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Post new topic Approx 9 months for tube in Carvin BX500
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Author Topic:  Approx 9 months for tube in Carvin BX500
Jack Stoner


From:
Inverness, Fl
Post Posted 31 Aug 2017 5:37 am     Reply with quote

The 12AX7 tube in my Carvin BX500 seems to be only lasting about 9 months (plus or minus). I like the BX500 and don't plan on replacing it.

The OEM tube lasted about 9 months and started to get some slight distortion. I replaced it with a NOS RCA 12AX7 and it lasted about the same length of time. I tried a Tung Sol 12AX7 and it developed a mid "honk" after a couple of weeks. I installed a Groove Tube 12AX7 last week so I can't say how long it will last. When the OEM went bad I didn't really think much about it as a tube can go bad "anytime" from my experience as an amp tech, but a trend of only lasting about 9 months concerns me.

My friend and retired broadcast engineer, Blake Hawkins (forum member) wondered about heat or plate voltage on the tube. The Carvin schematic doesn't detail what the B+ voltage is for the plate, however, the capacitor on the power supply B+ output is a 160 volt cap so I wouldn't think (too high) plate voltage is an issue. It has air vent slots on the front panel and a fan on the rear panel so its operating with the designed air flow. It is mounted in a 2 space Carvin rack case with the front and back open and always located where there is adequate air flow. I'm going to keep a close eye on the heat (I haven't noticed any heating) and maybe drill some air vent holes on the chassis top panel where the tube is located.

Anyone else noticing any "premature" tube problems?
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Jack Stoner


From:
Inverness, Fl
Post Posted 31 Aug 2017 6:23 am     Reply with quote

This is the response from Carvin in reference to the tube "wear". I'm not sure if its just a "brush off".

Quote:
Hello Jack,

If your looking at the schematic,pin 1 of the 12AX7 is where your plate voltage is, measure it with the meter and should be

Around 70-72.7Volts.

BX500 has a FAN constantly blowing cool air in at immediate turn on and an opening at the opposite of the fan to blow out

Warmer temp created by the heat sink and tube.

What your experiencing with Carvin BX500 preamp tube is just normal wear and tear for what the tube is design for with the

rest of the preamp circuit and to compensate for Class D power output section.


Best regards,

Warren


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Michael Remming


From:
Kimberly, Idaho, USA
Post Posted 31 Aug 2017 7:27 am     Reply with quote

Jack, Is there any noticeable difference in tone between the RCA tube and your new replacement. I'm going on 2 yrs with the BX500 still have the factory tube.
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DG Whitley


From:
Gone
Post Posted 31 Aug 2017 7:29 am     Reply with quote

I just do not believe this is "normal" wear and tear. I have known tubes, even in taxing uses, last much longer than that. I had an all-tube color TV that lasted over 20 years without replacement. My opinion, I think it has either been a slack design putting too much on the tube or something else going on with the circuit.

I also have a little bias here, I have never liked class D equipment. Something about the sound.
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Jack Stoner


From:
Inverness, Fl
Post Posted 31 Aug 2017 9:45 am     Reply with quote

Michael, so far the Groove Tube seems to be similar to the RCA.

My initial thought is the "low" plate voltage may be contributing to the tube starting to go relatively early. If it had a higher plate voltage it may not be as noticeable. I remember typical circuit applications in old RCA tube manual's that used 250VDC or more for plate voltage. I have an old ARRL (Ham Radio) manual and it lists 300VDC for a 12AX7 plate.
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Tom Wolverton


From:
San Diego, CA
Post Posted 31 Aug 2017 9:46 am     Reply with quote

If this was a common problem with the Carvin BX500, I would think the bass player community would be talking about it a lot. Anything on the bass forums about this? You might have a lemon. I do not think this is a normal life cycle for a tube.

I did read somewhere that the BX500 did not do well with crummy AC power and some folks recommended a power conditioner. I don't use one with my BX500, however.
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Jack Stoner


From:
Inverness, Fl
Post Posted 31 Aug 2017 10:10 am     Reply with quote

I doubt that a bass player would hear the very slight distortion, if they use the tube option, as its more in the higher frequencies. Bypassing the tube is clean so its not something else in the circuit. I'm a former amp tech and probably more critical or sensitive to problems than some others.

