5F1 No Sound Problem-It just quit!

[wsscott]

Sluckey, Shooter and anyone else that's interested, I need some help again.  I built this Ted Weber 5F1 kit about 1 1/2 years ago, and its worked fine since the beginning.  It was my first amp kit build, and was excited it worked. So I decided yesterday to shorten the speaker wire connecting the amp’s RCA output going to the 4 ohm 8” speaker, just to neaten things up. When I built it I just left the wire full length since I was anxious to hear if it worked and never went back to shorten it. So I un-soldered the wires at the speaker connection, cut them to length, and soldered them back on. Then when I put everything back together and plugged in the guitar, no sound! I’ve run a test tone directly through the speaker and I get sound, so I must have done something to the wiring of the circuit!

I've checked the readings on all the resistors in the circuit and they are fine.  I used a DMM set to ohms to check the wiring from the input jack to Pin 2, Grid, on the 12AX7 and it showed 0.00, so that connection is good.  So signal is getting to the preamp.

I put a DMM probe to test continuity on the RCA plug connecting to the speaker.  It seems odd, but if I touch the shield and the lead on the plug it shows continuity.  Is this right?  Is it because the wire connects up in the speaker?  There's some static/crackle when I move the plug around in the RCA jack.  I did check the wiring in the RCA plug housing and its good and the leads don't appear to be touching to short out.

I did the same thing with the RCA jack, and got the same type of reading.  It shows continuity.  Is that because the Output Transformer's wiring is connected to the jack, and so the jack's input and ground lugs are "connected"?

The tubes look fine, ie. they light up.  Thoughts?  Thanks.

[sluckey]

Did you test the amp just prior to fixing it?

[wsscott]

Well, I had played it a couple days before and it was fine.  It was only moved to my work table. 

I can generate a 1K test tone with my iPhone, and sound will come out the speaker of the amp when I turn it on, but it doesn't change volume when I turn the volume control.  Weird.  If I plug the guitar in I don't get any sound.  And I've plugged the guitar into my other amp using the same cable and its fine.  Could it be something with the volume control.  When I built this amp I modified it so that I have a separate on/off switch, and then just a volume pot.

I did the chopstick test on the tubes and no issues.  In chopsticking around the circuit the filter caps have some crackle but all the solder joints on the whole circuit look good.

[sluckey]

It's easy to mess up shielded cable on a phono plug. Replace that shielded wire with regular wire.

[wsscott]

Its not shielded wire, just regular wire.  Just 2 separate non-solid core wires.

[wsscott]

Ok, so I pulled out my little scope to see what's up.  First I moved the probe to the center terminal of the Volume Pot which connects to Pin 7, and there's no signal.  So I then connected the probe onto Pin 7 and no signal.  I also connect the probe to Pin 6 and no signal.  So no signal is coming out of Pin 7 or Pin 6.  So its fine into Pin 2 and out of Pin 1.  Does that indicate a tube problem?

[dude]

Cold solder joint on the phono plug, take wire off, scrape the shield clean, solder.
Don’t leave the amp on without the speaker working, no load on OT “will blow it”
Usually it’s the last thing you did is where the issue lies.
What are the voltages of the 12ax7

[wsscott]

Well, I tried replacing the 12AX7 with 2 other tubes and nothing changed.

As to Dude's comment on the cold solder joint on the RCA plug, it seems the problem is way before the Output since the signal isn't being seen at either Pin 7 or Pin 6 and also not at the Volume Pot.

[dude]

Check the input Jack, maybe a wire came lose or off. Do you have voltages on 12ax7, pin 6 and 1?

[wsscott]

I'm showing about 208 VDC on each Pin 1 and 6.

[tubeswell]

If it worked fine before you changed it, then it was something you did when you changed it that caused the problem.

[wsscott]

The only change I made was to un-solder the 2 wires coming from the RCA output jack, cut them shorter, and then re-soldered to the terminals on the speaker.  I actually re-soldered the shorten wires twice, thinking maybe it was a bad solder joint.  If somehow I reversed the wires on the speaker terminals, would that make a difference?

I checked the signal with my scope where it comes out of Pin 1 and goes into the 0.022uf cap, and read a clear sine wave.  But there is no signal coming out of that cap going to the input on the Volume Pot.  And there's no signal coming out of the Center Terminal on the Volume pot going into Pin 7.  Could it be a bad cap, or is that normal for no AC signal to be shown coming out of a cap?

[sluckey]

Either the cap is bad or there is a short to ground on the wire going to the VOL pot. Check resistance between that wire and chassis.

