SoLow Watt build

[pullshocks]

This project lost traction  for a few months, but I made some progress today.  I had a bit of luck that I had a step drill bit that worked for the ceramic tube sockets.


Could I get a reality check on OT primary impedance for the 6F4P tubes and 8 ohm speaker.  Looks like my options with the Hammond 125 E are
22.5K ohms
17.6K ohms
12.8K ohms


Per Tubenit's schematic B+ should come in at 255 V with the Hammond 269.  (The schematic attached here has the modified B+ rail discussed in my earlier thread.


Thanks and hope everybody had a good Christmas.


Pullshocks / Mark

[tubenit]

Will follow this with interest!  Please continue to post your progress.

With respect, Tubenit

[shooter]

next time, either pull or tape off the pots, those chunks n dust like pots

[vampwizzard]

im in the middle of a build for one of these as well. 269 PT/1750E OT.Found some NOS RCA ECL84s. Ill post some pics tomorrow.

[DummyLoad]

see attached. B+=250V ; Rk=120Ω ; Ra-a 17.5KΩ ; about 3-1/2V to drive to full power for a whopping 5.4W.

add another bottle and use a pair of 6CM6 for the outputs - should get you <=10W with 250V to the plates with a Ra-a=10KΩ. 6CM6 are low cost 9pin 6V6.   

closest to that with the 125E into 8Ω is 17.6KΩ - should be close enough, no?   

try both? but do have fun.   

--pete

[pullshocks]

Thanks Pete, that helps a lot.  I will use the 17.6K for sure.


Vampwizard--look forward to seeing how you are laying yours out.





Here's what I came up with in DIYLC.  It looks a little different placing the actual parts in the actual 17x8x2 chassis, especially the output transformer.  But I'll get it worked out.


Pullshocks / Mark

[DummyLoad]

in the take 4 PDF - you have left-over wires and hardware: they gonna rattle around an short $#!^ out.... 

hold out for take 5 - un-da un-da unda un-da un-da unda...

--pete

[vampwizzard]

I worked off the original layout.. it'll come together with some terminal strips. Had trouble finding a 2.2uf cap so got that big honkin one, only to discover I had one in my collection that wouldve fit better. d'oh!

Also, reused one of those hammond boards like folks were discussing not too long ago.

[pullshocks]

in the take 4 PDF - you have left-over wires and hardware: they gonna rattle around an short $#!^ out.... 

hold out for take 5 - un-da un-da unda un-da un-da unda...

--pete

Between the extra stuff on the layout, and the aluminum chips, this build is probably doomed.


Haven't listened to "Take 5" for a long time, forgot how much of it is drum solo.  I remember going to hear the DBQ back in the 60's with some of my high school band mates.  We all thought Joe Morello was a pretty cool drummer.

[pullshocks]

This one got side tracked by a couple other projects (sound familiar to anyone?) but I have made a little headway recently.


I love DIYLC but before making my turret board I'm doing a reality check with what I call the "poor man's Visio".


My least favorite part of amp building is wiring heaters.  For some reason, dealing with the 2 pairs of wires at every socket is ultra frustrating for me.  Here's my alternative.  It took just as much time, maybe more, and has the drawback of leaving some wire untwisted. 

[Willabe]

Hummmm, interesting with the heaters.

Where did you get those insulated turrets? Those are nice 1's. 

[pullshocks]

Hummmm, interesting with the heaters.

Where did you get those insulated turrets? Those are nice 1's.

Sluckey posted the link for those turrets (Thanks Sluckey).  Concord Electronics 1127-01-1516 Insulated Turret Solder Terminal, 4-40, 25pcs | eBay  Seller shipped quite promptly.


My first time using them, so far so good but I have not tried to solder to them yet.  I shined up the turret with some scotch brite and I'm hoping it goes ok.


Another source at a good price for insulated single turrets is Angela, but they don't call them turrets Terminal Posts Brown .632" Tall (angela.com) 

[pullshocks]

Hummmm, interesting with the heaters.

