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jefrs

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Everything posted by jefrs

  1. It may take a moment to get the head around it but that 5V(?) high-current heater is also firing hot electrons onto the rectifier anode. They are supposed to do that and why a valve rectifier cannot be run off an ordinary heater circuit. SS diodes are more efficient. Increasing B+ will alter the bias requirement. Cathode bias can cope with a lot of variation but the one in there is intended for 5881, Rk (and load/OT) on EL34 is different. Keep an eye on the EL34, see how they glow. Can you safely measure B+ and bias volts? I don't think valve amps are designed as energy efficient appliances.
  2. Yes it has its own heater but it also forms part of the power supply, it is not wasted. That current drawn from the heater goes into the HT. It is supposed to do that. That heater is also acting as a cathode. It is not just heating the cathode but supplying power too. That is why it has its own high current supply from the PT.
  3. By drain on the heater, do you mean the valve rectifier heater? - that is supposed to be supplying power, and quite a lot of it, to the amp. It is a feature, not a fault. If you mean the other valves' heaters then that is taken off a separate winding on the P/T, it is unaffected by the rectifier whether valve or solid state. Now the pre-amp bottles run their heaters in series from a supply of +13.5VDC on mine (due to my high mains volts again) but the output valves have 6.3VAC supplied heaters.
  4. Yes, it has been running a long time ;) - but it ain't dead yet! One of the first things I had to do when I got mine, many moons ago now, was connect the fan because there was a wire off at the PCB. Then I observed it made the amp hum and had to re-route the fan wires as twisted-pair. The fan supply should not be taken from the middle of the PCB near a valve base. Part of the heat 'problem' is lack of sufficient vents and running the bottles 'upside-down'. The fan is a "computer" fan, my PC has additional fans (controlled by temperature) due to old house dusty environment and cats. The main CPU now has two mounted in tandem. How did you attach your aux fan? The 6550 is interesting. They can take an amp to 100W territory. Did you do any mods to increase power or just 'stick them in'? I find my BC30 already too fearsomely loud for a lot of use. I have no real idea what its output is with the =C=6L6GC but I suspect well over 30 watts. Hence no wish to increase power further. Ime not all 6L6 types sound alike, there is a lot of variation. So worth playing with different makes to find ones one likes. I reckon I've got my BC30 well into Bluesbreaker territory now.
  5. You can get a plug in solid state rectifier that directly replaces the 5AR4/etc bottle. example http://www.watfordvalves.com/product_detail.asp?id=3071 Expect to get more volts with solid state diode rectifier.
  6. My standby switch actually exploded out of its back. It is an AC mains switch being asked to perform at >400VDC, no wonder it arcs. Completely the wrong tool for the job. It is also an odd size. I used some big "repair washers" to fit my MV there. You do not need a standby switch, you /can/ bypass it, spade connectors are easy to do this to. Just use the on/off switch and silence the amp with the input socket (it shorts = silence) like you are supposed to do. The standby switch is a fashion accessory. The Chinese rectifier in mine also made pop-corn noises, that was arcing over too, rubbish bottle. Sovtek 5AR4 very good and can do EL34 The stock EH ECC83/12AX7 were ok except I strangely prefer the cheaper Sovtek flavour (from same factory in Saratov?), although the EH were best for, and are still on, the reverb. I have since completely re-valved the other ECC83 with a variety of bottles, notably Harma's "Retro" Mullard clones (also suspected from Xpo-Pul/Reflektor) I found some "wrong value" components in mine too, but not a cathode resistor. Bad luck. Possibly worth checking a few others. You may toast any 6V6 (max plate 315V) except JJ-6V6GTS = JJ-6V6S. The JJ appears to be a 5881 with a low current heater that sounds like a 6V6, it can take something like 800V on the anode plate, I'm not sure how much current (power) it can handle though but I've not managed to blow my pair up. Putting 6V6 in does not make the amp much quieter, cranking it up will likely take 6V6 over 26 watts, which won't do them any good at all. The 6V6 has a low power or "half current" heater because it was intended for use in vehicles with pathetic 6V batteries. Note - I am getting 420VDC because I've got very close to 250VAC on the PT set to its max 240VDC. That means I get a higher bias point too. The fixed bias point for 6L6 is -37V at 450VDC, so -34V is a good compromise for 6V6/6L6 at this B+. EL34 would want -36V. The cathode resistor self-adjusts somewhat, it's about getting that bias point almost-right that counts. We are running much higher B+ than most spec sheets, hence greater bias volts. See plate characteristics graphs ;) BTW - using the full winding of the OT, 16ohm into 16ohm speakers, sounds crisper and clearer than the 8ohm tap (into stock pair at 8ohm) FWI - "pin 1" should already be connected to cathode so it should take any octal incl EL34 although a change of cathode resistor might be advisable. Although imo the =C=6L6GC sound more open, clearer and "bigger" than the EL34 mod BC30 I know.
