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Any idea's on a visual Grid / Cathode fuse indicating device ??
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pentalab
Fellows
I have several fuses installed in most of these linear's. Like a fast 750 ma 3agc style grid fuse... located between chassis.. and negative terminal of 0-1 Amp grid meter. Also have a 3 A cathode fuse [also a 3agc] in the center tap of the fil xfmr circuit [ along with the 60 x 6A10 6-A , rotary switched bias diodes. ] This 3 A cathode fuse has a 1k-25 watt ressitor across it. Question is... what's an easy way to obtain a visual indication /led/neon/etc.... to let us know said fuses are blown ??? Most of the "visual indicating" type fuse holders I have seen... all require use of the line voltage being fed into em... ie: 120 vac, etc. I was thinking.. perhaps a 1-2 ohm resistor across each fuse holder... such that the v drop across the resistor [with fuse open] will trigger a led/etc. [this would amount to installing a typ grid/cathode electronic overcurrent device..... except it would only work IF the grid/cathode fuse opens]. Origionaly, we installed a 100 K- 2w-MF across the grid fuse.... believing if the grid fuse blew.... the resulting bias developed.. would cutoff the tube... and tube couldn't be driven. In actual practise... with the typ globs of drive... you can still drive the tube... albeit.. in class C ! So we subsequently removed the 100K resistor. Now,,, if the grid fuse opens... zero output... input swr rises from flat.. to 2:1. ..... No DC grid current. Here's the real concern. With the grid fuse blown open.... there is NO return for DC grid current... fine so far. But what happens to the drive RF [800 ++ watts worth] ?? Are we burning up the cathode.... and /or the grid ..or both ???? In all cases these RF decks/ power supplies are remote located 30- 50' away from the station. The station can however... read all 4 bird line sections... and also continuously monitor the PEP refelcted pwr [part of the high swr shut down circuit]. On a related note.... IF either HV fuse blow open..[1st one in one leg of sec of plate xfmr... the 2nd in the B+].... and with tube driven... ALL the electrons flow to the control grid... instead of the anode... and you guessed it... the grid current will skyrocket.. [read it will blow the grid fuse asap] We have seen cases whereby the HV fuse + grid fuse will blow open... cuz of an antenna problem... and high swr [trips on high reflected pep power] circuit has NOT tripped the amp offline. We have also seen cases, where the high reflected power circuit trips 1st.... and trips both linears offline.. [which also applies -10 vdc to the ALC of the xcvr. ] All depends sometimes... whether it's an arcing ant/badly installed connector. [resulting in an INSTANTANEOUS wide open/dead short... infintite swr] OR just a high swr... between 2:1 and 5:1 In some cases the B+ sandfilled fuse blows..... some times BOTH sandfilled fuses blow. For some added protection... we are thinking of an adjustable spark gap between load cap ...and chassis..... set to fire at a V higher than the normal low swr V.... and a point well below the rating of the 5 kv vac load cap. I think Rich may have done this ? I don't want the expensive ceramic vac load cap to ever internally arc. Sri for the drivel....... does anybody actually read any of my posts....... or is this all old news ??? Later... Jim VE7RF |
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Tony King - W4ZT
pentalab wrote:
FellowsI'd protect this with diodes and leave that fuse out. It wouldn't be a good idea for the cathode to ground circuit to open and essentially float the grid. Also have a 3 A cathode fuse [also a 3agc] in the center tap of the fil xfmr circuit [ along with the 60 x 6A10 6-A , rotary switched bias diodes. ] This 3 A cathode fuse has a 1k-25 watt ressitor across it.1k is a little low, use something like 15K to 20K across the fuse. When the fuse blows you will have an increase in bias that will cut the tube off and will develop a voltage proportional to that needed to cut it off. Question is... what's an easy way to obtain a visual indication /led/neon/etc.... to let us know said fuses are blown ???If the cutoff bias is high enough, a neon bulb with a series resistor could be used as an indicator. continued below... Most of the "visual indicating" type fuse holders I have seen... all require use of the line voltage being fed into em... ie: 120 vac, etc. I was