Hoffman Fender Blues Junior Conversion 6v6

[purpletele]

Good evening,

Do you guys have a modified schematic or layout for the 6v6 tubes in lieu of the EL84's?  Looking through the past threads, it looks like quite a few people make that change but I am not seeing anyone post a configuration.  I am all set up I just want to triple check.

Thank you,

[sluckey]

Your biggest challenge will be to provide enough bias voltage for the 6V6s (approx. -30v to -40v)

There are two easy, low cost ways to get the voltage you need. Modify the existing bias circuit to look like the one on the bottom left of page 5 of this pdf...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

And another option... Get the smallest 120 to 12.6v transformer that Radio Shack has. Connect the 12.6V secondary to your 6.3V filament string. This will give ≈60VAC on the primary. Now just connect the primary in place of your present bias winding.[/quote]

I'd also change the screen resistors to 470Ω/3W.

[purpletele]

Sluckey,

Thank you for the info.  I hoard info, its printed and placed in binders.

I failed to establish my base situation.  This is essentially a complete resourced build.  My first from scratch, I may get the chassis hardware loaded and then hold off.  I have a Marshall 74x coming that I may jump in front of this Blues Junior due to the immediate assistance and guidance that will be available for that project. I have gone through four of the Blues Junior's and revived a dead one and super charged them all.  I am also an electrical engineer that spent a career in Construction Management.  Not sure how that worked out yet.

I have had the three chassis' constructed by Terry at Seaside in Canada.  Beautiful work.  The custom chassis fits into a Mojotone Deluxe/Blues Junior cabinet. I have the Hoffman Blues Junior Kit, and I have a new TP24 and a TO22 for the xfmrs.  I was anticipating changing the screen resistors to 470/3W, but I am surprised if I need to boost as suggested.  I will research some more.  I have a Billm Mod for the PCB to switch to the octal sockets and there is a bias board with that which thought was an enlarged resistance pod, for the lack of the actual term.  Maybe the bias board that I have for the PCB is a booster?  and that is what you are suggesting here.

I have a career of making things difficult for myself, but I am always satisfied when I can complete a project as imagined.  I will review the info provided and see what I am missing.  It certainly doesn't seem too difficult.

Thank You,

BV

[EL34]

The Blues Junior has it's own dedicated 20vac winding that is used to create bias voltage and run the solid state stuff in the reverb circuit
If you have one of my BJ boards, then you are only using the 20vac winding for bias
And it is not set up with a bridge rectifier

I would first twist the bias pot all the way and see how much negative bias voltage you have at the max setting

[sluckey]

If you have one of my BJ boards, then you are only using the 20vac winding for bias
And it is not set up with a bridge rectifier

Exactly. That's the whole point. That bias supply will put out a maximum of -28V. That's more than enough for EL84s but it's not enough for 6V6s.

The ideas I proposed will put out a maximum of approx. -50V and that can be adjusted down to make the 6V6s happy.

[purpletele]

Sluckey/EL84,

You guys rock!

Thank you.

BTW, can I donate to the fund to support the forum?

PS.  Doug, I am going to order some parts.  Can you throw in one of you Blues Junior Labels?  I would like to have the label to read right side up.

[EL34]

You donate by buying parts from me 


Add a note to you order that you need a blues junior gold label

[purpletele]

el34/Sluckey,

I am embarrassed a bit that I didn't see the specific question and answer a few months before, so thank you for being polite. 

I got the spec's on a Hammond Filament Transformer from Jeffs who had gone through the same thing in April.  I have the same transformer on the way.

Thanks again

BV

[purpletele]

Hey Guys,

I am just about complete with my Hoffman Blues Junior Build.  I started from scratch etc.  I received some great advice and help from JeffS who has recently completed the conversion.

I have the amp up and running and it has a slight distortion and is operating at about 1/3rd of the potential volume.