  7. Do please note that my main "tone opening up" mod was to put 820k shunts around both R5 and R6 (both 1Meg), reducing it to 450k. The logical reason for using a 1W resistor here is that they are longer and can thus "stand off" a higher voltage, not that they pass much current, even if they had the entire HT across them, they would only see 0.18W P=E^2/R. My C4 "bright cap" is now on a bright switch. However the CH2 "bright cap" C6 is disconnected. Do check component values here, I found some on the PCB that did not match the circuit diagram. Also, for interest, my B+ appears to be 420VDC with the cathodes at 34.3V wrt ground. I also separated the HT from the heater wires, and twisted them. And another note that I separated the speaker socket ground so it does not return through the main PCB. Why would you pass that current through the main PCB? It's supposed to go to the O/T. These things do make the BC30 hum less. I did post up a circuit diagram here, with my mods on it but it was only as large are permitted and so may not be entirely legible. I can read it, it pops up very large and you can determine the original circuit. That's at post#145 - http://forum.gibson.com/index.php?/topic/43505-blues-custom-mods/page__view__findpost__p__758439 I have A/B'd mine with an EL34 mod BC30 and imo not much to choose between them, very adjacent. Actually they only modified for EL34, not the R5/R6 shunt so it still sounded congested. In theory, the EL34 is supposed to make it a trifle louder. In practice the little mods I made to mine made it too loud, too easy to drown out the rest of the band, hence the MV. =C=6L6GC
  8. I have used JJ-6V6GTS aka JJ-6V6S in there with no problem at all but have not / would not crank it right up. No red plate there. Note our mains is almost 250VAC so the 240Veuro PT will be running HT B+ a wee bit higher than spec hence 34.3V seen. Usual and current op valves are =C=6L6GC it's not just loud, it's bloody loud! I have taken chassis pics but cannot remember if I ever posted them on here. It will be the same as yours with a bit of tidy-up plus some damage from dropping. Let me know if you really, really need them?
  9. This is a pretty old thread now but I have just been notified of a reply. Any questions re BC30 mods? 1) I changed my ISP and hence email addresses, thus notifications got lost in the aether. 2) What was the Epiphone forum is now a Gibson forum. I lost the linky. Re - Greenbacks. They do not break in but just keep sounding better and better as they age. Re - speaker cable. I tend to use mains twin-flex. Mains twin-flex is often better than fancy hi-fi speaker cable, both electrically and more robust. Do solder the connections on because they need to be low resistance, crimp connections do have a significant contact resistance. They work fine in the car where they belong, and there the contact resistance is insignificant or does not matter - or you have a Mercedes and they solder them to the wire anyway (lederhosen have belts and braces ;) Re - the reverb tank. It is a Belton unit. They bought out Accutronics, They don't come much better than the one that's in it. The Belton site is a mare's nest to figure out the correct part number. There should be a part number stamped into the OEM tank. Get the specification from that. I think it is an 8ohm feed. You need to know orientation, insulation, impedance and output. You can ring changes on length, number of springs and decay time etc. The reverb is also dependent on the send level i.e. you get more reverb as you crank it up (becomes more obvious with an MV added). Also note that if you play live and loud - turn the reverb off and let the room do it or it all turns to mush.
  10. I was suggesting the schematic does not always show the correct component values fitted to the PCB. Can you read my diagram? PM me if you need a more legible one.
