Just starting my Blues Jr. III project

[Tbone55]

Any recommendations for pre-amp and power tube replacements for this amp?

[sluckey]

Power tube is bad. I doubt the 12AX7s are bad. Unlike the EL84s, some 12AX7s are very hard to see the filaments glowing. Should be able to see them in a dark room.

You need to connect a speaker when you have this amp turned on!!!

Please confirm... Have you disconnected the feedback wire from the speaker jack?

[Tbone55]

Power tube is bad. I doubt the 12AX7s are bad. Unlike the EL84s, some 12AX7s are very hard to see the filaments glowing. Should be able to see them in a dark room.

You need to connect a speaker when you have this amp turned on!!!

Please confirm... Have you disconnected the feedback wire from the speaker jack?

Yes, I have disconnected the feedback wire from the speaker jack. The V1 pre-amp is an Electro-Harmonix. V2 & V3 are JJ's. The power tubes are also JJ's.V2 & V3 are the ones where it's difficult to see the filaments glowing. Oops! I didn't have a speaker hooked up! Did I damage the OT?

[sluckey]

Did I damage the OT?

It's possible, especially while the amp was squealing.

[Tbone55]

Did I damage the OT?

It's possible, especially while the amp was squealing.

What's the best way of checking? Measure the resistance between the output wires going to the jack?

[sluckey]

Best way to check is play your guitar through the amp.

[Tbone55]

Doh! Why didn't I think of that. 

Will it make a difference with the one power tube not working properly? I'll be getting some new tubes sometime today.

[sluckey]

It will make a difference. Wait until you have two EL84s plugged in.

[Tbone55]

Thanks. Just on my way to pick up some replacement resistors. I'll probably have the tubes by this afternoon and can then try out the amp.

[Tbone55]

I've been to several electronics supply stores and none of them carry the 2.2K/3W 1% +- resistor I need. I picked up a wire wound 2.2K/5W 5%+- resistor and a 2.2k/2W 5%+- resistor. Will either of these do for now or should I just leave the original one I put back in that is reading 4.8k for now until I can get the right one? I looked at the BOM but there's no tolerance shown for this resistor or whether it's a metal film resistor, It just says 2.2k, 3W resistor.

Thanks.

[sluckey]

Use the 5 watter and don't look back.

[Tbone55]

Use the 5 watter and don't look back.

OK. Will do. Don't have my tubes yet. Am going to do a bit of house keeping on my work area.

Thanks.

[Tbone55]

Alrighty then, received my new EL84's and installed them. Changed around my input and speaker output jack so they're now in the right place. Also changed the 2.2k/3W resistor in the power section with a ceramic 2.2k/5W. Couldn't find a replacement 2.2k/3W. Made up a speaker cable to connect to the speaker in Blues Jr. I had another solid state amp that I thought I could use but the speaker is only 4ohm and I've wired up the OT for 8ohms.

Fired up the amp. No smoke, no smells, very quiet. Maybe too quiet? Plugged in my guitar and got sound from the speaker. Good right? No. The sound coming out was horrible! Very distorted, but not in a good way. Not sure if I can even describe it. The amp is dead quiet when not playing. I decided to check all of the voltages. Here are my results.

B+ 356v

Bias Voltages    V4 -28.2v, V5 -28.2v

Output Transformer    V4 Pin7 355v, V5 Pin7 355v

Bridge Rectifier AC voltage   258v

Pre-Amp Tube Voltages:
     
V3 Pin6 223v, Pin1 224v, Pin 3 63.9v, Pin 8 63.8v

V2 Pins 1&6 162.0v,  Pins 3&8 1.637v,  Pins 2&7 7.1mv

V1 Pin 1 177.3v,  Pin 3 1.414v

Filament Voltages:

V1 6.95v
V2 6.96v
V3 6.97v
V4 6.98v
V5 6.98v

Is there anything in these results that could be causing my problem? Is something wrong with the OT? At this point I have no idea.

Help!

[sluckey]

Bias Voltages    V4 -28.2v, V5 -28.2v

Adjust the bias pot for -15V on pin 2 of V4 and V5. Sound any better?

[Tbone55]

Bias Voltages    V4 -28.2v, V5 -28.2v

Adjust the bias pot for -15V on pin 2 of V4 and V5. Sound any better?

