First build 5F2-A: low output

[JDodd]

I just finished my first amp build: a Princeton 5F2A from a Weber parts kit. No smoke, but the output is ridiculously low. It's barely detectable until you hit 9 o'clock, and even at full volume it isn't loud enough for a bedroom amp. I've gone through and tested for bad solder joints, shorts to ground, bad resistors, and the usual things, but now I'm going to have to approach it more logically. Any suggestions for troubleshooting this symptom?

[loudthud]

First thing to look at is the Voltages at each tube socket terminal. This tells us things are hooked up and operating as they should. Errant Voltages can also point to wrong value resistors. Also measure the Voltage at the plus side of the large filter capacitors and post the readings.

Another common problem is a short at the speaker jack. Because the output transformer winding is a low resistance, testing with a meter is tricky. The easiest thing to do is connect a meter across the speaker's terminals with the amp connected, but turned off. You should measure a low resistance, less than 1 Ohm. Now unsolder one of the output transformer wires at the speaker jack. The resistance reading at the speaker should go up to about 75% of the rated speaker impedance. If it doesn't, you have a short at the speaker jack.

[Enzo]

You built it, you have been looking at it for some time, it looks familiar to you. You might test all the parts, but you could have a part misplaced. When i work on amps, I usually have the controls facing me. Some guys work on them with the controls facing away. WHichever way you do it, turn the amp around 180 degrees. Turn your layout drawing around also. So they face the same way. Now your familiar amp is no longer so familiar. Now go through the layout drawing part by part, and verify each is indeed the value it is supposed to be. Very easy to install a 470 ohm resistor where a 470k resistor belongs. Also it is easy to have a cap and a resistor next to each other and you installed them in each other's places. They look like they belong, but are actually in the wrong places. I find by turning the amp around, I am less likely to skip something due to familiarity.

Avoid the temptation to tell yourself, "Well I KNOW these three parts are good, I just tested them yesterday."

NEVER think up reasons not to check something.


Then in terms of circuits, I agree with LT^^^, go down the row of sockets and check voltages.

[loudthud]

Another common first timer mistake is connecting a wire on the backside of the eyelet board to the wrong eyelet. You can check them easily with your Ohm-meter from the front side, just press the probes on the two eyelets that are supposed to be connected, the resistance should read less than one Ohm.

[Justin Thomas]

Check each and every resistor to make sure the measured value is the same as the marked value... depending on what kind you used, colors are easy to confuse, and manufacturers DO make mistakes...

Justin

[JDodd]

Thanks for all the suggestions. I'll work through them all and update the post with what happens.

Much appreciated!

[mikepukmel]

Check each and every resistor to make sure the measured value is the same as the marked value... depending on what kind you used, colors are easy to confuse, and manufacturers DO make mistakes...

Justin

Oh man, and those awful blue background resistors, don't know about youall, but I can NOT read the band colors on them, even under a magnifying ring light. I would have never gotten anything right without measuring 3x before soldering.

[mikepukmel]

Thanks for all the suggestions. I'll work through them all and update the post with what happens.

Much appreciated!

Photos of your amps guts if you can, would help.

[JDodd]

Everything seems fine across the speaker jack, as you described. Negligible resistance normally, then up to ~6 ohms with one of the transformer wires removed.

I did add a pair of 100 ohm resistors to ground across the 6.3 VAC tap on the power transformer along with the filaments and the pilot light. This wasn't on one of my schematics, but was on another one and seems to be something they assumed I knew. It didn't seem to make a difference, though.

Pin voltages are as follows:
12AX7B
1) 128.5V
2) < 1mV
3) 1.2 V
6) 128.5 V
7) < 1mV
8) 1.18 V

6V6GT
3) 294 V
4) 241 V
5) 1.25 V
8) 14.75 V

All filaments are measured at 6.8 VAC.

I have gone through and checked values for resistors by color code (I have trouble with orange and red), and metered in place (when possible) and disconnected (when not). They all seem correct, but I am continuing to double and triple check.