[dude]

Someone else might chime in on the cap issue, if you have another replace it.
The last thing you did is usually the culprit if the amp played before the change. Just unsoldering that RCA Jack an back, maybe the ground side is soldered and holds tight but is cold inside. Don’t just reheat it, take the hack off again, clean a good spot on the ground side, sand it shinny same with center. Before you solder it disconnect on end to the speaker, check for continuity in both sides of wire, maybe a bad wire. Or just replace the wire. You may have an issue on the speaker end, maybe one of the connectors has a weak connection from being move around when you shorten the wire.

[wsscott]

Sluckey-I think you may have it.  I checked continuity/ohms at the input on the Vol pot and chassis GND and there was continuity! So to be sure, I checked it through the wire connecting the input of the Vol Pot and the output on the 0.022uf cap, and there is continuity, ie. closed circuit 0.000 ohms.  But if I check it from the wire on the input of the Vol Pot to the input on the cap, there's no continuity.

So is this a sign of a bad cap?  I can't imagine how that 1 short wire from the Vol Pot to the 0.022 cap can be grounding out. Its just a jumper wire with nothing in between the 2 ends.   

[sluckey]

Not a bad cap. There is a short. It's very simple. You should be able to see it with your eyes. If not, just remove the wire. Now check resistance from top of VOL pot to chassis. Still grounded? Also check resistance from end of that cap to ground. Still shorted?

Is this wire a shielded cable? If so, replace with a single conductor wire.

[wsscott]

Well the short is at Terminal 1, the input, on the Vol. Pot.  I un-soldered the wire from the cap the terminal, and connected probes at that terminal and at the chassis and it showed closed circuit, ie. 0.000 ohms and also buzzed for continuity.  I checked the wire itself from the pot end to the cap and it showed a closed circuit.  No short when testing the Center terminal on the pot.

So it looks like its the pot.  Does that make sense?  I've never heard of a pot going bad this quick.  I didn't use the Audio 1 M Vol/on/off pot that came with the kit, and bought a major brand pot instead.  I have an Audio Log 1 M in my stash.  Should that be okay?  I assume that's what's in it now, unless its a Linear one.

[shooter]

use about any pot for testing, and ya, modern pots are real easy to damage from over-heat, or slag dripping into the pot

[wsscott]

Fixed!!  I put in the new pot this morning, and it sounds great!  Thanks to everyone for your help in diagnosing the problem.  Wahooooo!

[wsscott]

I decided while I had things opened up to check the bias on the 6V6GT.  And it looks like its running pretty hot if I measured things properly.  Below is the information:

Voltage Drop at Cathode Pin 8--23.83 DCV
Cathode Resistor value--470 ohms
Actual Plate Current between Cathode and Plate--359.8 DCV

So bias is 23.83/470= 0.0507 Amps or 50ma

Shouldn't it be around 30ma?
If so, I was thinking of changing the 470 ohm R for a 1K, and maybe replacing the 25uf cap with 100uf to see how it affects the tone.  Thoughts?

Would this have had any effect on the Volume Pot going bad?   Thanks.

[sluckey]

Max dissipation is 14 watts. You're dissipating 18.24 watts. Too hot! Yes, increase cathode resistance and recalculate. 14 watts is your target. This is not related to the pot failure.

[wsscott]

Thanks.

[wsscott]

I'm waiting for the resistor to come in.  Lost somewhere in the USPS.  I was wondering though, just as a test of sorts, if I could run the amp through my Variac and set the voltage to 115V vs. the current wall supply of 123, and see how that impacted the bias.  I was thinking of doing this just to see if it was closer to the spec. bias/tube dissipation as designed when the voltage was 115.  Thoughts?

[sluckey]

You may proceed.

[wsscott]

Well the initial order of components has yet to arrive via USPS and was last tracked in MD on January 18, 2021.  Sort of like that old Kingston Trio song, "The Man Who'll Never Return", Charlie lost on the Boston MTA!  Maybe I'm dating myself.  So I placed another order, this time for delivery by FedEx and it arrived in 3 days!  So I finally had a selection of resistors to swap out for the 470 cathode bias resistor that was making the amp run too hot.  I used a 680 ohm, and also replaced the cathode bias cap with a 50uf vs. the 25uf.  The bias turned out great first try.  Its reading tube dissipation of 13.7 watts at 97.9%.  The Plate Current dropped to 37.85ma at 120VAC vs. 50.7ma!  So I'm very pleased.  Thanks for everyone's help. Stephen

[shooter]

and was last tracked

I'm still waiting on a G-tuner I ordered last May, I do get a monthly "still on back-order" we've made it back to the 18th century with flush toilets 

[wsscott]

The parts came today that were shipped on Jan. 14!