Tube Amps / Music Electronics
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|4/25/2000 1:10 AM|
|J.D||Sundown GOV and RMS functions|
Curious if anyone have had any experience with these
designs? I am looking for different ways of taming the power of a 50w amp a bit, and I checked out the Sundown schems at the Ahola site,
anyone know the correct value of the RMS pot?is it 200ohm 5W or? It looks like some kind of variable Cathode bias, how about the GOV control, does that one cancel out the signal? would this work in a Class AB amp? is the Sundown a class A or Class AB amp??
|4/25/2000 9:17 AM|
The Sundown is a class AB amp. The Guvnor control is a 500K pot wired across the p/i as a "choke" or "strangler" type mv. It pulls the signals from the p/i together and they cancel out since they are 180 deg. out of phase. Shorting across the tone-cut cap in a Vox AC-30 will produce the same MV effect, Matchless used a similar circuit in their DC-30. The RMS control is a 200 ohm 5W wire-wound pot. It is indeed a variable cathode resistor. At its full power setting it connects the cathodes directly to ground. At the lowest power setting it places a 200 ohm resistance in the cathode circuit. The amp has a fixed bias supply as well as the variable cathode resistance which will bias the amp a bit colder in the lower power settings, creating a bit more distortion.
I have seen this control burn out in Sundowns. If a tube shorts to cathode it will often take the pot with it. Hope this helps.
|4/26/2000 12:51 AM|
Do you think this design (RMS)would sound any good compared to for example a "regular" Pentode/Triode switch for making a Bassman head amp a little "smaller" ?
I am not looking for more distortion , just a little less volume and maybe a little sweet compression.
|4/26/2000 3:18 AM|
I have owned several of the Sundown amps, and have found that using the Govenor and RMS controls are a nice compromise for lowering the volume. Some people like the sound that using these controls gives, others don't like it, it is really a matter of personal taste.
Rick is right concerning the RMS pot burning out, I own an Artist 50 combo that came with the pot burned out. Dennis Kager, the designer and builder of Sundown amps, told me once a while back in a conversation that he calculated the power handling value of the pot incorrectly. The pot really needs to be more like 10 watts. Unfortunately, these seem to be pretty rare, and if you do find one, they tend to be very expensive. YMMV.
I've tried pentode/triode switches in amps, and didn't find that it made a huge difference volume wise. Sure triode mode was less loud than pentode mode, but triode mode got darker too, so it affects the tone.
I guess the best thing for you to do is try it, and maybe you'll like it, maybe you won't.
|4/26/2000 5:54 AM|
||Replacing 200ohm/5 watt ww pot w/SS circuit?|
There have been posts here about using various ss devices in the cathode of an output tube instead of a resistor. I wonder if anything like that would work here...
|4/26/2000 6:20 PM|
The RMS controls I have installed on a few customer amps, seem to be (as you stated) a matter of personal preferences. I found myself liking it all the way up or all the way down, but the middle positions were so/so at best. I eventually started just adding a switch that kicked in a resistor and a capacitor to make it full/ and low power. The triode/pentode switch along with this switch, gives more range to the wattage and tone as well. The choice of the bypass cap (as expected) also changes the tone as well. A good protector is a diode across the device (banded end towards ground), which protects the device if a power tube arcs out. I saw this on one of McIntire's amps, on the power tube cathode resistors. I always fear solid state devices in the power tube cathodes. The Music Man amps often failed due to the failures of the driver transistors in the power tube cathodes. Perhaps the durability of HexFets could tolerate this better. An adjustable VBE multiplier circuit in the cathode return of the power tubes might work. I don't much care for the Governor control sound in Sundowns.
|4/26/2000 8:59 PM|
This will work for protecting 1 ohm 1W resistors since a .7V drop would equal 700mA or .49W dissapation acroos the resistor. With a 200 ohm resistor the voltage drop across the resistor would likely be 10V or more. The diode would limit the voltage across the resistor and effectively short the cathodes to ground.
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