Where did you get those insulated turrets? Those are nice 1's.

If you were talking about the heater wiring, those little mini two-turret boards were made in my shop

[sluckey]

I would tin the turrets before putting any components on them. Just a dab of flux and a tiny bit of solder, just enough to wet the surface.

[pullshocks]

I would tin the turrets before putting any components on them. Just a dab of flux and a tiny bit of solder, just enough to wet the surface.

Thanks.  I appreciate the tip.

[acheld]

The Concorde ones work fine, and the eBay seller is very fast.   I may try the Angela ones, though -- a) I like the 6-32 mounting and, b) the two prongs make it easier to mount two components securely, and to make changes later if needed.

[pullshocks]

Had to pause work on this project to reduce work area chaos, and adapt for working in a seated position.  I like having the rack for pliers, nut drivers, and screw drivers right above the chassis in progress.  The drivers are kind of crammed together, but they are in arms reach.


As far as the build itself, should I use shielded cable for the run from V1 to the volume control and the run back f (Edit--just doing regular wire from v1 to the coupling cap and from the coupling cap to volume control. but how about from the volume control to V2? 


How about for the runs from the treble control to the effects loop, and from the loop return jack to the PI? 


How about from the PI to the PPIMV and back to the output tubes? 


Thank you

[tubenit]

Too much shielded wiring will squash the tone and touch sensitivity to the amp in my experience. 


I tend to use shielded wiring from the input jack to V1-2.


I tend to use shielded wiring on long runs next to areas that I think will possibly impact the noise at idle when not playing but amp is turned on.  An example is that if I have to run a long signal wire by or across B+ wires with high voltage, I'll likely use shielded wiring.


I would probably try to not use shielded wiring with the PPIMV and see if you still have a quiet amp. 


Lots of this is trial and error with each particular amp.


With respect,  Tubenit

[pullshocks]

Thanks Tubenit.   Used shielded cable only on the run from input jack to V1 . 


Almost ready for the light bulb tester, so we'll know soon if more shielded cable is needed.


From the "that's 8 hours of my life I'm never getting back" department:  space is pretty tight around the mid boost switch for possible (probable) rework.  So I made a little bracket and mini eyelet board for the 390 pF mid boost cap to make it easy to install the correct value cap when I get it. I have a 300 and a 100 in parallel now.  Not to mention avoiding overheating the mini toggle during desoldering/resoldering

[tubenit]

That looks great!  Please post a sound clip when you get her all done!


With respect, Tubenit

[pullshocks]

Ah the thrill of of a build working first try!  Well, there was a brief "oh shit--no sound" moment as the speaker jack/plug was initially not making good contact, but a quick wiggle took care of that. 


Other than that, everything seems to work as it should, and it is nice and quiet.  No noise issues to detract from playing.  I followed the Merlin grounding scheme as well as I could.   


Preamp tubes are supposedly NOS 12AV7, but the mfg info has been removed.  The PI/PA tubes are from Russia.


Looking forward to getting to know this new amp.


Thanks to Tubenit for the design, and to Sluckey, Dummyload, and other forum members who helped along the way.

[vampwizzard]

very nice! ill have to finish mine up.. been sitting on the bench too damn long!

would love sound samples if you got them!

[pullshocks]

EDIT  Schematic revised to show 1 ohm resistors at power amp cathodes.


EDIT 2:  This post turns out to be  kind of a wild goose chase.  I *may* have the problem identified.  Stay tuned for future posts when I get the amp put back together.



After playing the amp for a few hours maybe 10 on/off cycles, and rolling some speakers,  the popping sound became much worse.  This occurred briefly the first power up, and seemed to go away when I twisted the speaker plug in the (Cliff) speaker jack.
It  progressed to a persistent, loud popping, starting 30-50 seconds after power up.
Pulling the first 2 preamp tubes does not eliminate the sound, appears to be a PI or PA issue (triode and pentode in same bottle).



