Arcing rectifier -- seeking sage advice

[Prismatic Sky]

Hey everyone!

This past week I purchased a used Yamamoto A-08s amplifier. This came after months of research and careful decision making. It's also my first foray into the tube world, after owning solid state gear for decades. As such, I'm sorry for any silly or irrelevant points I may bring up, as they will be extensions of my very basic understanding of valves and their circuits (I'm learning experientially).

Here's the situation. After a very careful unboxing and clean-up, I'm happy that the amp is visually in mint condition. I finally got to the point of powering it up last night, and immediately the rectifier tube (80) began to arc and flicker. I did a few hours of research, and it appears to me that it was not what people call 'tube flash', but rather arcing, or 'cathode stripping'. I let it go for about 3 seconds to see if it would stop, but then made the quick decision to power it back down... then immediately started researching.

From what I gathered, it seems that this is caused by the tube being 'overtaxed' from two factors -- first, on a cold startup, it is only able to operate at a fraction of it's 'warmed up' ability. Second, and compounding the issue, is that the tubes highest demand period is right at startup, when it need to charge a large 'empty' capacitor. Again, I'm sure this assessment is begging for corrections or clarifications.

In addition to any general advice people may have, I have a few specific questions:

1. What are the chances that this compromised or damaged any other component in the amp? From what I gather, some people just let the tube arc at startup and let it subside after it warms up, but I'm not so brave myself... so hopefully this means that it is 'safe' for everything else in the amp.

2. Could this issue have anything to do with the fact that I'm powering it through a Furman "IT-Ref 20i" isolation transformer? I know that each AC leg is split to carry 60v each in this design, as opposed to the 'regular' 120v + neutral... It just had me thinking, what if there is an incompatibility there somehow?

3. Could this be actually be a normal occurrence for a healthy tube that hasn't been powered on for some time? I don't know when it was last used / powered up... but if it's been years, perhaps time has some relevant effect. If this is the case, should I keep using the tube, or is it dead?

I turned to this community first because you guys seem like you really know your stuff -- thank you for any and all help!

 

[PRR]

I'm powering it through a Furman "IT-Ref 20i" isolation transformer? I know that each AC leg is split to carry 60hz each in this design, as opposed to the 'regular' 120hz

So take that gizmo out of the picture and try "normal" power, as the designer likely intended?

BTW, you mean Volts not Hertz.

And this should not make any real difference on properly-designed gear.

I think you have a bad rectifier (why else have a socket?) OR a shorted capacitor. If it was mine, I'd set up a video camera to post to YouTube, and leave it powered-up until smoke clears. (But there may be much wiser suggestions from other folks here.)

 

[rayma]

No arcing happens if proper design has been done. See if the seller will give you a new rectifier (if he said it works).
If the amp was sold as-is, the best thing to do, may be to just buy a new rectifier tube. Even testing the rectifier tube
in a tube tester may not be helpful, as operating conditions will differ.

Of course you could slowly bring up the circuit on a Variac and see what happens, which is what many do with
older equipment in unknown condition. Is the amp set up for 120VAC line input? Japan is 100VAC line instead.

It may be helpful to post some photos of the amp. Cathode stripping is completely different from arcing.

 

[jjasniew]

I guess if it was me I'd use a variac to more slowly bring up the AC line input to proper line voltage, while simultaneously watching the voltage across the B+ caps with a voltmeter.

If you dont have a variac, or are unwilling to expose any circuitry inside involved with the B+ caps to physical contact (which would be a very wise decision, BTW) it's game over until you can bring it to a qualified service technician who can do this for you safely.

It may be after a few soft AC ramp-up cycles using the variac dial, it becomes able to come up to operating without the arcing you've witnessed at the flick of the on switch. No guarantee however that this "hasnt been powered on for some time" factor is the root cause.

 

[jhstewart9]

So take that gizmo out of the picture and try "normal" power, as the designer likely intended?

