Blue Tube Flash

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Blue Tube Flash

Postby vez on Wed Jun 14, 2017 9:19 pm

Is it normal for a momentary blue flash in my 6L6 tubes when I throw the standby switch on? One seems to do it more than the other (duel ended power amp). They are 6L6GC with 467 and 466 volts on the plates with .0361 and .0285 amps respectively. Thanks!
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Re: Blue Tube Flash

Postby pdf64 on Thu Jun 15, 2017 10:47 am

Blue glow or blue flash, ie would it be apparent in bright sunlight?
What make/model/age/maintenance status amp?
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Re: Blue Tube Flash

Postby vez on Thu Jun 15, 2017 12:56 pm

pdf64 wrote:Blue glow or blue flash, ie would it be apparent in bright sunlight?
What make/model/age/maintenance status amp?

Hi pdf64, thx for replying. Yes it is a blue glow not like arc flash but, it lasts only about 1/10 sec.
It likely wouldn't be seen under sunlight.
It's a '67 Super Reverb. All filter caps were replaced a couple years ago. Other than that it's never been maintained. I found the bias voltage for these tubes was at -62. I reset it the factory spec -52. I have no idea how long it's been at -62v. It gets played very little. The amp sounds great very strong.
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Re: Blue Tube Flash

Postby Vortexion on Thu Jun 15, 2017 2:19 pm

As I'm sure you already know, a steady blue glow is normal, and especially visible with beam tetrodes (like the 6L6). I'd say that this brief, visible "surge" when you come out of standby is just evidence of the natural inrush current. Nothing to worry about.
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Re: Blue Tube Flash

Postby vez on Thu Jun 15, 2017 4:19 pm

Vortexion wrote:As I'm sure you already know, a steady blue glow is normal, and especially visible with beam tetrodes (like the 6L6). I'd say that this brief, visible "surge" when you come out of standby is just evidence of the natural inrush current. Nothing to worry about.

Thanks Vortexion, I was thinking that's all that it is. But I thought I would ask just in case it was an obvious flaw as it seems more pronounced in one tube compared to the other.
I have a technical background but, I'm somewhat new to fixing tube amps and I'm learning a lot. Safety is a priority. I constructed a probe to dissipate the filter caps and I use only one hand when reaching in into the chassis. The tremelo circuit is acting up on this amp so I'm tackling that next.
Thanks again,
Rick
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Re: Blue Tube Flash

Postby pdf64 on Fri Jun 16, 2017 11:32 am

As mentioned, blue glow is normal, and it's normal for different tubes to have different degrees of it.

As with any amp that has a GZ34 / 5AR4, the best use of standby is not to use it.
GZ34 provide a nice slow smooth soft start for the HT, ramping it up gradually over ~40 seconds.

vez wrote:...It's a '67 Super Reverb. All filter caps were replaced a couple years ago. Other than that it's never been maintained. I found the bias voltage for these tubes was at -62. I reset it the factory spec -52. I have no idea how long it's been at -62v. It gets played very little. The amp sounds great very strong.

That's a big change in bias voltage! Did you check that the power tube dissipations at idle were reasonable, before and after? Did it sound bad before?
The voltages noted on schematics are for guidance only, and apply to that amp with those tubes at that mains supply.
The bias appropriate for a given real tube may differ from its type bogie by +/-20%

Was the bias supply ecap replaced? If so was one with an appropriate voltage rating used, bearing in mind that it must be subject to >62V?

A grounded mains cable should be fitted to amps that are intended to be operated.

In order to keep its ecaps 'formed', it may be beneficial to operate the amp at least for a few minutes every few months.
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Re: Blue Tube Flash

Postby vez on Fri Jun 16, 2017 4:08 pm

pdf64 wrote:As mentioned, blue glow is normal, and it's normal for different tubes to have different degrees of it.

As with any amp that has a GZ34 / 5AR4, the best use of standby is not to use it.
GZ34 provide a nice slow smooth soft start for the HT, ramping it up gradually over ~40 seconds.

vez wrote:...It's a '67 Super Reverb. All filter caps were replaced a couple years ago. Other than that it's never been maintained. I found the bias voltage for these tubes was at -62. I reset it the factory spec -52. I have no idea how long it's been at -62v. It gets played very little. The amp sounds great very strong.

That's a big change in bias voltage! Did you check that the power tube dissipations at idle were reasonable, before and after? Did it sound bad before?
The voltages noted on schematics are for guidance only, and apply to that amp with those tubes at that mains supply.
The bias appropriate for a given real tube may differ from its type bogie by +/-20%

Was the bias supply ecap replaced? If so was one with an appropriate voltage rating used, bearing in mind that it must be subject to >62V?

