New Tubes for my Supro Amp ?? Tube savvy question

[KevWind]

Originally Posted by rockabilly69

I know it seems like it would be an easier solution to have Sweetwater look at it. But me personally, I would just buy a new pair of tubes and call it good. If you have any problems after that, then maybe pursue having them, or better yet, a good Denver factory authorized tech look at it. The reason being, is shipping an amp can actually be hard on it, and you don't know how it's going to be treated both ways! And we're talking about glass tubes!!!

Thanks for the reply and some very good points. Shipping can sometimes have negative consequences .

Unfortunately because I bought it at Sweetwater which has a great 2 year warranty . I did not register the purchase with Supro (which in retrospect was dumb) Because Supro comes with a 1 yr. warranty but extends it a year if you register it . So now if there was actually something besides just a bad tube Sweetwater will cover it, but a Supro authorized repair shop would not.

But I still may have a local amp tech troubleshoot it (if I can find one) before shipping it off

[aknow]

Originally Posted by Steve DeRosa

This might help with your decision, Kev:

http://www.thetubestore.com/Resource...-Reviews-Chart

As I'm sure you recall I'm a big fan of the Preferred Series tubes as a whole - their 7025's and EL84's turned my Bugera V22 into a real tone monster - and IME the tubestore.com folks are dead-on with their evaluations, so I'd trust their 6L6GC to be everything they say it is. That said, there are some very interesting things coming out of Tung-Sol these days, including a reissue of the near-impossible-to-get 7581A:

http://www.thetubestore.com/Tubes/6L...Tung-Sol-7581A

A direct replacement (with appropriate re-biasing) for 6L6 applications, this one is rated at 35W (versus 25W for a typical 5881 and ~30W for most 6L6 variations), and in its original '60s incarnation was TMK the secret weapon of the big-name acts using piggyback Bassman/Showman/Twin-based rigs; should give you tons of clean headroom - great as a pedalboard platform or for rhythm/cleaner lead styles straight into the amp...

In either event, I'd see a tech first - based on your test, make sure there isn't something else going on in there...

Thank you Steve!

[KevWind]

Originally Posted by clintj

Occasionally, a tube will die after just a few hours. Nature of the beast with newer production. Also, once a tube redplates like that it's generally trashed so you're right in shopping for new ones now.

I'd schedule a little tech time with that amp, just to see where it biases with the new tubes. Cathode biased amps are often touted as "plug and play", but different sets of tubes idle differently and some amps just run tubes really hot - Voxes and some Fenders like the Pro Junior are examples.

After some more research I think you are correct the tube was Redplating

It looked like the second picture down in this article body but a little brighter orange
http://blog.thetubestore.com/filamen...-vacuum-tubes/
Apparently even though this is a cathode biased amp it is still suffering from redplating .

[Big Band Guitar]

Tubes by their very nature will fail in time. There are a limited amount of ions that can boil off a cathode. I have seen tubes with the glass envelope melted.

I gave up on tubes long ago. Nothing to do with the sound. Mostly the weight. I sold my Fender Deluxe when I ran out of my old stock of 12ax7. Replaced it with a Loud Box Mini. I am not sorry one bit. I like the lighter weight and the sound.

If you can find them NOS tubes from the 40's through the 60's were made with better quality control.

[KevWind]

OK so a quick update on my Supro Titan and it is good news /bad news .

So I decided to take the advice of folks here and get the amp checked out , before just getting new tubes. And thanks again for the replies ....

Since it is still under the Sweetwater warranty I decided to send it to them especially since they offered to pay for the shipping . Which at 54 lbs total probably would have been $75 +

So I sent it back they checked and said it was in fact something other than a bad tube so they have now sent it on to Supro (still under warranty ). And I do have another good tube amp to play in the interim.... So the that is the good news---- the bad news ,...... well not bad news per se. But simply I am a bit disappointed that a USA amp built like a tank, went defunct so quickly .

[muscmp]

nothing nor no one is perfect. you would rather it go bad under warranty instead of out of warranty. when you get it back, do a burn in as it may be new parts. get a list of exactly what was wrong and what was replaced. the rest of the amp, including the speaker, will already be ready to go, so, should sound great.

play music!

[BT55]

Originally Posted by Mr. Jelly

I had to read this thread. I'm so old I remember going down to the hardware store to test my tubes on their tube testing machine. I'd steal the tubes out of old radios that were being thrown out.

When I was a kid we took tubes to be tested at the drug store.

[KevWind]

Originally Posted by muscmp

nothing nor no one is perfect. you would rather it go bad under warranty instead of out of warranty. when you get it back, do a burn in as it may be new parts. get a list of exactly what was wrong and what was replaced. the rest of the amp, including the speaker, will already be ready to go, so, should sound great.

play music!

No I understand there has always been a certain % of failure in new audio products and I am glad it happened now, but just thought (or hoped) a new tube amp would run for years before needing work
Although I am assuming it will come back with new factory Tubes , I still may get a set of the Tung-Sol 7581 A's that Steve suggested just to try.

[Steve DeRosa]

Originally Posted by KevWind

...Although I am assuming it will come back with new factory tubes, I still may get a set of the Tung-Sol 7581A's that Steve suggested just to try.

You will need a bias job with the Tung-Sols - hope you've got a tech nearby who knows what he/she is doing when it comes to tube gear...

