5F4 build issues

ok_state_blues

TDPRI Member
Joined
Mar 21, 2021
Posts
94
Age
33
Location
USA
I’m not sure what the voltages should be exactly, but they at least make sense. Your bias voltage could probably be raised a bit to warm up the power tubes (say… -35V). Are you still troubleshooting a problem or just making sure this is in the ballpark?
I just want to make sure I am in the ballpark. Also to make sure nothing seems out of line tube power wise. I will address the bias voltage issue. It passes sound so far and doesn't smoke and is extremely quiet. Very happy up to this point.
 

ok_state_blues

TDPRI Member
Joined
Mar 21, 2021
Posts
94
Age
33
Location
USA
I’m not sure what the voltages should be exactly, but they at least make sense. Your bias voltage could probably be raised a bit to warm up the power tubes (say… -35V). Are you still troubleshooting a problem or just making sure this is in the ballpark?
Here is where I am working from.
 

Attachments

  • super_5e4a_schem.pdf
    44.4 KB · Views: 11

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,016
Location
Salt Lake City
What @andrewRneumann said. You’ve done strong work troubleshooting, and thanks for that schematic. For our reference here's a slice of it:

BA4ADE91-E09D-4FA2-A958-8973286D552B.jpeg

As you’ll know the schematic negative bias values aren’t always useful as a guide to biasing; are you up to speed on Rob's bias calculator and his safe ‘OT resistance method'? Best way to discuss bias is in % max plate dissipation (typical target 50-70%).
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,617
Location
Cincinnati, OH, USA
I just want to make sure I am in the ballpark. Also to make sure nothing seems out of line tube power wise. I will address the bias voltage issue. It passes sound so far and doesn't smoke and is extremely quiet. Very happy up to this point.

Ok good. @King Fan is right about the bias. Unless you have a way of measuring current through the tube, the bias voltage is just a guess. I have read that some people increase the voltage until visible red-plating and then decrease it from there. That method violates my German sense of precision. :twisted:

So, why the 12AT7 instead of 12AX7 for V3? Was that because you want a little more current headroom to drive 6L6's if you needed to?
 

ok_state_blues

TDPRI Member
Joined
Mar 21, 2021
Posts
94
Age
33
Location
USA
@King Fan I will walk through that page and apply what I learn. I am unfamiliar with the OT resistance method, but from another forum and reading in a Dan Torres book, I added in 1R 1% 3 watt resistors to ground so I could measure things accurately. Maybe I should bias conservatively since the 6v6GT's are the original Hammond tubes that shipped with the chassis.

@andrewRneumann yes that was the thinking with 6L6 tubes. But now with 6v6 power, I guess I have a little more wiggle room. But shouldn't a 12AT work better at that position as a PI?
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,016
Location
Salt Lake City
...but from another forum and reading in a Dan Torres book, I added in 1R 1% 3 watt resistors to ground so I could measure things accurately.

Great! No need to do the OT resistance or associated math. Your 1R resistors are even easier, safer, quicker.

You're been ahead of me on this bias thing and may not need any of this info. But in case it could help, as you likely know then, you measure the mV across the 1Ω resistor and Ohm's law means that number in mA is your cathode current. It's perfect to plug that value into the *Plate Current* part of Rob's calculator. (Cathode current is within a few % of plate current, and in the 'safe' direction -- slightly cool). Here's the first part if you get 398 plate volts on glass tube (GT) 6V6s; if you want to be conservative, use Rob's 12W '6V6' setting instead:

1642535741317.jpeg


Now pretend we got say 22.5 mV = mA across one of the 1Ω resistors. Bingo, you're at 64%.

1642535826424.jpeg


Knowing this, you can turn your bias knob up and down and 'bias by ear' with your mV meter reading across the resistor, targeting dissipation between say 50-70%. The one trick is you *do* want to remeasure plate current after you change bias, and recalculate both the first box and the second one; they interact. But it usually takes just one or two runs to find a safe number that sounds best to you.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,617
Location
Cincinnati, OH, USA
@andrewRneumann yes that was the thinking with 6L6 tubes. But now with 6v6 power, I guess I have a little more wiggle room. But shouldn't a 12AT work better at that position as a PI?

I guess it depends on what you are going for. Theoretically, the 12AT7 can drive a heavier load. (The 6L6 should have 100K grid leaks when using fixed bias.) I only asked because the schematics all had 12AX7s in that position. This is definitely a case where you are free to try out either tube and pick the one you like.
 

ok_state_blues

TDPRI Member
Joined
Mar 21, 2021
Posts
94
Age
33
Location
USA
Ok so after some bias pot turning here are my manual calculations per our dear uncle. Did I do this right?
 

Attachments

  • IMG_20220118_220157259~2.jpg
    IMG_20220118_220157259~2.jpg
    38.4 KB · Views: 14

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
5,363
Location
california
after some bias pot turning here are my manual calculations per our dear uncle. Did I do this right?
Yep. It is in a safe range.

Usually it is expressed as *% of maximum plate dissipation*. You just divide the plate dissipation by the rated wattage of the tube to get the percentage.

Congrats on the Hammonator 5F4. Nice looking addition. Your patience and persistence has been a good learning experience.:cool:
 




Top