  11. Yes, your thread has grown rather well. Thankyou. I've got my amp just about as I like it now, it is very versatile and sounds excellent (even compared to another nicely modified for EL34 (non-MV) BC30 that I know ;-) I have made some unnecessary mods to it, like it doesn't really need my bright/gain switch gizmo, but the MV, the tone-stack and the "open" mod do work, as do that wretched rectifier and a general tidy of the wiring for noise. The choice of speakers is important too. I might try some other valves like EL34 in it, but it doesn't need to be any louder. I don't plan any more modifications to it, but one day I might tidy up the front panel where I've put the MV in. Please give compliments to Pytor.
  12. One other thing I did was take the PCB out. This is quite tricky and, yes, I managed to drop the chassis in the process. I lost the input jack corner and VR2. VR2 was easy to replace with a standard PCB mount pot. I had to Dremel off the corner of the PCB complete with the input jack and replace the jack chassis mounted, and re-build the input circuit PTP and route the wiring around it. Strangely this also cured some of the mains hum because the BC30 is grounded all over the place to the chassis. Lesson: the BC30 PCB is quite thin and fragile - don't take it out unless you must.
  13. re the post above here's a run down of the modifications. I may have incorrectly stated these earlier: my eyesight is not good on small print and was working from memory. I have now updated that diagram from the circuit board and refreshed my notes... so hopefully... Connect fan to heater circuit (it was open circuit from new) Remove self-destruct switch (standby) and use hole for Master Volume Tidy all wiring, see diagram notes Re-valve, rectifier replacement is a must-do as the chinese Electron-Tube 5AR4 is noisy, spitty, rubbish. Other valves see previous posts and discussions Actual modifications, note I have undone some of the first ones e.g. part of the tone stack mods, and C16 reverts to stock - - - > Bright caps C4 on bright switch DPDT, C6 lifted (not used) 820k Shunt on both R5 and R6 to "open up" tone and allow amp to "breathe" (higher signal level through lower R) On other half of bright switch a switchable 6k8 shunt across 10k R15 making 4k when on, to give "fat" gain at V2A. Note this fat gain switch is on V2A CH2, not V1B as previous stated. CH2 mid control R14 replaced with 100nF poly cap, changes mid frequency (keep removed 223) Tone stack slope resistor R22 replaced with 33k (note C16 reverts to 471. previous mods undone). Tone stack additional cap 223 inserted between C18/VR6 and VR7/R23, this gives a more conventional tone control whilst allowing the interactive/independent function. Do note I have shown actual factory values of pots etc in green where these are different to those shown on the standard circuit diagram. Master volume Linear B250k (Alpha) potentiometer (no, not variable resistor) replaces R39 with wiper onto lifted leg of C27 before phase inverter. Shielded cable grounded one end only on PCB. Other modifications 2-off Celestion Classic G12M-25 speakers 8ohm with series/parallel switch (as per Marshall cab)from stock it's a harsh amp, ymmv. Castors, no-brainer! Tilt-back legs (Long Fender), it should be obvious that this is a very loud directional amp. If the player stands over it they cannot hear its volume and will play too loud, drowning out the entire band: tip it back!
  14. Ok, I am posting my mods circuit diagram, recently updated and corrected. It's quite large so I don't know how long I can keep it on here... Click on the thumbnail to enlarge. It's about as big as anything we can post on here, I hope you can read it.
  15. Hi, 1) the thing I mentioned about putting a jumper across the 1Meg resistors does "open up" the sound on both channels, you still get the top end but more mid and bass, the amp seems to breathe more easily. I also posted a mod to the cathode resistor of V1B the CH2, I added this when I removed the bright switch mod from CH2. This does change the gain and fatten it up on CH2. 2) the MV mod to R39 that I posted uses a knurled Alpha 250k VR and some shielded wire like you might use for a humbucker. This is before the phase inverter. If you want a post-PI like a Marshall then you would need a dual-gang pot and to research Marshall amps. 3) There is a (cough) manual for the BC30, you can get the circuit diagram of the interweb. 4) if there is one valve that is must-change, then it is the chinese rectifier, I got a Sovtek 5AR4. The Sovtek 6L6WXT+ bottles are actually not too bad at all, and the EH12AX7 could be worse. I think the first mod I did to it was change the speakers, because it was so hard and harsh I fitted Celestion Classic G12M-25 Greenbacks. The original Eminence pot-luck types sounded something like Vintage-30, which are fine at high volume in a closed cab but otherwise very harsh and nasty. Better but even so, this did not fix its problems to my liking. I keep meaning to open the amp up and check my mods that I've put in and removed and put in again, my notes I'm afraid are a bit of a mess. From memory:- Speakers Rectifier Re-valve Bright caps, switches V1A CH1 V1B CH2 Fat gain V1B CH2 (no bright on CH2) MV R39 Tone control (several false starts) Shunting the 1M pair of resistors on CR-filter to drop frequency And Castors! I think I found a single CR that I also shunted but I cannot remember which one but that it made the most significant difference. I need to do this and post the "final" mod details...