Adjusted bias voltage to around -15.0v. Sounds really good now. How do I go about determining what the optimal bias voltage would be? Should I play the guitar and adjust the bias voltage while I'm playing and back it off when it starts to sound bad again or is there a more scientific method?

[sluckey]

There's a better way to set the bias. I just wanted to get you in the ballpark. We'll cover that in a minute. But first, now it's time to tackle that feedback wire. Play the amp a bit to get an idea how it sounds and how loud it is. Now, using your third hand (I use gator clip leads) touch the feedback wire to the tip lug of the speaker jack. Does the amp sound more refined but a bit lower in volume? If so, switch the amp off and solder the feedback wire to the speaker jack.

But, if the amp howls, screams, or just sounds nasty, we will need to swap a couple wires. Make these checks and report back for further instructions. The amp may startle you but it will not bite you.

[Tbone55]

It does sound kind of loud right now and bit edgy. I'll do as instructed and post back in a few minutes. 

[Tbone55]

There's a better way to set the bias. I just wanted to get you in the ballpark. We'll cover that in a minute. But first, now it's time to tackle that feedback wire. Play the amp a bit to get an idea how it sounds and how loud it is. Now, using your third hand (I use gator clip leads) touch the feedback wire to the tip lug of the speaker jack. Does the amp sound more refined but a bit lower in volume? If so, switch the amp off and solder the feedback wire to the speaker jack.

But, if the amp howls, screams, or just sounds nasty, we will need to swap a couple wires. Make these checks and report back for further instructions. The amp may startle you but it will not bite you.

Did as instructed. It sounds a lot smoother with the feedback wire in place, more harmonic. It's just a tad lower in volume but it was pretty loud as it was with the amp volume barely turned up and the guitar volume at around 3. I'm not getting any howling or squealing whatsoever, just a bit of amp hiss. What are the next steps?

[Tbone55]

Go make the neighbors mad. 

I like your attitude dude!

[Tbone55]

Thanks man. It really helps to have folks surround you with a positive attitude. When people like to see you succeed, anything is possible. I'll try to post a sound clip if I can figure out how to do it. I already can tell that the amp has a lot more presence to its sound than before. It sounds like a much bigger amp. Can't wait to get it really dialed in!

Woohoo!

[sluckey]

Now you're gonna set the bias. Most of the internet believes you should bias a fixed bias amp at 70% of max dissipation (power). So that's what you will do. The EL84 has a max plate dissipation of 12 watts. 70% of 12W is 8.4W. You will measure the voltage on the pin 7 (plate) and then you will measure the voltage on pin 3 (should be millivolts). Now multiply these two numbers. This is your plate dissipation. If you're lucky it will be 8.4W. If not you will adjust the bias pot while monitoring the voltage on pin 3. Increase that voltage a bit if you were low, decrease the voltage if you were high. Now recheck the voltage on pin 7. Multiply these two new numbers. Continue doing this until your calculated number is 8.4W. May take a few cycles of adjusting and rechecking both voltages to get it right.

When you have this tube running at 8.4W, switch over to the other tube. Measure pin 3 and pin 9, then multiply. Hopefully the tubes are well matched and this tube will also be running at 8.4W. Fat chance! If it is slightly lower than 8.4W, then you are done. If it is higher than 8.4W, readjust the bias to make this hotter tube run at 8.4W and recheck the other tube, which should be less than 8.4W now.

Read this and reread if necessary before tweaking the bias pot. Then just do it. It's really simpler than it seems. Report back.

[Tbone55]

Yeah, I have a Line 6 DT50 and a Marshall Valvestate but they're way to big to lug around. The BJ fits the bill. This conversion turned a good amp into a GREAT amp. Besides, I need something to keep me busy during the winter months and I need to keep the noodle working. I was building guitars for the last few years as a hobby and I guess I needed a break from that.

 

[jamaio]

Glad to see you are almost at the finish line!!

Before I rebuilt my Blues Jr it sounded small and boxy, I had to crank the bass and treble to get a decent tone. You will find with Dougs improvements to the circuit, you can turn the bass and treble down, try starting at 5 or below and adjust until you find the sweet spot. You will need to relearn how to get the best sound from the amp.

Have fun playing!!

