Bassman 5B6 PT Choice

[plumcrazyfx]

I'm trying to use transformers that I have sitting around.  I have a Hammond 272FX (173ma) and a Hammond 272HX (230ma).  Which should I use for a cathode biased Bassman.

[HotBluePlates]

The 272FX (173mA) is plenty big enough for making 30w with a 3.5 - 4kΩ OT.

I might even want a bit of extra "winding resistance" in the form of small-ish resistors between the PT wires & rectifier plates.

[plumcrazyfx]

That helps.   The Hammond Bassman power transformer is well over 200ma.  Just wanted to check.

[HotBluePlates]

Bassman 5B6 ...  Which should I use for a cathode biased Bassman.

... The Hammond Bassman power transformer is well over 200ma.  Just wanted to check.

Except which "Hammond Bassman power transformer" are we talking, because Hammond has 7 different ones for domestic 60Hz use.

The lowest-wattage one is likely the tweed 5F6-A Bassman PT, which Hammond marks 290DX.  But that amp is fixed bias and 45+ watts, while you said, "Bassman 5B6" and "cathode bias."

A cathode-biased pair of 6L6s won't make much more than 30w (and often quite a bit less) so less-voltage & less-current are acceptable.


I looked at the 272FX and saw it was 600v CT at 173mA.
300v x 1.414 = 424v minus rectifier-drop ---> about 390-400v.

I looked at a 6L6GC data sheet (top graph of page 6) and saw that with 350-400v on the screen, the 6L6 can pull its plate down to 75-100v and pass over 200mA peak.

A typical 50w output transformer, like Hammond's 1650N has a 4.3kΩ primary impedance.

The 6L6 plate might swing a maximum voltage of 400v idle - 100v peak = 300v peak across the primary.
We can start by assuming the power section stays Class A:  4.3kΩ / 2 = 2150Ω to one side.

300v peak / 2150Ω = 140mA peak ---> we don't need all the current the PT can deliver
300v peak x 140mA peak = 42w peak ---> 21 watts RMS ---> about right for the 5B6's 5881 output tubes


You can have slop in any direction, and you won't really notice a difference in power/loudness or sound.  Your biggest defining feature is "cathode bias" and for that we don't really want high voltage (and shouldn't expect "high power output")

[plumcrazyfx]

So, I'm finally getting back to this after nearly a year.  Iwould like to use a transformer that I have rather than buying something new.  I have the Hammond 1760JB with a 6k rather than the 4.3k of the 1750N in the example.  I plugged in the new values for that output into the calculations above and am I right that I would most likely be looking at 15w RMS?  I want to make sure this is an acceptable OT for this build.

[tubeswell]

So, I'm finally getting back to this after nearly a year.  Iwould like to use a transformer that I have rather than buying something new.  I have the Hammond 1760JB with a 6k rather than the 4.3k of the 1750N in the example.  I plugged in the new values for that output into the calculations above and am I right that I would most likely be looking at 15w RMS?  I want to make sure this is an acceptable OT for this build.

I think it'll be fine. The early tweed Bassman was low powered

[plumcrazyfx]

Thanks.  I'm not looking for more power, really.  I have a 64 BF Bassman for that.  I just wanted to make sure.