5881's bias for 5f6a

[tweed5259]

Hey what should I bias reissue tung sol 5881's in my 5f6a?
I have them at 34mv now.

thanks.

[tubeswell]

Based on this old TUNG Sol 5881 datasheet, I think they can take quite a bit more than 34mA (When you say 34mV, I take it you are measuring total tube current over a 1R cathode resistor?). What voltage(s) are your Plates BTW?

[MWJB]

30-35 will be fine. The small bottle 6L6 5881 historically were 23-24W, x.7 = 16.5W. Assuming when you say 5F6A you are running a tube rectifier and a typical 5F6A PT, your B+ won't be much over 475v (As Tubeswell says, checking this would be good). Therefore, .034*475= 16.2W. 15-16W is pretty common.

[tweed5259]

ok I have it biased actually at 38mA but the plates are at 443v so if I'm doing my figures correctly, that should put me at...70% of what the plates can take?



PS - doesn't that voltage seem a bit low for a 5f6a?

it's setup as per the hoffman 5f6a layout.

[tubeswell]

yup 16.1W is 70% of the max rating. Is the 38mA reading over a 1R cathode resistor? (don't forget to subtract the screen current)

Interesting to see how high the plate voltage could go. Do you like the sound?

[tweed5259]

is that voltage too high? what's the average voltage for a tweed bassman clone of today? it sounds pretty good to me, late break up though, around 7 or 8 on the volume with single coils.

[tubeswell]

Well the tubestore gumpf claims a 400V max rating but goes on to say that you can run it higher if you watch your biasing. So I'd say where you've got it is probably fine. Plate voltage can generally be raised or lowered around a specified "max" rating (within reason) as long as you ensure that tube current is such that you don't exceed the max dissipation. Of course this will affect the sound in different ways

[tweed5259]

I don't understand, if in the 50's the 5f6a's were supposed to run on 5881 tubes, yet the voltages for the PT in a Tweed Bassman run voltages at 400 or above in most cases normally. What's the reasoning for this?

[tubeswell]

... And the datasheet for the 1962 Tung Sol 5881s specks a plate voltage of 360 in Class AB1 with a quiesecent plate current of 88mA. Doesn't mean that you can't run higher plate voltages with appropriate plate current to get the right dissipation. The tweed era bassman ran higher plate voltages and lower idle current, and nothin' bad happened to the tubes. The modern reissue 'tung sol 5881' specks a max plate voltage of 400V and a lower idle current for Class AB1, so its swings and roundabouts. Your plate voltage is fine and your plate idle current is probably within ballpark so I wouldn't worry. Unless the plates start glowing red, just enjoy the tone.

[MWJB]

Fender ignored the data sheets with regards to max screen & plate voltages...a big factor in their success.

If any manufacturer was really making a 400v max plate voltage 6L6/5881 today they should be shot...there are so few amps that they could be used in.

Typical 5F6A voltages, with a 5AR4 & 30-35mA on the plates are usually around the 470vdc mark, I have seen 500-ish with colder bias & 514 on one particular 5F6A reissue.

[TD_Madden]

don't forget the higher wall-voltage now as well.......

[tweed5259]

don't forget the higher wall-voltage now as well.......

yes, and speaking of which, do different PT's make the amp sound different, figuring the OT and Choke surely do, but is it a worthy expense to buy MM, or Hammond Power Transformers, or do they have not so much effect on the sound of the amp if you've already got normal 5F6A voltages... 440v-470v?

[MWJB]

If you already have your target B+ voltage and you have adequate B+ current handling (150-200mA) and 6.3VAC heater current (5A), then no, there isn't much point in changing the PT.

"figuring the OT and Choke surely do," How much difference do you think 2 chokes of the similar current handling and inductance will make? Think that you'll hear it?