Maybe a tube is red plating in my Dynaco... I have no idea.

nickrobotron

nickrobotron

una bella tazza di caffe
So today I just got new tubes in from Doug's Tubes. I went with the Tung-Sol 6550's.

They were supposed to be a matched quad, but looking at the labels on each tube I noticed that the plate currents are 49.1, 50.0, 50.0, and 50.1. The transconductance numbers are 5120, 5020, 5100, and 5120. Is this normal?

When I look at the tubes in the amp I see that one tube in particular is red in the middle where the other three are not. I tried to take a picture of it, but I can't capture it in film. Is something wrong?

BTW, they sound great! I'm concerned I'm going to damage a tube though.
 
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Also, this is the first time I've actually heard these amplifiers. There could be something else wrong with them. They have been restored and look beautiful. I'll try to post pictures soon.
 
Yes there is something wrong , move the tube to the other amp and see if the tube still red plates, if it does its a bad tube if it doesn't place another tube in the socket the red plating tube came out of ,and if it red plates the amp needs repair .
 
The red plate followed the tube. It must be a bad tube.

Can this hurt my amp? Should I call up Doug and ask for a new tube?
 
Thanks for the help, Mark.

Questions have been answered by Mark in a PM.

This is such a bummer though. I feel like I've been waiting so long to get these guys going, and I'm hung up AGAIN.

They sound so stinkin good though, even with a bad tube. I'll retire them for the time being and hope Doug is a nice guy and gets me a new tube FAST!
 
Let us know how that goes and if you want to run some quick tests with a tester or two I am probably not too far away from you.
 
Where are you Dukiedook? I'm in Merriam, KS.

So Doug from Doug's Tubes is cool. I put the bad tube in the mail for him and he put a replacement tube in the mail for me today. In the meantime...

I have an older Electro Harmonix tube that apparently tested "good" fairly recently. I put that in the amp and biased it again and am listening to the amps again. It sounds fine, but could this hurt the amp? To have a pair of tubes in the amp that don't match?
 
Can't hurt anything as long as it biases okay and doesn't redplate.

Looks like your new amps are Mark III's. They run a really high plate voltage and a lot of new production tubes can't take that. KT-88's are rated for higher plate voltages.

The HK Citation II runs output tubes hard too , but not as high a plate voltage as the Mark III's, and Jim McShane didn't recommend those TS 6550 for it. He recommended EH & GL KT-88's.

Maybe some kind of bucking transformer could be used to bring the plate voltage on the Mark III's down.

Mark III ( 400-0-400 ) & ST-70 (360-360)

I think the ST-70 transformer would give ideal plate voltages especially with todays 120+ line voltages and you would get lot's of headroom with the high mA rating used in mono.

Maybe the new KT-120 would be a good tube for Mark III's, but how will they sound?

Randy
 
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Prairie Village, if you need to test a tube or two send me a PM and we'll se what we need to do. Usually it is optimal to try and match up the tubes as closely as possible in a push pull setup. If you watch the bias pretty carefully like Shadow said you should be OK running a different tube.
 
A lot of people recommended the Tung-Sol 6550 for the Mark III's. Where were you guys before I bought them?! Haha.

Prairie Village? Cool. I work at Latteland on 79th and State Line. Come in for some coffee, on me, and we'll talk audio.
 
To compensate for the high plate voltage you can try running the bias a fair bit lower than stock recommendation.

It may not sound good at lower settings, but from there you can turn the bias up till it sounds good and see where your at. Hopefully it will be lower than before!


In the long run best to get the B+ down for decent tube life. These and others have short OPT tube life because of high B+. They were after max power back in the day and some NOS could take more abuse! Some tubes sound best run hot too under certain conditions !

Antec makes affordable PS trans like the 325-0-325 shelly_d used in his ST-70 repair recently. They are only $40 each and you could leave stock PS trans. to run the tube filaments, & bias. One actually has more than enough mA capacity to run Both Amps. You could just hide it behind the amps. May have to adjust the 6.8K R a bit.

It's also good to add screen resistors to output tubes for longer life as Dave Gillespie has mentioned in his paper. Really necessary if the capacitance in the PS was beefed up.

Diode mod mentioned before too is a good one except use UF4007 or better.

Anyway you will get long life out of TS's if you lower B+ and I don't think a lot of the other new production tubes (KT-88's included) will last that much longer anyway as they too are run too hard.

