Tweed Bassman smoking resitors. Preamp tubes not light up.

[silat]

Here is a pic of the Layout with the resistors circled. Preamp tubes are not lighting.

[Platefire]

If cirrcled resistor is smoking something must be wired incorrectly. Re-check your wiring with layout. Also if you have a tube rectifirer, pull it to remove DC voltage to stop short and check heater voltage to pins 4/5 and 9. Shold be about 6.4 ACV between heater pins. If these pins are getting correct voltage, the heaters should be lighting up.

[sluckey]

You have 3 unrelated problems. Divide and conquer.

[silat]

here is a copy of my Power Transformer which is International. I hope it is not because of the PT.

[silat]

That last one could not be seen.

[sluckey]

These 3 unrelated symptoms are not likely due to your PT.

Divide and conquer means tackle one problem at a time. Pull the rectifier tube to remove any B+ voltages and fix the filament issue first. That should be a very simple fix. You specifically said preamp tubes are not lighting. That implies that the big tubes are lighting. If so, then you have a wiring error on the filament string, probably between the big tubes and the first little tube, unless you made the same mistake on all little tube sockets.

[robrob]

After you get the filament issue solved power up the amp without the preamp tubes in place. If the resistor still overheats or blows then you have a short to ground. The resistor that's blowing supplies the voltage to power your preamp tubes. Is it possible something is making contact with the chassis underneath the board? How did you raise the circuit board off the chassis?

Use a magnifying glass to closely inspect your tube sockets for tiny solder bridges or single strands of wire that are making contact with an adjacent pin.

[silat]

I know what you all are saying, but I have checked and rechecked and the filament string looks right.I even ran through with a layout and a yellow highlighter, Unless the ADD is kicking in and I am seeing everything backwards. I have ran through them so much I am tired of looking.

[silat]

I forgot to mention I am not getting the right voltages  for the preamp tube and I am getting some; about 3 to 3.3.

[John]

These 3 unrelated symptoms are not likely due to your PT.

Divide and conquer means tackle one problem at a time. Pull the rectifier tube to remove any B+ voltages and fix the filament issue first. That should be a very simple fix. You specifically said preamp tubes are not lighting. That implies that the big tubes are lighting. If so, then you have a wiring error on the filament string, probably between the big tubes and the first little tube, unless you made the same mistake on all little tube sockets.

Do what Sluckey said. Pull that rect. tube. Fire up amp. Measure filament voltages. Remember to measure ACROSS the pins, not from each pin to ground. If you have 6.3 or so on the power tubes, but zero or only a volt or 2 on the preamp tubes, you have a wire not connected, or not soldered right. Or your artificial center tap, if you're using one, isn't connected right.

but I have checked and rechecked .....  I have ran through them so much I am tired of looking.

And that's why you're missing something obvious. I've missed all kinds of stuff, even after rechecking 10 times. It wasn't till I left it and came back the next day I saw it, and wondered how I ever missed it.

[silat]

I did what sluckey said from the beginning. I have got 6.75  on all of the filaments  on the preamps, but no tubes lit up on the same??? That is weird!

[Willabe]

Are you sure you didn't wire up the preamp tubes backwards?

Looking at the back of the tube socket inside the chassis, pin 1 is to the left of the double space between pins and then goes clock wise.

If they are correct, try turning the amp on in a totally dark room. Some preamp tubes you can hardly see the filaments light up.

It has happened at least once before where a guy had a preamp tube that would not light up and it turned out that the socket was wired up correctly but 1 of the heater pins was broken inside of the socket where he couldn't see it.

He found it by turning the chassis over so the sockets faced up and with the preamp tubes out he turned on the amp and put his meter probes into the socket where the tubes pins would go, he found no acv.         

[Platefire]

That voltage reading looks OK. Try turning out the lights with it on in the darkness, I have had tubes that was hard to see glow in the day time with the lights on. Also if you have any extra pre tubes, try a different tube.