thinking.. perhaps a 1-2 ohm resistor across each fuse holder... such that the v drop across the resistor [with fuse open] will trigger a led/etc. [this would amount to installing a typ grid/cathode electronic overcurrent device..... except it would only work IF the grid/cathode fuse opens]. Origionaly, we installed a 100 K- 2w-MF across the grid fuse.... believing if the grid fuse blew.... the resulting bias developed.. would cutoff the tube... and tube couldn't be driven. In actual practise... with the typ globs of drive... you can still drive the tube... albeit.. in class C ! So we subsequently removed the 100K resistor. Now,,, if the grid fuse opens... zero output... input swr rises from flat.. to 2:1. ..... No DC grid current. Here's the real concern. With the grid fuse blown open.... there is NO return for DC grid current... fine so far. But what happens to the drive RF [800 ++ watts worth] ?? Are we burning up the cathode.... and /or the grid ..or both ???? In all cases these RF decks/ power supplies are remote located 30-Spark gaps are good! Sri for the drivel....... does anybody actually read any of my posts....... or is this all old news ???I enjoy reading your posts Jim! Best thing I have seen is to use one of the triode control boards and use the protection circuitry they provide. You will find there's hardly any failure that will can not be detected and reacted to almost instantly. You might have to make a few adaptations to meet the need for QRO++ but the principle is the same. This is especially good because it can trip the amp off line so you don't have high drive going into a malfunctioning amp. 73, Tony W4ZT |
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FRANCIS CARCIA
I put a 50K 10 watt resistor across my fuse in the fil. C.T. so the bias is at cut off if it ever blows.
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Tony King - W4ZT wrote:
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pentalab
--- In ham_amplifiers@..., Tony King - W4ZT <w4zt-
060920@...> wrote: It wouldn't be a good idea for the cathode to ground circuit to open and essentially float the grid. ### Not quite. The GRID RING is bonded to the chassis !! This is not a case like Rich using 1/2 watt resistor's as "grid fuses" [ which WILL leave the grid floating] ### same deal on a YC-156 and also a socketed 3000/6000. ### I use several 6 A diodes between chassis and B-... [and also across all the meter's] . When the grid fuse blows the cathode can't float more than +/- .7 V. of rotarythe fil xfmr circuit [ along with the 60 x 6A10 6-A , 25switched bias diodes. ] This 3 A cathode fuse has a 1k- fuse. When the fuse blows you will have an increase in bias thatwatt ressitor across it.TONY SEZ a little low, use something like 15K to 20K across the will cut the tube off and will develop a voltage proportional to that needed to cut it off. ### partially agreed. Trbl is... you can STILL drive it.. and get say 40% output [Class C] I tried a 10-100k across the grid fuse as well. IF the grid fuse blows.... the bias developed will do exactly the same thing as if the cathode fuse blows. [cathode fuse with 1k to 100 kw across it] u still get 40% output. ### You can remove the resistor across the grid fuse with NO problem at all. Works better too. With no path for dc grid current... u get zero watts out. Still don't know where the 800+ watts of drive has gone ?? series resistor could be used as an indicator.TONY SEZ if the cutoff bias is high enough, a neon bulb with a ### I'm gonna measure the cutoff bias on the existing 100 k 2 w mf in the vac t/r set up.... on RX. and measure it. ## tnx. I didn't know whether to take my marbles home or not.LOL and use the protection circuitry they provide. You will find there'shardly any failure that will can not be detected and reacted to almostneed for QRO++ but the principle is the same. This is especially goodbecause it can trip the amp off line so you don't have high drive going intoa malfunctioning amp.