I've read great things for the sonics of the TS 6550 & on diyaudio one fellow in Europe was getting 2000 Hr's out of his before they started to fade in his big diy amps.

Sorry I missed that about the TS's when you were asking, but as I said I think many other tubes will have be trouble much too soon for you.

They will be a great sounding amps as you have found & with a few mods will go for many Hr's!

I've also read on the forms . I know of one or two people on diyaudio saying they got rid of their Mark III's as they went through output tubes.

Douglas has been one to suggest a different PS is needed a few times on the forms in the past too.

Enjoy!

Randy
 
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I'm going to have to do some more research in order to understand all of that.

I'd like to do whatever modifications possible to extend the life of my tubes and future tubes, but I don't understand any technical talk at all.

I set the bias to 1.40 instead of 1.56. It was just an arbitrary number that I came up with, but it's lower than 1.56 and it still sounds good.

Thanks for the advice, shadowdog. Without doing any in depth modifications, what do you think would be the best place to start?
 
You know, I don't even know what's been done to these amps. I should take some good pictures and post them. The amps have been restored. Maybe some modifications have already been done.
 
Yes pic's should help.

A variac could help keep B+ down as line voltage can get really high which can be really multiplied. I have seen 127 V where I am, though it goes down to 117V so I use one

Slowly bringing the voltage up over say 1 min. with the variac should help tube life too.

Fans can help tube life especially on hot days.

Try lowering the Bias even lower to say 1.25 and see how they sound, slowly bringing them up till they sound good. See where your at.

You can't hurt anything with a low bias and the lower you go the longer the tube life!

A good start would be the screen resistors to the output transformers as they are a good mod for output tube life . I've seen 100 ohms, 270 ohms , 470 ohms used so I don't think it's too critical. Then the diode mod.

I asked for opinions on Bob Latino's Dynaco form. I know he uses TS KT-120's in his bigger amps.

Link to manual and other Dynaco ones.

http://www.tubes4hifi.com/downloads.htm

Diode mod: UF4007 diodes are quieter

http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod




Randy
 
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I set the bias to 1.40 instead of 1.56. It was just an arbitrary number that I came up with, but it's lower than 1.56 and it still sounds good.


OK, here's the skinny on biasing. What you do NOT want to do is bias an amp such that the product of the plate current multiplied by the plate voltage is going to exceed the tube's maximum allowable plate dissipation. So, if your plate voltage is, say, 500 volts on a 6550, then you do not want a plate current that will cause the tube to exceed 35 watts at idle. So anything over 70 ma would be a problem with a plate voltage of 500 volts. In most class AB amps, idle plate current is well below this figure. Most amplifiers, unless used close to clipping or maximum power, will sound fine with much lower plate current then original spec. Many tubes will show a slight red plate when biased at or very near their maximum plate dissipation. But if you have a plate voltage of 500 volts and a plate current of 50 ma for a 6550 and it is still red plating, the tube is probably faulty. If you look in any RCA tube manual and check the specs for certain applications of output tubes, you will find that unless the amp is a true class A amp, most will have idle current quite low and idle plate dissipation well under maximum.
 
Shadowdog said:
Yes pic's should help.

A variac could help keep B+ down as line voltage can get really high which can be really multiplied. I have seen 127 V where I am, though it goes down to 117V so I use one

Slowly bringing the voltage up over say 1 min. with the variac should help tube life too.

Fans can help tube life especially on hot days.

Try lowering the Bias even lower to say 1.25 and see how they sound, slowly bringing them up till they sound good. See where your at.

You can't hurt anything with a low bias and the lower you go the longer the tube life!

A good start would be the screen resistors to the output transformers as they are a good mod for output tube life . I've seen 100 ohms, 270 ohms , 470 ohms used so I don't think it's too critical. Then the diode mod.

I asked for opinions on Bob Latino's Dynaco form. I know he uses TS KT-120's in his bigger amps.

Randy
Click to expand...

Is there a tutorial for doing the screen resistors? You might as well have told me to rebuild the transmission in my car... Haha. I'm sure it's not that tough, but I wouldn't know where to begin.

I'm going to research the KT-120's. Maybe they'll be my next tube after my 6550's die. I wonder if modifications would need to be made before putting those tubes in.

I'm going to post some pictures of my Dynacos tonight.
 
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