No Sound From Amp - Hoffman Blues Jr Conversion

[Tbone55]

I had already posted earlier about this amp and how really loud it was. Now I'm not getting any sound at all. I checked the speaker by plugging it into another amp and it works fine. The power tube filaments glow and the pre-amp tubes are lit although not as much as the power tubes.

So far I've checked the voltages at each of the filter capacitors at the following points with these results.

Current Readings       Original Readings
B+ 364v                          364v
Z   12.76v                       357v
Y   11.85v                       320v
X   11.08v                       287v

Power Tube Output  @Pin 7
Current      Original     Tube
364v           364v          V5
366V           364V         V4

Power Tube Output @Pin 9
Current       Original     Tube
10.03v        242v          V4
9.71v           242v          V5

B+ voltage going to the first resistor is 366v but when I measure it after the resistor it's only 9.36v. This explains why there'ssuch a low voltage on Pin 9 at the power output tubes. Is this resitor the problem because there's not enough plate voltage to induce electron flow? This the first amp I built and the first time I've had to try and troubleshoot a problem. Any help or advice would be greatly appreciated. Thanks.

[sluckey]

B+ voltage going to the first resistor is 366v but when I measure it after the resistor it's only 9.36v.

Resolder the 2.2K/3W resistor.

[Tbone55]

B+ voltage going to the first resistor is 366v but when I measure it after the resistor it's only 9.36v.

Resolder the 2.2K/3W resistor.

Thanks Slucky, will do that. 🤗

[shooter]

fwiw,
makes things easier to follow by keeping everything in the same thread.  the symptoms might change, the "fix" will solve the original amps problems.

[Tbone55]

Thank You for that info. I was going to post on my original post but wasn't sure. I'll definitrly remember that for next time. Cheers!

[Tbone55]

Hey Slucky, resoldered the connections but no change. I'll have get a new resistor and see if that does the trick.

Thanks for your help. Cheers!

[sluckey]

Measure the resistance of that 2.2K. Check the resistance of each side of the 2.2K to chassis.

[Tbone55]

Hi Slucky,

It's measuring 318k across the resistor and infinity on either side to chassis ground.

[EL34]

Did you do this?
https://el34world.com/Forum/index.php?topic=17701.0

[Tbone55]

I haven't gone through that whole process because I localized where the problem was coming from. Since I wasn't getting any sound I figured the problem was likely in the power output section of the circuit. You've provided a great methodology for tracing a problem with your animated video which is extremely useful. I can certainly go back through the entirecircuit if that's what's needed. What's the best way to discharge the capacitors other than waitn for them to discharge on their own?

Thanks for your help.

[sluckey]

It's measuring 318k across the resistor and infinity on either side to chassis ground.

If you don't have a 2.2K/2W handy you could use anything from about 1K up to 4.7K temporarily. But the final repair should be to use a 2.2K/2W or 2.2K/3W.

[mresistor]

 What's the best way to discharge the capacitors other than waitn for them to discharge on their own?

Heres a simple tool you can make to discharge filter caps safely. 

[Tbone55]

It's measuring 318k across the resistor and infinity on either side to chassis ground.

If you don't have a 2.2K/2W handy you could use anything from about 1K up to 4.7K temporarily. But the final repair should be to use a 2.2K/2W or 2.2K/3W.

Thanks Sluckey,

I've just finished going through the steps to check the wiring. Everything checked out good. I took a snapshot of my board as well as the layout drawing I used to do it. I'm attaching it for reference in case you see anything on it seems out of whack. The only incorrect resistor reading I got was for the one on the volume pot. It's supposed to 120k and I'm only getting 67k. I don't have a meter to check the capacitors.

I'm not able to attach any images. It's telling me they're too large. How do I make them smaller?

Thanks again for your help.

[sluckey]

The only incorrect resistor reading I got was for the one on the volume pot. It's supposed to 120k and I'm only getting 67k. I don't have a meter to check the capacitors.

No worry. It's in parallel with the volume pot and the measured value will depend on the setting of the volume pot.

I'm not able to attach any images. It's telling me they're too large. How do I make them smaller?

