No bias on the 6550 tube Dynaco ST 70

I have a ST-70 with modified driver board from tubes4hifi running EL34 and everything working good for over one year. I also replaced the transformer with the new stock that has more juice to run the 6550 tubes. Then my friend lend me 4 of his 6550 tube for my ST-70. When I redo the bias on each 6550 tube and set them to 0.5 volts. Everything sound and look ok. I play the music for 30 min and recheck the bias, one of the tubes read 0V (no bias). I turn the amp off and let it cool down completely. Turn it back on, recheck the bias on the tubes. They are all fine, then half an hours later, the same tube has no bias (measure 0V). Did anyone experience what I have? Is it something wrong with the 6550?

Thanks in advance

Eugene

One step at a time ...  when you have a good reading on the bias, try wiggling and tapping the tube to see if it goes weird. That would usually indicate a good tube, but a weak connection. Keep a finger handy on the power switch as any red plating can kill a tube fast.

Try swapping another tube into the socket that's giving you fits and see if the problem follows the tube. Easy enough then to either tag the tube as bad or look for another issue.

Next off, try swapping in your old set and see what happens.

I'd also then retension the sockets (all of them) as they probably need it anyway. Not uncommon for an open circuit or cold solder joint to show itself once the amp heats and the metal expands. Once that's done, double check to see if you're getting anywhere.

From there you got to dig around inside if you're comfortable with that. If you've tracked it to a particular socket, try poking and wiggling all the connections with a meter in place so you can see if the bias changes. Not to forget - there's high voltage in there, so careful to use an insulated stick and "keep a hand in the pocket" to avoid shorting yourself to ground.

From there, it tends to get a bit more technical.

Be sure to post back with the results.

After I put the EL34 back to the amp, I play music for an hour. Check the bias and all the reading show good. I think the driver board should be ok without cold solder joint but I will carefully check them again.
I will retention the sockets (all of them). Try the wiggling and tapping of the tube and let you know the result.

After I retention all the sockets. The issues are all gone. The 6550 didn't lost the bias reading anymore.

It's always the simple stuff ... unless it's not ...  ;-}

You might want to do the retensioning every time you swap tubes unless they're identical replacements. Come to find out, all pins aren't created equal. My KT120's have slightly larger diameter pins and are a tight fit when swapping them in. Go back to the KT88's and they work, but you can tell they're not as tight. Chances are good they could go open as things heat up, so it's just plain common sense to give em a quick adjustment before firing it up.

Now, about those 6550's ... what kind and waddaya think about them?

Thank you for your tips

I can sit down and enjoy my music without pulling my hair.

I've brought this up elsewhere, but it might be worth repeating on this thread. My electronic micrometer doesn't lie. The old U.S.-made Tung-Sol 6550s have ever so slightly a skinnier pin diameter than the Russian tubes. Barely measurable but it's there. The New Sensor tubes will blow out the socket holes -- not by much but enough to red-plate an American TS 6550 when Pin 7 can't see its mother, and there's another $150 down the drain.
So, socket retentioning is everything when yer tube-rolling. It's a pain, but worth doing.
Eugene, glad you've retained some hair.

Hi EugeneHiFi

I am running Dynaco MK3s with 6550.....I like the Sound better than just about any EL34 KT77
KT66 KT88........... I get you just setup your ST70....but how do you feel about the sound?
Be for I pulled the Choke out of my B+ I could not relly tell....that the 6550 sounded like it has a flater frequency respose...the other tubes sound colored....more bass maybe.....I have went back an forth....but 6550 sounds best...well too me!
thanks for any an all input on tube sound............



thanks for the site Bob........................

Deepee99, it is very true about your conclusion on the pin diameter. Just the little different between the measurement on the pins can create so many nightmares

Socket retentioning is a must on my to do list now every thing I am doing tube rolling.

Hi zx,

I have been listening to my ST70 with the 6550 for about 2 months now. Just before I tell you what I think about the sound of 6550 compare with the EL34. I have to warn everyone; once I listen to 6550, there is no way to look back to use the EL34 anymore.

The 6550 sound so much warmer and have wider soundstage especially I am using the ST70 to push my JBL horn. I can listen to saxophone and jazz all day long without getting sick.

Now I have 4 RCA NOS EL34 for sale Anybody interested ??

Zx, one thing I suggest you to try is using 12bh7 on the 2 outside tubes. The change is noticable and make my ST70 sound even sweeter....

On my MK3 I run the Stock board.....only one 6AN8 input tube...Glad the 6550s work for you..... all so..
What 6550 are you runing in your ST70?....................thanks





thanks for the site Bob...............................

EugeneHiFi wrote:Hi zx,

I have been listening to my ST70 with the 6550 for about 2 months now. Just before I tell you what I think about the sound of 6550 compare with the EL34. I have to warn everyone; once I listen to 6550, there is no way to look back to use the EL34 anymore.

The 6550 sound so much warmer and have wider soundstage especially I am using the ST70 to push my JBL horn. I can listen to saxophone and jazz all day long without getting sick.

Now I have 4 RCA NOS EL34 for sale Anybody interested ??

Zx, one thing I suggest you to try is using 12bh7 on the 2 outside tubes. The change is noticable and make my ST70 sound even sweeter....

Eugene, there's a "sticky" on this board for folks with stuff to sell or want to buy. You might consider a post there. You shouldn't have much trouble selling them. Another place to go is Canuck Audio Mart, lots of good tube people there.
Avoid Audio Goner. They are worse than FleaBay and take a hefty slice of your transaction.
I go back and forth on output tubes; I have a good stock of KT-120s and KT-88s. but there is something just magical about the old New Jersey Tung-Sol 6550s. Can't put my finger on it, but there it is.
I run the KT-120s in winter, when I need the heat. With summer here, time for the 6550s, which means re-tensioning the sockets, which means another day is shot. ARRRghhh. But oh well, this is our obsession.