[Tbone55]

Glad to see you are almost at the finish line!!

Before I rebuilt my Blues Jr it sounded small and boxy, I had to crank the bass and treble to get a decent tone. You will find with Dougs improvements to the circuit, you can turn the bass and treble down, try starting at 5 or below and adjust until you find the sweet spot. You will need to relearn how to get the best sound from the amp.

Have fun playing!!

Hey Thanks kindly. I know exactly what you mean. I haven't finished setting the bias yet and already the amp sounds so much bigger than before. This is what I was looking to achieve. I could have left it alone but I'm a believer that if you can improve something why not do it. Striving for improvement is part of the human drive. If not, it would be a pretty dull world to live in.

Thanks for your support. 

[Tbone55]

I found a site that is loaded with very helpful info on amps. There are others here that are probably already aware of it but I thought it would be a good idea to share it anyways.

https://robrobinette.com/Amp_Stuff.htm

[sluckey]

Rob's website is often referenced. Some good stuff. He's a current member here. robrob is his member name.

How did the biasing session go? What numbers did you get? How's it sound? I was concerned that my instructions might be too wordy and a bit confusing. Truth is, you'll be able to do it faster than I wrote it. Here's a procedure written by Lord Valve. He's was a big usenet asshole (his words, not mine). May want to bookmark this if you ever intend to bias another amp...

     http://www.duncanamps.com/technical/lvbias.html

[Tbone55]

Now you're gonna set the bias. Most of the internet believes you should bias a fixed bias amp at 70% of max dissipation (power). So that's what you will do. The EL84 has a max plate dissipation of 12 watts. 70% of 12W is 8.4W. You will measure the voltage on the pin 7 (plate) and then you will measure the voltage on pin 3 (should be millivolts). Now multiply these two numbers. This is your plate dissipation. If you're lucky it will be 8.4W. If not you will adjust the bias pot while monitoring the voltage on pin 3. Increase that voltage a bit if you were low, decrease the voltage if you were high. Now recheck the voltage on pin 7. Multiply these two new numbers. Continue doing this until your calculated number is 8.4W. May take a few cycles of adjusting and rechecking both voltages to get it right.


When you have this tube running at 8.4W, switch over to the other tube. Measure pin 3 and pin 9, then multiply. Hopefully the tubes are well matched and this tube will also be running at 8.4W. Fat chance! If it is slightly lower than 8.4W, then you are done. If it is higher than 8.4W, readjust the bias to make this hotter tube run at 8.4W and recheck the other tube, which should be less than 8.4W now.

Read this and reread if necessary before tweaking the bias pot. Then just do it. It's really simpler than it seems. Report back.

Good Morning, I have completed the procedure you outlined above and have the following results to report.

V4 EL84 Final Voltages:

Pin 3 (Grid) 25.7mv
Pin 4 (Plate) 335v
Plate Dissipation 8.6095W (71.7%)

V5 Final Voltages:
Pin 3 (Grid) 16.7mv
Pin 7 (Plate) 337v
Plate Dissipation 5.6279W (46.8991%)

Well, so much for the tubes coming as a matched set. What now?

Thanks!

[sluckey]

Well, if you bought those tubes as a matched set, you can beg customer support to replace them with another matched set.

Or, you can buy a bucketful and do your own matching.

Or, you can play it like you stole it. And enjoy.   

[Tbone55]

Well, if you bought those tubes as a matched set, you can beg customer support to replace them with another matched set.

Or, you can buy a bucketful and do your own matching.

Or, you can play it like you stole it. And enjoy.   

Well, there's one more thing that's happening. When I tried the amp with the feedback resistor hooked up I had the meat control to the max and there was no problem. I didn't try backing it off from max. Now that I soldered it to the tip connection of the speaker output when I try to back it off (turn counter clockwise) I get what sounds like feedback but at a low frequency. Does this mean I need to disconnect the feedback and do without it?

Thanks!

[Tbone55]

Well, there's one more thing that's happening. When I tried the amp with the feedback resistor hooked up I had the meat control to the max and there was no problem. I didn't try backing it off from max. Now that I soldered it to the tip connection of the speaker output when I try to back it off (turn counter clockwise) I get what sounds like feedback but at a low frequency. Does this mean I need to disconnect the feedback and do without it?

