Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tbone55 on February 10, 2019, 09:00:36 pm
-
Well, I just started working on my Blues Jr. III amp rebuild. It's a nice amp and sounds decent enough but I just can't stand the inside looking more like a computer than an amp. It has the potential to be so much better and Doug's kit is just the ticket. I'm going to try and document as much of the build as possible and hopefully this will help others down the line. Here are some of my initial steps. I wanted to post some photos but they appear to be to large to post.
1. Inventory all parts that came in the kit.
2. Measured all resistors to verify values.
3. Placed the resistors and capacitors on a board clearly marking their values.
4. Bagged remaining parts into groups, clearly labelled
As per Doug's advice, I plan to take it slow and easy. Steady wins the race.
Thanks and stay tuned. I'm sure I'll be posting questions.
-
Those seem to be good, practical steps. As a relatively new builder myself, this is a great, supportive forum with lots of knowledge floating around. Looking forward to the progress updates.
-
Slow and steady, it's all in the details.
If you're using google drive or something similar, post shareable links to your pictures in your posts here.
:icon_biggrin:
-
Those seem to be good, practical steps. As a relatively new builder myself, this is a great, supportive forum with lots of knowledge floating around. Looking forward to the progress updates.
Thanks. I'll try to post some pictures as soon as I figure out how to resize the files. It's definitely important to take small steps and review what you've at each stage. As Doug said, if you do everything the way it should be done the amp should work. If it doesn't you've gone wrong somewhere.
-
Slow and steady, it's all in the details.
If you're using google drive or something similar, post shareable links to your pictures in your posts here.
:icon_biggrin:
Thanks. I may have to do that. Appreciate the response.
-
Here are some images of the build so far. This is the first time I'm trying to share photos through Microsoft One Drive so I hope it works. Here's the link.
https://1drv.ms/f/s!AloVFOBmUO_sa35nFBYztVTNpV4
My next step will be to drill the holes in the board and the chassis. I think I've located the chassis in a good location but please let me know if there's a better spot. I'll wait to hear back before drilling the holes. You can see in the images where I've located the board and marked out the holes. Take a look at the difference between the original output transformer and my replacement. I'm using a Hammond transformer that is a replacement for a Fender Deluxe Reverb amp. I just have to drill another hole to mount it because it's bigger. A lot more options for wiring speakers, hope I can figure it out.
-
pics look great.good luck with your mods.measure and measure some more,dont wanna do redos or do overs.
-
pics look great.good luck with your mods.measure and measure some more,dont wanna do redos or do overs.
Thanks. Will definitely be double checking everything as I move along. More pics to come.
-
On my Blues Jr conversion I mounted the board about ¾" from the edge of the chassis. Looking at the pictures Doug has posted in the build section, he has the board closer to the edge of the chassis similar to your layout.
As long as there are no clearance issues or conflicts with mounting the OT you should be good.
Edit: Just looked at the pictures again, the way you have the board positioned is backwards. The Caps need to be near the PT.
-
On my Blues Jr conversion I mounted the board about ¾" from the edge of the chassis. Looking at the pictures Doug has posted in the build section, he has the board closer to the edge of the chassis similar to your layout.
As long as there are no clearance issues or conflicts with mounting the OT you should be good.
Edit: Just looked at the pictures again, the way you have the board positioned is backwards. The Caps need to be near the PT.
Thanks for your input. I placed the board temporarily that way just to get an idea of the location. I went with Doug's placement based on the images in his instructions. The OT is mounted now and I've drilled the holes in the board and chassis. Finished lacing up the turrets but have to buy some solder to finish the job. Also put the tube board together as well. Will wire the filaments as well when I get my solder tomorrow. Will post some pics soon.
-
Sounds like you are off to a good start.
I made a few mistakes along the way, I looked at Doug's picture of the under board jumpers and matched mine exactly. After I thought my build was complete, when I started to adjust my bias I had no voltage reading. On Doug's first board he has the bias pot on a separate board, hence the missing jumper. I did not want to remove the board so I ran a jumper on top the board, no one will ever notice!! There was another ground I missed and my ground bus on the back of the pots broke. The break in the ground bus was a hairline fracture that I could not see. Everything worked great on my bench, then when I installed the chassis in the cabinet I had no sound at all. I took the chassis out and on my bench it started working again. It took me a while to find the broken wire, I poked everything with a wooden dowel while injecting a tone. My amp sounds great now and has preformed well for the past 2 years.
Look forward to seeing more pictures of your build.
-
Thanks, looks like Murphy's law paid you a visit. I'm hoping to avoid that but the best laid plans........you know how it goes. I'm keeping a log of each step as I go along to keep me on track and to hopefully help me review my steps. If I run into any problems during a step I can make a note of it. That way it might help troubleshoot problems down the road. Before I laced up the board I assigned a number to each group or pair of turrets that required lacing. As I did each lace I ticked off that group of turrets then used my multi meter to check for continuity. Now I know I have the turrets laced up right. After I solder the turrets I'll check for continuity again. Yes it is tedious but if you go along and just assume that everything's connected properly, well,, that's when good old Murphy comes along. It doesn't mean that I still won't encounter a problem, but maybe just one less to deal with. That crack you had is a perfect example.
Cheers!
-
Here are some pics that show how I deal with lacing the board jumpers and installing under-board jumpers. I use a highlighter to keep up with jumpers on complicated boards. There are three pics on this page that deal with board jumpers. May give you some ideas. BTW, the highlighter method also works well for installing components and board interconnecting wires.
http://sluckeyamps.com/VAC15/ac15.htm
-
That is one hell of an Amp. What I'm doing is amateur hour in comparison. Beautiful work! As per your example I used a highliter to mark the underboard wires. Double checked each one afterwards. The wiring for this board changed due to the bias pot being added so there were a couple of wires in different places. I used a white wire for the bias pot connection as the original example provided by Doug (Blues Jr conversion) didn't show this wire.
Thanks for your post. Hope to hear from you when I have questions.