Just as a sanity check, I played a 5W Fender Champ in a local store over the week and it is louder on 2 as mine is on 12 (max).

Again, I appreciate everyone's input on this.

[eschertron]

Everything seems fine across the speaker jack, as you described. Negligible resistance normally, then up to ~6 ohms with one of the transformer wires removed.

I did add a pair of 100 ohm resistors to ground across the 6.3 VAC tap on the power transformer along with the filaments and the pilot light. This wasn't on one of my schematics, but was on another one and seems to be something they assumed I knew. It didn't seem to make a difference, though.

Pin voltages are as follows:
12AX7B
1) 128.5V
2) < 1mV
3) 1.2 V
6) 128.5 V
7) < 1mV
8) 1.18 V

6V6GT
3) 294 V
4) 241 V
5) 1.25 V
8) 14.75 V

All filaments are measured at 6.8 VAC.

I have gone through and checked values for resistors by color code (I have trouble with orange and red), and metered in place (when possible) and disconnected (when not). They all seem correct, but I am continuing to double and triple check.

Just as a sanity check, I played a 5W Fender Champ in a local store over the week and it is louder on 2 as mine is on 12 (max).

Again, I appreciate everyone's input on this.

All the HT voltages seem low to me. Anyone else?

[mikepukmel]

Escherton, yeah, dug around and found an old post from ... you guessed it

http://music-electronics-forum.com/a...n-voltages.gif

Depends on the transformer, of course, but looks like the plate should be closer to 350 on the 6V6.

JDodd: maybe this will help.

What transformer do you have, make, model, etc? Do you have a data sheet for it? If you yank out all of the tubes and read the voltages unloaded what do you get?

Any chance you can post a few photos of the chassis guts? The people here are really sharp at finding stuff.

Another question: its a Weber kit, on his schematic it says you can either use webers solid state WY3GT or the standard 5Y3GT tube,

https://www.tedweber.com/media/kits/5f2a_schem.jpg

(Sorry EVERYONE for the dozen edits, Im reading through the Weber web site, some cool stuff there)
Weber's 5F2A BOM says your transformer is this one:
https://www.tedweber.com/media/import/w022772sch.JPG

It says that there are two sets of secondary wires, Red-White/Red-White will give you 600Vac, Red/Red will give you 660Vac, huge difference. Also, this transformer has a center tap on the 6.3V secondary so you don't need 2x 100ohm resistors. You might want to double check that wiring.

surprised that the Weber schematic doesn't have voltages on it. <frown>

[mac dillard]

Plate voltage on 6V6 and the 12ax7 are low. Measure the voltages at points A, B, and C. If the transformer is wired for 300-0-300 and you are using a new production 5Y3 you should be getting about 390 VDC at point A. About 340 VDC if using NOS 5Y3.

[Tom Phillips]

All the HT voltages seem low to me. Anyone else?

Yes. They do seem a little low. However, they are still within ~20% of the original Princeton voltages and they are not low enough to be the primary cause of the very low volume problem that JDodd described. Bit...the fact that the voltages are lower than we would normally expect may be a secondary result of the primary issue that is causing the low volume.

It would be a good idea to test the amp with another speaker.

[JDodd]

Here is the Power TX


I measured across the pairs with no load and get Red 595 VAC, Green 6.88 VAC, and Yellow 5.8 VAC.

and the Output TX (in case it matters))


I measured the voltages at A, B, and C as follows
A - 368.7 V
B - 235.8 V
C - 201.3 V

A seems fine, but B and C look significantly low. I guess that will be a good place to start looking.


And I am using a 5Y3GT tube, (not the WY3GT). I have a mix of parts from Weber and from Hoffman as I purchased things in three different orders (in case my patience wore thin ;-) ), so sorry if this causes any confusion.

I will address the other suggestions as I can. Thanks again for the interest.

[DrGonz78]

Please show pics of your build so we can look over the wiring and connections points. Show some close up ones with detail then some that show the whole thing for good reference.