Newcomb E-17 "guitarified"

[Jennings]

Recently came across this cute little 1950s interloper from the States, and decided to take it under my wing.  It's a 17W PA amplifier, with neat metal preamp valves and a pair of big bottle 6L6G output tubes. 


I renewed the caps and drifted resistors, tested and replaced the tired valves, replaced the speaker selector and sockets with hard wired 1/4" jacks (one for 8 and 16 Ohm taps), and replaced the "MIKE" input (sorry Mike) with a 1/4" jack too.  33k grid load resistor and 1M to ground to gear for guitar.  Works fine, and sound reasonable...but it really doesn't get going until the volume is at 11 or 12 o'clock, and even then is lacking some raunch with guitar.


What way do you think I should go next?  Revise that triode input stage for gain?  Modify the feedback from the OT secondary?  Happy to take suggestions to get this more lively!


Schematic attached from the bottom of the chassis (I wish modern amps did that!)...note that I have an extra "Bass" control on my model.  It's a pot wired as a VR with a cap between wiper and input lug...placed between the node of the 330k mixer resistors and the .022uF coupling cap to the preamp pentode.

[shooter]

measure the VDC across the 250 ohm cathode R in the PA
measure the VDC at the "B+ tap" for the 6L6 Anodes


do math;
(VDC anode - VDC cathode) * (Vdc cathode / 250ohm)


that will give you the ballpark dissipation (watts) of the 6L6's


add a bunch of pedals up front til you find.... ok, i like that, then you can use that info to "build those pedal additives" into the amp

[Jennings]

Yes, good point…let’s see what I have going on with these 6L6G. Looks like I have 323 and 317 VDC at each respective 6L6 anode. 22 VDC across the shared cathode resistor. So my dissipation calculates to 26.49 and 25.96 Watts respectively. So 88.3% and 86.5% of 30W max respectively.

[shooter]

so it should be lOuD enough


have you rolled as many speakers as you can beg, borrow or steal for testing??



put a gain pedal up front

[Calboy]

I think the dissipation is half of what's posted since there are 2 tubes, so about 13w per tube. 

Might try no nfb or 47k to 8ohm tap.

Are the 6l6 wired up as triodes, with plate connected to screen?  If so, maybe rewire.
Or maybe not.
The 5z4 tube is similar to a 5y3 in current capacity and typically a 5y3 is not used with 6l6.
I think some builders use a switch for triode mode for half/reduced power feature.

[Jennings]

Ah yes, you're right!  Looks like I'm running around 13W dissipation, so about 69%.  Good spot.  I'm running 6L6G valves, which tested good and came in the amp.  So they're probably 19-23W max rated.  So I could ease off the cathode resistor a touch to squeeze a little more total volume out of the amp then.  They're wired as pentodes stock.


I feel most of the apparent lack of audio/guitar volume is probably related to a combination of preamp taming (it is a PA design after all as stock), and also perhaps the taper of the volume potentiometer.  But I've yet to really try this with any speakers apart from my small bench testing unit...which is a high-wattage 4" job, literally just for testing amps I'm working on, but it does give me an indication versus other amps I run on the bench.  I'll try it with a proper cab when the other half goes out  Definitely not a raunchy kind of circuit as stock.  My first thought was to perhaps at least remove the NFB and go from there in the preamp...and a quick desolder of the wire and test shows it definitely helps, so that's a start.  I'll give that a go with a proper speaker setup late on and see where to go next.


Thinking ahead I'll ponder that first gain stage too...I've located the data sheet, which at first glance seems to suggest I won't be running at higher gain in stock format with my circuit values.

[Jennings]

Well, I have some great news...I did some "real" speaker testing, and still thought the amp was lacking.  Remember it's currently virtually stock, except I've removed the NFB.  And then it hit me!  That 6SC7 is grid biased as stock in this circuit!!!  Silly me had removed the cap when I installed the 1/4" input jack...DOH!!!  So input cap re-installed and tested again and the amp is much livelier and sounding great.