-
I have a question. The tube board for this amp that I see in Doug's conversion has turrets located at the one end to attach two 47 ohm resistors to but the tube board I received has eyelets, fairly large. Is there any advantage either way?
Also, with the resistors installed, what should my resistance reading be from the outer lug to the outer lug? I'm expecting to read the total of the two resistors (94 ohms) as they are in series but that's not what I'm getting. Did I possibly damage the resistors? I'm only reading 23 ohms.
Thanks.
-
I have a question. The tube board for this amp that I see in Doug's conversion has turrets located at the one end to attach two 47 ohm resistors to but the tube board I received has eyelets, fairly large. Is there any advantage either way?
I don't know why Doug changed to eyelets, but I suspect it has something to do with mounting the tube board. The eyelets are lower profile which would allow the board to be mounted with shorter standoffs, possibly allowing tube shields to be used. Why not ask Doug? Here's a link that shows the newer boards with eyelets...
http://el34world.com/Hoffman/Blues_Junior_Tube_Board.htm
Also, with the resistors installed, what should my resistance reading be from the outer lug to the outer lug? I'm expecting to read the total of the two resistors (94 ohms) as they are in series but that's not what I'm getting. Did I possibly damage the resistors? I'm only reading 23 ohms.
Outer lug to outer lug would be 94Ω, but only if you don't have the filament leads connected or tubes plugged in.
-
Thanks Sluckey,
I had the tube board info printed out but didn't notice the image was the same as mine. I've got all my information printed out but I think it's information overload in my case. The larger eyelets will make it easier for soldering the 6.3vac leads from the PT to the board.
As far as the resistors go, is it because the filaments are open at the end of the run to the pre-amp tubes that I can't get a proper reading? You've confirmed the resistance I should see.
Thanks.
-
Do you have tubes plugged into the board? Do you have any wires connected to the board?
-
Do you have tubes plugged into the board? Do you have any wires connected to the board?
Yes, I have the tube board wired up with the tubes installed.
In the meantime, here is a link to some more pictures I took of what I've done so far. I wired up the V2 preamp tube as Doug did to get more push from the tube. My understanding is that only half of the tube is actually being used in the original design. I didn't wire it up exactly the same way as Doug did with very short pieces of connecting wire. I did one connection the same way but the other two I ran the wires around the outside of the ceramic base. You can see it in one of my pictures. Should I have just used really short jumpers or is the way I did it ok?
I believe the next step will be to install the turret board and connect the transformers as well as the speaker jack and the footswitch jack. Also have to hook up some ground connections. Don't want to forget those!
Cheers!
https://1drv.ms/f/s!AloVFOBmUO_sa35nFBYztVTNpV4
-
This meticulousness may be overkill for smaller amps but absolutely necessary for any level of complexity in future electrical work. Im jealous of your work habits. Super impressive.
As far as longer leads go.. you have to balance ease construction with length of conductors. In signal wire, length of unshielded wire starts introducing noise into the circuit. Im pretty sure youll be ok here, but wait for more knowledgable critique.
Keep it up! looking forward to more pictures.
-
Your way works too. Doug is the master. I do it his way. I remove that center shield pin on my sockets specifically for putting jumpers between tube pins. Here's a pic with cross jumpers on three sockets...
http://sluckeyamps.com/dual_lite/big_guts.jpg
-
This meticulousness may be overkill for smaller amps but absolutely necessary for any level of complexity in future electrical work. Im jealous of your work habits. Super impressive.
As far as longer leads go.. you have to balance ease construction with length of conductors. In signal wire, length of unshielded wire starts introducing noise into the circuit. Im pretty sure youll be ok here, but wait for more knowledgable critique.
Keep it up! looking forward to more pictures.
Thank kindly. I'm trying my best not to run into problems down the road. Appreciate your input.
-
Your way works too. Doug is the master. I do it his way. I remove that center shield pin on my sockets specifically for putting jumpers between tube pins. Here's a pic with cross jumpers on three sockets...
http://sluckeyamps.com/dual_lite/big_guts.jpg
Thanks kindly. I didn't realize you could remove the center shield pin. I don't know what that center pin is for but if I did I might have removed it. I'm still trying to learn more about how tubes work as well as how the whole circuit works. Hope to be sending more pics later today after I've worked on the amp some more.
-
That center pin is usually connected to ground and serves as a ground shield as well as for connecting some components from tube pins to ground. It's very useful in radio frequency circuits but not needed at audio frequencies. I just use a Dremel with a cone shaped grinder to remove them. I've heard that sometimes those center pins are holding the socket together but I've never seen one like this. Should be easy to determine with a quick look. You can even remove them after the socket is wired but you must use a steady hand.
-
Thanks, I may remove that center shield pin later on. For now I'll leave it as is. Hopefully it won't cause any problems the way it is.
-
Bit late posting my update of the build. I finished hooking up all of the tube sockets to the board, installed the filter caps, bias resistor, bias diode, bridge rectifier. Hooked up the output transformer, power transformer, ran all of the grounds. I had to extend the a/c leads of the PT that go to the bridge rectifier because they wouldn't reach. I used automotive wire of the same gage (copper stranded) but I'm now wondering if it is good enough to use for this. The leads of the PT said the wire was rated for 600V, the automotive wire only says 60V. Does the rating have to do with the ability of the covering to not get burned through by the voltage pressure? I'll get some proper rated wire and replace these. The speaker jack is all hooked up as well. The other input jack next to it was for hooking up a footswitch to turn the FAT switch on and off. I haven't hooked this up yet.
Next step will be to install the remaining components onto the board, then hook up the pots to the board. In one of my pictures you can see the names of the components that have already been installed written on some masking tape. Some that I installed aren't there because there are others with the same values that still need to be installed. When all of them are installed I'll put a piece of masking tape with their value and quantity on my wooden board. The one thing I kept checking for was continuity wherever possible after soldering my connections. This should help weed out any bad connections up front rather than trying to chase them down afterwards. It may not guarantee it, but should help reduce problems of this type.
