[adydula]

I read in some design papers for the older Dynaco ST70's which I know Frank must have memorized inside and out..that the bias resistor is there to make life easier to set the bias current for idle...instead of having to put an ammeter inseries etc...

So could you set the bias, remove the resistor would this work until you lost a tube or change them..ie have to put the resistors back in or use an ammeter?

Yea I know u must thing I am nuts..but i havent got a whole lot to do right now!!!

All thes best!
Alex
 

[rlee8394]

Alex,

You, could, but you'd have to replace the resistor with a short to ground. If you're looking for a way to save resistors in the future, and you want to do something useful, just mount a couple of fuse holders under the chassis, move the resistor connection from ground to one end of the fuse holder, and connect the other end of the fuse holder to ground. Insert a 200  ma or 250 ma fast-blo fuse for protection. That way the fuse will save the resistors in the future. You could also use a resistor with a higher power rating as well.

Ron

[trebejo]

This is like my favorite thread, ever...

[adydula]

My soldering iron is 'talking' to me.......use me! use me! oh come on plug me in and get it over with!!!!

Alex

[adydula]

How about this:

Dayton DNR-16 16 Ohm 10W Precision Audio Grade Resistor

http://www.amazon.com/Dayton-DNR-16-Precision-Audio-Resistor/dp/B0002MQFF0/ref=sr_1_2?ie=UTF8&qid=1288025330&sr=8-2

or

16 Ohm 1/2W Flameproof Resistor 10 Pcs

http://www.amazon.com/16-Ohm-Flameproof-Resistor-Pcs/dp/B0002KR0HO/ref=sr_1_16?ie=UTF8&qid=1288025510&sr=8-16

Alex
 

[adydula]

Ok,

I got an opinion from a person that really knows Dynaco stuff....and thinks that adding a fuse in series with the bias resistor would affect the sonic attributes of the amp??? Hmmmm.

Alex
 

[gjs_cds]

I still can't figure out of this is a joke thread, or if someone is seriously considering this...

Just replace the freakin' tubes and the resistor, then move on with your life!

[mark funk]

Alex,  Call Frank! He well send you one! Also get new tubes (4). It was just a resistor this time. A bad tube could take out a transformer also. Ya it's fun having a tub amp. 


                                                                                     

[adydula]

Hey Guys Chill out just a little!!

I have been building stuff for years, but am always curious to what other people, engineers especially that tell me things about stuff I am interested in.

I have moved on and 4 NEW tubes are in the Ultravalve!

I also replaced the resistor and its fine.

But I do not really want to keep replacing resistors even if this is part of the game.
There should be a better way, but at what cost sonically??? Maybe nothing at all.

And YES I did really get a reply from a person that is highly regarded that thought this was a good idea but indicated fuses could impact the sound. The exact words were "fuses have their own sound".

Now I for one think this is ...well lets not go there.

Good news is I have a tube tester on the way from mr ebay and it will be neat to be able to plug some valves in and see if i can find the offending tube that caused the resistor meltdown to begin with.

Thanks for all the comments, advice and mirth!

All the best!
Alex
 

[Brett Buck]

Hey Guys Chill out just a little!!

I have been building stuff for years, but am always curious to what other people, engineers especially that tell me things about stuff I am interested in.

I have moved on and 4 NEW tubes are in the Ultravalve!

I also replaced the resistor and its fine.

But I do not really want to keep replacing resistors even if this is part of the game.
There should be a better way, but at what cost sonically??? Maybe nothing at all.

And YES I did really get a reply from a person that is highly regarded that thought this was a good idea but indicated fuses could impact the sound. The exact words were "fuses have their own sound".

Now I for one think this is ...well lets not go there.

  And right you are, the usual audiophool gibberish.

   But back the the real world - putting in a fuse seems a little silly - resistors are cheaper than fuses. And the sort of failure you have had is the first time I have ever heard of in about 40 years of doing this stuff.

     Brett

[adydula]

Hi Brett...

Yes audiophool gibberish....

What got me to thinking this was a more frequent occurence (is bias resistors going bad due to a output tube short) was just searching on the all knowing internet for "fried bias resistors'. There are many hits and many people with varying amps, same basic failure popping up.

So being the inquisitive guy that I am  I decided to "work" this thread to see what pop'd up as far as fact and fiction.

If anything it give us something to do while we are not able to just listen and enjoy the music!!

Nice Meeting you!

Alex
 

[JWJ356]

Back in the 'old days' the typical circuit had a fuse in the cathode to ground leg.  In the Keroes 60W Williamson circuit (Radio & Television News, 1955) 1/8A fuse was specified for each KT88 cathode leg.  It was suggested to remove the fuse and tie a milliammeter across the fuse clip to check the bias current.  That was too awkward for me so I wired in a phone jack in each leg so I could make the connection to the meter.  Works for me!

[adydula]

JWJ356!!

1955!!! Wow now were talking!!

Amazing that we still deal with issues from waaaaaaay baaaaaack theeeeeeen!

Neat, this was a neat little nugget of tube amp trivia, thanks for it!

All the best!
Alex

[JWJ356]

Here is the article:
http://homepage.mac.com/tottoritakayasu/yamaneko/audiodata/williamsonto330.pdf

I used the Dynaco 431 transformer instead of the Acro 330 (a bit less expensive).  The fuses are in the cathode to ground circuit and with the series phone jacks accessible on the rear chassis skirt.

[adydula]

JWJ356...

This is so cool!

Many thanks, I just printed the article out and will read it tonight, have a few laughs, maybe a beer and listen to some good music..and think about 200 or 250 ma fuses!!

Alex
 

[adydula]

Well I got my tube tester today and put all the output tubes in and checked them, they all tested fine, gas, emissions, shorts etc.

So I guess whatever happened to cause the resistor to 'open' or change values was when one of these tubes get really hot under load.

I think the only was I will really know is to put a fuse in line to gnd and put them back in etc...but for now I am just going to listen with the new tubes, think about it some more.

Hey the tube tester was and is a really neat nostalgic trip back in time, and it even smells old!

Alex

[Listens2tubes]

Hey Alex what model tube tester did you get? I've got a few myself.

[adydula]

Hey listens2tubes!

I got a used Knight Kit 600 series, 1960's vintage.
It works very well for its age.

I am trying to get a mutual cond. type but they are going for much more....etc..

This one is a gas emssions, good/bad, open short tester.

Ya gotta love those up down switches...

I grew up with this stuff. and being an amateur radio extra class 'ham' I played with many tubes, built almost every Heathkit there was!!

When Solid State came around many of us said thank God that we dont have to deal with the heat, the shorts, etc....and now....WTF over we are going back in time!!

Holy Crap, NOS tubes for $hundreds of dollars!!

OMG!
LOL
Alex

[Listens2tubes]

I'm sure this should be it's own thread ...but...I've got a B&K 707 & 747, Hickok 533A and Mercury 2000. Keep your eyes on ebay, craigslist and even audiogon for the rare deal. Many only need calibration though you probably know better than I what parts don't age gracefully and should be changed.

Neal