Blues Junior Conversion: Power but no sound

[ylelevier77]

Hi there, I'm new to the forum and to amp building as well I've got a tweed champ build under my belt and decided to tackle the Hoffman Blues Jr. conversion kit. After completing the build and doing a continuity check, I powered it up to find no signal/sound coming through the speaker. I rechecked my connections using a grayscale printout of the wiring diagram and a highlighter to check each connnection off. I'm guessing it to be a grounding issue. With the amp on, I tapped the connections with a plastic dowel and heard some scratching sounds and pops when I poke at a couple of the pin connections for V1 and V2. I should also mention that I upgraded both transformers to Allen Amps' TP26 (PT) and T020B (OT). Any help would be greatly appreciated.
Here are my voltage readings:
V1(ECC83S): Pin 1- 168V Pin 6- 167V
V2(ECC83S): A/B - 159V
V3(12AX7): Pin 1- 209V Pin 6- 201V
V4: Pin 7- 351V Pin 9- 315V
V5: Pin 7- 350V Pin 9- 314V

[pompeiisneaks]

Pictures of the amp would help a ton, is it 100% true to Doug's layout and schematic or any mods?  It's usually nice to link to them here but I'm able to look them over if I have the pics, as are any other here, just makes it faster when they want to look it over to click the links in your post instead of hunting them down :)  The voltages look good, in a 'general' way but I don't know the expected voltages on that amp per se.

For anyone else, here's the link: http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

~Phil

[ylelevier77]

Sorry about the delay. I tried to post pics in the original post but had an issue with the file size.

[ylelevier77]

one more.

[pompeiisneaks]

I don't see anything glaringly obvious, what is your bias voltage?  (basically measure where it says C- at either point, or both, make sure the bias voltage isn't too far off (you could be at cutoff).  Also triple check your input wiring and the shielded cable that you didn't accidentally melt the shield through to the ground so you're grounding out the input (I've done this two or three times).  Just test with a continuity tester to see if the input point is grounded with a jack plugged in. 

~Phil

[ylelevier77]

Rad. All great suggestions. I'll check the bias voltage and input continuity just as soon as I can. Funny. I thought it might be something related to the shielded cable. I wired those connections before I viewed Hoffman's technique. I made sure to order more cable just in case ;-) Thanks for the help.

[PRR]

Put head in speaker. NO! sound, or low hum/hiss?

A miswired (open or short) speaker jack is a popular cause for NO! sound.

[ylelevier77]

Definitely low hum/hiss. As I mentioned above in the original post, I can hear scratching or popping through the speaker if I poke a couple of the pins for V1 and V2. I should be able to get bias voltage readings as well as a check on the input wiring today. I'll report back with my findings as quickly as I can. Thanks.

[ylelevier77]

Phil, I just got a chance to do some quick investigation. First the bias voltage is 8.15VDC and second and more importantly, when I checked continuity on the shielded cables they all checked out except for the one on the treble pot. Meaning that when I touched the hot with one probe and the chasis with the other the meter beeped continuity. Looks like you called it. I'll do a quick check by temporarily replacing the shielded wire with a standard one...just as soon as my kid goes down for a nap ;-) Fingers crossed!

[sluckey]

First the bias voltage is 8.15VDC

I hope you mean this is -8.15vdc. Must be negative. To be safe adjust it to be about -12v initially.

If this voltage is positive, better stop everything, pull the output tubes, and make it negative before proceeding.

[ylelevier77]

Sorry. Yes -8.15VDC. Dumb question, but to adjust you're referring to tweaking the 50K trimmer correct?  I only ask because my tube voltage readings are already kinda high and that's with the trimmer maxed. Meaning if I turn it in the opposite direction my tubes are gonna run crazy hot. This may have to do with the upgraded transformers I installed (Allen Amps TP26 &T020B) In any case do you think I should get different trimmer? 100K? Or is there a better solution?

[sluckey]

Your tubes will run cooler if you increase the bias voltage from -8v to -12v. Running cooler means less current flowing through the tubes. But less current through the tubes means the plate voltage will be higher. That's OK. What is the plate voltage on your output tubes? What are the voltages on all pins of the output tubes?

I don't see a TP26 listed on Allen's website. Can you provide some specs for the PT or a link?

Pull the output tubes. Crank the bias pot to one end and measure the voltage. Crank the pot to the other end and measure the voltage. Hopefully you will have a reading less than -12V and another reading greater than -12V. Set the pot to -12V for now. I'm only referencing -12V because Hoffman has -12.3V on his schematic. I'd trust his voltages.

None of this has anything to do with your no sound issue.
 

[ylelevier77]

So here are my voltage readings with the bias at -8.15V:
V1(ECC83S): Pin 1- 168V Pin 6- 167V
V2(ECC83S): A/B - 159V
V3(12AX7): Pin 1- 209V Pin 6- 201V
V4(EL84): Pin 7- 351V Pin 9- 315V
V5(EL84): Pin 7- 350V Pin 9- 314V

I'll try your recommendations just as soon as I get a chance. Thanks for the help. Much appreciated.

[pompeiisneaks]

Yup per sluckey, you've got an 'ok' but probably too hot bias point, which means you'd definitely have output but would end up eating power tubes pretty soon.  Dialing that back will help with tube longevity, but the continuity to ground is a clear indicator your input isn't getting signal to V1.

~Phil

[ylelevier77]

Success!!! It was indeed the a short in the shielded wires. Re-wired using Hoffman's technique and worked like a charm! I also brought my bias voltage down to -12.15V. Thanks for all your guys' help. This forum is such an asset!. I'm sure I'll be back!  ;-)

[ylelevier77]

Guys, one thing I'm noticing is that I'm still hearing my guitar signal with the volume all the way up but the master volume all the way down. Is that from bad grounding somewhere?

[ratgon]

I'm so stoked you posted and got some help! But don't let it stop you from adding to our Facebook messenger amp building thread. :)

[pompeiisneaks]

Guys, one thing I'm noticing is that I'm still hearing my guitar signal with the volume all the way up but the master volume all the way down. Is that from bad grounding somewhere?

It could be, but it could also be that the pot doesn't quite get down to 0 resistance on the path to ground, you'd probably want to check that, if its got some resistance left, that means a little still goes through the other path to the next stage.  Measure the resistance from the wiper to ground when the pot at max volume and at minimum volume to see the max and min resistance on both halves of the dual ganged pot.

~Phil

[PRR]

> still hearing my guitar signal with the volume all the way up but the master volume all the way down

You have to ask: is this a normal way to play?

You have a HIGH(?) gain preamp chain, and you are asking a 99-cent pot to shut that racket to zero.

If I put a garden spigot on the output of my pressure washer, I'd expect it to dribble a bit. (Or burst!)

I might want to shut a pressure washer to zero, but an amp?

There may be an imperfection in parts or wiring. But if it is not limiting your playing, I would not fret.

[ylelevier77]

PRR, its more than a dribble. It's very noticeable. Let me explain more clearly. With the normal volume at max or even at half and the master at zero through to 2, their is a noticeable amount of signal and the tone is that of a cocked wah-wah. Make sense?

[EL34]

Did you do this already?

http://el34world.com/Forum/index.php?topic=17701.0

[ylelevier77]

Yeah. That was the first thing I did before posting onto the forum. The missing "ingredient" was plugging into the input jack when checking the grounds. I'm wondering if for my new issue I need an oscilloscope to do a proper signal trace? I'm thinking it's not properly grounded at he the master volume pot. I'll check my solder joints again just as soon as I have time.

[EL34]

In your pictures, I don't see any wires going to your filter caps?

[ylelevier77]

They're run under the board.