Good advice. Take that thing out of the system. I'm thinkin' you simply have fried 80 rectifier. Replace it ASAP.
Be sure the amp is fused.

People stuff the damndest things in looking for some kind of improvement.
One of the craziest I came upon occurred while selling HP TE. A large standards lab here had bought one of our HP 140 scope systems.
They called us about a problem. Not far from our Regional Office, I went to have a look. The scope was cycling in & out of operation.
I asked where the power was coming from. There was a large Sola Constant Voltage Transformer under their test bench.
The regulation system in the scope was in oscillation with the Sola, each wanted control.
Scope plugged into regular power, problem resolved.

Some Sola's also put lots of odd order distortion on their output spigot. It appears to be load dependent.
The Sola maintains the RMS level, not the waveform.

 

[daqvin_carter]

I would make sure the amplifier has not been altered or at least see what the supply capacitors are.
I looked up a schematic and see 47 microfarad for a first cap and a 47 mictrofarad after the choke.
Seems like a reasonable design.
If you see more capacitance than that, you should expect broken rectifiers.

 

[6A3sUMMER]

The capacitor uF values for the type 80 rectifier are usually not listed in the data sheets.

The 5Y3 rectifier is very similar to the type 80, and was considered to be the 'modern' replacement for the 80.
The various 5Y3/GT data sheets list 10, 20, and 32uF.
(I thought that 47uF sounded a little too high for both the 80 and the 5Y3/GT)

Those date sheets also call for source resistances of 30 to 50 Ohms.

If the former owner modified the used amplifier, either the B+, or the B+ load current . . . that might cause a problem.

Your Mileage May Vary.

 

[chrisng]

I thought that 47uF sounded a little too high for both the 80 and the 5Y3/GT

+1

Schematics can be found on this thread:

Yamamoto A08S amplifier kit build

 

[Prismatic Sky]

Woah -- awesome responses from everyone -- can't believe how helpful the community is!

I've carefully read everyone's suggestions, but for the sake of simplicity I'll respond more generally. Covering a few basics -- the amp was factory built in 2016 by Yamamoto, and I don't think it was ever modified (the owner I purchased it from would never have done this, but who knows about the first owner). The guy I bought it from has been really helpful - I messaged him yesterday about this, and he told me that he used the amp up to the day before he shipped it to me... so I guess that rules out the possibility of the capacitors needing to be reformed after having sat for a long time.

After explaining the situation, he also told me that the rectifier is fine, and that what I'm seeing is just "gas", and that he experienced the same thing. This confused me because, errrr, I thought this was a "vacuum" tube... but maybe there are trace amounts of gas in some tubes? Now I'm questioning whether what I saw was actually arcing or not... I only had a few surprised seconds to observe before I shut the amp off, but I'm remembering what I saw appearing as more gentle and 'gas like' than what I would imagine internal electrical arcing to be... There was no loud snapping or popping, just sort of a flickering bluish light maybe mixed with the orange of the elements... and I would imagine arcing to make a snapping, popping sound.

If it turns out that I actually wasn't seeing arcing, I'm embarrassed and apologetic for leading everyone astray here with a false assessment.

I've decided that, later today, I'll bypass the balanced isolation transformer as a couple people here suggested, and plug it directly into my wall outlets. Then, I'll take a video of the tube as I turn it on again, and I'll find a way to post it here so you guys can visually see what is happening. Wouldn't that be wonderful, if in fact nothing is wrong...

 

[rayma]

You would definitely know if it were arcing, and often the fuse will blow.

 

[Prismatic Sky]

So, I'm curious, what else would cause flickering, blueish light? I read that it could be electrons bombarding the glass and exciting impurities? Is this 'common'?

 

[rayma]

Yes, this electron scatter is harmless and common.

 

[euro21]

I looked up a schematic and see 47 microfarad for a first cap and a 47 mictrofarad after the choke.