A grounded mains cable should be fitted to amps that are intended to be operated.

In order to keep its ecaps 'formed', it may be beneficial to operate the amp at least for a few minutes every few months.

Hi pdf64, the power tubes are running about 4 watts lower now. 16.88 and 13.28 watts and it sounds a whole lot better. But I also had to replace the tube in V2 because I was about 100 volts light on the V2 plate. I had no idea why I was losing this voltage and checked everything and e eventually had to replace the tube which is a bummer because that tube was a Telefunken with super high strength reading on the tube tester. So, I replaced the V2 with an old British GE and then set the bias on the power tubes then then I measured the dissipation on the power tube and listened to the amp and it sounded much better. Before it sounded sterile, a lot like my buddy's Hotrod Deluxe (lol) now it sounds like SRV! I know the bias made a huge difference because the number one channel (doesn't use V2) and it now sounds a lot much better.
If you mean a "grounded main" , I installed a three prong cable back when I replaced the filter caps andalso replaced the ecap at the bias pot with a 100uf/100v Atom.
Yesterday I fixed the the tremelo. It was a broken ground wire to the chassis.
Now I think I have to replace at least two sockets and a vol pot.
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Re: Blue Tube Flash

Postby pdf64 on Fri Jun 16, 2017 5:11 pm

Ok, good job :thumbsup:
vez wrote:...the power tubes are running about 4 watts lower now. 16.88 and 13.28 watts and it sounds a whole lot better...

Surely they should be running hotter after the bias voltage (ie at g1) was reduced from -62 to -52?
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Re: Blue Tube Flash

Postby vez on Fri Jun 16, 2017 7:52 pm

pdf64 wrote:Ok, good job :thumbsup:
vez wrote:...the power tubes are running about 4 watts lower now. 16.88 and 13.28 watts and it sounds a whole lot better...

Surely they should be running hotter after the bias voltage (ie at g1) was reduced from -62 to -52?

My bad, running about 4 watt higher now not lower.
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Re: Blue Tube Flash

Postby vez on Sat Jun 17, 2017 1:02 am

I had a scratchy pot that was making a loud clutter noise. I pulled it and sprayed it with CRC contact cleaner.
I've never used it before. I must have used too much of it because the pot is now seized. Have you guys ever used it? Apparently it pure solvent with no lubricant.
https://www.lowes.com/pd/CRC-Contact-Cleaner/999964587
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Re: Blue Tube Flash

Postby pdf64 on Sun Jun 18, 2017 8:30 am

A squirt of a cleaner that leaves a lubricating deposit behind may revive it; that's the type pots and switches generally need.
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Re: Blue Tube Flash

Postby vez on Sun Jun 18, 2017 4:32 pm

pdf64 wrote:A squirt of a cleaner that leaves a lubricating deposit behind may revive it; that's the type pots and switches generally need.

I disassembled the pot it's a 1M log 1966 Stackpole. The shaft is frozen in its bushing. The wiper and the conductive wafer looks ok. I ordered some detoxit. If that doesn't work I'll soak it in some penetrating fluid like Liquid Wrench. I People are asking $250 for 250k pots from the same era on Ebay. Who are these people?
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Re: Blue Tube Flash

Postby OldSchoolDave on Thu Jun 22, 2017 11:54 am

vez wrote:People are asking $250 for 250k pots from the same era on Ebay. Who are these people?


Rather than wonder about who is offering to SELL, wonder about who is willing to PAY those prices. If there weren't a market, there wouldn't be advertised product.

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Re: Blue Tube Flash

Postby vez on Fri Jun 23, 2017 5:01 pm

OldSchoolDave wrote:
vez wrote:People are asking $250 for 250k pots from the same era on Ebay. Who are these people?


Rather than wonder about who is offering to SELL, wonder about who is willing to PAY those prices. If there weren't a market, there wouldn't be advertised product.

Dave

Dave, I'd like to believe it's just some deluded seller. But you're right, it's the buyers that are deluded. It's like they assign some unreal importance if they pay a ridiculous sum. And ill bet these are they same people that haggle on Craigslist over a $5 golf club.
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Re: Blue Tube Flash

Postby Vortexion on Fri Jun 23, 2017 5:14 pm

vez wrote:I'd like to believe it's just some deluded seller. But you're right, it's the buyers that are deluded. It's like they assign some unreal importance if they pay a ridiculous sum.

Ain't that the truth.

vez wrote:And ill bet these are they same people that haggle on Craigslist over a $5 golf club.

Would you accept $3?

And how much to ship to the UK?
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