  16. I would be disinclined to use a pre-amp valve anode resistor such as R4 as a discharge route because they generally do not have sufficient power rating. Find the anode of one of the output valves and discharge from there, through the transformer primary. The cap connections are inaccessible but the valve base connections are easily got at, especially if you think the circuit may be hot. Do remove the mains lead, my favourite, ouch! This may not be necessary since the BC30 does discharge itself, but it is nice to check with the voltmeter before plugging yourself in.
  17. Try putting C5 and C6 back down and shunting the 1Meg R5 and R6 with an 820k in parallel to each (R5 and R6 become 450k) and the fundamental pass frequency is halved, also the potential divider is halved (more signal to next stage), the sound opens up with increased bass but no loss of top end, not dark and not harsh. There are problems with the TMB tone controls, it is trying to be too many things at once: read my 18 April notes above, it can be fixed. Have we mentioned that changing the 5AR4 valve helps it a lot?
  18. Oops! I've maybe advocated modifying the cathode resistor on V2A above but the switch may actually be on V1B. I have modified both at some time and I'm afraid that my diagram has so much scribble on it now that it is hard to tell. When I look under the hood next I'll sort that out and re-post. I have now A/B'd the BC30 clean alongside my lovely sounding WEM Dominator 25 (with original Mullards) and I can say that we are defiantly in the same ballpark now tone-wise, about 97% I put this down to the "20W Marshall 2022" mod to R5 and R6 changing the frequency response across their 470pF into the second stage. My strange mods to the T-bridge also helped a little, the rest is down to the speakers.
  19. Tone control: more-or-less stock now, but the slope resistor R22 is now about 33k and I've added a 22nF cap from its junction with C17/C18 to the bottom of the bass pot, and R23 which is now about 10k. This makes it more like a conventional T-bridge, and more usable (for me) the knobs do something. I've noticed similarities to the WEM Dominator /"18 watt" Marshal and the 20W Marshall "2204"... Ch2 midrange tone cap C14 is 22nF, the 18W Marshall and WEM Dominator single tone knob types use a 10nF here, just thinking... Anyway tried a few different values incl 100nF in there now, makes little odds. Harshness and lack of bass: R5 and R6. Hmm, take a look at a 20W Marshall, these are voltage dividers with their VR but their fundamental frequency is governed by the caps C3 and C5 =470pF - no wonder this amp is so harsh... The Marshall 2204 uses a 470k here, that halves the frequency [F=1/(2piCR) ]. Ok, so I shunt the 1Meg with 820k, makes about 450k, and lo! lovely bass and fullness of voice emerges. I recommend this one! Enjoy!
  20. Can anybody please explain that output transformer ? Is it really 22kohm - 0 - 22kohm ? Is that 44,000ohm for plate-to-plate, OMG ? Svetlana reckon 6L6GC Ra-a ought to be 5600ohm. Somebody please tell me they got the decimal point wrong on the transformer label ! What on earth is going on there ? [traditionally RL = 2Ra (single valve), but halve that for push-pull, hence RL > Ra-a approx] PLEASE ? anyone...?
