5F10 FUN

[screefer]

Slow progress.

My first build was a Mojo 5F2a kit with pushback wire. Man do I miss that stuff!!

Heading off for the long weekend so I thought I would leave the sockets unsoldered.
Best to study it before I repeat my diode snafu.


The writing on the filter caps sure doesn't survive much handling, eh?

Here are the latest pix. Please chime in if anything is haywire.

 

[Esquier]

I home-brewed a Harvard circuit into my buddy's Pro Jr. He has a nice 12" Mojo tweed cabinet. I cut up its chassis a dozen different ways to accomodate all the new things such as the Hammond transformers and a proper Fender lamp. I crafted a terminal strip setup for the bias circuit alongside the PT. I put a .0033uF cap on the tone control so it's more versatile. It turned out really nicely and now I'll build my own but with a 5E3 style cab but only a 10"er inside

 

[screefer]

"Life is what happens when you're busy making other plans"

I finally got a window to get back to this.
I'm all soldered up and just need to make a speaker cable for my GA10 SC64.

I first checked continuity between all jacks and power cord ground.
I then plugged into my light bulb limiter and the pre-tested 60 watt bulb did nothing. The amp lamp came on and no smoke or noise.
I unplugged and re tested the 60 watt bulb and tried again and nothing.
When I built my first amp (5F2a) I used a 150 watt bulb and it was very dim but at least showed life. I decided to go with the lower 60 watts as I was trying to form the caps as Rob suggests. I don't have a 15 watter.

Am I doing something wrong here?

 

[CascoSieg1]

Great progress!
I think of the light bulb limiter as a good tool to mitigate problems that could otherwise be catastrophic or dangerous. Once I've checked power supply voltages are in the ball park, first without tubes (and the voltages will of course be much higher with no load) and then with tubes (where they should be much closer to spec), I switch over to wall supply. Sometimes my when I use the bulb limiter it doesn't glow at all... I don't worry about it.

 

[screefer]

Sometimes my when I use the bulb limiter it doesn't glow at all... I don't worry about it.

Thanks for this!
I had never heard of a LBL not glowing and Google had nothing.
I will proceed..

 

[chas.wahl]

When I built my first amp (5F2a) I used a 150 watt bulb and it was very dim but at least showed life. I decided to go with the lower 60 watts as I was trying to form the caps as Rob suggests. I don't have a 15 watter.

Am I doing something wrong here?

View attachment 989716

Hmm -- the bulb in that photo seems to say 100W. Can't read the one on the table.

 

[screefer]

Hmm -- the bulb in that photo seems to say 100W. Can't read the one on the table.

You're right my mistake.
The one on the table is 150 w.
I put it in this morning and...nothing.
The 150 w. glowed dimly on my 5F2a.
Any idea why??

 

[Snfoilhat]

You could grab a known-good amp and run it through the limiter for a sec just to confirm that everything about it is as it should be. I would guess that the reason you get no glow at this stage is because the amp with no tubes is just pulling the small amount of current the indicator light and bias supply draw.

With a 150W bulb, I can hardly see a glow even with a full tube complement on my smaller amps, the ones that consume something like 40 - 60W total

 

[screefer]

You could grab a known-good amp and run it through the limiter for a sec just to confirm that everything about it is as it should be.

This is with my 5F2a. It took a few seconds to glow dimly.
I left it for longer on the F10 and nothing.
So it must be the lack of tube draw?

 

[screefer]

With the LBL on I get
AC 121.6 v. to the fuse and switch.
6.9 v. to 6AT6, 12AX7, and both 6V6 heaters.
Rectifier tube heaters are 5.45 v.
HT is 301.6. I'm using the 275 v. secondaries

So all seems to be good and I will proceed as per Rob's procedure.
Sorry for the newb angst but I just don't have the whiskers....yet.

 

[screefer]

So I think I may have pooched my PT.
I switched a 60 watt rough service bulb in my limiter which was the lowest wattage I had.
These bulbs draw less and I'm thinking about forming the caps.

I then put the rectifier tube in. The LBL bulb glowed and right away went dim. I measured 365.9 on B+1, 365.7 on B+2, and 364.6 on B+3.
The preamp plates and the OT to the power tubes were about the same.
Powered down,

I then installed the preamp tubes. Powered up and the LBL glowed and right away went dim.
On the 6AT6 I measured 0 on pin1, 1.564 on pin2, and 157.4 on pin 7.
On the 12ax7 I measured 217.6 on pin1, 1.36 on pin3, 217.8 on pin 6, and 49.5!!! on pin8.
All the above are DC.
I then measured the DC bias voltage on the control grids (pin 1 in this case) and got -20.
I turned the 10k trimpot down to the lowest...-25 v.