I have stable voltage readings with B+ at 400 V.  I set the bias at 18 mA.  I am utilizing 6v6's

I went through multiple clean layout sheets and highlighted and checked things off.

I checked and measured all of the resistors.  I checked the capacitor layout and component ratings.

I went through with a tone generator and a signal amplifier and the signal wires are barking like they should


If anyone has any advice to what direction I should take or what area I should focus I would appreciate it.  You can tell that the amp wants to sound real smooth.  It just needs to be let out of its box.

Thanks

http://i284.photobucket.com/albums/ll1/otis810/Vallis%20Musical%20Instruments/Amplifiers/Hoffman%20Blues%20Junior/2015-07-23%2010.55.30_zpsrgdnwoqv.jpg

[EL34]

Hard to say


It almost always come down to something not wired correctly


This post can help with that
http://el34world.com/Forum/index.php?topic=17701.0

[John]

I have stable voltage readings with B+ at 400 V.  I set the bias at 18 mA.

I think your bias might be too cold. IF, and a big IF, I'm thinking it through right, you've got ~ 400 volts * .018 amps. That means you're dissipating just 7.2 watts out of the pair of tubes. I'm not subtracting screen current so that's a little off.


And please, don't go cranking up the bias until one of the other guys chimes in. I could be totally wrong and don't want you hurting your tubes because I gave bad advice! :) But if your bias is way off, that could explain the lack of volume. Sluckey, Willabe or someone will along shortly and they all know wayyyy more than me.

[purpletele]

Hard to say


It almost always come down to something not wired correctly


This post can help with that
http://el34world.com/Forum/index.php?topic=17701.0

I am cerrtainly a proponent of the highlighter system.  I think I am on round 5 by now.  I pulled the knobs and checked stamped values there as well.  I may have to set it aside and come back after a few days with fresh eyes.

I went through and checked resister values and placement.  I have certainly not torched anything but should I be considering that I might have a bad capacitor in the signal path?  Since the power system is stable and I can control the bias, should I asssume that the power side is functioning properly?

Thanks

[purpletele]

I have stable voltage readings with B+ at 400 V.  I set the bias at 18 mA.

I think your bias might be too cold. IF, and a big IF, I'm thinking it through right, you've got ~ 400 volts * .018 amps. That means you're dissipating just 7.2 watts out of the pair of tubes. I'm not subtracting screen current so that's a little off.


And please, don't go cranking up the bias until one of the other guys chimes in. I could be totally wrong and don't want you hurting your tubes because I gave bad advice! :) But if your bias is way off, that could explain the lack of volume. Sluckey, Willabe or someone will along shortly and they all know wayyyy more than me.

John,

Thanks, I did have the bias set too low, It ended up at 24 mA, but it didn't solve my volume/slight distortion issue.  I may have to  look at gettting some hands on advice for a fee so I can understand the steps to resolve this bug.  There not much to the circuit, but it can drive you to obsession. 

[jojokeo]

The photobucket picture shows a nice build from a distance but doesn't show what's needed to be seen. So no help there for anyone. Also no schem or layout to look at? That means no more reliable help unless you can provide more info. and it's an exercise in futility and a waste of time for anyone even trying. I for don't have time for things like this from others that can't help us help you. This should go without saying but is seen way too often and should be common sense.
 
 That said, I tend to agree with EL34 but again, how does anyone know we can't see anything?

[purpletele]

Jojokeo,

I appreciate your input.  I am new to this environment of communicating technical issues on tube amps.  Learning the steps to request help as I learn about tube amp circuits.  I am organizing my documents so that I can layout my questions properly.

Thanks

[DummyLoad]

use the existing bias winding. see attached.


not tested but should work fine.


--pete

[DummyLoad]

simulations...


--pete

[jojokeo]

I appreciate your input.  I am new to this environment of communicating technical issues on tube amps.  Learning the steps to request help as I learn about tube amp circuits.  I am organizing my documents so that I can layout my questions properly.

