Fisher SA-300B Set-up and Bias Adj. Question

[dcgillespie]

Gullaz -- The negative voltage at the grid is only a "typical" voltage, and varies based on the setting of the bias control, which itself is adjusted to get the correct cathode current -- which is the goal you are after. It is the correct output stage current draw -- as determined by a target voltage produced across a specific resistance in the cathode circuit (50 ohms) -- that sets the tubes up for proper operation -- not a predetermined negative voltage. The required negative voltage to produce the proper current draw varies from tube to tube. That's why an adjustment is provided.

Dave

 

[audiodon]

The rectifiers didn't turn on again.
After wiggling some things, the rectifiers came on when I grabbed and wiggled one of the two 15 ohm resistors. I reflowed all four of those solder joints.

I also rebiased down to about 58 ma/tube.
That's about 6.5 volts cathode to ground.

We'll see now how it behaves.

 

[larryderouin]

Don't ya just LOVE those sneaky intermittent's, Don. Just when you think you got it all solved and 100%, it stands up and kicks you in the ass. I've gotten to the point where I reflow all joints now on P2P and look at them under a magnifying loupe with boards then hit the obvious ones. The traces are so brittle that you end up fixing more traces than cold joints.

 

[heyraz]

You said "the rectifiers didn't turn on" which implies to me the problem is in the socket(s). If the problem is an open or intermittent wirewound resistor, you should have seen the tubes lit. If it's the resistors, just replace them and be done with it.
I read some time ago that tube sockets (especially rectifier tube sockets) cause problems and should be regarded as "components" that wear out requiring replacement.

 

[audiodon]

I did retension the sockets.
I think the problem was an intermittent path to ground. No, not to ground, but one of the joints was bad. I reflowed all the sockets on the rectifiers also. That part had escaped out of my mind.
If it continues to happen, by all means I'll go to the next level, but I admit that I'm not ready to put the cage on yet.

 

[heyraz]

Here's some photos of my progress so far:
I changed the flimsy blue bias pots to Mil. Grade types. The solder tabs on the pots are kind of weak, so I mounted a tag board above them and ran wires from the pots up. Now the pots are safe from damage. In the front, I mounted a plate to support extension tubes that will eventually run out to the amp's end cap. Two holes will be drilled through that end cap and plugged with metal covers to resemble the covers used to adjust AC and DC bias. Once done, I will be able to adjust the bias for each channel by removing the covers to access the individual pots.

New Bias circuit:

Quick sketch of new bias schematic:

The Diode Mod:

Finished (for tonight) new split bias circuit:

Still so much to do.....

 

[heyraz]

Here's a Fun Question:
Anyone know what these two potentiometers on the left are called when used to minimize heater hum as in the SA300/SA300B?
(Hint:It's not a "Doozie")

 

[audiodon]

So you changed the single in-line 10uf 350V cap to one on each bias branch.
What advantage does that offer?

One of the most fun things about this hobby, and about this thread in particular, is watching how others do the same thing. It's very educational. I enjoy seeing your workmanship.

 

[heyraz]

So you changed the single in-line 10uf 350V cap to one on each bias branch.
What advantage does that offer?

Maybe none. But if you look at the schematic as I've drawn it, each channel has it's own cap before the potentiometer as well as on the tap. I'm hoping the extra caps minimizes any effect one channel may have (if any) on the other.
And...there's nothing wrong with overkill.

What I don't get is why (for a 30v circuit) the caps are rated for 250v (what my unit came with).

 

[larryderouin]

Kept the bean counters happy with a limited inventory. Plus semi skilled assy line workers. I've seen stuff like that in most all of my Fishers from the 1951 coronet (R-3 chassis) all the way to the 800-C's. The Solid State stuff uses lower voltage stuff but there are a couple instances where a 100V cap is used in a 6V circuit.

 

[heyraz]

C'mon Guys---What's another name for those hum adjustment pots???
(I'll post the answer 12 am Eastern)

 

[heyraz]

Each of them are called Hum-Dingers.