The place I play has good AC power. I also use a Furman surge protector.
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Glenn Demichele


From:
(20mi N of) Chicago Illinois, USA
Post Posted 31 Aug 2017 11:07 am     Reply with quote

How about mechanical damage? Drop your amp a lot?
I have a bx 500 tube problem, but haven't bothered to fix it yet. Every once in awhile I'll get a "rushing water" noise which quits and returns with the bypass switch. If I whack the top of the amp with my open hand, it goes away for a couple weeks. I'm suspecting socket issues because I have replaced the tube. Try tightening the socket with pliers, deox-It, or remating the tube.
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Jack Stoner


From:
Inverness, Fl
Post Posted 31 Aug 2017 12:40 pm     Reply with quote

No mechanical damage. Its housed in a two space Carvin rack case and I have a custom D2F cover that I use. I'm the only one that handles it and its never been dropped.

I have found the fan is running but not doing much, which could be one issue. According to Carvin support the fan is supposed to be "blowing cool air in" but I can't feel any air or if I place a piece of thin paper over the fan inlet it does nothing (doesn't suck it against the fan grille). Its supposed to be sucking in air and expelling it out the front air outlet. The is nothing coming out the front (which is another issue - there is a front panel circuit board that restricts air getting to the front outlet, which seems to be a design deficiency).

The tube socket is not an issue. It is tight inserting a tube and there is no movement after the tube is seated. The tube is connected via a cable (and connector) to the main board. If you have a problem with the tube I would suspect the connectors/cable.
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Mike Wheeler


From:
Delaware, Ohio, USA
Post Posted 31 Aug 2017 4:43 pm     Reply with quote

Jack, I've had problems with those little (actually mini) fans in the past. I remember one had a low drive voltage (wasn't spinning fast enough-bad xsistor), and another one was dust bunnies clogging the air flow path.

If there's no apparent defect, maybe you could find a better performing replacement fan for it. At any rate, you should feel SOME airflow, as you well know.

Good luck solving the issue.
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Best regards,
Mike
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George Macdonald


From:
Vancouver Island BC Canada
Post Posted 31 Aug 2017 5:31 pm     Carvin BX-500 Reply with quote

I had a Carvin BX-500 for two years, and while I didn't have any problems with it, I have gone back to a GK MB-200 with a Telonics 15" speaker l can dial in a good sound with the MB-200 easier than I could with the BX-500. Also, I really like my Nashville 112 with a double T speaker in it. I just don't need the power of a 500 watt amp.
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Jack Stoner


From:
Inverness, Fl
Post Posted 1 Sep 2017 2:28 am     Reply with quote

I've checked the voltage for the fan and its correct. The tube plate is getting the correct voltage (71VDC). I searched the net for an "exact" replacement but can't find one. I've found some others that may work but they are ball bearing type where the OEM in the BX500 is a sleeve type (quieter). The fan is Cooltron FD6020S12W5-71-2N.

George, I'm the opposite - I was using an MB200 and switched to the BX500 because of the tube. I have an excellent sound (easily) dialed in. Its so good that our lead player who is a "must have Fender tube amp" likes the BX500.
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Dennis Detweiler


From:
Solon, Iowa, US
Post Posted 1 Sep 2017 7:23 am     Reply with quote

There are several tube preamps that can be used in front of a transistor amp. Two that I have used in the past are Peavey Tube Sweetener (stereo/mono) and the ART Studio Tube Preamp (microphone or instrument). Currently, I'm playing through the Revelation (3 tubes) preamp, but still have the Peavey and ART.
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1976 Birdseye U-12 MSA with Telonics 427 pickup, Revelation Preamp, TC Electronic M-350 Processor, Carvin HT 400 Stereo Rack Head, 15" BW, Hilton Pedal, 1949 Epiphone D-8. And, too much extra gear.
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Jack Stoner


From:
Inverness, Fl
Post Posted 1 Sep 2017 7:31 am     Reply with quote

I have several of the ART's. They are not true class A tube preamps, but rather the typical "starved plate" operation which is basically just a buffer. I originally used them in my home recording studio but now have "real" tube preamps. I tried one of the ART's with my steel and was not impressed with it and after finding out it was a starved plate design even less impressed. I know nothing about the Peavey, but not interested in an external tube preamp anyway.

