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Old 16th October 2006, 06:44 PM   #1
pjpoes is offline pjpoes  United States
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Default meixing 805 tube amp problems

Hi guys, so I decided to chance purchasing a meixing tube amp from China and see how it went. So far not good. The amp arrived damaged. I'm going to try and resolve in a monitary sense the problem, I need a new 805 tube, so hopefully the seller will send me new tubes. However, I know near nothing about diagnosing problems and could use some help.

First off the amp came with one of the 805's damaged, a spring on the top was off and rattles inside. I found this after plugging everything in first. The amp was not hooked to any load, but the amp blew the fuse right away. Would the broken 805 tube cause that, or is something else wrong as well. Please let me know. I may end up having to pay a shop to fix this if I can't resolve it better on my own.
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Old 17th October 2006, 12:40 AM   #2
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Hello pjpoes, you asked:

Quote:
Would the broken 805 tube cause that?
Absolutely yes. That spring inside is one of the filament support springs. If it's floating around inside, the filament is broken and is shorting the tube elements. It's definitely junk. When you replace it, you may very well find that's all it needs.

However, I would carefully check the other 805 with an ohmmeter for shorts between the elements as well as the filament. And it wouldn't be a bad idea to test the other tubes too. Assuming there's no other physical damage, that's probably all your trouble.

Victor
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Old 17th October 2006, 02:13 AM   #3
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thankyou, do I just test the correct pairs, or all contacts. Thanks.
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Old 17th October 2006, 04:17 AM   #4
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On the 805s the filaments will be a diagonal pair and will read very low, basically a short. No other pins should give any readings to the filaments or each other. This is true whether your 805s have a top cap or not. Top (plate) cap should read open.

Since I don't know which amp you've got, I can't be more specific about any other tubes. If yours has the 300B drivers (?), the same goes for them. The filaments will be the two larger pins.

Victor
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Old 18th October 2006, 12:55 PM   #5
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I dont have the 300B drivers. That would be the amp I ordered, but not the amp I recieved. I'm quite unhappy with the company I ordered from and the situation I am now in. I appear to have no recourse what so ever and simply don't know what I want to do at this point. I could go to Ebay or Paypal and complain, problem is, I recieved a product, its even a product of equal value and similar design. Its just not the specific model I ordered. I debated between this amp and the other one anyway, so I'm tempted to cut my losses and leave it alone. I'm afraid at this point that the best course of action I could get is having to pay to ship this back, at over 200 dollars, and then have him send me another amp, which may come damaged worse.

Though I dont have another set of 805 tubes at this point, I should in the next few days, and then I can test for sure that its just the tube. As is, it appears it was the one 805 tube. It tests as a short, but you say it should, but when I have every tube accept the one bad 805 tube in the amp, it doesn't blow the fuse. Put that tube in, and it does. My thoughts is that it must be that tube, and that replacing that tube will fix the problem. I purchased, at my own expense, two 805 tubes for about 90 dollars including shipping. I believe they are identical tubes, the Sheguang, or however its spelled, brand Chinese made tubes. They appear to be of so so quality overall, but they are the only new production 805's I can find, and NOS are rare and pricey it seems.

I intend to write up a full report of my experience with the company that I ordered from and was wondering if this forum had a speciifc place to warn consumers about bad sellers. I will say, I dont even know that its fully the seller, infact I highly doupt it is. My guess is its the overall situation, needing to ship a small heavy item from China to the US durring a time of high security in both countries makes it difficult to get through. Essentially, better shipping cartons are needed, such as wooden crates. Let me know on that and I will fully write up what I have gone through to import my own amp.
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Old 19th October 2006, 02:15 AM   #6
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You can find 805s here in the US, NOS USA mfr.

Also check with Electro-Harmonix, they may have reliable tubes, and they are in NYC... many other sources out there.

Not so expensive usually.

Depending on where you are in NY - think that is where your info says, you can likely find any number of Hams or audio hobbyists to help you diagnose the tube amp... where are you?