Here's the link, https://1drv.ms/f/s!AloVFOBmUO_sa35nFBYztVTNpV4
Suggestions and comments are greatly appreciated. :icon_biggrin: :guitar1
-
The other input jack next to it was for hooking up a footswitch to turn the FAT switch on and off. I haven't hooked this up yet.
That's a bad location for that footswitch jack. I really don't think you'll be using a FAT footswitch but if you must have one, move it down near V1.
-
Thanks for the reply. I don't think I'll be using the FAT switch either. I've already installed the mini-switch for it on the faceplate but haven't hooked it up to the board. For now I think I won't wire up the input jack. Would you mind telling me why this isn't a good location for it?
I have another question regarding hooking up the OT transformer leads to the power tubes. Does it make a difference which colored lead goes to each power tube, pin 7? The schematic shows the blue lead going to V4 and the brown lead going to V5. If this is reversed, will it cause a problem?
Thanks.
-
It's always a good idea to keep preamp stuff far away from power transformers, power amps and other noisy high current components.
It does make a difference which OT leads connect to the power tubes in an amp such as this that has a NFB loop. Get'em wrong and the amp may squeal, howl, or just have other odd sounding symptoms. And you can't know for sure which is the right way, so it's best to not trim the leads to length until you can turn the amp on and test it out. If you need to swap them you will have enough length to do so.
If you have already trimmed them to length, don't panic. You have a 50/50 chance of having it right. :icon_biggrin: But if not there's another easy way to accomplish the same thing. We'll talk about it more when/if a swap is needed.
-
Okay Thanks. I have the correct colored lead hooked to the correct power tube according to the schematic. Guess we'll find out soon enough if it's gonna work. Do you think I'll have a problem with the wire I used to extend the a/c leads going to the rectifier? I had mentioned it in the earlier post.
Cheers!
-
Probably not. I would not put any auto wire in my amps though.
-
A bit about conductors: You size the conductor to deal with how much current it can handle, and you use insulation rating to deal with how much voltage you need to handle. I had never really seen 60V wire but it makes sense in mostly digital or super low voltage situations like cars and solar (48V systems max) vs the NEC classified "Low Voltage" systems typically designed up to 600V like most wire we use for amps (but goes up to 1000V in the code).
What happens when you exceed the voltage rating is that you lose the insulation properties and it basically becomes bare wire. When you exceed the current rating you get a fun fire starter/wire melter.
Youre going to need 600V rated wire most likely. Even if the 60V wire works, thats pretty darn precarious.
-
A bit about conductors: You size the conductor to deal with how much current it can handle, and you use insulation rating to deal with how much voltage you need to handle. I had never really seen 60V wire but it makes sense in mostly digital or super low voltage situations like cars and solar (48V systems max) vs the NEC classified "Low Voltage" systems typically designed up to 600V like most wire we use for amps (but goes up to 1000V in the code).
What happens when you exceed the voltage rating is that you lose the insulation properties and it basically becomes bare wire. When you exceed the current rating you get a fun fire starter/wire melter.
Youre going to need 600V rated wire most likely. Even if the 60V wire works, thats pretty darn precarious.
Yep. Totally agree. I'm not going to take that chance. Will be buying some today. Thanks!
-
I have a question. The tube board for this amp that I see in Doug's conversion has turrets located at the one end to attach two 47 ohm resistors to but the tube board I received has eyelets, fairly large. Is there any advantage either way?
It says this on the library page
Note: When assembling the Hoffman Turret boards, use the documents under each board
Do not use my builds below to assemble a Hoffman board -(my picture builds)
You may look at my builds below as a rough reference only
Things always change after I do a build
That's why I do these test builds first
You use the documents for this build here to assemble
https://el34world.com/schematics.htm#Hoffman_Pro_Junior_Conversion (https://el34world.com/schematics.htm#Hoffman_Pro_Junior_Conversion)
-
Hi Doug. Please clarify something for me. Shouldn't I be using the Blues Jr. Conversion, not The Pro Junior Conversion Instructions? I referred to your Blues Jr conversion, the Hoffman amp build instructions, the Blues Jr tube board instructions as well as all of the layout, schematic, hook up tips and one other pdf for my build.
Thanks.
-
Hi Doug. Please clarify something for me. Shouldn't I be using the Blues Jr. Conversion, not The Pro Junior Conversion Instructions? I referred to your Blues Jr conversion, the Hoffman amp build instructions, the Blues Jr tube board instructions as well as all of the layout, schematic, hook up tips and one other pdf for my build.
Thanks.
There is no Blues Jr instructions or Pro Jr instructions
Did you read the very top lines of the Install Instructions?
https://el34world.com/Hoffman/instructions.htm
see the screen shot below
-
Yes, I did. Those are the instructions I used. I used the Blues Jr conversion you did as well, to see how you positioned the board, ran the wires and what color wire you used for the hookups. It was a good visual aid. I've been very meticulous in my build and layed out a step by step process which I have been documenting from the start as well checking and double checking my work. As you said, if everything is done right the amp should work. I totally agree.
Thanks.
-
Yes, I did. Those are the instructions I used. I used the Blues Jr conversion you did as well, to see how you positioned the board, ran the wires and what color wire you used for the hookups. It was a good visual aid. I've been very meticulous in my build and layed out a step by step process which I have been documenting from the start as well checking and double checking my work. As you said, if everything is done right the amp should work. I totally agree.
Thanks.
I was answering your question about different sets of instructions for different boards
As it says at the top of that document, the instructions apply to all my boards
-
Understood.
Thanks.
-
A new update in the build. I'm getting so close to the finish line. Here's a link to some pictures of the build. When soldering the components to the board I used both the layout diagram and the schematic as a cross reference to make sure I put the correct component in the correct place. I also checked each of the components off of the BOM as they were installed. This just verified that I had used all of the components and didn't wind up with "extra" pieces at the end that shouldn't be there. Next it's time to hook up the POTS to the board.