Glad you took it as it was intended & not meant to offend. Just stating that we need help to assist you properly & timely. Good luck!

[jojokeo]

use the existing bias winding. see attached.

not tested but should work fine.

I was thinking the same thing Pete (but not along with the drawing & sims  + I've never used a neg voltage doubler for something like this).

[purpletele]

Hey guy's,

I put some documents together as well as a link to some close up photos of tube connections and component connections.  This is regarding a my Blues Junior Project with slight distortion/Low volume.

I have attached a PDF Description of Symptoms.  I have also attached a PDF of the schematic with mods in red.

I really appreciate any input that anyone may have.  I would like to thank all of you in advance for taking the time to help solve these types of issues.  It is quite remarkable really, the amount of knowledge that is out there, and the willingness to share that knowledge.

Please let me know if this presentation of information is adequate and if there is any information or photos missing.

[u<iframe width="480" height="360" src="http://s284.photobucket.com/user/otis810/embed/slideshow/Vallis%20Musical%20Instruments/Amplifiers/Hoffman%20Troubleshooting"></iframe>rl][/url]

[John]

I like your presentation!


Not to hijack the thread, but I am curious; what function does C13 provide? I have yet to use feedback, so don't know much about the circuitry for it.


I'm sure you already tried this, but if not, if I were troubleshooting this for the first time, I would :


Measure heater voltages at each tube, just to be sure.
Bypass the tone stack (just to be sure)
Bypass the volume pots.
Both of those you can just clip in an alligator lead from the "entrance point" to the grid of the following stage.
Disconnect the FB circuit, and if easily done, lift the ground to C13 and R26 (because I'm not familiar with FB)


Sluckey and others can give you better advice, I'm sure. But this'll give you something to do in the meantime. 

[sluckey]

I see that you added a cathode follower to drive the tone stack rather than follow Hoffman's circuit. I would first eliminate the CF and wire V2 triodes parallel just like Hoffman did.

Next I would split node X into two parallel nodes, call them X₁ and X₂ just for reference. Each node would be identical with it's own resistor and cap. Now use X₁ to feed V1 and use X₂ to feed V2.

I would also temporarily put a 100pF cap across V3 pins 1 and 6 to see if that helps.

And temporarily disconnect the NFB to see if that helps.

[jojokeo]

I have something that is quick and easy:

After the 4th gain stage you show a 430k : 50k pot Master Vol voltage divider - this alone knocks down your signal to 10.4 % and that's with it being all the way turned up on "10". Make this a 500K audio taper / logarithmic type.

You also show a cap to ground being .001uF - this is also bleeding off a lot of your high-end portion of the signal that is there there too - disconnect it. It's possible it's drawn incorrectly and it's just a treble bleed cap? But I would lower it to 250pF or even 100pf if that's what it is? Whichever gives you not too bright of a response when you're at it's lower settings.

The phase inverter's tail is something I'm not used to seeing with it going through a cap before ground and the added resistance the way the feedback is being inserted. Seems overly complicated without needing to be.

*very nice information provided and exactly what helps to quickly identify what has got to be your issue?

[sluckey]

After the 4th gain stage you show a 430k : 50k pot Master Vol voltage divider - this alone knocks down your signal to 10.4 % and that's with it being all the way turned up on "10". Make this a 500K audio taper / logarithmic type.

Good catch! That should fix the low volume issue. That 430K:50K was probably OK when driving EL84s. But 6V6s require more signal voltage than EL84s to drive to full power.

I see that you added a cathode follower to drive the tone stack rather than follow Hoffman's circuit.

I looked at your pics and V2 seems to be wired parallel just like Hoffman's schematic. So why are you showing a cathode follower on your modified schematic? Is this a mod you are considering but have not done yet?

[purpletele]

I see that you added a cathode follower to drive the tone stack rather than follow Hoffman's circuit. I would first eliminate the CF and wire V2 triodes parallel just like Hoffman did.