Right now I'm waiting for a response from Carvin support on how to order a new cooling fan. That is the first step as the one in there now is barely moving any air and not enough to do any cooling. Heat is an enemy of components, including tubes.
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Dennis Detweiler


From:
Solon, Iowa, US
Post Posted 1 Sep 2017 8:01 am     Reply with quote

Peavey no longer makes the Tube Sweetener, but can be found sometimes on ebay. I use mine in my small home studio gear to fatten and enhance old recordings onto CD. Nice thing about them, you can adjust the amount of tube compression. Good piece of gear for recording or mastering. The ART seemed to warm my steel tone slightly, but nothing like the Tube Sweetener.
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1976 Birdseye U-12 MSA with Telonics 427 pickup, Revelation Preamp, TC Electronic M-350 Processor, Carvin HT 400 Stereo Rack Head, 15" BW, Hilton Pedal, 1949 Epiphone D-8. And, too much extra gear.
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Erv Niehaus


From:
Litchfield, MN, USA
Post Posted 1 Sep 2017 8:15 am     Reply with quote

Dennis,
I also have a Peavey Tube Sweetener but never considered it to be a preamp.
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Dennis Detweiler


From:
Solon, Iowa, US
Post Posted 1 Sep 2017 8:45 am     Reply with quote

Not a preamp. It's to add tube compression. I always used it for the first connection out of my guitar, before going to my volume pedal. Then to my delay and reverb. Then to amp. I replaced the two channel AX7's and used AT7's. I could run the Sweetener volumes full on without overdriving the signal.
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1976 Birdseye U-12 MSA with Telonics 427 pickup, Revelation Preamp, TC Electronic M-350 Processor, Carvin HT 400 Stereo Rack Head, 15" BW, Hilton Pedal, 1949 Epiphone D-8. And, too much extra gear.
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Jack Stoner


From:
Inverness, Fl
Post Posted 1 Sep 2017 10:29 am     Reply with quote

With the help of forum member Blake Hawkins we did some investigation into the BX500.

1. The fan is OK. As it turns out it is thermally controlled and the speed (voltage) will vary. At cold start up the fan voltage was 4.5VDC. I wrapped some towels around it for about 15 minutes to make it warm up (a little) and the fan voltage increased to 5.5VDC. Thus it appears the fan and suspected cooling (heat) is not an issue.

2. I measured the filament voltage on the 12AX7 tube. Normal operation is 12.6V (AC or DC). The measured voltage is 10.5VDC thus this could lead to an earlier "weakening" of the 12AX7 tube.

Bottom line it appears it is working as designed, although a questionable design of the tube filament voltage.
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Jack Stoner


From:
Inverness, Fl
Post Posted 16 Sep 2017 2:47 am     Reply with quote

Update on the low filament voltage. There was an engineering change (resistor) that increased the filament voltage up to the 12.6VDC range, according to Carvin Tech Support.

The Rev Level of my main board is "L" and it does not have the revised resistor value. R232 was changed from 10K to 15K in the filament voltage regulator circuit.

Carvin is sending me a couple 15K resistors, free. However they are "SMT" (Surface Mount Technology) and I'm not sure I want to tackle that. I have a cheap SMT workstation with hot air to desolder components but have never tried it. Considering where R232 is located I may just leave it alone. I have enough new (useable) tubes to last nearly 3 years.
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Bill Terry


From:
Bastrop, TX
Post Posted 16 Sep 2017 6:35 am     Reply with quote

Back a few years ago, I did eval board development for a semiconductor company. I've done a lot of SM resistor swaps and I got pretty good at it, but you have to have a good loop to work under and the right pair of tweezers. They tend to stick to the tip of the iron if the solder doesn't flow well.

I could do as small as 0806 without too much trouble. The x-factors are what else may be in the way, i.e. how much elbow room, and how much coffee you've had.. Smile
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Jack Stoner


From:
Inverness, Fl
Post Posted 16 Sep 2017 8:42 am     Reply with quote

Here is a picture. R232 is next to the RTV glob.
In the circuit one end is connected to the Base of the transistor and the other end to ground.



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Bill Terry


From:
Bastrop, TX
Post Posted 16 Sep 2017 9:20 am     Reply with quote

Yeah, that one's a little crowded Jack.. Smile I don't blame ya.
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