Hoffmann 5F6A bias circuit

[Bangy]

Hofman 5F6A bias circuit.

I have read the extensive discussion on the forum started by Ed Chamberly. Is it safe to say. . .

That the 1K/3W Bias resistor can be used to tweak to bias range of the pot or to find the proper bias in the middle of the pots rotation?

Is this the purpose of this addition to Leo's 5F6A bias circuit?

Can someone also explain the following:

Leo has a 56K resistor in the circuit, we add a variable resistor (50K pot), a 1K resistor, a 47K resistor to replace the 56K 'Leo" resister. How does this match work out? I know I need a fundamental lashing, likea nun with ruler over the knuckles--But please not so close to Easter. . . . (That was a joke)...

Can a 1W or 2W 1K resister be used safely?

Bangy
An Appreciative enthusiastic amp builder

[sluckey]

Leo's bias circuit was not adjustable. It was cheap and also used a selenium rectifier. Old tech. Hoffman's bias circuit is far superior.

[Bangy]

I dont dont doubt that Sluckey.

I am using Dougs Bias circuit.  I am just trying to understand it.

[HotBluePlates]

That the 1K/3W Bias resistor can be used to tweak to bias range of the pot or to find the proper bias in the middle of the pots rotation?

Is this the purpose of this addition to Leo's 5F6A bias circuit? ...

The 1kΩ (R36) and the 47kΩ (R35) could be adjusted to center-up bias pot range.  The 1kΩ definitely makes it easier to get less-negative voltage (-35v instead of -55v).

Leo has a 56K resistor in the circuit, we add a variable resistor (50K pot), a 1K resistor, a 47K resistor to replace the 56K 'Leo" resister. How does this match work out?

The original 56kΩ resistor went to ground, and helped determine the actual bias voltage going to the tubes.  The original 56kΩ was replaced with a 50kΩ pot (R34) to make the bias adjustable.

But the pot by itself allows bias to be adjusted to -0v.  That's bad, and allows the output tubes to redplate & melt.  So there's no value in ever going to -0v.

The 47kΩ (R35) was added to the 50kΩ pot to prevent adjustment to -0v.  It's also smaller than the original 56kΩ, so now you have range above and below the original non-adjustable bias voltage.

Can a 1W or 2W 1K resister be used safely?

Probably not.  During switch-on the dissipation of that resistor will surge over 3w, though it will dissipate less than 1.5w most of the time.  Best to just use the indicated part.

[Bangy]

Thank you HBP.
 

All Questions answered!

Now is there such a thing as a 3W carbon comp resister?

[sluckey]

Now is there such a thing as a 3W carbon comp resister?

That would probably be very hard or impossible to find. 2 watt was the largest common size CC resistor, but there were some larger one made in the '50s and '60s. They were special cases and were not a common stocked size.

Hoffman used all 3W metal oxide flame proof resistors in his power supply circuits even if a 1/2W would be sufficient. That 1K/3W resistor in his bias circuit is overkill, but that is the smallest wattage 1K resistor he stocks that would be sufficient. That's why he used it, not because the circuit requires a 3 watt resistor. A two watt resistor is plenty sufficient. 2 watt carbon comps are very common if you want to use CCs.

[Bangy]

Thats great! Thanks Sluckey. I have 2W CC wired up already. . .

[Willabe]

Now is there such a thing as a 3W carbon comp resister?

We know you want to get as vintage as possible with this amp build but, you really don't want CC R's in the power supply (PSU) or in the -bias supply.

They can catch fire and their noisy/hissy. 

[sluckey]

I use CCs exclusively in all my restoration projects, even in the power supply. I love the metal oxide and use them in my new builds but they just look crappy in a vintage amp. Those CCs gave us the golden tone we've been trying to emulate.

I've seen quite a few burned CCs, some so badly that the resistor body was cracked or even crumbled. I've even watched them smoke, but I've never seen one actually flaming. Are you really concerned with noise/hiss in the power supply with big ole filter caps at the end of every dropping resistor?

I'm not advocating the use of CCs over modern improved low noise, flame proof resistors, but I do believe they have there place, especially restoration. Heck, you can get stupid money for CCs on eBay.

Imagine not using CCs in this old Harvard, or Magnatone, or Hammond restoration...

     http://sluckeyamps.com/harvard/harvard_02.jpg
     http://sluckeyamps.com/magnatone/m09.jpg
     http://sluckeyamps.com/hammond_2/after_2.jpg

[HotBluePlates]

This build is an attempt to get as close as possible to my favorite amp I have ever owned, its a 59 Bassman 5F6A.

Then don't forget the extra bolt through the L-bracket attached to the power transformer.



It's there to help hold the chassis, given the massive output transformer...

[sluckey]

Plexi50 did a good restoration of a 5F6A a couple years ago. Even used the paper sleeving from the old filter caps to hide the new caps. Do a search on this forum if you're interested.

[Bangy]

I am fabricating that bracket on the PT.  It helps to have a vintage chasiss sitting right new to me.

I tried the paper sleeves over the caps, but I didnt like the aesthetic in the end. I think it was because I couldnt find the actual values like 250/6, or 8/150--tough finds.  And since I am using Mustards and not Astrons, I thought I need to draw the line somewhere.

Thanks for the Plexii50 thread lead.

[Bangy]

The amp is complete and sounds unreal! Comparing them side by side to the Original will come.  I do however have this in my head as I play the original 5F6A everyday.  A couple things to share-- the color codes on the Fender layout for the OT primary leads were not the correct phase for the classictone OT. I had to switch them, which took that dreaded high pitched squel away and left me with a masterful tone.

Dougs Bias circuit was right-on PERFECT or so it seems as my bias meter arrives today.  I have been playing the amp at lower AC until I can properly check the bias. Anyway at 50% on the bias pot the plates were upper 300's voltage. I have since spoke to other amp builders who have been using this bias circuit for years.  It is great and so simple!

Anyway, I used all the same value caps and resisters with no mod's other than the bias circuita, but the electrolytics were 8 uF at 150V just like the layout I didn't sway. I used the old technology Selenium rectifier, and Doug's cloth wire which 'smells like Fender' and copied Lili's wiring of my 5F6A.  And yes I did manufacturer the brass grounding plate which sits under the pots---and the Amp is dead quiet.

A word to the new guy who is making a 5F6A - bear in mind this is my 4th build and this is where it all came together for me.  Finally. Not saying I am past mistakes- that comes upon death hopefully. . .

Diagnosing the circuit after the build was so easy, but it came from multiple builds, from stupid questions to Sluckey HBP, Tubenit-whoever was answering questions that day--and hours of studying circuits. Take your time and enjoy the build, treat each solder joint as it's own world and don't move on until it's perfect. If you are not looking at the finish line your build will come out better.

Thank you SLuckey, HBP, Tubenit and everyone I am missing who has helped me. We are all very lucky to have this forum. And most of all thank you Doug for doing what you do.
 

Next-another 2 bassman's

Then . . . Either a JTM 45 or an AC-15.

Bangy

[Bangy]

Bias Pot 50%, Tung Sol 5881 PT Mojo752

5U4
110VAC-385V Plate - 20mA
120V-423 Plate - 23mA

GZ34
110V - 400 - 23.7 mA
120V - 433V - 30mA