  21. Um, the amp is Class AB1 in both modes (doh!) it is switched either triode of tetrode, even EL34 are run as tetrodes. You /can/ design a push-pull to run as as pure Class A, but no guitar amp does that. If a valve switches off at /any/ point of its cycle then it is AB. If the valve does not switch off then you don't get high power, guitar amps are all about high power output. Maybe because the BC30 is 230VAC and UK mains measures 245V, I'm getting about 430V HT and -37V on cathode. Svetlana 6L6GC data indicates this could be in the order of 50W. Given that my Greenbacks are only 98dB and this amp is incredibly loud in a live situation (we have to keep it turned down or we drown the band), I think we're getting well over 30 watts. Triode mode is rather loud too but tonally different. The eq is pretty weird and very interactive. I have virtually undone all the mods I did to it. My normal setting is treble 9 o'clock, middle and bass around half-past 10 o'clock. The pots are large-value log where most T-bridge use lower value linear. Turning the middle to max removes it from the circuit to all practical intents. The gain channel can be run fairly clean by dropping the gain knob, then you get the use of that middle/presence control, I use it almost maxed, gain 11 o'clock, volume to suit. N.B. I have a Master Volume, and I really really recommend this mod. Svetlana =C= 6L6GC are huge sound-wise. For clean, I don't think you need any more headroom than these but they do have a big bottom. JJ 6V6GTS=6V6S, a high-voltage almost 5881 look-alike type. No noticeable loss of volume compare to stock Sovtek 6L6WXT+. Maybe not as much headroom but punchy and sparkly clean, tight bottom. To me, 5751 spec don't look like a good substitute for 12AX7/12AT7 in the PI. I have used a Mullard ECC81 (12AT7), and quickly put the ECC83 back in :-( To get extra clean try an ECC83-STR or E83CC (i.e. a super-clean low-microphonic type) in V1, a higher than normal gain selected type here will also push the drive channel a bit more. The amp is cathode/self-biased. Without re-biasing you can safely use any octal 5881=6L6WGB, 6L6GC, EL34 (& KT77/88?, 6550) etc provided that they can take the HT volts which is over 400VDC. I have used JJ 6V6S in there. But, but you will not get much increased power just by swapping the bottles, you would have to re-bias specifically. Of interest, the mains transformer has a tap for grid bias, as per the SoCal50. I opine the Beastly30 needs no additional volume, thanks. Regarding the bright/boost switch. I used a sub-miniature 2p2w toggle. Originally I used it as a bright on both channels but changed it over to a bright switch on channel 1 and a boost switch on channel 2. Bright is up, it puts the cap back down (for my mods see above). I am still undecided whether up or down for fat boost on channel 2 is best, but used with the channel switch you get four settings, roughly jazz, country, rock and blues, and then end up using fat boost with the gain down for jazz. I will take the opportunity to reiterate that changing the Chinese rectifier for a Sovtek or practically anything-else is a good thing. Imo the stock speakers are harsh and this is a harsh amp, so good old woody Greenbacks suit it, because they're not. Master volume, master volume, master volume. Yup, this really, really tames the amp. It allows you to crank the pre-amps and take that shriek off. BTW I am posting these notes for folks like me what like fiddling with their amps because out-of-the-box the BC30 is a bit of a PITA. Can anybody explain that output transformer ? Is it really 22kohm - 0 - 22kohm ? Is that 44,000ohm for plate-to-plate, OMG ? Svetlana reckon 6L6GC Ra-a ought to be 5600ohm. Somebody please tell me they got the decimal point wrong on the transformer label ! What on earth is going on there ? PLEASE ?
  22. Guitslinger et al, I found that it is possible to exchange caps and resistors from the top of the board if you can flow a little tin/lead solder down the new component legs, the capillary action flows it onto the board, but beware the original solder can explode away from the legs, fortunately the two types are mixable. Regarding C15 Cathode decoupling caps are chosen to give to give a bottom roll off frequency to the valve section. A large cap value will give a lower frequency than a smaller one. I have seen values of 220µF used to get 1Hz through whereas 22µF would only pass 10Hz. [ref ISBN 978-0-7506-5694-8] The drive channel uses V1B and V2A. V2A has no decoupling cap. When I added a 22µF across its R15 I got a lot more bass. The cathode decoupling cap will also increase the voltage on the cathode, which affects its gain. I used values of 2.2µF, 4.7µF, 22µF, 47µF but could not hear much difference and did not like what I heard, so I removed them again. However, putting a 22µF cap in and reducing the gain may well work, perhaps with