I then went back to recheck the 49.5 at pin 8 and while probing, the LBL came for a quick glow and then right back to dim.
I must have shorted something?
I powered off.

When I powered back up, I have heater voltages and preamp voltages (the 49 v. on pin 8 should be around 1.5?) but no B+ or HT 275 v. voltages.
With just the 5Y3 and Preamp tubes in the PT doesn't get at all warm

Did I pooch the PT?

 

[dan40]

I seriously doubt it. The LBL will save you from costly mistakes such as shorting a meter probe. Check to see if you have proper AC voltage from each of your red secondary wires connected to the rectifier socket. If those voltages look good, you may have a bad rectifier tube.

 

[dan40]

Sorry...I see that you already checked your secondary voltages. I would be very surprised if you toasted your PT while hooked to a 60 watt bulb. You did have your meter set to AC when checking the secondary voltages didn't you?

 

[screefer]

Sorry...I see that you already checked your secondary voltages. I would be very surprised if you toasted your PT while hooked to a 60 watt bulb. You did have your meter set to AC when checking the secondary voltages didn't you?

Sorry for the late reply. Thanks for your reply.

Yes the meter was set to AC.
I get 121 wall v. on the fuse and switch but that is before the tranny. I get 6.3 v. on all heaters 5v. on rectifier heater.
0 v. on HT secondary AC and DC Filters.

With just the 5Y3 and preamp tubes in, should the tranny be warm?
It is not.

I definitely was hooked to the LBL at all times.

 

[wangdaning]

You should get AC from the secondary on the tube pins, tube removed of course. Be careful with this. If you get nothing try resoldering, if still nothing, seems something died.

I mean on the rectifier socket if that was not clear.

 

[dan40]

With just the 5Y3 and preamp tubes in, should the tranny be warm?
It is not.

Some transformers will run warm while in service and others will not.

 

[screefer]

I decided to start over.

I swapped the 60 w. bulb for the 150 which gave me success on my first build.
(I know, shouldn't make a diff)

I removed the 5Y3 and preamp tubes, plugged into the LBL, and here is what I got.
AC in 121.6
5Y3 heaters 5.4
All other heaters and lamp 6.8.
HT secondaries 299 v.

With 5Y3 in, all Filters, preamp plates, and OT to 6V6 sockets are around 365

With preamp tubes in.
6AT6 pin 1 0
pin 2 1.8 (Attached schematic says 1.3)
pin 7 178.5 (schem says 160)

12AX7 pin 220.2 ( s sez 170)
pin 2 0
pin 3 1.62 (s sez 1.5)
pin 6 246.5 (s sez 207)
pin 7 22.0 (can't find on schem)
pin 8 49.5 (s sez 44.5)

DC bias voltage at pin 1 -29 with 10k trim pot turned down.

 

[screefer]

I installed the 6V6s and plugged my speaker in and switched on with the LBL.
With Volume and Tone down, it made a loud hum so I shut it off.

It was not a squeal but I thought I should reverse the brown and blue OT wires to the power tubes which I did.

Turned it on and no sound at all. Plugged in Guitar and no sound. turned off

Upon inspection, I noticed that the red OT wire had popped out of the B+1 eyelet probably when I was rerouting the brown and blue leads.
The wire from the voltage divider was loose as well in that same eyelet

What happened is I had tacked the B+ Cap and under board lead, leaving room for the two wires, and forgot to finish soldering the added wires because it looked soldered.

Here are the before and after shots

Fired it up and it still hummed so I swapped the brown and blue back to their original pins. It still hummed so I shut it down.

This loud hum is similar to the hum of an unplugged guitar cable. Both the tone and volume pots were at zero.
I will now go through all my ground points and check for continuity. All tubes are new.

I just hope the loose red OT wire didn't cause damage because I did fire it up with that wire unattached.

 

[2L man]

I removed the 5Y3 and preamp tubes, plugged into the LBL, and here is what I got.

HT secondaries 299 v.

With 5Y3 in, all Filters, preamp plates, and OT to 6V6 sockets are around 365

This look but strange? If transformer output is 299-0-299VAC there should be at least 400VDC on first filter capacitor when no current flow. To me 5Y3 look weak!

 

[screefer]

This look but strange? If transformer output is 299-0-299VAC there should be at least 400VDC on first filter capacitor when no current flow. To me 5Y3 look weak!

299 X 1.414 = 422

You are right. It is a new JJ 5Y3S.

I have a new spare 5Y3 and will pull all tubes and start over.

Thank you