[KevWind]

Originally Posted by Steve DeRosa

You will need a bias job with the Tung-Sols - hope you've got a tech nearby who knows what he/she is doing when it comes to tube gear...

Even on a Cathode biased amp I thought they did not need that ? Or is that because it is changing from the 30 watt 5881 , to the 35 watt 7581 A?

[clintj]

Cathode bias is designed by taking the tube data sheet, plotting a few lines, and doing some math to come up with a bias resistor that will work with the ideal tube. Some designers will even just copy an existing circuit and hope it works. Thing is, tubes are handmade. There can be quite a variance from one to the next, and they don't fall in the same good tolerances as they did when factories would churn out thousands upon thousands for commercial and military use back before the late sixties, when transistors started to take over. I've swapped a set of 6V6 in one cathode biased amp and gone from 86% idle (safe) to 94% (danger zone). Most swaps within type should work pretty well, but it's prudent to check.

If you swap tube types, all bets are off. There's no guarantee the circuit will support the new tube, because the new type may require more or less bias voltage to run safely. A good example is 6V6 and EL84. Both are 14W tubes, but a 6V6 needs almost twice the bias voltage. If you could plug it in to an EL84 designed amp, it would have a very short life before it melted down.

[KevWind]

Originally Posted by clintj

Cathode bias is designed by taking the tube data sheet, plotting a few lines, and doing some math to come up with a bias resistor that will work with the ideal tube. Some designers will even just copy an existing circuit and hope it works. Thing is, tubes are handmade. There can be quite a variance from one to the next, and they don't fall in the same good tolerances as they did when factories would churn out thousands upon thousands for commercial and military use back before the late sixties, when transistors started to take over. I've swapped a set of 6V6 in one cathode biased amp and gone from 86% idle (safe) to 94% (danger zone). Most swaps within type should work pretty well, but it's prudent to check.

If you swap tube types, all bets are off. There's no guarantee the circuit will support the new tube, because the new type may require more or less bias voltage to run safely. A good example is 6V6 and EL84. Both are 14W tubes, but a 6V6 needs almost twice the bias voltage. If you could plug it in to an EL84 designed amp, it would have a very short life before it melted down.

Thanks for the reply and I completely understand why different types would be a problem... Not arguing here just trying to figure it out, the Tung-Sol 7581 A is stated to be an upgraded 6L6 CG direct replacement .. form the Tubestore web site " The 7581A tube can be used in any 6L6 based tube circuit." And is in listed under the Tubstore's ---- 6L6 / 5881 Tube Types category. BUT since the Supro will come back with new factory installed tubes it is not like I have to rush out and buy any replacement right now ,so I will do further research

[muscmp]

Originally Posted by KevWind

Thanks for the reply and I completely understand why different types would be a problem... Not arguing here just trying to figure it out, the Tung-Sol 7581 A is stated to be an upgraded 6L6 CG direct replacement .. form the Tubestore web site " The 7581A tube can be used in any 6L6 based tube circuit." And is in listed under the Tubstore's ---- 6L6 / 5881 Tube Types category. BUT since the Supro will come back with new factory installed tubes it is not like I have to rush out and buy any replacement right now ,so I will do further research

kev: they have indicated to you what the problem is and that is why they are giving you new tubes? i would also try the new tubes prior to any changes.

play music!

[KevWind]

Originally Posted by muscmp

kev: they have indicated to you what the problem is and that is why they are giving you new tubes? i would also try the new tubes prior to any changes.

play music!

Ok perhaps I was not clear enough

So to recap

The symptom was the amp would play fine for somewhere between 30 mins and an hour then would go way down in volume and start humming.
And one of the power tubes was visually "redplating" (glowing hot orange in a narrow 1/8 inch vertical strip , top to bottom of the tube.)
So I shut it down and let it cool

After the amp cooled , I tested the continuity on the power tubes and the continuity was fine and within spec on both tubes

I sent the amp to Sweetwater and the tech confirmed the exact same symptoms . And suspected that the issue might be a faulty resistor or circuit in the amp

Sweetwater's agreement with Supro is that they can do minor repairs but any major repair (circuits ect.) has to be done by Supro

So Sweetwater is sending the amp to Supro and will update me on what Supro finds .

I am simply assuming that Supro will replace the tubes given that at least one has replated at least twice now... But that has yet to be determined

Sorry for any confusion.

[muscmp]

Originally Posted by KevWind

Ok perhaps I was not clear enough

So to recap

The symptom was the amp would play fine for somewhere between 30 mins and an hour then would go way down in volume and start humming.
And one of the power tubes was visually "redplating" (glowing hot orange in a narrow 1/8 inch vertical strip , top to bottom of the tube.)
So I shut it down and let it cool

After the amp cooled , I tested the continuity on the power tubes and the continuity was fine and within spec on both tubes

I sent the amp to Sweetwater and the tech confirmed the exact same symptoms . And suspected that the issue might be a faulty resistor or circuit in the amp

Sweetwater's agreement with Supro is that they can do minor repairs but any major repair (circuits ect.) has to be done by Supro

So Sweetwater is sending the amp to Supro and will update me on what Supro finds .

I am simply assuming that Supro will replace the tubes given that at least one has replated at least twice now... But that has yet to be determined

Sorry for any confusion.

perfectly clear. thanks for the update. bet it will sound great once it returns.

play music!