### I have looked at both triode boards in the past. Some of it would need extensive mods, etc. As far as a grid over current device goes... the ones depicted in the handbooks work very well... albeit they all add varying bias.... cuz of the grid current flowing through a resistor [ vdrop used to trigger the 2n222 etc... then a 4pdt latching relay.... hotswitching amp offline, latching to itself... turning on a led etc. ] I have seen and built Orr's plate over current device same deal. [ just don't use the vac relay.. used to open off the HV ] ### During a "glitch".... it's a whole nuther ball game. They all use series diodes... such that during normal operation... slightly excess grid/plate current will simply kick amp offline. During a glitch.... with huge currents flowing... the safety diodes on the plate/grid protection devices will turn on..limiting voltage to a safe value.. so the grid/plate overcurrent protection device doesn't get fried. ## IF plate grid current are slightly over XXX.... the fast fuses take a long time to blow. During a glitch.... the fuse blow REAL fast.. and protect the tubes grid, etc. The eletronic stuff... while fast... still has to activate a mech relay. U can get small mech relays that will op in <2 msecs however. ### I'm going to try and incorporate BOTH fast fuses and electronic grid/plate current protection... I'll let u know. ### also a 2nd high reflected power kick out device b4 the big amp would help.... help to kick the IPA + xcvr off line... IF the grid fuse in the big amp blew. Take Care........ Jim VE7RF
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pentalab
--- In ham_amplifiers@..., FRANCIS CARCIA <carcia@...>
wrote: the bias is at cut off if it ever blows. ### agreed. Basicly anything from 1 k to 1 meg is fine. When the CATHODE fuse blows..... the amp can still be driven a bit.... so a 2-10 w resistor will work fine. ### IF u use a CATHODE fuse....make damn sure u stick a resistor across the fuse holder... like u described. ### I once had a 4-1000.... and while re-installing it... forgot to hook up the B- to the pos junctions of the plate + grid meter's........ a real disaster !! The cathode will try and assume full plate V... and the bypass caps at the cold ends of the fil choke starting snapping away !! ### also... in schemes like the L4B... where they use +130 vdc [and +90 vdc on cw position] to cut off the tube[s] on RX is bad news. When the 3pdt t/r relay is in 'mid air'... the cathode is floating for a split second ! I rewired it with a simple 100 k-2 w mf in the center tap.... and use the same contacts... to just short out the resistor on TX. gotta run.... Jim VE7RF |
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Tony King - W4ZT
pentalab wrote:
<snip> ### Not quite. The GRID RING is bonded to the chassis !! Thisagreed ### same deal on a YC-156 and also a socketed 3000/6000.yeah... and how sweet it is! ### I use several 6 A diodes between chassis and B-... [and also across all the meter's] . When the grid fuse blows the cathode can't float more than +/- .7 V.That's good and also the 1K safety resistor makes it all good ;) And that still takes us to electronic protection...fuse. When the fuse blows you will have an increase in bias that will cut the tube off and will develop a voltage proportional to that needed to cut it off.Also have a 3 A cathode fuse [also a 3agc] in the center tap ofTONY SEZ a little low, use something like 15K to 20K across the ### You can remove the resistor across the grid fuse with NO problem at all. Works better too. With no path for dc grid current... u get zero watts out. Still don't know where the 800+But the price is nominal for them... use the pieces you need. Take a little time looking at Paul Hewitt's board (WD7S). He has done some nice work and all his trimmers are multi turn pots. As far as a grid over current device goes... the ones depicted in the handbooks work veryI think you're thinking in the right direction. Paul uses solid state relays in primary leads but I'd think they would be difficult to find and very expensive for really big stuff. Of course they can open the circuit quicker than anything else. Only problem is, with that huge capacitor bank, speed on the primary might not be the saving grace. ### also a 2nd high reflected power kick out device b4 the big amp would help.... help to kick the IPA + xcvr off line... IF thePaul does do some neat sensing and comparing input and output. worth looking at! Love reading your technical details! 73, Tony W4ZT |
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pentalab
--- In ham_amplifiers@..., Tony King - W4ZT <w4zt-
