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The "Little Millie" SET spud amp!


tube fanatic
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On 1/31/2020 at 9:42 AM, tube fanatic said:

 

Nice to see you on here again Mike!  You are correct- the p/s is external (the same design as the Little Sweet Potato supply), and the grid of the 50L6 is directly driven.  I suggest that anyone contemplating building their own read the Little Sweet Potato thread above as it outlines the design philosophy and provides some technical details.  In the Little Millie, I'm using only a resistor for filament ballast as opposed to using a tube.  It was a design choice based on what the guy asked for.  The three power resistors dissipate a great deal of heat, so lots of ventilation is needed (or, alternatively, chassis mount resistors installed with heat conductive grease).  I was able to get the schematic drawn last night so here it is for those who are interested.  If anything doesn't look  correct, let me know!  And, if adjustment of the high frequency characteristics is not desired, R1/C1 and R2/C2 can be eliminated.

 

ONE MORE REMINDER ABOUT THE VOLTAGES USED IN THIS AMP- DON'T BUILD IT IF YOU LACK EXPERIENCE IN WORKING WITH LETHAL VOLTAGES SAFELY!

 

Maynard

LITTLE MILLIE PNG.png

 

I have noticed that earth ground and signal ground are connected at one point in the above schematic...

 

An alternate grounding scheme may eliminate or prevent hum in the amp depending on interconnected equipment. See below;

 

This circuit will break the  possible current paths between earth ground and signal ground. The cap and resistor values are not critical, but should remain within a range of values.

 

I use a 4.7nF X cap because I happen to have a huge stash of them (they were free). Alternate value X caps can be used up to 0.1uF with good results too. 

 

The resistor can range from 75 ohm to 150 ohm and still perform its function. I use this circuit in all my amp builds (SS & tube) and have yet to have a hum problem regardless of what equipment is connected. 

 

Earth_Signal Ground_v1.0.0_080621.jpg

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Yes, the alternate grounding scheme may be of some use although this amp is dead silent in every system in which it has been tried.  The Millie is quite a contrast to the SET monster you posted the picture of!  Good to have you on here Nick!

 

Maynard

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  • 2 weeks later...

Would there be any pitfalls in swapping output transformers with Hammond 1627SEA? Same impedence on the primary (2.5k). Primary inductance is 20H vs the approximate 10H of the 125ese.

I’ll attach the link to hammonds info below. As it is, I see the spec sheet for the 125ESE warns to use this series use the 1627-1640 trannies for “HI-FI” 20-20khz output.

 

http://www.hammondmfg.com/1627.htm

 

cheers,

Jake

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9 hours ago, Jazzmaster Jake said:

Would there be any pitfalls in swapping output transformers with Hammond 1627SEA? Same impedence on the primary (2.5k). Primary inductance is 20H vs the approximate 10H of the 125ese.

I’ll attach the link to hammonds info below. As it is, I see the spec sheet for the 125ESE warns to use this series use the 1627-1640 trannies for “HI-FI” 20-20khz output.

 

http://www.hammondmfg.com/1627.htm

 

cheers,

Jake


It would be fine to use the 1627 although it may not be much better than the 125ESE in this application.  The bandwidth of the latter is extremely wide due to being operated at such a tiny fraction of its output power rating.  Personally, I would build the Millie as shown and keep the size and weight down.

 

Look at George Anderson’s measurement of the 125CSE to gain some insight about transformer bandwidth as a function of output power:

 

http://tubelab.com/articles/component-testing/budget-output-transformers/

 

 

Maynard

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