 

[larryderouin]

Hum-Bucker's

 

[heyraz]

Hum bucker's are guitar pickups

Humdinger's take the place of two 220 ohm resistors to ground on the heater circuit. The wiper is grounded so you can dial in the least hum
http://www.valvewizard.co.uk/heater.html

What I'd like to know is what a "SheBang" is. As in "The Whole Shebang!".

 

[dcgillespie]

Maybe not unlike "The Whole Nine Yards" , or "Minding your Ps and Qs" -- but I know what the origin of those are!

Dave

 

[heyraz]

Could be a Mexican food, like "The Whole Enchilada" The next time I'm in Taco Bell I'll ask for one....should be interesting.

 

[fred soop]

If it's a 600-T or the similar power amp in any of the consoles, the proper name would be "garbage".

 

[larryderouin]

Dictionary.com:

she·bang/ʃəˈbæŋ/ Show Spelled [shuh-bang]
noun
1. Informal. the structure of something, as of an organization, contrivance, or affair: The whole shebang fell apart when the chairman quit.
2. a primitive dwelling; shack; shanty.

--------------------------------------------------------------------------------

Origin:
1860–65, Americanism; origin uncertain (perhaps alteration of char-À-banc , though sense shift unclear; shebeen, often cited as the source, is implausible both phonetically and semantically)

Wiktionary:
shebang
Definition from Wiktionary, the free dictionary

English[edit]Etymology 1[edit]First seen in 1862 meaning a “temporary shelter”. For instance in the US Civil War (1860-65), the small self-constructed shelters made by Union soldiers in the Confederate Prisoner-of-War camp at Andersonsville, south of Atlanta, Georgia, were known as "Shebangs". [1]

The usage as “temporary shelter” perhaps from shebeen (“cabin where unlicensed liquor is sold and drunk”), pre-1800, chiefly in Ireland and Scotland, from Irish seibin (“small mug”), diminutive of seibe (“mug, bottle, liquid measure”)

Modern sense "the whole shebang", origin unknown but meaning "all of the parts" of what is being referred to.

Alternative forms[edit]chebang, schebang, sheebang
Noun[edit]shebang (uncountable)

1.Any matter of present concern; thing; or business.  [quotations ▼]
1869, Samuel Clemens (Mark Twain), letter to publisher:[2] I like the book, I like you and your style and your business vim, and believe the chebang will be a success.1934, Robert E. Howard, Sluggers on the Beach: "Before I'd share anything with you," he said bitterly, "I’d lose the whole shebang."

2.(obsolete) A vehicle.[3]  [quotations ▼]
1871, December 14, Samuel Clemens (Mark Twain), “Roughing It” (lecture), printed in Fred W. Lorch, “Mark Twain’s Lecture from Roughing it”, in American Literature, volume 22, number 3 (November 1950), pages 305: […] So they got into the empty omnibus and sat down. Colonel Jack says: “...What is the name of this.” Colonel Jim told him it was a barouche. After a while he poked his head out in front and said to the driver, “I say, Johnny, this suits me. We want this shebang all day. Let the horses go.”

3.(archaic) A lean-to or temporary shelter.  [quotations ▼]
1862, Walt Whitman, Journal, December: Their shebang enclosures of bushes.1889, Bret Harte, The Heritage of Dedlow Marsh They say that old pirate, Kingfisher Culpepper, had a stock of the real thing from Robertson County laid in his shebang on the Marsh just before he died.

 

[Wharfcreek]

Here's another one: The whole 'Kittenkabootle'........ what ever that is....... WC

 

[heyraz]

Thanks guys-I googled "shebang" myself.
But now you know what a "humdinger" really is! I found a craft beer by that name a couple of years ago.

WC-'Kittenkabootle' might be on the menu over at Lucky Chan's (sorry-bad joke that I couldn't resist).