Next I would split node X into two parallel nodes, call them X₁ and X₂ just for reference. Each node would be identical with it's own resistor and cap. Now use X₁ to feed V1 and use X₂ to feed V2.

I would also temporarily put a 100pF cap across V3 pins 1 and 6 to see if that helps.

And temporarily disconnect the NFB to see if that helps.

John,

Thank you for the testing methods.  I will be charting voltages again, and essentially re-checking everything.  I like the isolation technique.

Sluckey,

Thank you!  You picked up on an error on the schematic that I did not see.  I started with a colleagues schematic that separated the Hammond transformer.  I am going to redo the schematic starting from Hoffman's original.  My intent original intent was to wire this amp up as designed with the addition of 6v6's if it didn't create complications.  I did not wire up a cathode follower, I'll fix that on the schematic.

I will try a 100p cap across v3 pins I and 6 to see what happens.  I will disconnect the NFB


JoJo,

I was looking at the master volume, but probably not seeing  it the way you do. I will try a 500K Audio taper /Logrithmic type.  As far as the .001uF cap, it might be drawn wrong, I will double check.  I think it is important for me to re-check and correct the schematic.  I certainly was not trying to be 'Clever' with any modifications.  My approach is stock with upgrades after running. 

I will review the phase inverter tail. Once again, not trying to be 'Clever', I just want electrons to flow in the right direction without restrictions.


Thank you both for hanging in there with me.  I have some work to do.  I am going to start from scratch on the schematic so that I don't mis-represent mods that I did not perform.  I will go through each line of advice and check it out.  I will let provide an update.

BV

[purpletele]

After the 4th gain stage you show a 430k : 50k pot Master Vol voltage divider - this alone knocks down your signal to 10.4 % and that's with it being all the way turned up on "10". Make this a 500K audio taper / logarithmic type.

Good catch! That should fix the low volume issue. That 430K:50K was probably OK when driving EL84s. But 6V6s require more signal voltage than EL84s to drive to full power.

I see that you added a cathode follower to drive the tone stack rather than follow Hoffman's circuit.

I looked at your pics and V2 seems to be wired parallel just like Hoffman's schematic. So why are you showing a cathode follower on your modified schematic? Is this a mod you are considering but have not done yet?

Sluckey,

Thanks, sorry it is my error from not editing the schematic to match exactly.  Correcting now.

[sluckey]

I will review the phase inverter tail. Once again, not trying to be 'Clever', I just want electrons to flow in the right direction without restrictions.

There's nothing wrong with the PI tail.

[purpletele]

I have modified the original Hoffman schematic to include the few modifications that I have done.  I have attached it for further reference.

I'm looking around for a new 500K Master volume pot.  I'll let you know the results once I find one and install it

Thanks again.

[EL34]

I stock 500ka pots here
http://hoffmanamps.com/MyStore/catalog/PotsKnobs.htm

[purpletele]

Doug,

I see another order in my future.

[purpletele]

After the 4th gain stage you show a 430k : 50k pot Master Vol voltage divider - this alone knocks down your signal to 10.4 % and that's with it being all the way turned up on "10". Make this a 500K audio taper / logarithmic type.

Good catch! That should fix the low volume issue. That 430K:50K was probably OK when driving EL84s. But 6V6s require more signal voltage than EL84s to drive to full power.

I see that you added a cathode follower to drive the tone stack rather than follow Hoffman's circuit.

I looked at your pics and V2 seems to be wired parallel just like Hoffman's schematic. So why are you showing a cathode follower on your modified schematic? Is this a mod you are considering but have not done yet?

JoJokeo/Sluckey,John,

I got great results with just 50% of your recommendations so far!  I pulled the C9 cap completely as recommended and and that opened up the amp quite a bit.  The distortion seems to be gone and volume has increased.  This is without even testing or replacing the 50Kl Pot.  I placed a 100pF cap across V3 6/1, but didn't test it by itself.  I will go back and see if that makes a difference.  Ordering some new parts.  I will test the MV with a 470K Ohm resistor tomorrow night.