060920@...> wrote: TONY SEZ.. And that still takes us to electronic protection...### My concern with electronic protection is.... if it malfunctions... when u need it ! I have seen these high energy diverter's.. [used to crow bar HV supplies, used in commercial large HV supplies like John Lyles describes at times] malfunction.... then u have the charred remains of a huge diode stack ! ### Having said that... it would be simple to incorporate two of everything... redundant protection schemes... and incorporate "test switches" on em..... so u can actually test em on site... without having to drive ur expensive tube into 'too much plate + grid current' ### I have looked at both triode boards in the past. Some of ita little time looking at Paul Hewitt's board (WD7S). He has donewould need extensive mods, etc.But the price is nominal for them... use the pieces you need. Take some nice work and all his trimmers are multi turn pots. ## agreed. He did have one UNIQUE twist. His board would sense the HV [ I think from the HV multiplier resistor's] and if NO hv present.... would inhibit the T/R relay's......very slick. ### IF my HV fuses blow... the amp is still online, being driven.... and a split second later, the grid fuse blows. His scheme would keep it shut down... until the HV was XXX volts. TONY SEZ .. Paul does do some neat sensing and comparing input and output. worth looking at! ### I'm gonna check his site again. I really don't like the SS relays in the 240 V primary. To work right... they should be zero cross Voltage on turn on.... and zero CURRENT cross on shut down. The 80 A rated ones I have seen require aprx a 100+ square inches of aluminium plate [very thick].. as the heatsink. They also have leakage across em. In Canada, they can't be used as the sole disconnect... they gotta be supplemented by at least a fuse. My electrician buddy got zapped last yr from one... from the leakage. ### Like u say.... the SS relays/ mech relays in the primary are a poor way to go, to shut off ... "follow on energy".... and totally useless to eliminate the B+ energy stored in a bank of lytics. ### Come glitch time, the idea of opening up the primary 240 V with a SS device freaks me out... ditto with a contactor... even a big one. Notice on the backs of 240 V breakers .. like the P+B "controlled magnetic hydraulic breakers" [ these things are 500% better/faster than any standard heat activated breaker] they all have slots on the rear. My buddy sez they are... 'arc chutes' designed to divert the arc out the back.. and away from the contacts. Most breakers will have a Max KA interupt rating on em... like 100 ka, most are designed to handle a dead short.... and assume the supply line from the street WILL pump out a huge amount of fault current. ### Even the fast breakers in the 240 primary are not fast enough during a glitch. To eliminate the "follow on energy" from the xfmr + diodes + caps..... I used the pair of sandfilled HV fuses. 7900V /50 ohm glitch R = 160 A 160A flowing through a 3 A rated HV sandfilled fuse blows extremely fast... and quenches the arc asap... as the sand turns to glass!! The other reason the fuses are filled with sand is to void 95% of the air inside the fuse to begin with. [little or no air left to ionize] ### The 100 A breaker in the 240 V line always remains intact... the faster fuses always beat it to the punch. Love reading your technical details!### Tnx.... none of it is rocket science... just a lot of experimenting over the yrs... and bumbling through things. I still contend.. most of this stuff coulda been done at least 30 yrs ago. ### My buddy phones me last night in an uproar... his 15 kw linear is putting out zero watts... and input swr has risen........ turns out it was the grid fuse gone open...... from the day b4.... when he just about vapourized his 15 m ant ! ### QRO is one thing..... ALL this stuff downstream is another. His 160m GP has a 30 ohm Z..[his mfj shows 1.7:1 swr right at the feed point.] He wants to use a 4:1 balun on it... using a T- 200 core !! ["to match it"] The simple solution was to use an L network... minus the cap of course. A simple 4.9 uh coil hooked directly across the feedpoint coax resulted in a flat swr. F-12 calls it a hairpin... Hy-gain calls it a beta match. It works on yagi's..... and now it also works on verticals + Groundplanes.... simple... one component. Now we remove the temp 12 ga 4.9 uh coil.... and replace it with a permanent 1/4" OD copper tubing coil....nothing to blow up.... end of problem..... end of story. Later... Jim VE7RF
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On Sep 29, 2006, at 4:56 PM, Tony King - W4ZT wrote:
