Gemtune GS-01 blows second rectifier tube

[KlipschFan61]

No specific comment on the OP or his issue.
I'm just commenting in general. This is the 3rd or 4th thread I've seen come up in a week about amps eating rectifier tubes. Different brand , era amps. It just seems odd they all happened so close together

I've got a collection of dead rectifiers. The one that broke my heart the most was a nice NOS RCA 5U4GB.

Once I got this amp dialed in, I just leave it alone and let it do its thing.

 

[FlaCharlie]

I've got a collection of dead rectifiers. The one that broke my heart the most was a nice NOS RCA 5U4GB.

Once I got this amp dialed in, I just leave it alone and let it do its thing.

If you're using a rectifier that's designed for a 4uf cap and your amp has a 47uf first cap, your amp is not "dialed in".

Of course it's your amp and your choice. But if you choose not to bother with this issue you should not be surprised if the rectifier failures continue. And hope that you don't have to deal with a worst case scenario, which is that when the rectifier fails it takes out the power transformer, which seems to be a weak link in these amps anyway or they wouldn't run so hot.

I have a lot of tube amps, mostly vintage and a few DIY, and I've never had a rectifier tube fail. Come to think of it, I've never had any tube die suddenly.

When you have numerous tube failures in the same amp, there is a problem with the amp.

 

[KlipschFan61]

If you're using a rectifier that's designed for a 4uf cap and your amp has a 47uf first cap, your amp is not "dialed in".

Of course it's your amp and your choice. But if you choose not to bother with this issue you should not be surprised if the rectifier failures continue. And hope that you don't have to deal with a worst case scenario, which is that when the rectifier fails it takes out the power transformer, which seems to be a weak link in these amps anyway or they wouldn't run so hot.

I have a lot of tube amps, mostly vintage and a few DIY, and I've never had a rectifier tube fail. Come to think of it, I've never had any tube die suddenly.

When you have numerous tube failures in the same amp, there is a problem with the amp.

Like I said...I've got it dialed in and it works flawlessly. I received excellent support from the seller and I'm totally satisfied with the performance of this little gem. Thank you for the excellent dialogue Charlie. You obviously have lots of experience with fire bottle power.

 

[Captain Slack]

I was also wondering if the Captain bought the amp from the seller Gemini Doctor. This was the seller willing to send a replacement rectifier for my amp.

It's been almost a year and this is the second tube it's blown. I doubt he'd offer a replacement at this time.

 

[Captain Slack]

Finally had some spare time to open up the GS-01 today. It wasn't hard, but a few tips for anyone who might need or want to peak under the hood for this amp:

1. Access to the internal components is through the bottom plate. The four feet on the bottom are NOT screwed into the chassis and do not need to be removed.
2. The six screws which hold the bottom are a weird type that's somewhere between a Phillips & a flathead. They're made of a soft material and could strip easily.

So, here's what this things looks like on the inside:

There are several types of capacitors. None of them seemed to be bulging or leaking. I didn't notice any cold solder joints, either, but that doesn't mean there isn't one.

Here's some closeups of the various caps:

The two large one's above the rectifier tube.

The two on either side of the rectifier. I checked and they are the same value.

The rest.

So, where should I from here? Obviously, I need a new rectifier tube and I plan to order a 5U4GB per Palutris' recommendation. Should I order some beefier or better quality caps while I'm at it?

 

[KlipschFan61]

I do not believe using a 5U4GB is the answer but keep us in the loop as to how you make out.

 

[KlipschFan61]

It's been almost a year and this is the second tube it's blown. I doubt he'd offer a replacement at this time.

Who was the seller? Have you tried to ask them for a replacement tube? You'll never know unless you ask.

 

[triode17]

I bought a Gemtune GS-01 new from Amazon this past January. Two days after hooking it up, the rectifier tube blew. I can't remember which tube it came with, but it was an older NOS one I replaced it with a SovTek 5AR4 and it worked beautifully.

Notice when you go to the link you provided, the brochure page shows a 5AR4 which starts slowly and can take a 50 uF filter cap. The 5U4C is directly heated and comes on fast and can only take a 4uF cap, which may be failing because of inrush current. Also, newer designs tend to use low ESR caps, so they load down the power supply harder to start with. What is the value of the first cap. after the rect. tube?

 

[FlaCharlie]

Finally had some spare time to open up the GS-01 today. It wasn't hard . . .
So, here's what this things looks like on the inside: So, where should I from here? Obviously, I need a new rectifier tube and I plan to order a 5U4GB per Palutris' recommendation. Should I order some beefier or better quality caps while I'm at it?

