Build Report: Tubenit's SoLowWatt CPI

[boodle]

I have built Tubenit's SoLowWatt CPI amp in an old Bendix radio chassis and cab.

I added a MOSFET source follower to the circuit (I think also based on a Tubenit schematic) to decrease the loading of the tone stack and maintain some of the gain that would be lost by using the CPI vs LTPI.  Using a MOSFET saved me an additional tube as part of my build criteria was as few tubes as possible for a P-P low watt amp.

I will eventually get to some sound samples, but don't have any yet.  [Don't hold your breath for too long as life is busy at the moment.]

I thought the community might like to know that the schematic works and sounds fantastic.  If I were to change anything I would:
1. drop the Tone Stack removal switch as it doesn't really give more gain with the source follower right before it.
2. raise the pentode screen resistors from 1k5 to 2k2 or 2k7 as an experiment.  May still do so.
3. drop the Trim control as the amp sounds great with all pedals going in the front end, even delays and verbs, clean or driven.  Because it is so good with pedals in the front, I have tried the effects loop yet.  Wanted to play guitar instead!  A good sign.

I decided to build the amp point-to-point and the result is a rat's nest (of sub-par planning), but a low noise amp even so.

Will post more pictures in a couple more posts

[boodle]

The control panel was laser etched on plexi.  The haze effect on the plexi was from wet sanding the back with sandpaper.

The OT is a intercom transformer that I got in a box of old transformers.  Thankfully, it was labeled.  I measured the voltage ratio and determined that at an 8 ohm secondary load the primary was at 14,900 ohms.  Close enough to the 17,500 ohm plate impedance for a P-P pair of ECL84 tubes as stated on a "Miniwatt" datasheet from Jan. 6, 1960.

[boodle]

Pics of loaded chassis

[boodle]

A friend with a CNC machine that he uses to build fantastic acoustic guitars cut the name plate out of maple for me.

[PRR]

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[boodle]

PRR,
Thanks for putting up the PDF.  [FYI to all: the second page must be a leftover bit of work from some other project.]

[PRR]

> second page must be a leftover bit

Sorry, didn't see it. Reposted with just the one sheet.

Some folks don't have JSchem at hand and this gives them something to read.

[boodle]

RPP, I was intending to delete the page, but obviously forgot.  Thanks for repost.  Makes perfect sense.

[bmccowan]

Nice job - love the cabinet. And any rats nest that does not hum, buzz, spark, blow fuses, or catch fire is a good rats nest. I am amazed at some of the rats nests in old hi-fi equipment I have bought. Most still working after years of neglect.
I need to play my So-Low ECL-84 more and ask some questions. It sounds great at low volumes (which is of course what I built it for) but seems to get a bit harsh at higher volumes.

[boodle]

bmccown,
Mine gets sharper as the MV and gain are both turned up, but I wouldn't call it harsh.  Listening in isolation, it might get fatiguing, but put on a song and play with it and that extra top is just the ticket to be heard.
Also, the CPI version, even with the source follower, might simply push the pentodes less...?

[bmccowan]

Earlier today I played mine for a bit, and fiddled a lot with the interplay between the volume and MV. Then I read your post. What you describe is exactly what I experience. Given the types of amps I build and work on, I rarely have a MV, so it just may be my lack of experience with them. When I find the sweet spot, twiddling the V, MV, treble and mid pots, its a great sounding amp.