I do have a question. I bought some wire to extend the leads of the Power Trans high voltage output (256vac) that goes to the a/c leads of the diode rectifier. I bought the wire on line as I didn't want to buy a 100ft reel at my local electronics supplier. The information stated that the wire was rated for 600v, 200c temp and that the insulation was silicone. Now that I received it, nowhere on it does it say 600v, only the gauge and temp rating. Shouldn't the voltage rating be on the insulation? I don't feel comfortable using it. Any thoughts?
here's the link: https://1drv.ms/f/s!AloVFOBmUO_sgRJCbOOHP-Gm2Kmn
Thanks.
-
Well, I just finished up the conversion. Now it's time for the true test but I won't be doing that tonight. Time to rest a bit and do a bit of reading about firing it up. If anyone has any advice on this I'd appreciate it. I've attached a link to the finished amp.
https://1drv.ms/f/s!AloVFOBmUO_sgRjOdl-pxidGC5L5
-
I've been reading the information on firing up the amp for the first time and it seems that I should have been doing some testing at certain points in the build. I'm going to make up a current limiter before doing anything else and would like some advice. Should I unsolder my PT and OT leads in order to check for the output voltages at the secondary sides or would it be ok to leave them hooked up while using the current limiter? I'm going to go over the circuit again and double check the wiring and resistor values but don't have the capability of measuring capacitor values so I'm going by the values on them and comparing to the layout/schematic/BOM. I'm pretty confident that I've hooked everything up correctly but want to be cautious at this point. Any and all advice would be greatly appreciated.
Thanks.
-
I've gone through everything once again from the very beginning, the turret board lacing and jumper wires to all of the tube connections, component connections, pot connections, jack connections, transformer connections and power on/off switch. Good thing I did to because I found two capacitors that were located in the opposite locations they should have been. Everything else appears to be in the correct location and the connections appear to be solid. I also checked the ground connections and they all show 0.1 ohms resistance. I'm now ready to make the current limiter to use for the start up. All tubes are removed. It certainly paid off re-checking everything again. Not sure whether I should unsolder the PT & OT output leads to test the transformer voltages. :dontknow: I guess if I have enough confidence in my work it shouldn't be a difficult choice. :icon_biggrin:
-
That looks like you are doing a super nice job on it. Keep us posted on the completion.
-
Not sure whether I should unsolder the PT & OT output leads to test the transformer voltages.
I wouldn't
-
That looks like you are doing a super nice job on it. Keep us posted on the completion.
Thanks! I certainly will.
-
Not sure whether I should unsolder the PT & OT output leads to test the transformer voltages.
I wouldn't
Yeah. After some more thought I don't think I will. I think I've done as much as I can to eliminate any errors. Need to be confident.
-
That amp looks GREAT. It would be nice to have a variac, but one idea is to plug your current limiter into another amp or device that draws similar current first. This way you know what to expect. Mine didn't work right at first so it was a WTF moment instead of cool voltage readings. Once again NICE JOB!
Yes it would but I think I've done as much double and triple checking that I can and I have to have some confidence in what I've done. What was the problem you ran into?
Thanks.
-
The current limiter itself. Fire it up man! You've put in the time. I was just referring to know what the dim bulb will look like when you do.
Well, I do have a solid state amp I could use but would that be the same as a tube amp?
-
The current limiter itself. Fire it up man! You've put in the time. I was just referring to know what the dim bulb will look like when you do.
I have a solid state amp. Would that be the same as using a tube amp?
-
I have a solid state amp. Would that be the same as using a tube amp?
The light bulb can't tell if it's tube, solid state or a table lamp.
Just plug it in and turn it on.
-
Yes, lamp-limiter. Better a bright bulb than a burned-down house. Any lesser burn-up is no big disaster.
-
It was probably a bad idea on my part honestly. If you studied the limiter like you did this amp, I have a feeling its all gonna work fine. You clearly put in a lot of effort. Sorry if I glitched your confidence.
j
Hey, no problem. Your suggestion was good. I'm going to try it on my solid state amp first. Better to have some idea of what to expect than none at all.
Thanks.
-
I have a solid state amp. Would that be the same as using a tube amp?
The light bulb can't tell if it's tube, solid state or a table lamp.
Just plug it in and turn it on.
I think you've got more confidence in my build than I do. :icon_biggrin:
-
Yes, lamp-limiter. Better a bright bulb than a burned-down house. Any lesser burn-up is no big disaster.
Totally agree!
-
Even if your amp has a dead short across the ac power cord the amp will just look like a wall switch to the lamp limiter and the bulb will be full bright. No damage done. You don't want to see a full bright bulb. For that size amp I would recommend a 40w to 60w bulb. If you use a 200w bulb as U.Doug uses, the lamp won't even glow if the amp is good, but you can actually play the guitar through the amp (with a much strangled sound). Once you do the lamp limiter test and verify that the bulb just glows dimly, it's time to put the limiter away and plug the amp straight into the wall.
You can check voltages while plugged into a limiter but you must realize the voltages will be low and will be useless as far as troubleshooting goes. So just check voltages if you want to get an idea if the rectifier is working but don't bother making an elaborate voltage chart for the whole amp.
-
Here's my lamp limiter with my November amp plugged in. I chose the November because it has the same topology as your amp, except for the tube rectifier. It has three 12AX7s and two EL84s and one 5Y3. I used a 25W appliance bulb and a 60W standard bulb. Notice the bulbs are nowhere near full brightness. Yours should look very similar.
-
Here's my lamp limiter with my November amp plugged in. I chose the November because it has the same topology as your amp, except for the tube rectifier. It has three 12AX7s and two EL84s and one 5Y3. I used a 25W appliance bulb and a 60W standard bulb. Notice the bulbs are nowhere near full brightness. Yours should look very similar.
