Keywords: medium and low μ tripod parallel drive 300B
Medium and low μ triode parallel drive 300B
RC coupling drive level
In the European and American home appliances, in order to save non-ferrous metals, try to cancel all inductive components, only a few micro-inductances are no exception, let alone Ten Thousands of electric inquirable audio coupling drive transformers. At home and abroad, there are a large number of 300b, 845 coupling, using RC coupling, in order to meet the demand of the drive, each has a trick, and the following introduces.
First, medium and low μ triode parallel drive grade
300B amplifier generally does not adopt negative feedback, the reason is that the low internal resistance output tube is linear, the amplifier front stage is also used in the medium and low μ small power tube, as long as the design is reasonable, there is an ideal indicator, the drive signal peak 60VP- P or so, the output power is about 7W.
On the 6SN7 RC coupling parameter table given by the US Slavic Company, when the anode supply voltage is 250V, as long as the input signal peak is close to the gate negative voltage value (not reached the gate negative value), the output signal nonlinear distortion has increased to 4% close to 5%. When the gate negative pressure is -4.75V, enter the 1.4VP-P signal peak, the peak value of the output signal can reach 22.4V (gain is 16 times), the nonlinear distortion is only 1.3%; the input signal peak increases to 4.3VP-P The output signal peak is 68 Vp-p, and nonlinear distortion rises to 4.5%. This test results indicate that the 6SN7 is not exceeding the gate negative pressure even in a standard A-class state, and the nonlinearity of the anode current has been extremely obvious. The input signal positive half period peak enters an anode current saturated zone curved surface, while the signal negative half cycle peak enters the anode current cut-off region bending portion, thereby increasing the distortion. This phenomenon increases with an anode supply voltage to increase the amplitude of the input signal. Therefore, theoretical linear zone is actually extremely short, 6SN7 can output 52VMS (73.5VP-P)'s limit status is premised on nonlinear distortion greater than 5%.
In many cranes, in order to reduce the large signal distortion of 6SN7, a higher anode voltage is often used, so that the anode current linear region is extended. However, the limit of the limit anode supply voltage of 6SN7 is UAMAX330V, in order to expand the linear zone of the anode current, the 6SN7 two-striped in parallel applications. The tube is connected in parallel, when the anode voltage, the gate negative pressure does not change, the anode current is the sum of the two tubes, and the respective linear anode current allows the variation range to increase. The cross-conversion S increased in parallel, and the anode internal resistance Ri decreased as a single tube 1/2, and the magnification coefficient u = s · ri remains unchanged. Obviously, the parallel transport cannot improve the voltage gain, just improve the nonlinear distortion degree of large signal input state.
According to the gain K = URA / RA + RI of the RC coupling amplifier (RA is an anode load resistance, Ri is an electronic pipe internal resistance). The lower resistance of Ra can be used, the gain K is constant, which will make the high-end frequency of the amplifier improve. The picture shows the 300B amplifier produced by a plant, the factory index is: output power 2x8W, nonlinear distortion degrees 0.1%, input sensitivity 500mV, frequency ring 13Hz-38kHz-1db.
The 300b state state in the circuit is: anode voltage: 400V (air-loaded up to 450V), static gate negative pressure: -89V, static anode current: 60mA, load impedance: 3kΩ, output power: rated 8W, maximum 11.5W Nonlinear distortion of <3%. the="" resistance="" of="" r11="" can="" be="" changed="" in="" the="" range="" of="" 400="" to="" 450v,="" so="" that="" the="" 300b="" static="" anode="" current="" is="" not="" less="" than="" 60="" ma,="" no="" more="" than="" 80="" ma.="" when="" reaching="" 60="" ma="" anode="" current,="" the="" gate="" negative="" pressure="" differs="" depending="" on="" the="" anode="" voltage.="" when="" the="" anode="" voltage="" is="" about="" 4.00v,="" the="" r11="" resistance="" is="" about="" 1430="" ω;="" the="" anode="" voltage="" is="" about="" to="" -100v,="" and="" the="" r11="" resistance="" is="" about="" 1650="" ω.="" for="" the="" 300b="" of="" the="" west,="" the="" load="" impedance="" 3kω,="" the="" anode="" voltage="" is="" 400="" ~="" 450v,="" the="" anode="" current="" is="" 60-70ma,="" which="" is="" the="" best="" state="" in="" linear.="" if="" the="" gate="" negative="" pressure="" is="" adjusted="" to="" be="" -87v="" or="" more="" (eg,="" -84v)="" under="" the="" above="" anode="" voltage,="" the="" anode="" current="" is="" increased="" by="" 80="" ma,="" the="" load="" impedance="" 2.5="" kq,="" can="" output="" power="" of="" 12.5="" w,="" but="" the="" nonlinear="" distortion="" increase="" is="" 5.5.="" %above.="" it="" can="" be="" concluded="" by="" the="" above="" operating="" state,="" and="" the="" effective="" power="" of="" 300b="" output="" 8w,="" the="" gate="" should="" have="" a="" driving="" signal="" not="" less="" than="" 75vp-p="" (53vrms),="" according="" to="" the="" characteristics="" of="" 6sn7,="" the="" pre-stage="" distortion="" will="" exceed="" 5%.="" therefore,="" the="" amplifier="" driving="" stage="" is="" used="" in="" parallel="" with="" 6sn7.="" according="" to="" the="" value="" of="" the="" icon="" element,="" the="" 6sn7="" is="" connected="" in="" parallel,="" and="" the="" 75vp-p="" signal="" peak="" drive="" 300b="" can="" be="" output="" as="" long="" as="" the="" driving="" level="" has="" 5.35vp-p="" input="" signal="" peak.="" in="" order="" to="" reduce="" the="" prior="" stage="" distortion,="" the="" level="" is="" added="" to="" the="" driving="" stage="" and="" the="" front="" stage,="" and="" the="" negative="" feedback="" loop="" does="" not="" involve="" the="" 300b="" output="" stage.="" the="" 300b="" output="" stage="" is="" a="" no="" feedback="" output="" is="" based="" on="" its="" own="" linear="" linear.="" although="" the="" test="" data="" indicate="" that="" there="" is="" 3%="" nonlinear="" distortion,="" the="" μ="" μ="" 三="" 输="" 输="" 输="" 级="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属="" 属.="" the="" 300b="" single-ended="" class="" a="" amplifier="" is="" more="" common="" in="" parallel="" applications="" in="" parallel="" applications,="" except="" for="" 6sn7,="" it="" is="" often="" commonly="" used="" in="" parallel="" drives="" such="" as="" dual-triodes="" 5687,="" 12bh7,="" such="" as="" high="" cross-conversion="" triodes,="" low="" internal="" resistance,="" the="" dynamic="" range="" is="" greater,="" the="" first="" choice="" for="" driving="" 300b,="" even="" if="" there="" is="" no="" negative="" feedback,="" there="" is="" a="" good="" test="" data="" and="" listening="" effect.="" domestic="" 6n6,="" 6n12p="" and="" 5687,="" 12bh7="" features="" similar="" to="" the="" application.="">
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