Hoffman sets a generous maximum individual file size 2048KB. Use an image editing program to reduce the file size. Or change the resolution settings on your camera to produce a smaller file size.

[mresistor]

TBone   save the pics somewhere on disc    then you can upload them using Attachments and other Options at the bottom the reply window. 


Click Options then chose Attach File     and Chose file from the disc, hard drive, cloud, or where ever else you saved the pics to


then post     


the pics will post a thumnail in the reply for people to see  (or smaller image)  which users then can click to enlarge. 


Note: I do this on a Win 10 or 11 PC and it's easy ..  if you are using a smartphone then I hope you can figure it out..  smartphones are a whole different world

[Tbone55]

Thanks Sluckey, I didn't realize the value of the resistor would change based on the volume setting. I'll try something else with images to load them.

Cheers!

[Tbone55]

Thanks mresistor.

I took the pictures using my android tablet. I'll see if I can download them onto my computer that has Windows 10 and see if that works.

Cheers!

[Tbone55]

I imported the images into my PC but can't do anything with them. I've tried to change the settings on my tablet for the camera but there aren't any to reduce the image resolution. Any other thoughts?

[AlNewman]

There's apps for your phone.

I use Croc photo on my android.  It's free, but ad supported.  Pretty intuitive, you can adjust by pixel size or percentage.

I think you can also resize in ms paint on your computer.

[tubeswell]

voltages at each of the filter capacitors at the following points.

Current Readings       Original Readings
B+ 364v                          364v
Z   12.76v                       357v
Y   11.85v                       320v
X   11.08v                       287v

Power Tube Output  @Pin 7
Current      Original     Tube
364v           364v          V5
366V           364V         V4

Power Tube Output @Pin 9
Current       Original     Tube
10.03v        242v          V4
9.71v           242v          V5

There’s some sort of unintended current path to ground between the reservoir cap and the smoothing (screen supply node)


Check behind the board.


Or one of the EL84s ist kaput.

[sluckey]

There’s some sort of unintended current path to ground between the reservoir cap and the smoothing (screen supply node)

Did you see these two posts?

Measure the resistance of that 2.2K. Check the resistance of each side of the 2.2K to chassis.

It's measuring 318k across the resistor and infinity on either side to chassis ground.

[Tbone55]

There's apps for your phone.

I use Croc photo on my android.  It's free, but ad supported.  Pretty intuitive, you can adjust by pixel size or percentage.

I think you can also resize in ms paint on your computer.

I actualy imported the images into Microsoft 3D Paint because I thought  I could resize it but didnt seem to work. Does it have to ms paint?

[AlNewman]

What were the circumstances in which you went from a working amp (albeit too loud), to frying a 2 watt resistor?  Did you do some soldering, were you testing something at the time?  There seems to be some information missing.

You could replace the resistor, and fry it again (or worse), if you don't find the root of the problem.  (You're working without a safety net if you don't have a current limiter.)

[AlNewman]

There's apps for your phone.

I use Croc photo on my android.  It's free, but ad supported.  Pretty intuitive, you can adjust by pixel size or percentage.

I think you can also resize in ms paint on your computer.

To tell the truth, I think I remember doing it once on my computer, and it was a pain in the ass.  The phone app I have has always worked fine for me.

I actualy imported the images into Microsoft 3D Paint because I thought  I could resize it but didnt seem to work. Does it have to ms paint?

[shooter]

 
the price of "convenience"


I take pics with camera's
I process data with computers
I talk n txt with the flip-phone
I listen to music on analog amps


i can re-size any viewable image with win10 paint

[Tbone55]

What were the circumstances in which you went from a working amp (albeit too loud), to frying a 2 watt resistor?  Did you do some soldering, were you testing something at the time?  There seems to be some information missing.

You could replace the resistor, and fry it again (or worse), if you don't find the root of the problem.  (You're working without a safety net if you don't have a current limiter.)

I had removed the chassis to try out a different speaker. I installed the other speaker , re-installed the chassis and the amp worked. I didn't like the speaker I had put in so I removed the chassis, replaced the speaker and put the original speaker back in. When I went try it no sound came out of the speaker. The amp was dead silent as if it wasn't even turned on. The only thing I thought was that may during the removal an installation the the amp chassis I touched a component and it did something to it.