Thanks so much for those pictures. I was going to use a 100W bulb I had but I think I've got a 40W or 60W kicking around. Haven't been able to finish my limiter as I need a female plug. I was wondering why it's being called a current limiter? Isn't it really a power limiter? Isn't the lamp creating a voltage drop so their isn't actually 120v being fed to the amp? It's a series circuit so isn't the current the same going to the amp as it is to the lamp? All I remember is E= IxR and P=ExI
-
That's all correct. Most of the electronics world call it a current limiter though. The idea behind that is the fact that whenever you put a resistance in series with any device, you limit/lower the current that can flow through the device. Resistance is the property of a device which opposes current flow. The light bulb is being used as a resistor in this case. The smaller wattage a bulb is the higher it's resistance. Small bulb will only allow a small amount of current to flow through the amp Big bulbs allow greater current to flow.
The limiter part comes into play because the bulb will only allow so much current to flow. Think about this. A 40W bulb connected directly to 120V will only allow .3A to flow. So, even if your series connected amp is a dead short (looks like a wall switch to the light bulb) the bulb will still only allow .3A to flow.
Some examples of using current limiting resistors are neon lamps. They must have a series current limiting resistor because when they fire (light up) they become a dead short and the resistor is needed to prevent fires in the wiring. Same thing for a LED. They must have a CL resistor or other current limiting device or they will burn up when they light up. Zener diodes will always have a series resistor unless they are being used in some small signal clamp/clip circuit.
Just like a hammer, we could have named it something else but it's still a hammer! :icon_biggrin:
-
That's all correct. Most of the electronics world call it a current limiter though. The idea behind that is the fact that whenever you put a resistance in series with any device, you limit/lower the current that can flow through the device. Resistance is the property of a device which opposes current flow. The light bulb is being used as a resistor in this case. The smaller wattage a bulb is the higher it's resistance. Small bulb will only allow a small amount of current to flow through the amp Big bulbs allow greater current to flow.
The limiter part comes into play because the bulb will only allow so much current to flow. Think about this. A 40W bulb connected directly to 120V will only allow .3A to flow. So, even if your series connected amp is a dead short (looks like a wall switch to the light bulb) the bulb will still only allow .3A to flow.
Some examples of using current limiting resistors are neon lamps. They must have a series current limiting resistor because when they fire (light up) they become a dead short and the resistor is needed to prevent fires in the wiring. Same thing for a LED. They must have a CL resistor or other current limiting device or they will burn up when they light up. Zener diodes will always have a series resistor unless they are being used in some small signal clamp/clip circuit.
Just like a hammer, we could have named it something else but it's still a hammer! :icon_biggrin:
Thanks for the explanation. Curious minds always want to know. :icon_biggrin:
-
The basic point is that the bulb MUST be less power than your wall-wires can supply, so that IF you have made a dead-short, the house won't burn down. On that theory you "could" go a 1,000 Watt bulb, since even old wiring can support 1,800 Watts short-term.
The *incandescent* (includes "halogen") lamp is "special". Its resistance rises quickly as it gets hot. So with a small series load the load may get most of the voltage. If the series load is heavy, the filament hots-up, resistance rises, and now the lamp is taking most of the applied voltage.
No lamp will protect against all mistakes. A small PT, shorted, may still not pull enough current to excite the lamp, and will slowly burn-up. 10r (instead of 100r) 1/4W heater centering resistors will emit smoke without much lamp-light. Transistors *always* fail before fuses or lamps (part of their magic).
-
HELP! I got smoke!
Well, I guess I should have expected something to. go wrong. After all, what are the chances that everything would work right off the bat. Here's the situation. I have one resistor, the 2.2K/3W that is attached to the B+ and Z points that starting smoking right away and started to glow red. I didn't see any other components that looked or smelled like they were in distress. I didn't really have any time to watch the lamp as I had to turn the amp off very quickly. I tried turning the amp on again to see if I could spot any other trouble but had to turn the amp off because the resistor started to glow.
I could really use some guidance on this. Thanks.
-
Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.
-
Sorry to hear you smoked some parts. I had a few issues with my built but no smoke.
Hope you resolve the issue quickly.
-
Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.
I didn't see your post until now but I think I've found the problem. Resistor R49 (10k/3W) which should be connected between point X and point Y was connected correctly. It was connected from X to the next lug which I believe dropped the resistance in that section of the circuit below what it should be and the voltage coming from B+ would have been a lot more than it should have been. That toasted R47 (2.2k/3W). Funny thing is now that I measure its resistance its gone up to over 4k. I imagine the internal substance that is used to create the resistance changed by being overloaded. I'm going to have to replace both of these resistors.
One other thing I found was on one of the preamp tubes (V3). I hadn't noticed that pin 3 and pin 8 have a jumper wire between them. I didn't notice this until I was going through the schematic and comparing it to the layout. I just had a connection going from pin 8 to the 820ohm resistor. I'll have to fix this as well.
Any and all feedback welcomed.
-
Sorry to hear you smoked some parts. I had a few issues with my built but no smoke.
Hope you resolve the issue quickly.
Yeah well, it was too much to expect it to work off the bat. Hopefully once I change a couple of resistors it will work. Unfortunately I won't if anything else got damaged until I do. Guess you can't make an omelet without breaking a few eggs. :icon_biggrin:
-
Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.
Worth repeating.
-
Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.
Worth repeating.
Yes, I did that. Meter is reading like it's an open circuit. Readout is blinking a steady 1,000kohms.
-
Remove the 2.2K that burned. Power up again and check for B+ at the + terminal of the FWB. Also check for negative voltage at pin 2 of V4 and V5. And make sure every tube is lit up.That's about all you can check with that 2.2K out of the circuit. What voltages do you have?
-
Remove the 2.2K that burned. Power up again and check for B+ at the + terminal of the FWB. Also check for negative voltage at pin 2 of V4 and V5. And make sure every tube is lit up.That's about all you can check with that 2.2K out of the circuit. What voltages do you have?
I had to remove not only the 2.2k resistor but also one of the 10k resistors because it was not hooked up properly. I can't reconnect it because the leads are too short now to do so. It's the resistor that goes between X and Y. Sorry, but what does FWB stand for? Can I still power up the amp with these two resistors missing? What voltages should I be checking?
Thanks.