Thanks!

 I probably would leave the feedback in on this particular amp, and trouble shoot it. I don't think it would cause low freq. oscillation, but swap tubes on the brown and blue leads from the output transformer.(looking at your pic you would put blue on the right el84, and brown on the left el84 If that doesn't fix it, double check all your values in the feedback circuit. I.E. resistors and pots. I'm really not sure about this one, but maybe the wire going from the meat pot, to the input cap might need a little more length to get it further away from the PS.

Thanks. I replaced the lead from the meat pot to the input cap with a longer one that runs closer to the pots and then over the board to the capacitor. When I turn the meat control control clockwise I get a high pitched squeal. I'll try interchanging the leads to the EL84's from the OT next. Won't get around to doing it until tomorrow. Heading off for an open mic night to play with the friends. Will let you know tomorrow.

Cheers!

[sluckey]

Many replies ago you had mentioned you would have to splice some wires onto the OT to be able to swap the brown and blue plate leads. I mentioned there was another way to accomplish the same thing, but to set it aside until the time comes. Sounds like you have arrived!

Rather than swap the OT blue and brown wires, swap the white wires that connect to pins 1 of the EL84s. Looks like you will need to actually replace one of those white wires, but that's still faster and neater than splicing the OT wires.

[Tbone55]

Many replies ago you had mentioned you would have to splice some wires onto the OT to be able to swap the brown and blue plate leads. I mentioned there was another way to accomplish the same thing, but to set it aside until the time comes. Sounds like you have arrived!

Rather than swap the OT blue and brown wires, swap the white wires that connect to pins 1 of the EL84s. Looks like you will need to actually replace one of those white wires, but that's still faster and neater than splicing the OT wires.  (edit...fixed quote...sluckey)

Hallelujah! Worked like a charm. The amp sounds amazing! I'll be playing with this amp all day long. Can't wait to get a chance to take it out to our next open mic. The presence this amp has now is blowing me away. Note definition is clearer and the controls actually do something. WOW! I can't thank you enough for the tremendous help you've given me. I would also be remiss if I didn't include the rest of the great people who responded to this post and helped me along the way.

Being that I have a curious mind and always like to know why things work they way they do, can you explain to me what reversing the white leads did that made the squeal go away? If you could point me in the direction of information that explains this I would love to read up on it. The education I've received with the help of you folks here is truly wonderful.

I hope that I will at some point be able to contribute back to this forum and help someone else out as much as all of you have helped me.

Sincerest Thanks!

P.S. Have a drink on me. 

[sluckey]

Read this...

http://www.aikenamps.com/index.php/what-is-negative-feedback

… and then google "guitar amp negative feedback loop".

[Tbone55]

Read this...

http://www.aikenamps.com/index.php/what-is-negative-feedback

… and then google "guitar amp negative feedback loop".

Thanks. I bought a basic amp book that should be here today. I have a feeling I've probably learnt more from here than I'll get from the book.

 

[Tbone55]

Reporting back with some observations. The amp is LOUD! With my guitar volume at 3 I only have to turn the volume and master volume to 2 or 3 and its already pretty loud for practicing. The gain comes on pretty early. Turning the volume past 3 already starts to give quite a bit of break up. If I wanted the break-up to occur a little later would I look at replacing V2 with maybe a 12AT7 or a 12AY7 tube? I don't really need this much gain so early. I'd like to be able to use my guitar volume to dial back the volume to clean up the sound and raise it to get some dirt. I have pedals (tube screamer, SL Drive). Otherwise, do I just keep the volume low and raise the master volume for clean sound and just use my pedals for more gain for solos or more crunch?

[sluckey]

Sounds like you're ready for a champ. That BJ ain't no parlor amp. 

[Tbone55]

Sounds like you're ready for a champ. That BJ ain't no parlor amp. 

Oh no, another amp? 

You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? 

[jamaio]

Oh no, another amp? 

You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? 

My Blues Jr does not get used much anymore, I only play at home. My second build was a 5f1 Champ, it has become my go to amp. I had an old solid state amp that I converted to a Champ!!

[Tbone55]

Yep. Turn down the master!

It's only between 2 & 3! 

[Tbone55]

Oh no, another amp? 

You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? 