The only thing I found wrong was the power resistor connected to the first filter capacitor where you have the B+ voltage from the rectifier was reading 314k when it should be 2.2k. The B+ voltage read 355v on the side where the resistor was connected to the filter capacitor and rectifier but on the other side of the resistor it read around 17.8v. Because of this all the voltsge readings at the other three filter capacitors read around the same value. The voltages at pin 9 of the power tubes read the same as well.

With the grid voltage being so low (pin 9 output tubes) than what it should be wouldn't this be the cause for no sound? The tubes aren't producing the required output signal? My knowledge is limited so my understanding of how this is supposed to work may be way off.

I appreciate your help.

[shooter]

had removed the chassis

how many connectors did you have to undue n redo?


if you've got the pcb amp with PCB connectors, look for cold solder at ever pin on every connector attached to the pcb
any loose pins, verify the foil isn't cracked at the solder pad.

[sluckey]

REPLACE THE RESISTOR! WHATCHA WAITING FOR?

[tubeswell]

^what he said^


And pull the EL84s out while you're at it before you switch it back on - in case its something to do with bad screens

[Tbone55]

REPLACE THE RESISTOR! WHATCHA WAITING FOR?

Going to my electronics store today to buy the resistor. Stores were closed yesterday due to our long weekend. Will update once I change it out. 😁

[sluckey]

Does that resistor show any signs of overheating?

[Tbone55]

Does that resistor show any signs of overheating?

No but I haven't seen the underside of it yet. I picked up some resistors but they didn't have the 2.2k 3w one. I bought a couple of 2.2k 5w wirewound resistors, the same as what's in the amp now and 2k 5w ones as well. I remember having to replace the original 2.2k 3w resistor that came with the kit back when I built it because the tubes were really glowing red. I forget the term used when tubes do this.

Should I source a 2.2k 3w resistor instead?

[sluckey]

no

[Tbone55]

no

Ok then. I'll use the 2.2k 5w one. Should I pull the power tubes out Before I restart the amp?

[sluckey]

ARRRGH!!!      

[Tbone55]

2.2k 5w residtor installed. Checked it's value, right on 2.2k.

Am now powering the amp in real time.

YES! WE HAVE LIFT OFF!

The amp seems to be a bit noisy but it's working. Will now go through and measure all of the voltages.

Thanks!

[Tbone55]

Voltage Measurements

                    V4                       V5
Pin 9          319v                   319v
Pin 7          332v                   335v
Pin 2         -14.8v                 -14.8v

Where can I find the information for biasing the EL 84's? I thought zi had it in my documentation but can't seem to find it.

Thanks.

[Tbone55]

From what I remember the bias plate voltage should be a lot more negative than this. I just turned my bias pot to its full negative position and have -22.8v reading from pin 2 and the amp has really quieted down.

Am I in the ballpark of where I should be?

[sluckey]

Adjust the bias pot until you have 25mV on pin 3 of the EL84s.

[Tbone55]

Okay. Bias voltage adjusted.

Pin 3 V4 -25mv
Pin 3 V5 -22.8mv

The amp is working just like it was before. It's really loud. Can't set it   past 3. I used a Hammond 1760H transformer rated at 20watts for the output but I didn't think it would be that much louder. Should I base the biasing on it having 20w output instead? If I remember correctly you want the max plate dissipation to be around 70% of the output for biasing.

[sluckey]

The bias should be set for 70% max dissipation of the tube. EL84s are a 12W tube so 70% would be 8.4W. According to your numbers, V4 is running at 69% and V5 is running at 64%. I'd button it up. Live with the loudness.

[Tbone55]

The bias should be set for 70% max dissipation of the tube. EL84s are a 12W tube so 70% would be 8.4W. According to your numbers, V4 is running at 69% and V5 is running at 64%. I'd button it up. Live with the loudness.

Thanks for your input. It's a little monster. I don't know why it's so loud but it makes for a great clean platform for pedals. 😁