-
You can still check the voltages I suggested. When you pulled that 2.2K that removed all other B+ voltages in the amp. The FWB is your full wave bridge rectifier. Black square thing with four leads. Check the B+ on the positive lead of the FWB. Check the negative bias voltage on pin 2 of each EL84 and adjust the bias pot for maximum negative voltage at pin 2.
Make sure every tube is lit up. That's about all you can do until you replace that 2.2K resistor.
You say the 10K was connected wrong? Please explain. I'm looking at your completed pic and the 2.2K and both 10K resistors appear to be connected properly. What am I not seeing?
-
You can still check the voltages I suggested. When you pulled that 2.2K that removed all other B+ voltages in the amp. The FWB is your full wave bridge rectifier. Black square thing with four leads. Check the B+ on the positive lead of the FWB. Check the negative bias voltage on pin 2 of each EL84 and adjust the bias pot for maximum negative voltage at pin 2.
Make sure every tube is lit up. That's about all you can do until you replace that 2.2K resistor.
You say the 10K was connected wrong? Please explain. I'm looking at your completed pic and the 2.2K and both 10K resistors appear to be connected properly. What am I not seeing?
Hey thanks for your response. Had to get to bed last night so I didn't see your response until this morning. I'll check the voltages as soon as I've had some breakfast. I should be able to check the voltage at B+(DC voltage), the A/C voltage at the FWB, the filament voltages, input voltage to on/off switch and PT and the bias voltage at pin 2 of the power tubes. The voltage at B+ is also the voltage being supplied to the OT as well. If I've got any of this wrong let me know.
Now that I look at the 10K resistor I see that it was hooked up correctly. I thought it wasn't because I had soldered it to the lug where the 82K resistor is attached but that lug is connected to the other 10K resistor so the connection was ok. I thought I had to connected the one 10k resistor to the lug where the other 10K resistor was attached but that isn't necessary.
Thanks so much for your help.
Edit by sluckey… fixed quote.
-
One thing I noticed in your final picture is I don't see a ground connection to the negative bias, and cathode resistors ground rail. I guess it is under the board. Although I don't think not having bias would make it draw too much current, if you had no cathode connection either. Probably not the problem, but wondering if you did in fact make that connection. Also, I would take the power tubes out untill you verify your voltages and negative bias. I have done a couple of these conversion's. It will pay off in the end. Hope you get it worked out.
Hey, thanks for the input. You'll have to forgive my ignorance as most of this is pretty new to me. The ground connection to the negative bias you asked about is a jumper wire that is under the board. It is connected to the ground side of the filter caps and to the two capacitors (47/100v) attached to the bias pot.
The cathodes of the power tubes are each connected to a 1ohm resistor and these are connected to the same line as the jumper wire above that is the ground connection for the bias pot, bias pot resistor and the two 47/100v capacitors above.
I hope I've explained myself well enough but please ask if I haven't. Thanks again.
-
More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.
Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.
-
More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.
Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.
Thanks. Will check those as well.
-
More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.
Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.
Thanks. Will check those as well.
Finished checking all jumper wires and filter caps. Jumper wires all 0.1 ohms, all filter caps infinite resistance. Found a lead from one of the bias caps that I hadn't trimmed down sticking out. Lifted the board a bit to get a better look. The lead does not touch the chassis or any other turret connection. The cap is right next to where the jumper lead from the filter cap ground goes to the first bias cap of the bias circuit. Not sure if I should unsolder the pot leads or tube leads and cut the cap lead that's sticking out. I don't think it's causing a problem.
Thanks.
-
Hey, thanks for the input. You'll have to forgive my ignorance as most of this is pretty new to me. The ground connection to the negative bias you asked about is a jumper wire that is under the board. It is connected to the ground side of the filter caps and to the two capacitors (47/100v) attached to the bias pot.
The cathodes of the power tubes are each connected to a 1ohm resistor and these are connected to the same line as the jumper wire above that is the ground connection for the bias pot, bias pot resistor and the two 47/100v capacitors above.
I hope I've explained myself well enough but please ask if I haven't. Thanks again.
Yes. Perfect. No ignorance here. Looks like a good job. Probably something simple. I had the same thing happen to me on my first push pull amp. Same resistor. Problem is, I don't remember what it was.
Have checked all of the jumper leads and they are good. The filter caps are good as well.
-
Tony, looking at your final build picture it's hard to see the connections at the FWB, the + goes to B+, the - goes to the turret that has an under board jumper to ground, the 2 ac connect to the red wires from the PT. Is this correct?
-
Tony, looking at your final build picture it's hard to see the connections at the FWB, the + goes to B+, the - goes to the turret that has an under board jumper to ground, the 2 ac connect to the red wires from the PT. Is this correct?
Yes, the B+ of the FWB goes to the B+ line at 47uf/500v filter capacitor and 2.2k/3w resistor turret lug connection. The - of the FWB is connected to the turret lug that has the jumper wire going to the negative ground of the filter caps to chassis ground. The two red wires are from the PT with one lead going to each of the A/C terminals of the FWB.
I did have to extend the PT a/c leads going to the FWB because they wouldn't reach and I used the proper wire, soldered them together and then put heat shrink around the joints. Could this be doing something?
Thanks.
-
In the picture it appeared that one ac connection also went to Z, the ac connections only go to the PT red wires. I had to extend those wires on my build, should not be an issue.
-
I have looked and looked comparing to layout, and I can't find any mistakes. What about the output secondary. Would that cause it to pull that much current that fast, or would it have to be a short on the primary side of the output? :dontknow:
Will it fire up with that resistor out? If so measure voltage at pin 2 of both el84's. That resistor feeding your power rail won't affect your bias supply working. Make sure you have -v. Then let someone else verify this, but I think you could pull the first 10k in the B+ rail, reinstall the 2.2k, to isolate the output section. Bring power up, (EL84's out) If it doesn't overcurrent the resistor, and voltages look good, then its a short or mistake in preamp section. Please let Sluckey or PRR or someone say this is okay BEFORE doing so!