The original schematic shows 10uF.
https://www.diyaudio.com/community/threads/need-help-troubleshooting-yamamoto-a08s-kit-build.295621/


https://6moons.com/audioreviews/yamamoto2/45_2.html
https://www.canuckaudiomart.com/det...o_special_rectifier_socketrare/images/621281/

 

[Prismatic Sky]

Yes, this electron scatter is harmless and common.

Wow, thank you... I never knew... Figured for sure something was blowing up . Silly me -- great news to hear.

I've only had personal experience with EL34s and a little 6N8S buffer in my preamp, so I'm a tube newb. Maybe the smaller tubes don't exhibit this effect, or it's less pronounced?

 

[20to20]

Wow, thank you... I never knew... Figured for sure something was blowing up . Silly me -- great news to hear.

The blue glow is not a problem but the arcing you described should never happen. Did you flip the power on and immediately within a second or two see the arcing? Or did you do what some would do and flip the power on for a few seconds 5-10 to see if it smoked, flipped it back off to catch your breath, then flipped it back on after a few seconds off, when it all seemed OK? A hot start.

 

[Prismatic Sky]

The blue glow is not a problem but the arcing you described should never happen. Did you flip the power on and immediately within a second or two see the arcing? Or did you do what some would do and flip the power on for a few seconds 5-10 to see if it smoked, flipped it back off to catch your breath, then flipped it back on after a few seconds off, when it all seemed OK? A hot start.

I just turned it on for 2 or 3 seconds, then right off again... Not 'on-off-on'. I'm not sure if I did see arcing at this point though... More of a blue flicker / swirl that made no noise. I'm going to video another turn on tonight and will report back.

 

[6A3sUMMER]

Troubleshooting without all the symptoms and details . . .
Is like shooting an arrow at a target when you are blindfolded.

At Post # 16, we do not have enough details to do any troubleshooting, if there even actually is a problem.

I did notice that one of the schematics that is "supposed to be the original", right after the tube rectifier, you will see a single capacitor symbol that is labeled:
400V, 2X10 uF (The one symbol represents a total of 20uF). That is connected to the input of the choke.
But the picture that is right next to the schematic shows a pair (2) 400V 10uF capacitors side by side.
I bet the rectifier sees 20uF.

The same schematic also shows a 400V 100uF capacitor right After the choke.

Attention to details, Please!

 

[Prismatic Sky]

Troubleshooting without all the symptoms and details . . .
Is like shooting an arrow at a target when you are blindfolded.

At Post # 16, we do not have enough details to do any troubleshooting, if there actually is a problem.

I did notice that one of the schematics that is "supposed to be the original", right after the tube rectifier, you will see a single capacitor symbol that is labeled:
400V, 2X10 uF (The one symbol represents a total of 20uF).

Attention to details, Please!

Ya I agree -- bad science on my part. It was tricky because I myself am working with little experience and only a few seconds of observation... And I was hesitant to power it back on in an effort to minimize potential damage.

I'm about to go test it again though, with video this time. Thanks for the help!

 

[Prismatic Sky]

Ok, I just tried it again, with my wife acting as the videographer. The only change from the first startup was that it's directly plugged into the wall this time. Quite obviously, it's a bad tube. I'm actually not sure what's happening with the left top side of the tube -- why is it glowing so brightly here?

I'm fine with finding a new tube... Not a big deal. I'm assuming this one somehow was damaged while being jostled during shipping, even though it was packed really well. My biggest concern now is the possibility that this may have damaged or degraded something else... Thoughts on the chances of this?

This time I had it on for about 30 seconds so I could observe and video, but I'm thinking now that perhaps I should have shut it back off immediately... Not sure, new territory for me.

 

[jjasniew]

Good instincts - that cant possibly be how a good tube is supposed to behave. Was it shipped in place - in socket? Package may have taken a good, solid landing somewhere long the way, which could have moved some structure inside it slightly.