[sluckey]

Do you have a jumper between pin 4 and 5 on each of the preamp sockets? I'd like to see a hi rez pic that clearly shows your preamp tube sockets.

[silat]

Sluckey, I do have a jumper between pins 4&5 on the preamp tubes. I will get a high-res up in a minute.

[silat]

Here is a high rez.

[sluckey]

YOUR SOCKETS ARE WIRED BACKWARDS!

See the wide gap between two pins? That's the key. The first pin CLOCKWISE of the gap is pin 1 (NOT PIN 9!). Count pins in a clockwise direction until you get back to the gap.

[silat]

ROTFL! Thank you! It was my ADD. You knew it all along Sluckey.

[sluckey]

This error probably explains the seemingly unrelated burned resistors too.

[Willabe]

YOUR SOCKETS ARE WIRED BACKWARDS!

In reply #11.

Your not the 1st and won't be the last to do that.

When you get the sockets rewired check the heater acv again, I bet it will be closer to 6.3 because 0.9A's just got added to the heater load.

[DummyLoad]

This error probably explains the seemingly unrelated burned resistors too.

counter top
clock bottom


all you need to remember is counter top.


i designed a PCB for an amp where the silk-screen is on the bottom...on the drawing board it was on the top. yeah, i meant to do that. <whistling at the sky>


--pete

[silat]

So I kept compounding my error, thinking it was right, when it was the wrong the whole time. I just could not see that it was completely backwards. Ok thank you all! Hopefully after the change everything will work correctly.

[silat]

I got the filaments back right all of them are not lit up no more smoking resistor with the rectifier back in. I now have no sound. 

[silat]

I meant to say the preamp tubes are all lit up. and there is no more smoking of the resistors.

[eleventeen]

There is no tube amp builder in recorded history who has not made that mistake.


There is no tube amp builder in recorded history who wonders whether he/she did that the next thing they build if there are any "startup" problems. You double-triple-quadruple check this. Never underestimate the human brain's capacity to get this wrong.



What many like to do, is to wire up the fils first, then fire them up. This proves our "count" and prevents the true catastrophe, which is to wire up THE WHOLE AMP that way.

[silat]

yeah if i only could find out why I have no sound now.

[Willabe]

You'll find it.   

[sluckey]

yeah if i only could find out why I have no sound now.

I see five resistors that are wrong values! Two of them are sound killers. I've circled these resistors and annotated what value they should be. The values of the two that I labeled 'KILLER' are 1000X bigger than they should be. Either one would suck almost all the sound out of your amp, and when combined will totally kill all sound.

That's a high percentage or errors. You should practice reading color code values correctly. And learn how to use your ohm meter properly to verify correct values. If you can't do this you are doomed.

[lego4040]

Bassman's are a Badass Amp. Sluckey, Willibe ,Dummyload , etc advice are golden and helped me on every build.

[silat]

Thanks Sluckey! I can read  my Ohm meter even though it does not look like it at this point.

[silat]

I know the one should have been a 10K But I burnt them up trying to find the problem. I had none left.

[sluckey]

Using an 8.2K in place of a 10K on the B+ rail is not a show stopper. But using a 470K instead of a 470Ω or an 820K instead of an 820Ω on a tube's cathode is definitely a show stopper. Providing there are no other errors in this amp, using the correct value for the two resistors I labeled as "KILLER" should get you some sound.

I still recommend replacing all five of those circled resistors with the correct value. And I recommend verifying the correct value for every resistor visually and with your ohm meter. Most (not all) resistors in that amp can be accurately measured in circuit. Some will measure a lower value if there is another resistive path parallel to the resistor. A resistor will never measure higher than it's actual value if there are no stray voltages present while measuring.

I'm not trying to sound condescending toward you. I'm trying to point you in the direction of making this amp work. I usually speak directly to the point with no sugar coating. Everything I've said needed to be said. I hope you receive it in the way I meant it.