, its a good idea to cover the chassis with electrical insulating tape. Glass-coated (a.k.a. vitreous) wire wound resistors are the most suitable type of resistor for this application. If a glass-coated resistor comes apart during a major glitch, it won't be throwing chunks of shrapnel around--like a less-expensive rectangular ceramic-cased resistor often does. Metal-case power resistors should not be used in this application. If a glass-coated glitch resistor is damaged during a glitch, it should be replaced with two such resistors in series to reduce the peak V-gradient per unit of length during a problem.

If the positive HV arcs to chassis ground--due to lint, a hapless insect, a VHF parasitic oscillation, or moisture--the negative HV circuit will try to spike to several kilovolts negative in the typical 1500W amplifier. In the real world, this type of glitch is not an uncommon occurrence. Anything that gets in the way of the negative spike may be damaged. Since the grid-current meter is normally connected between chassis ground and the negative HV circuit, the meter can be exposed to kilovolts at hundreds of amperes.

The easiest way to protect a current meter is to connect a silicon rectifier diode across it, or across its shunt resistor. Usually, only one diode {cathode band to meter negative} is needed in parallel with a DC meter. In some circuits, it is best to use two diodes in parallel [anode to cathode] with the meter movement to protect against positive and negative surges.

It may take more than one diode to protect a meter shunt resistor. A silicon diode begins to conduct at a forward voltage of about 0.5V. To avoid affecting meter accuracy, the operating voltage per glitch protection diode should not exceed 0.5V. For example, a 1 ohm shunt, at a reading of 1A full-scale, has 1V across it. Thus, two protection diodes in series would be needed to preserve meter accuracy. Similarly, if the shunt resistor for a 1A full-scale meter is 1.5 ohm, the maximum shunt voltage is 1.5V--so three diodes are needed.

Glitch protection diodes should not be petite. Big, ugly diodes with a peak current rating of 200a or more are best. Smaller diodes--and the meter they were supposed to be protecting--can be destroyed during a glitch. Suitable glitch protection diodes are 1N5400 (50PIV) to 1N5408 (1000PIV). In this application, PIV is not important. The 1N5400 family of diodes is rated at 200a for 8.3mS.

During an extremely high current surge, a glitch protection diode may short out--and by so doing protect the precious parts. Replacing a shorted protection diode instead of a kaput meter is almost fun.

To reduce the chance of the negative HV circuit spiking to several kilovolts, connect a string of glitch protection diodes from the negative terminal on the HV filter capacitor to chassis. At 200a, each diode will limit the surge voltage across it to about 1.5v. Typically, three diodes are needed--thusly limiting the negative spike to about 4.5 volts. Diode polarity is: cathode band toward the negative HV. With one simple wiring change, the same string of diodes can also protect the grid I meter and the anode I meter. This dual protection technique is incorporated into the Adjustable Electronic Cathode Bias Switch on Figure 7.

https://www.qsl.net/yt1vp/Introduction.htm

[robi]

Glitch protection
During a major glitch, the anode (plate) current meter is subjected to a current surge as the HV filter capacitors discharge. Such a current is typically several hundred peak amperes - not exactly courteous treatment for a 1-A meter. However, the peak current will be much higher if a resistor is not used to limit the short circuit current that can be delivered by the HV filter capacitors. The current-limiting resistor is placed in series with the positive output of the filter capacitors. A 10 Ω, 10-W wire-wound resistor is adequate for up to 3 kV and 1 A. Since the current-limiting resistor will be dissipating many kilowatts during a major glitch, it should be a rugged glass-coated (ie, vitreous) type. If a glass-coated resistor opens during a major glitch, it won't be throwing large chunks of shrapnel around - like a rectangular ceramic-cased resistor often does.