Chopsticking while the popping is going on, the whole amp seemed microphonic.  I was not able to find any component or wire that was worse than any other.  Microphonic everywhere.
I checked voltages.  At node A right after turn on, DC volts would rise very briefly to 320, then back down to about 290, and then slowly drop to about 240 and readings became erratic as the popping started.
I pulled all tubes and rechecked voltages, and found steady readings at all nodes.. 
Node A 289.7
Node B 288.1
Node C 287.2
Node D 287.1
Node D 287.6
I did my best to clean the 6F4P pins and the sockets (Deoxit on the socket, wire brush on the pins).  I have 4 of the 6F4P on hand, and tried different combinations.  Mostly the same pattern of loud popping.  I noticed whatever tube I have in socket V3 had the microphonic behavior, not the tube in socket V4
I reflowed the connections to the V3 and V4 tube sockets, and replaced the 1 ohm cathode current test resistors.  Picture attached.
The Hammond 125E OT seems to have continuity
Currently, I can start up the amp with no popping, just a faint hum from the speaker.  Checking voltages I see the following steady readings:
Plates V3-239.9 V4 236
Cathodes both 9.5
Screens V3 215 V4 197
Grid V3-13 mV V4 9.6 mV
The differing screen voltages stuck out but the big thing was the 1 ohm test resistors showed no cathode current coming from V3.  I forgot to write down the reading for V4
This leads me to conclude I have a bad socket at V3, specifically pin 7 (cathode).   If there were a good connection, there should be cathode current.  If I understand correctly, the cathode voltage reading on V3 is coming from the V4 cathode, via the 2, 1 ohm test resistors.  Why it is not still popping, I can’t explain.
Not sure if the big difference in screen voltage indicates a bad connection on pin 9 also, or if that is a side effect of the bad cathode connection.
Before I swap out the tube socket, is there anything else I should look at or test?
Thanks very much
 
 
 

[pullshocks]

To recap, I  recently finished a build from the SoLow Watt design that has been built by several on the Hoffman and AmpGarage forums.  It is a push pull amp.  I used 6F4P tubes instead of ECL84.  Schematic attached in post above.  All new parts, except the tone caps and PI coupling caps.  I got the 6F4P from a member on the Wattkins forum, don’t know anything about them.  Output transformer is a Hammond 125E, wired for 16K .
It worked first time, and sounded great for a couple hours.  Now it sounds like a long string of fire crackers going off, kind of like 4^th of July in my neighborhood.  In retrospect there were a few isolated pops at first.  After maybe 2 hours of playing and maybe 10 off/on cycles, the popping got worse and worse.  It would start 30-50 seconds after initial turn on.  While the popping is going on, voltages are unstable, and one or both of the 6F4P are strongly microphonic.  Tapping anywhere in the chassis or amp it is microphonic.
With a guitar plugged in, it makes sound, but sounds awful when the popping is going on. 
The post above covers  my initial attempts to trouble shoot.  I now think it is an issue with the tubes.  As of last night I found one of my 6F4P is shorted between grid and cathode.  With my remaining 3 6F4P, the popping starts after 30-50 sec of warm up.  In the dark, the 6F4P blue glow gets brighter as the pops happen.  The onset of popping seems to occur as the tubes get warmed up
The popping behavior occurs even with the 12AV7 preamp tubes pulled.
I am going to order up some NOS ECL84/6DX8 tubes.
To reiterate, the amp seemed to work well for a couple hours. But I wonder if the 6F4P just have a high failure rate, or if something about the build is stressing them.

[pullshocks]

Received the Sylvania 6DX8s.  No more firecrackers, and the amp now sounds about the same as it did originally.  Next, I'll see if anything changes with a few hours of playing time, i.e. whether the new tubes develop firecracker issues like the 6F4P did.  Hopefully not, as B+  node A lines up with Tubenits origninal schematic. as do the pentode plate, screen, and cathode voltages. 


The other nodes have different voltages, presumably due to the Node E that is paralleled off node B.    I am going to try rewiring the power supply back to what Tubenit had, i.e. disconnect node E, and run V1 from Node D.