Inside it's the same layout as all the other various brands that these are sold under. There seems to be some variation, such as some have meters and others do not. I'm guessing that the versions that use different tubes are essentially the same layout with perhaps a few different parts values but maybe not.

First thing to do is to find the value of the first cap in the power supply. There will be a wire or some connection coming from either pin 8 or pin 2 of the rectifier socket and going to a capacitor. What's the value of that capacitor?

Pin numbers are arranged clockwise when looking at the socket from the bottom.

OK, I enlarged the pic you posted and there's plenty of detail. You'll notice there are two yellow wires going to pins 2 and 8. Those are the heater wires from the 5v winding. Then there are two red wires on pin 8. One of them goes to the choke and the other goes to the first power supply cap. What's the value of that cap?

The one on the right is 150uf 450v so hopefully it's the cap on the left. I'm guessing 47uf 450v.

 

[FlaCharlie]

I know that the original version of these amps used a 5z3p rectifier, which has a 3A heater. They apparently switched to using rectifiers with 2A heaters, perhaps in an effort to reduce stress on the marginal power transformer. So it might be a best to stick with a 5AR4 which has a 1.9A heater rather than use an American 5U4GB which has a 3A heater. Since the modern 5AR4s seem to be significantly less robust than the old stock versions you should do a search for "5AR4 diode mod" which has been promoted by Eli Duttman for many years now. Adding diodes to the rectifier socket protects the tube from arcing yet you retain the benefits of tube rectification, especially the slow start feature of the 5AR4.

What I'm curious about is if they changed any other parts to compensate for the different voltage drops when they switched rectifiers. The original 5z3p, according to data sheets, drops 58v while the other rectifiers only drop 20v or less. It would be interesting to see what voltages are actually running in these amps.

The other tube they have apparently switched to, the Russian (cyrillic) 5U4C, is not the same specs as an American 5U4. It's the equivalent of the American 5z4. While its heater does draw only 2A and it does have slow startup characteristics, it is designed for a 4uf input cap and it can only handle 125mA of current vs 250mA for the 5AR4.

 

[mcgjohn]

I bought one of these amps sort of on a lark, just to see what you would get for a few hundred bucks. Was pretty surprised. pretty reasonable sound.

would also have to agree with FlaCharlie, in the tube issue. Once I made sure it worked, stuck in some NOS tubes and was off and running.

have been using an RCA 5U4G in the rectifier slot and all is well. also replaced the chinese 6SL7 with a NOS US version, which sounded a ton better.

the copy I got came with PSVANE HIFI grade EL34s, which are pretty nice tubes.

best

 

[KlipschFan61]

I bought one of these amps sort of on a lark, just to see what you would get for a few hundred bucks. Was pretty surprised. pretty reasonable sound.

would also have to agree with FlaCharlie, in the tube issue. Once I made sure it worked, stuck in some NOS tubes and was off and running.

have been using an RCA 5U4G in the rectifier slot and all is well. also replaced the chinese 6SL7 with a NOS US version, which sounded a ton better.

the copy I got came with PSVANE HIFI grade EL34s, which are pretty nice tubes.

best

I'm quite fond of PSVane tubes. They seem to have gotten better and better as time rolls along.

 

[FlaCharlie]

The pic, when enlarged, also confirms the use of a power transformer with a 110v primary. This is another concern.

Vintage amps were designed to run on 117v wall voltage so they run a bit higher voltages when plugged in to a typical 122v modern wall outlet. Running on voltage that is ~5v higher than the original spec does not cause any instant destruction but it does put a bit more stress on the amp and some people choose to reduce the voltage in order to eliminate that stress.

In this case, the Chinese amp would be running 12v higher! So if it was designed to run on 110v the secondary voltages will all be substantially higher than intended. Unfortunately, accurate, up to date, schematics for these are almost impossible to find so it's hard to know what all the voltages should be.

But we do know what the heater voltages should be so I'd suggest reducing the primary voltage enough so that you measure 6.3v, or preferably a bit less, on the heaters of the input and output tubes.

Since there doesn't seem to be any unused heater windings you would need to build a simple voltage bucker or use a variac to accomplish this unless you want to add resistors to the heater supply.

From the older schematics I've seen the input and output tubes are being run at conservative levels of plate dissipation. So if anyone is having an issue with those tubes failing I would suspect heater voltages that are much too high.