If the positive high voltage briefly arcs to chassis ground - because of lint, a tiny insect, an intermittent VHF parasitic, or an errant hair - the negative high-voltage circuit will try to spike to several kilovolts negative. In the real world, this type of glitch is not an uncommon occurrence. Anything that gets in the way of the negative spike may be damaged. Since the grid-current meter is normally connected between chassis ground and the negative high-voltage circuit, the meter can be exposed to kilovolts at hundreds of amperes. I heard about one grid current meter in a homebrew amplifier that exploded during a glitch. The glass from the meter landed on the floor.

The easiest way to protect a current meter is to connect a silicon rectifier diode across it or across its shunt resistor. Usually, only one diode (connected with its cathode band to the meter's negative terminal) is needed in parallel with a meter.

It may take more than one diode to protect a meter shunt resistor. A silicon diode begins to conduct at a forward voltage of about 0.5 V. To avoid affecting meter accuracy, the operating voltage per protection diode should not exceed 0.5 V. For example, a 1-Ω shunt, for a reading of 1 A full-scale, has 1 V across it. Thus, two protection diodes would be needed to preserve meter accuracy. If the shunt resistor for a 1-A full-scale meter is 1.5 Ω, three diodes are needed.

Protection diodes should not be petite. Big, ugly diodes with a peak current rating of 200 A or more are best. I have seen smaller diodes - and the meter they were supposed to be protecting - literally blown away by a glitch. After some bitter experiences with lesser diodes, I began using the 1N5401. In small quantities, the 1N5401 costs about 20 cents each. It is rated at 200 A for 8.3 ms, 3 A rms, and 100 PIV. Other diodes from the 1N5400 family will work as well. During an extremely high current surge, a glitch-protection diode may short out - and, by so doing, still protect the precious parts. Replacing a shorted protection diode instead of a blown meter is almost fun.

A brief high-voltage flashover can damage an indirectly heated cathode tube. Here's how: In many amplifiers, one side of the filament/heater is grounded. The cathode is connected to the negative HV circuit. If the negative HV spikes to several kilovolts, the cathode will arc to the grounded filament. At a minimum, this breaks down the insulation between the heater and the cathode. Sometimes the heater wire burns out - and sometimes the cathode arcs to the grounded grid. Either way, the tube is kaput. There have been many 8877s and other indirectly heated cathode amplifier tubes that died this way - all for lack of 60 cents worth of glitch-protection diodes.

So why don't manufacturers of such amplifiers protect the negative HV circuit from spikes? The answer is an electronic Catch-22. Even though it's likely that no amplifier manufacturer has ever seen a grid that was damaged by HF grid current, they seem to feel that electronic over-current protection for the grid is important. However, electronic over-current "protection" circuits for grids are not compatible with things that limit the voltage from the negative HV circuit to chassis ground. Thus, in an attempt to protect the amplifier tube from one perceived problem, designers leave the tube vulnerable to assassination from common occurrences.

To prevent the negative HV circuit from spiking to several kilovolts, connect a string of 200-A (or greater) glitch-protection diodes from the negative terminal on the high-voltage filter capacitor to chassis. Each diode will limit the voltage across itself to about 1.5 V. Typically, three diodes are needed - thus limiting the spike to about 4.5 V. The diode polarity is with the cathode band toward the negative high voltage. With one simple wiring change, the same string of diodes can also protect the grid-current meter and the anode-current meter.

https://www.robkalmeijer.nl/techniek/el ... index.html

[Kagliostro]

Gazie Roberto

Mi leggo tutto con calma

EDIT: Letto, Viene confermato quello che dicevi, son lì per protezione

Franco

[robi]

Mi chiedo però quanto servano davvero, dal momento che non li ho mai visti su nessun altro amplificatore.

[Kagliostro]

L'autore parla di valvole usate in trasmissione e problemi agli strumenti usati per il controllo del bias
Forse in quell'ampli, per qualche motivo, il problema si presentava sui prototipi e han pensato di risolvere così

Per quanto non so neanche se in quell' ampli ci sia uno strumento integrato per controllare il bias, più facile avessero problemi con il TU

Franco

[robi]

Infatti quei problemi me li immagino su un amplificatore in classe C, non su un classe AB.