a resistor in series like the 'fat' switch on the Blues Junior, or the gain pad on the Laney L5T. I do not use my bright switch so a fat switch is a likely next-mod candidate. I have changed R15 to 3.2k by putting a 4.7k shunt across the 10k in place. That gave the drive channel more bass. The cathode resistor sets its voltage. The cathode decoupling cap determines the lowest frequency The anode resistor determine the gain. Do not monkey with the DC decoupling caps C2, C9, C12 There is an odd thing going on here. If one uses half a double triode, you should use the best 'B' section, and if you have an extra gain section, you normally use a separate valve for it. So V1B should have been be V2A, V2A should have been V2B and V2B should have been V1B. It is not possible to re-wire the PCB ;-( Changing C15 from 22µF to 2.2µF should make the whole amp brighter, less bottom end. It may be more effective to alter its cathode resistor R21, mine is 1.5k not the 2.2k shown on the ver1 schematic. Modding C16 to 680pF gave me less shrill treble. I don't know if it is the Greenbacks but if I dare crank the bass round to 3 o'clock I get a huge Marshallesque bass roar, I normally operate the tone controls below 12 o'clock. Incidently I lifted (by de-soldering) one leg of C3 and C5 up but put C4 and C6 on a DPDT bright switch but note the values were changed from stock to schematic (was my board mis-assembled?) Values now - C3=C5=471 one leg lifted C4=C6=101 on bright switch (always off for me) These caps have been in and out so many times that I have fitted Harwin pins for them. The master volume mod does affect the tone in a good way too, and if I had to pick the single most important mod, this would be it. A bit muddled but hope it helps
  23. +1 Castors, I fitted some. Also note that the amp must be tilted back when playing with a band other wise to the sound flaps around the player's legs and he will have the amp set to drown everyone-else. to summarize my mods, and they are actually very simple - bright caps C3 and C5 lift or remove bright caps C4 and C6 lift or remove, or fit DPDT bright switch. Note correct value 101 = 100pF V2A cathode resistor R15 is 10k, gain is too great (lack of cap here means some negative feedback) shunt with 4k7 to make 3k2 N.B. tone stack C16 is 471 = 470pF, increase to about 680pF (I put 3 x 220pF in parallel). And I'm pretty sure I've put R23 back to 5k6 from the 10k I fitted N.B. Master Volume - replace R39 with linear B250k on shielded cable run (it went in hole left by standby switch) Other stuff I bypassed and replaced the standby switch N.B. I fitted a pair of Celestion Classic G12M-25 Greenback 8 ohm (series into the 16 ohm outlet). Warm tones. I've replaced all except the reverb bottles with better stuff (see earlier/other posts/elsewhere/TDPRI) The stock BC30 is considered very harsh and biting, by many. With these mods, this amps get compliments on how smooth and rich it is. Now it can go from warm smoky jazz, through heavily over-driven blues, to bitingly clean telecaster twang.
  24. I had a similar fault but I found the standby switch to be arcing. Because we have a 5AR4 rectifier in the BC30, we don't need no standby switch, the 5AR4 has a slow warm-up by design, to allow the heaters to get up before the HT (there's a problem called 'cathode poisoning' or 'sleeping sickness' caused by using standby switches - see article 'standby for the truth' by Hartley Peavey). So I bypassed the standby switch (actually I replaced it with a master volume control). I had already changed the stock Chinese Electron-Tube for a Sovtek, there was a very noticeable improvement. If you would go solid state rectifier check out the SoCal50 schematic (it's the grid-biased EL34 version of the BC30). The BC30 has the same power transformer and could easily be grid-biased too but I don't need it any louder, it's already louder then a Blues Deluxe (honest, we used it for a jam on Friday, I told one lad it was loud but he didn't believe and he drowned the band). We used to get solid state rectifiers fitted to an old valve base, so you could swap over, hmm, that Electron-Tube and some 1N4007 ($0.0167ea min order 1000) ... A Bassman AA864 is biased at -44V (at the pot) for 6L6GC. My BC30 is 35V (stock 250R) at the cathodes and 420V on the HT. I'm running =C= 6L6GC. What else... removed or lifted C3, C4, C5, C6. Made R15=3k2 and R23=5k6 and C16=660p (3x220p piggy-back) and C17=100n and replaced R39 with 250k master volume, ditched the speaker and fitted Greenbacks. These are all minor changes. Nice and smooth now, the stock job is very harsh. Now if I can fix the heater hum... which clot put a fan motor on it? Check your standby switch, imo they're not well made, eventually mine shot is springs and contacts out the back of the switch, yes it was live too, underwhelmed.
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