I've removed the 2.2 K resistor and am starting to measure voltages. I don't have any of the tubes installed. Stay tuned for voltage readings. There is one thing I did have to fix. I had to run a jumper wire on pre-amp tube V3 from pin 3 to pin 8. I only had a connection from pin 3 to the board. I wish I understood more about the different portions of the sub circuits. I guess a little bit of knowledge is dangerous after all. :icon_biggrin:
-
In the picture it appeared that one ac connection also went to Z, the ac connections only go to the PT red wires. I had to extend those wires on my build, should not be an issue.
Yikes! I think you may have found the problem! Should the a/c lead of the FWB be connected to Z? I've got it connected to Z and my PT a/c lead is connected to Z as well! The FWB a/c lead should not be connected to Z and neither should my PT a/c lead. I'm putting an a/c voltage on top of the DC voltage that's coming from B+. No wonder I toasted the 2.2k resistor!
I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?
-
I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?
I would replace that resistor, you may not get correct voltage readings.
Glad I could help.
-
I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?
I would replace that resistor, you may not get correct voltage readings.
Glad I could help.
Second that. I see it now. That dashed wire (underboard). Is a little confusing in that it runs beside the lead of the FWB in the pic, but does not connect.
Thanks. I needed that second opinion. I'm going to leave that resistor out and measure some voltages in the meantime without the tubes in.
-
One of the mistakes I made, I reversed the polarity of C1, once I noticed that I questioned the integrity of everything. Several members of the forum reassured me and said it would be fine to just correct the polarity without changing the cap. Is all a good learning experience and will help with details on future builds.
You will be happy once you hear your amp working.
-
Here are some voltage readings without the tubes installed now that I fixed the connection problem at the WFB.
B+ Voltage - 368v
Filament Voltage - 7.54v
AC Voltage at WFB - Lead 1, 156.1v, Lead 2, 116.2v (is this normal?)
PT Bias Voltage - Pin 2, V4, -29.0v, Pin 2, V5, -29.0v
Switch Voltage - Line connection, 121.6v PT Side, 121.6v
I didn't check the voltage at the bias diode connection to the PT. It should be around 20v. I'll check that later. Next I'll install the tubes and re-measure voltages. Can't do anything else until I get a replacement 2.2k/3W resistor but I think the amp will be fine now. More to come.
-
Put the smoked 2.2K back in and finish checking the amp. It'll work for now and wont cause any problems, but replace it soon.
-
Put the smoked 2.2K back in and finish checking the amp. It'll work for now and wont cause any problems, but replace it soon.
OK. I'll do that. Can't do it right now but will later today and will post back voltages again.
Thanks.
-
POWER SUPPLY 101... When working with a FWB such as found in this amp, measure the transformer AC voltage BETWEEN the red wires, not from one red wire to chassis. There is no center tap on the HT secondary so the voltages you measured don't mean much.
-
Here are some voltage readings without the tubes installed now that I fixed the connection problem at the WFB.
B+ Voltage - 368v
Filament Voltage - 7.54v
AC Voltage at WFB - Lead 1, 156.1v, Lead 2, 116.2v (is this normal?)
PT Bias Voltage - Pin 2, V4, -29.0v, Pin 2, V5, -29.0v
Switch Voltage - Line connection, 121.6v PT Side, 121.6v
I didn't check the voltage at the bias diode connection to the PT. It should be around 20v.
Yes you did. You have -29.0v on pin 2. That's good. No need to check at diode.
OK Thanks.
-
POWER SUPPLY 101... When working with a FWB such as found in this amp, measure the transformer AC voltage BETWEEN the red wires, not from one red wire to chassis. There is no center tap on the HT secondary so the voltages you measured don't mean much.
New voltage measurements with tubes installed and no current limiter.
B+ 314V
Bias Voltage - Here's where there's a problem
V4 -26.8v
V5 -15.1mv
Not sure why this is happening now when I was getting the same readings from both tubes before.
Filament Voltage - 6.98v
I checked the filament voltage at the tube board supply and at each tube and it was the same all the way through. All of the tube filaments glowed although it was a bit hard to see two of the pre-amp tubes (V2, V3) because they are a different make and are constructed a bit different making it difficult to see but the tubes were definitely warm.
AC Voltage to Bridge Rectifier - 248V
The amp is also squealing and the squealing gets louder when the volume control is turned up. It's not a steady squeal but more like a bouncing around squeal. I also have no output sound from the speaker I have hooked up. This doesn't bode well.
I have one other question. There were three input jacks supplied with the kit. They look the same but the only difference was that one of them had a longer threaded barrel. I realize now that I should have used the one with the longer barrel as the guitar input jack but does it really matter if used it as the output jack for the speaker connection?
Thanks everyone.
-
The amp is also squealing and the squealing gets louder when the volume control is turned up. It's not a steady squeal but more like a bouncing around squeal. I also have no output sound from the speaker I have hooked up. This doesn't bode well.
The wires from the output transformer need to be swapped, sluckey mentioned this earlier when you asked if it mattered which one goes to which tube.
It does make a difference which OT leads connect to the power tubes in an amp such as this that has a NFB loop. Get'em wrong and the amp may squeal, howl, or just have other odd sounding symptoms. And you can't know for sure which is the right way, so it's best to not trim the leads to length until you can turn the amp on and test it out. If you need to swap them you will have enough length to do so.
If you have already trimmed them to length, don't panic. You have a 50/50 chance of having it right. :icon_biggrin: But if not there's another easy way to accomplish the same thing. We'll talk about it more when/if a swap is needed.
I have one other question. There were three input jacks supplied with the kit. They look the same but the only difference was that one of them had a longer threaded barrel. I realize now that I should have used the one with the longer barrel as the guitar input jack but does it really matter if used it as the output jack for the speaker connection?
If I remember correctly, the longer input jack was used with 2 fiber washers to isolate it from the chassis, not sure if this will make a difference now.
Wait for someone else to answer your other questions and confirm my answers.
