Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on September 18, 2012, 12:18:33 pm
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Hello All,
I have a Hammond 261G6 power transformer I'm looking to use and was thinking I'd try an 18 watt stout. What I'd like to know is if this little PT would be suited to such a project. I'd be working with 250Vac bridge rectified which should net me around 350VDC and the 6.3V taps are rated for 2A for a pair of 6V6 and a couple 12ax7's shouldn't be a problem. I'm just not sure if 350 would be enough to get the job done since the EL84 stout calls for 350V according to the posted layout with voltages. I know generally a few volts here and there don't tend to make a difference but I'd be changing output tubes so I'm looking for a second opinion on this one.
Any info you guys can share would be greatly appreciated.
Cheers,
Neill
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It will work fine and 6V6GT tubes sound great at 350v.
With respect, Tubenit
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Excellent that's good to know. Just two more questions. Should I still use a choke in the power supply rail and what output transformer should I use for this. I was thinking either a tweed deluxe 8k primary or regular deluxe 6k6 primary. Im not sure which OT would be better suited though.
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You may use a choke or for a lower budget use a dropping R instead.
Both OT's will be fine. 8K is the usual for cathode bias, 6K6 for fixed bias.
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Ok perfect sounds like the 8k is the way to got then. One last thing I'd like to know is how the stout's tone control is supposed to work. As its drawn it looks to me like regardless of where the knob is set any of the signal going through the 10nF cap is getting sent to ground. Is this just a Bass cut and the 100k resistor going to the volume control just to prevent noise or something?
If someone could go over that one it would be appreciated.
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Why 6.6k for fixed bias and 8k for cathode? I would think that fixed bias would have more voltage on the tube so you would want a higher impedance.
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More power.
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Ok so I whipped up a quick frequency test using CircuitLab and i looks like the tone stack on the stout is a constant bass cut. There also appears to be a treble roll off but only when the pot is dimed. Now my simulation may be flawed but it would seem the tone stack doesn't have a whole lot of variation. Looking at it I would expect more of a rocking bass and treble control similar to a Big Muff tone stack but the simulation seems to say otherwise. If anyone else here can weigh in id like to know if im understanding how it actually works.
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More power.
So why would a 6.6k not be good for cathode bias for more power? You would think that since there is less voltage across the tunes you would want to push more current and the lower impedance would be an advantage also.
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> Why 6.6k for fixed bias and 8k for cathode?
It depends.
In cathode-bias the total amplifier current is (nearly) constant, so the peak current is limited by this. For most reasonable plate and screen voltages and acceptable dissipation, it works out that 8K or 10K is best.
In self-bias the peak current can be anything up to whatever the screen voltage will suck off the cathode. For 6V6 at higher plate and screen voltages, 6.6K is possible and will be a bit more power.
That's on paper or bench with exact loads. On loudspeaker the load is different. The above numbers are fine starting points, but specific builders may like other numbers.
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OK so another question has popped up in my research. Im wondering what the difference is between the hoffman stout and an 18watt liteiib with the zaphod cascade preamp mod. for reference the zaphod mod can be seen here
http://www.18watt.com/files/cascprll_boost.jpg (http://www.18watt.com/files/cascprll_boost.jpg)