pentalab wrote:When a high-R exists in the cathode circuit, as soon as a little current begins to flow, the tube cuts off. My modified SB-220 and TL-922 do this, and if a tube ever happens to short, nothing is damaged.FellowsI'd protect this with diodes and leave that fuse out. It wouldn't be a - note - In a stock SB-220 or 922, a shorted tube can destroy the filament transformer in short order. With 3-500Zs, 1k-ohm will limit anode current to about 25mA when the fuse opens.1k is a little low, use something like 15K to 20K across the fuse. When - editorial - A 250v-rated fuse in a circuit operated by 3000v is not good engineering practice. It takes c. 90v to light neon. A LED takes c. 1v. Also, with 1k=ohm and 3-500s, 25mA means the fuse is open, so one does not need an idiot light. Also, when the grid-I meter deflects backward, a tube has a filament that is shorting to the grounded-grid.If the cutoff bias is high enough, a neon bulb with a series resistor ... ... ... CorrectFor some added protection... we are thinking of an adjustable However, adding a rugged low-ohm low-L resistor in series with the spark gap helps to limit peak-I during a glitch.I don't want the expensive ceramic vac load cap to everSpark gaps are good! I enjoy reading your posts Jim! |
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On Sep 29, 2006, at 6:05 PM, pentalab wrote:
--- In ham_amplifiers@..., Tony King - W4ZT <w4zt-Tony -- Have you ever blown a grid fusing R and observed what happens? |
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Tony King - W4ZT
R L Measures wrote:
<snip> nope, no grid fuse if the grid is bonded to the chassis directly ;)### Not quite. The GRID RING is bonded to the chassis !! ThisTony -- Have you ever blown a grid fusing R and observed what happens? I was referring to B- floating off ground but if it isn't, then there isn't an issue. 73, Tony W4ZT |
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On Sep 30, 2006, at 10:53 AM, Tony King - W4ZT wrote:
R L Measures wrote:Nothing happens. ... R L Measures, AG6K, 805.386.3734 r@... |
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pentalab
--- In ham_amplifiers@..., R L Measures <r@...> wrote:
a little current begins to flow, the tube cuts off. My modified SB-220 and TL-922 do this, and if a tube ever happens to short, nothing is damaged. #### Partially agreed. When a grid fuse [inserted between grid meter and chassis... (or between grid shunt and chassis on a multimeter)... AND the grid fuse is shunted with a 100 K resistor..... AND the grid fuse opens, one would think the grid current [600-900 ma on a 6000A7] flowing through the 100 K resistor would create enough bias to cut off the tube ? It sorta does..... fact is.... with 800-1000 w of drive..... you still get 1/4 pwr output . We simply REMOVED the 100 k resistor across the grid fuse.... and now all is well. Now when grid fuse blows open...... zero DC grid current.. zero watts outa the amp. [where the 800+ watts of drive goes is beyond me. It either stays in the cathode.. or cooks the grid.... or both ??? ] ### On one of my L4B's a few yrs back [this one was unmodified], It was still using the +130 V scheme for cut off bias on RX. The contacts on the 3PDT t/r relay were bad... and the +130 v was still being applied on TX !! You could STILL get power output... [now in class C] ! Fix was to replace relay.... then rewire with a 100 K in the CT.... and relay contacts shunt the 100 K on TX.... end of problem. RL Drake's... and later Heath's "brilliant idea" of using +130vdc for cutoff bias on RX... was one of the stupidest moves ever...... 2nd only to using 6 x 200 pf caps + 2 x rf chokes on the 6 x grid pins !! ### BTW, the 100 w + 1 kw contacts of the bad T/R relay were perfectly intact. It's the center contact [with the +130 V} that goes bad eventually. When this center contact is in mid air... you have opened the cathode up for a split second.... and the cathode will want to assume full plate V ! RICH SEZ... - note - In a stock SB-220 or 922, a shorted tube can destroy the filament transformer in short order. ### say what ?? IF the fil xfmr draws too much current on it's sec,...... the fuse/small breaker on the PRI of the fil xfmr should blow ! Oh, I forgot.... in it's infinite wisdom... neither the SB-220 or the Drake L4B even had fil xfmr pri fuses........ they relied on the ..'big momma' 20 A slow blow ckt breaker in the 240 V pri!!! Fix... install a fil xfmr primary fuse. ## The L4B has an extra winding on the fil xfmr ... after being run through the 1/2 wave rectifier [ one diode, one cap.... how CHEAP could they have gotten ?] u get +27 vdc... to run the T/R relay. Fix is.... either change to a FWB..