-
Bias Voltage - Here's where there's a problem
V4 -26.8v
V5 -15.1mv
This is priority number 1! Pull the output tubes and fix that V5 bias voltage.
Also, disconnect the feedback wire at the speaker jack. Leave it disconnected until the amp is fixed.
-
I figured the leads probably needed to be swapped but just wanted some confirmation. I have the piece of wire I cut off. Can I join it back so the lead will reach V4?
I used the fiber washer that came with the kit for the input jack. The barrel of the input jack is smaller that the hole so the washer helps center the jack. There was only one with the kit.
Any chance reversing the leads will solve the tube bias on V5 and the no sound output?
-
Bias Voltage - Here's where there's a problem
V4 -26.8v
V5 -15.1mv
This is priority number 1! Pull the output tubes and fix that V5 bias voltage.
Also, disconnect the feedback wire at the speaker jack. Leave it disconnected until the amp is fixed.
OK. Will do. Should I reverse the PT leads to V4 & V5 or wait until I get the bias problem fixed?
-
You got bigger things to do right now. Forget the OT wires. Fix the bias. Disconnect the feedback wire. Reassess the situation.
-
You got bigger things to do right now. Forget the OT wires. Fix the bias. Disconnect the feedback wire. Reassess the situation.
OK. My order of operations
1. Verify previous bias voltage measurements with power tubes installed. V4 -28.6, V5 -15.4mv
2. Remove power tubes and re-measure bias voltage. V4 -28.6v, V5 - 28.6v
3. Re-installed power tubes and re-measured bias voltage. V4 -28.6v, V5 -15.2mv
4. Re-installed power tubes and measured supply voltage at pin 9. V4 242v, V5 242v
5. Measured output voltage of power tubes at pin 7. V4 361v, V5 361v
6. Removed power tubes and measured supply voltage at pin 9. V4 340v, V5 340v
7. Measured resistance value of resistors on pin 2 of each power tube. Verified value of 1.5k.
8. Measured resistance from pin 3 of power tubes to ground to verify the only resistance was the 1ohm resistor. Verified as 1ohm.
From the above I felt that there was a problem with the V5 power tube. To see if this was true I replaced it with the V4 power tube and the bias voltage read -28.6v. Looks to me like there's a problem with the power tube internally.
Would you concur?
-
Would you concur?
Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.
-
Would you concur?
Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.
Ok. Did as you suggested. The bias voltage read -2.0v. Not as small as the reading when it was in the V5 position but definitely way below where it should be. By the way, the bias pot was already turned as far as it can go to begin with (it came that way) so if I turn it the other way the voltage will go smaller (less negative). I'll purchase some new power tubes tomorrow. I'll also have to buy some new pre-amp tubes as well. I shut off the lights so I could see the filaments glow and the pre-amp tube in V1 was the only one I could really see lit well. V2 wasn't noticeable and V3 I saw a very, very, dim glow. I guess I should have taken voltage readings from them but will do so in order to have a record of it.
Would you agree I have a bad power tube for sure now? By the way, I'm not getting any squealing any more. No smells, no noise, just some heat off the filter caps and power tubes which seems normal. I won't plug into a speaker until I replace the pre-amp tubes. When I did before I wasn't getting any sound at all.
Thanks.
-
Would you concur?
Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.
If it was an original tube from the circuit board, it was likely already cooked. Don't think it was anything you did. They love cooking tubes, which is why doing this conversion is a good thing.
Yeah, apparently they weren't biased very well. The fixed resistor in the original circuit that biased them was replaceable with a bias pot but who wants to touch these circuit boards and burn it. I was going to mod the amp with a mod kit but the more I thought about the less I wanted to do it. This is what I was really looking to do. A bit frustrating right now but patience is the key. Tomorrow I'll buy the replacement resistor I need as well as some new tubes, probably replace all of them and keep the good ones for spares.
Thanks for being here.
-
Any recommendations for pre-amp and power tube replacements for this amp?
-
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?
-
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?
-
Did I damage the OT?
It's possible, especially while the amp was squealing.
-
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?
-
Best way to check is play your guitar through the amp.
-
Doh! Why didn't I think of that. :icon_biggrin:
Will it make a difference with the one power tube not working properly? I'll be getting some new tubes sometime today.
-
It will make a difference. Wait until you have two EL84s plugged in.
-
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.
-
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.
-
Use the 5 watter and don't look back.
-
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.
-
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!
-
Bias Voltages V4 -28.2v, V5 -28.2v
Adjust the bias pot for -15V on pin 2 of V4 and V5. Sound any better?
-
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?
-
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.
-
It does sound kind of loud right now and bit edgy. I'll do as instructed and post back in a few minutes. :icon_biggrin:
-
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?
-
Go make the neighbors mad. :guitar1
I like your attitude dude! :m8
-
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!
-
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.
-
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.
:occasion14: :guitar1
-
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!!
-
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. :guitar1
-
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
-
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
-
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!
-
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. :icon_biggrin:
-
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. :icon_biggrin:
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!
-
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!
-
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.
-
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. :icon_biggrin:
-
Read this...
http://www.aikenamps.com/index.php/what-is-negative-feedback
… and then google "guitar amp negative feedback loop".
-
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.
:icon_biggrin:
-
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?
-
Sounds like you're ready for a champ. That BJ ain't no parlor amp. :icon_biggrin:
-
Sounds like you're ready for a champ. That BJ ain't no parlor amp. :icon_biggrin:
Oh no, another amp? :icon_biggrin: :icon_biggrin:
You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? :laugh:
-
Oh no, another amp? :icon_biggrin: :icon_biggrin:
You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? :laugh:
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!!
-
Yep. Turn down the master! :icon_biggrin:
It's only between 2 & 3! :icon_biggrin:
-
Oh no, another amp? :icon_biggrin: :icon_biggrin:
You're right, the BJ is no parlour amp (now). Any suggestion on taming this beast a little bit (other than a Champ)? :laugh:
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.
-
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 (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.
-
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 (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.
-
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...
-
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?
-
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 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.