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Old 19th October 2006, 03:04 PM   #7
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Thanks everyone for the help. I have found various 805's and tubes said to be similar to the 805, such as the one Western Electric 3 something or nother, I need to recheck and ask some of you if its compatible. They aren't too bad, but for now I will try these chinese ones. Oh and before I forget, the amp is working now. It needed the one 805 replaced, the fuse obviously, and it had a lose wire inside. Everything is fine now, and testing within spec. The amp is making some pretty nice sounds right now, just waiting for it to break in and for that hardness on the edgess to fall away a bit.

I changed the 12ax7 with a sovtek LPS tube, a major improvement over the chinese 12ax7 that was in it, and something I had laying around from my guitar amps. The 12au7 also recieved a Sovtek tube, which also made an improvement. Now I need to change the 6l6 driver tube with something else. After that I intend to move on to the power supply caps and coupling caps, then possibly the resistors. I'm also working on a stepped attenuater for it, but I haven't finished it yet. Its going to use caddock resistors, well thats the plan, but I'm conserned it wont fit, those stick out a lot more than others.

As far as capacitors are concerned. For replacing the power supply caps with ones that are of better quality, will fit within the chassis, and offer a lot of value, I had been told to look at Panasonic caps. I have used them before in solid state projects such as my chip amps and Class D amps, but wasn't sure which of their high voltage rated caps would be the ideal choice. What series of Cap should I be looking at? Also, for the diode bridges, follow the same rules as with solid state diode bridges? Will Freds offer the same improvement or should I be looking elsewhere? Let me know, thanks all.
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Old 20th October 2006, 05:22 PM   #8
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If the amp has a solid state rectifier (?) I'd swap 'em out for soft recovery diodes. And also add in some sort of delay or soft ramp up for the B+ voltage.

The filter caps probably don't matter much, imho.
Bypass them with some good film caps, that's a good idea.

For the driver, if it is not already, change the 6L6 driver to triode operation! Should reduce some of the top end "splash" which it typical of pentodes/tetrodes.

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Old 20th October 2006, 09:44 PM   #9
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Thanks for the response. I have to be honost, I'm not particularly knowledgable about tube amp design, and I am not sure how to know if its wired for triode operation or not, the 6l6 that is. Nor would I know what is required to convert it.

I believe the caps are already bypassed by some small value film caps, so maybe an upgrade to a better bypass cap will improve sound a little.

Yes the amp does have a solidstate rectifyer. I wish it didn't, however I noticed that the Cary design that it claims to be similar to also has a solid state bridge rectifyer. There appear to be two of them side by side, grey bridge rectifiers of unknown origins. Would a Schotky bridge rectifier from the likes of IXYS be known as a soft recovery type, that was the kind I was thinking about going with. I notice they refer to the FRED as being a soft recovery type, so I will assume as much. Any idea what value I would need for a tube amp. I will look again, but I didn't see markings on the factory ones that I could understand. would the 27 amp 600 volt unit be enough for an 805 based tube amp. Would the 1200 volt be a better option just to be safe.

At the moment I have spent the last two days listening to Jazz, Classical, and Blues almost non-stop. Its gotten to the point where my Girl Friend has become annoyed with me and we had to make a deal so she could watch her Gilmore Girls (I know I know, dont ask). Tonight she is out with her friends for a bit, so I plan to spend that time listening to some old RCA Red Labels I have around. Thanks again, and let me know on some of those questions.
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Old 20th October 2006, 11:12 PM   #10
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Quote:
Originally posted by pjpoes

At the moment I have spent the last two days listening to Jazz, Classical, and Blues almost non-stop. Its gotten to the point where my Girl Friend has become annoyed with me and we had to make a deal so she could watch her Gilmore Girls (I know I know, dont ask). Tonight she is out with her friends for a bit, so I plan to spend that time listening to some old RCA Red Labels I have around. Thanks again, and let me know on some of those questions.

the above proves your one of us.

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