[+27 vdc]... OR change to a doubler. The doubler will put out +63 vdc.... and a resistor is simply inserted in one leg of the stock T/R relay's coil..... and presto.... you have a fast sped up mech stock relay.... not qsk... but faster VOX. The + 63 vdc ocv can [and is] be used also for a speed up circuit for a RJ1-A. ## Either way... this extra smaller sec... used for the T/R relay... should be FUSED. Fix... install a 3agc fuse holder... with the appropriate sized fuse. .... end of problems.... like as in this small sec ever shorted out..... you don't melt the fil xfmr [ "fused" with a 20 A breaker in the 240 V pri] RICH SEZ... With 3-500Zs, 1k-ohm will limit anode current to about 25mA when the fuse opens. ### when WHAT fuse blows? A cathode fuse ? If so, I agree.... and it should be increased to a 20-100 K unit..and complete cut off will /shoiuld occur. RICH SEZ - A 250v-rated fuse in a circuit operated by 3000v is not good engineering practice. ### the 250 V rated cathode fuse is NEVER gonna see 3000 V across it... and here's why. The worst case senario will be a HV to chassis short.. or a anode to grid flashover, etc. With reverse connected safety diodes [ I use a pair of paralled 6A diodes pointed one way... and a 2nd PAIR, pointed the other way] .. connected between CHASSIS and B-.... the fault current simply flows from chassis through the diodes... and back to B- ... completing the loop... and presto... the sand filled fuse in the B+ blows.. every time ! Also use reverse connected 6A diodes across BOTH the grid and plate current meter's. You will NEVER arc across the 250 V rated fast blow 3agc CATHODE fuse !! ##### Question is... what's an easy way to obtain a visual TONY SEZ... If the cutoff bias is high enough, a neon bulb with aindication /led/neon/etc.... to let us know said fuses are series resistor could be used as an indicator. RICH SEZ. It takes c. 90v to light neon. A LED takes c. 1v. Also, with 1k=ohm and 3-500s, 25mA means the fuse is open, so one does not need an idiot light. ####### When the big amp is 30' away... in a garage or a workshop, etc...."idiot lights" def are needed ! RICH SEZ...Also, when the grid-I meter deflects backward, a tube has a filament that is shorting to the grounded-grid. ### agreed. And the fix is to install reverse connected diodes across the grid meter/shunt.... then the diode will conduct.... instead of deflecting ur grid meter backwards. #### For some added protection... we are thinking of an adjustable spark gap between load cap ...and chassis..... set to fire at a V higher than the normal low swr V.... and a point well below the rating of the 5 kv vac load cap. I think Rich may have done this ? RICH SEZ ..Correct ### I don't want the expensive ceramic vac load cap to ever internally arc. RICH SEZ.... Spark gaps are good! However, adding a rugged low-ohm low-L resistor in series with the spark gap helps to limit peak-I during a glitch. ### Huh ? This is HIGH RF voltage across the spark gap [across the vac load cap]. What do u suggest.... a 50-500 ohm 90/150 watt globar ??? There's already a heavy duty glitch R in the B+ of the HV supply. Adding R in series to any spark gap may well slow down how fast the grid/cathode/ sandfilled HV fuses blow ??? Later......Jim VERF |
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pentalab
--- In ham_amplifiers@..., R L Measures <r@...> wrote:
### Not quite. The GRID RING is bonded to the chassis !! This fuses"is not a case like Rich using 1/2 watt resistor's as "grid which WILL leave the grid floating] RICH SEZ... Tony -- Have you ever blown a grid fusing R and observed what happens? ### WE are talkin bout a metal triode... complete with either a socketed grid ring... or a YC-156/YC-243... with a built in grid ring.... NOT a SB-220... with 1/4 or 1/2 watt 30 ohm resistor's installed between one grid pin and chassis. ## IF the 30 ohm resistors explode open... tube shuts down.... nothing happens.... except it's now a pain in the butt to change out the 30 ohm resistor's........ hence a rear panel 3agc fuse holder... with a fast 3agc grid fuse... installed between grid shunt and chassis..... takes 4 seconds to replace. In that above Rich modified SB-220... the grid is left floating in a vac....which isn't gonna harm anything... and no, the anode is not gonna arc to a now floating grid... then arc to the cathode. 73 Jim VE7RF |
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Steven Grant
What we are not seeing is when this fuse goes, the tube is already
bad!