My Blues Jr does not get used much anymore, I only play at home. My second build was a 5f1 Champ, it has become my go to amp. I had an old solid state amp that I converted to a Champ!!

Oh boy, another amp build. I've got a Peavey 110 solid state amp. Wonder if it would be a candidate for doing that. Where can you get the chassis for doing this? I'd be interested in doing it.

[jamaio]

Oh boy, another amp build. I've got a Peavey 110 solid state amp. Wonder if it would be a candidate for doing that. Where can you get the chassis for doing this? I'd be interested in doing it.

I used the solid state chassis, I stripped all the old parts and drilled holes where they were needed. You can see my build here:
https://el34world.com/Forum/index.php?topic=21444.msg227532#msg227532

I really like the Champ, I just replaced the speaker and I have a new 5751 preamp to try out. The fun never ends.

[Tbone55]

Oh boy, another amp build. I've got a Peavey 110 solid state amp. Wonder if it would be a candidate for doing that. Where can you get the chassis for doing this? I'd be interested in doing it.

I used the solid state chassis, I stripped all the old parts and drilled holes where they were needed. You can see my build here:
https://el34world.com/Forum/index.php?topic=21444.msg227532#msg227532

I really like the Champ, I just replaced the speaker and I have a new 5751 preamp to try out. The fun never ends.

It's almost like searching for the Holy Grail. I'm going to have a closer look at my solid state amp. I don't know if it has room for a 12" speaker. The one that's in there now is a 10". I'm really curious about your preamp.

[jamaio]

It's almost like searching for the Holy Grail. I'm going to have a closer look at my solid state amp. I don't know if it has room for a 12" speaker. The one that's in there now is a 10". I'm really curious about your preamp.

The original Champ had an 8" speaker, the 10" will be ok. I chose a cheep speaker when I first built my amp, it  was "ok" for a cheep speaker but it sounds much better now with a Jensen C12Q8. The 5751 preamp tube has less gain than the 12ax7, I like a clean tone with a little crunch. The 5751 sounds smooth when pushed to a natural breakup point.

The quest for tone is endless...

[Tbone55]

It's almost like searching for the Holy Grail. I'm going to have a closer look at my solid state amp. I don't know if it has room for a 12" speaker. The one that's in there now is a 10". I'm really curious about your preamp.

The original Champ had an 8" speaker, the 10" will be ok. I chose a cheep speaker when I first built my amp, it  was "ok" for a cheep speaker but it sounds much better now with a Jensen C12Q8. The 5751 preamp tube has less gain than the 12ax7, I like a clean tone with a little crunch. The 5751 sounds smooth when pushed to a natural breakup point.

The quest for tone is endless...

Yes, it is. Can't say how many hours I spent with my Line 6 PODHD500X trying to get the right tone to match different songs. Don't get me wrong, it's an amazing piece of gear, but you could spend eternity tinkering to get just the right sound. If I change the preamp tube should it be V2? Since I wired both halves to work is this why I'm getting breakup so early on?

[jamaio]

I have not tried swapping tubes in my Blues Jr. I believe you would swap out v1 for less gain. The 12ax7 has a gain factor of 100, the 5751 is 70 and the other 12a_7 tubes go down from there, google gain factor for preamp tubes and you will find a chart with all the info. Some people use a 12at7 in v1, it has a gain factor of 60. Experiment with different tubes and tube manufacturers. The 5751 I have is a Sovtek. I have JJ tubes in my Blues Jr.

You may find a combination that works or you may just keep building new amps until you’re happy!!

[Tbone55]

I have not tried swapping tubes in my Blues Jr. I believe you would swap out v1 for less gain. The 12ax7 has a gain factor of 100, the 5751 is 70 and the other 12a_7 tubes go down from there, google gain factor for preamp tubes and you will find a chart with all the info. Some people use a 12at7 in v1, it has a gain factor of 60. Experiment with different tubes and tube manufacturers. The 5751 I have is a Sovtek. I have JJ tubes in my Blues Jr.

You may find a combination that works or you may just keep building new amps until you’re happy!!

I've got an Electro Harmonix in V1, JJ's in V2 & V3, and JJ's for V4 & V5. You're right, like everything else about this gear, it's about trying it and letting your ear be the judge.

Thanks.