Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on September 09, 2023, 09:44:39 am
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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.
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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.
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That helps. The Hammond Bassman power transformer is well over 200ma. Just wanted to check.
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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 (https://www.hammfg.com/files/parts/pdf/290DX.pdf). 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 (https://www.hammfg.com/files/parts/pdf/272FX.pdf) 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 (https://frank.pocnet.net/sheets/093/6/6L6GC.pdf) (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 (https://www.hammfg.com/files/parts/pdf/1650N.pdf) 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 (https://www.hammfg.com/files/parts/pdf/1650N.pdf)
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")
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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.
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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
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Thanks. I'm not looking for more power, really. I have a 64 BF Bassman for that. I just wanted to make sure.