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What the grid fuse does is protect the filament transformer IMO Steven W4IIV At 05:29 AM 10/1/2006, pentalab wrote: --- In ham_amplifiers@..., R L Measures wrote: |
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pentalab
--- In ham_amplifiers@..., Steven Grant <w4iiv@...>
wrote: bad! ### Say what ? What do u mean "the tube is already bad" ? My buddy has blown the grid fuse about 8 times cuz of misc other problems. The tube is still pumping out 15 kw. Ditto with the grid fuses that have blown in my 2 x 3-500Z linears. ### simply change out the grid fuse.. and you are back in business. [prudent to investigate why the grid fuse blew open in the 1st place.... which could be anything from having a blown HV fuse 1st... in which case the grid current skyrockets. My buddy had an arc over on one of his ant's.... took out one of the HV sandfilled fuses... followed by the grid fuse. Another time, the grid fuse blew from accidental overdrive. STEVEN SEZ... What the grid fuse does is protect the filament transformer IMO
### The grid fuse protects the grid. When the grid fuse opens... their is NO DC path for DC grid current... tube shuts down... can't be driven... and zero watts power output. ### simply change out the grid fuse.. and you are back in business ! ### There is also NO path for dc grid current to flow back into the CT of the fil xfmr.... which then feeds the fil. Extremely high grid current couldn't damage a fil xfmr anyway. The dc grid current flows into the CT... then splits left and right... into each side of the fil, [via the bifilar] The entire secondary.. including the CT... is built like a tank... typ rated for 21-80 A, depending on tube type. later... Jim VE7RF |
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Steven Grant
At 08:17 AM 10/1/2006, pentalab wrote:
--- In ham_amplifiers@..., Steven Grant ?* In normal amateur service, i dont see how i could ever drive the amp hard enough to roast the grid ??? even if mistuned (2? 3-500Zs) ???? I dont tune with full power till im close to resonance btw (maybe a 3CX800) ???? I do agree that with the grid open, the tube is off ????? I have seen roasted filament transformers from filament to grid shorts ### simply change out the grid fuse.. and you are back in business. ? * OK, question - will the fuse blow if the tube becomes unstable (parasitic oscillation) ????? ### The grid fuse protects the grid. When the grid fuse ? Steven W4IIV |
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On Oct 1, 2006, at 2:29 AM, pentalab wrote:
--- In ham_amplifiers@..., R L Measures <r@...> wrote:An 80Vrms winding is on the unfused fil xfmr. When rectified it produces c. 110Vdc. During RX, 110Vdc is applied to the fil CT to cut the tubes off. When a fil-grid short takes place, the 110v is shorted to gnd and, if the amp is not switched off, the filament xfmr will eventually melt down.a little current begins to flow, the tube cuts off. My modified IF the fil xfmr draws too much current onBingo R L Measures, AG6K, 805.386.3734 r@... |
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On Oct 1, 2006, at 2:40 AM, pentalab wrote:
--- In ham_amplifiers@..., R L Measures <r@...> wrote:Correct. Jim. My thinking on not using a fuse holder is that if the fusing element blows, the reason is not a defective fuse, it has to be the whatever that caused the fuse to blow, so a new fuse is not the total repair. Thus, when the fusing element blows, the amplifier has to be disassembled anyway to find the real problem, so a fuse holder would be of no advantage. R L Measures, AG6K, 805.386.3734 r@..., , rlm@..., www.somis.org |
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pentalab
--- In ham_amplifiers@..., Steven Grant <w4iiv@...> wrote:
bad! What the grid fuse does is protect the filament transformer IMO### IF the tube fil shorted to the grid... u may well be correct. The CT of the fil xfmr would be tied to the chassis... via the neg terminal of the grid meter [through the grid fuse... then to chassis] This would short out the CT to BOTH halves of the fil xfmr sec... frying it........ unless either a grid fuse installed... or a fuse installed in the pri of the fil xfmr. later... Jim VE7RF |
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pentalab
--- In ham_amplifiers@..., Steven Grant <w4iiv@...>
wrote: STEVEN SEZ... * In normal amateur service, i dont see how i couldever drive the amp hard enough to roast the grid even if mistuned (2 3-500Zs)### partially agreed. IF u have a HV fuse... and it opens... the grid current will skyrocket..... hence another reason for a fast grid fuse. #### STEVEN SEZ... * OK, question - will the fuse blow if the tube becomes unstable (parasitic oscillation)#### Been a long time since I have experienced parasites.... seems to me, both the plate current and grid current went beserko. IF the HV fuse opened off... the grid fuse would blow instantly... ### Never had a problem with parasitics destroying a tube.. as long as glitch R installed... grid and cathode fuses... and a HV fuse. later... Jim VE7RF |
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