5F6A build

[renshen1957]

What should I drop the bias to with the 5881's? I'm feeling too lazy to plot load lines and calculate my bias (I do this enough at school with MOS and BJT designs). Anybody feel like doing me a small favor? I still need to clean the house and get up to school..

Hi Cliff,

5881 is a derated tube for industrial use (used in early computers, etc). Or as originally described, a 6L6G tube with the screen and plate ratings increased 20% Design Center Maximum was 400 Volts on the plate.

http://scottbecker.net/tube/sheets/127/5/5881.pdf Tung Sol original tube data sheet.

The 6L6WGB (6L6 WGB) is the miltary version of the tube (for use in Aviation), exactly the same size (about the size of a 6V6) originally made by Tung Sol, later by Philips/Syvannia

The 6L6GC has a plate Max of 500V and 450V Screens.

The 5881's are listed as equivalents to 6L6GB. Fender most likely used the 5581 to compensate for high vibration environement of 4X10' in a combo. 6L6GC's have a bit more power output capabilities.

The Tung Sol Re-issues are close, but I have a few of the vintage tubes (stubbies as these were called), and the sound in vintage Tweed Bassman is fantastic.

The original Tweed Bassman had a cap can on the outside of the chassis for space, and it wasn't a sweat to reach voltages as the GZ34 tube drop a little bit of voltage (5U4G dropped more, if your 5 volt windings had the current capacity).

When I get lazy I refer to:

http://www.dreamtone.org/Calc-Bias-Form.php

Gives bias rated at 70% of max plate dissipation

OR

http://www.webervst.com/tubes1/calcbias.htm

Best Regards,

Steve

[Cliff Schecht]

Thanks for the cool links Steve. I do know about all of the different 6L6 types and own a few examples of each type (well, lots of 6L6GC's just not many 5881's). I never thought about Fender using the 5881 because of the vibration resistance.

Using your links and some napkin math I come up with about -38V for the bias. Seems about right but it is hard to eyeball this number from the curves because the difference in grid voltage between the two transconductance lines I was using was 15V! Tung-Sol is making us work for those bias numbers .

[Zippy]

Steve's post provides an "aha"...

Cliff, have you considered plugging in a SS rectifier to get the voltages back up a bit?

[M Fowler]

In the old days we were using 7025 preamp tubes and 5881 power tubes in our amps (old Fenders in my case) because of low noise of the 7025s and durability of the 5881, as well as tone.

Both are military tubes, heavy duty use, spec.

Mark