The rectifier failures can be traced back to the mismatched tube / first cap values. One owner posted a schematic that showed that his amp actually had a 100uf first cap installed, which would be too high for even the 5AR4 or an American 5U4GB. Then you have the fact that modern versions of the 5AR4 just aren't as well made as vintage.

But if you do the diode mod, limit the size of the first cap to 47uf, and use a tube that is designed to handle 47uf you shouldn't have problems. A low cost 5AR4 would be my suggestion. That and making sure the heater voltages are OK.

 

[Captain Slack]

OK, I enlarged the pic you posted and there's plenty of detail. You'll notice there are two yellow wires going to pins 2 and 8. Those are the heater wires from the 5v winding. Then there are two red wires on pin 8. One of them goes to the choke and the other goes to the first power supply cap. What's the value of that cap?

The one on the right is 150uf 450v so hopefully it's the cap on the left. I'm guessing 47uf 450v.

Hard to tell visually, as the wire snakes underneath the bus bar above the socket. Continuity check says it's connected to the 150uf 450v above it (the big blue caps in the picture). Specifically, the one on the right. Both of the blue caps are identical value.

 

[FlaCharlie]

Hard to tell visually, as the wire snakes underneath the bus bar above the socket. Continuity check says it's connected to the 150uf 450v above it (the big blue caps in the picture). Specifically, the one on the right. Both of the blue caps are identical value.

Yikes! I would do the diode mod using UF4007 diodes. I believe that should be enough to protect the tube without changing to a lower value cap since the diodes alone should be able to deal with 150uf without a problem.

I assume that since you were able to do a continuity check that you have a multimeter. If so, once you get it fired up again you should check the heater voltage.

 

[gadget73]

Is the rectifier fed straight into those 150uf caps? Even in series to make a 75uf cap, thats more than a 5U4GB can deal with. No wonder its killing tubes. The diode mod won't fix that issue. That deals with reverse voltage abuse, not forward current abuse.

 

[FlaCharlie]

Is the rectifier fed straight into those 150uf caps? Even in series to make a 75uf cap, thats more than a 5U4GB can deal with. No wonder its killing tubes. The diode mod won't fix that issue. That deals with reverse voltage abuse, not forward current abuse.

I stand corrected, thanks for clarifying that Gadget. I've seen many posts suggesting the diode mod but I've never had to use it since all the rectifiers I have are old stock American and European.

I believe, from looking at some older schematics, that the PS is CLC and that B+ comes off the second cap. If so it's: 150uf -- 5H -- 150uf. The pic shows the PS splits into two legs after that. Each leg is RC with two 22uf 450v caps with each leg supplying one of the driver tubes.

Those light blue caps look pretty dodgy to me - definitely Chinese bargain bin specials. Probably best to replace them with real name brand parts and go with 47uf for the first cap. Again, I'd suggest keeping a 5AR4 in there because of the soft start and the lower current draw of the heater. The PTs they use seem to be somewhat marginal and can use any help they can get.

 

[Captain Slack]

Those light blue caps look pretty dodgy to me - definitely Chinese bargain bin specials. Probably best to replace them with real name brand parts and go with 47uf for the first cap. Again, I'd suggest keeping a 5AR4 in there because of the soft start and the lower current draw of the heater. The PTs they use seem to be somewhat marginal and can use any help they can get.

So, what should I replace the second of the two 150uf caps with? Another 47uf?

 

[FlaCharlie]

The value of the second cap can stay the same, it's only the value of the first cap that affects the rectifier. Try a Nichicon, Panasonic or C-D, they're all good. I'd use something rated at 105 degrees and the highest lifespan available. Also pay attention to measurements, don't get anything that won't fit in the same space as the originals.

Here's a thread I posted a while back about how to get free shipping from Digi-Key, one of the major U.S. parts suppliers. Shipping charges can push the cost of small orders up considerably. http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

 

[Captain Slack]

Update time!

Finally had a free moment to work on the Gemtune today. Got some new caps per FlaCharlie's recommendations as well as two new rectifier tubes: a JJ and a Telefunken 5AR4. I'm using the JJ as a test tube and the Telefunken for actual use.

Put the new caps in without any issues. Checked continuity, which was fine, and fired her up: no smoke and no arching tubes! Woo-hoo!

Here's the part I'm not sure about: someone asked me to check the heater voltage on the rectifier across pins 2 & 8. The amp was plugged into a power strip which was providing around 121VAC. The voltage I read was not a steady 5VDC, however. I don't know if that's normal or not, so I shot a quick video of what I was seeing. Have a look and let me know!