This has been a successful night of progress.  The amp is developing into what I had imagined, which is a real chimey amp that could have a rich thick growl.

That was really enjoyable to have those results.  Thanks for the help, that is quite remarkable.

BV

[tubenit]

I would have made the amp cathode biased & used a 270ohm/10w & 100uf/100v cap.  Not better but simply another way of approaching it.

With respect, Tubenit

[jojokeo]

I got great results with just 50% of your recommendations so far!  I pulled the C9 cap completely as recommended and and that opened up the amp quite a bit.  The distortion seems to be gone and volume has increased.  This is without even testing or replacing the 50Kl Pot.

This has been a successful night of progress.  The amp is developing into what I had imagined, which is a real chimey amp that could have a rich thick growl.

That was really enjoyable to have those results.  Thanks for the help, that is quite remarkable.

BV

Sweet! Good to hear you're on your way to dialing it in now.

[purpletele]

All,

My progress on the Blues Junior Amp has been limited to the great improvement of removing C9 from the MV circuit.  Modifying the MV to 500KA created a distorted sound.  I tried a 100KA pot and that too was distorted.  I switched it back to the 50KL Pot.

I have outlined the steps that I have taken in an attached outline.  It seems to me that the tone is there and just needs some power.  It seems logical that I may have a structural issue of a ground that may be limiting the output?  Should I be considering the shielded wire connections?  I was meticulous in the make up, but it is possible that I have made a mistake.

Is it possible that I have grounded or not grounded correctly on the make up of a shielded wire connection, and would that have an affect on the power?  What would the steps be to test that theory?

Thanks for any insights

BV

[John]

I have grounded or not grounded correctly on the make up of a shielded wire connection

Probably not the problem, since you do get some volume, but I've many times had the insulation on the inner wire melt while soldering, and ground it out on the shield part of the cable. I guess the best way to check for that is to measure from each grid on your tubes to ground. If you hit one that's only say 50k or so, that *might* be where your signal is going.


But usually, when I melt the insulation it's making great contact. 

[sluckey]

but I've many times had the insulation on the inner wire melt while soldering, and ground it out on the shield part of the cable.

Switch to RG-316 and never worry about that again.

[purpletele]

but I've many times had the insulation on the inner wire melt while soldering, and ground it out on the shield part of the cable.

Switch to RG-316 and never worry about that again.

I am looking at RG-316 now.  I am going to test that signal to see if that uncovers anything.

I imagine that the RG-316 does not have a grounding shield that needs to be pulled back and trimmed?

[sluckey]

I imagine that the RG-316 does not have a grounding shield that needs to be pulled back and trimmed?

Sure it does. The center conductor has PTFE (Teflon) insulation. Ever that opaque brown skin over the braid is some high temp plastic that won't melt unless you lay your iron on it for a while. This is how I prepare RG-316. Don't try this with RG-174!

[purpletele]

Nice!  Great tip

[John]

but I've many times had the insulation on the inner wire melt while soldering, and ground it out on the shield part of the cable.

Switch to RG-316 and never worry about that again.

I know! I've looked at it before, and when I run out of the 174 that'll be that. By that time, Doug will probably be stocking the 316, the little capitalist that he is. 

[purpletele]

Nice!  Great tip

[jojokeo]

My progress on the Blues Junior Amp has been limited to the great improvement of removing C9 from the MV circuit.  Modifying the MV to 500KA created a distorted sound.  I tried a 100KA pot and that too was distorted.  I switched it back to the 50KL Pot.

I have outlined the steps that I have taken in an attached outline.  It seems to me that the tone is there and just needs some power.  It seems logical that I may have a structural issue of a ground that may be limiting the output?  Should I be considering the shielded wire connections?  I was meticulous in the make up, but it is possible that I have made a mistake.

Is it possible that I have grounded or not grounded correctly on the make up of a shielded wire connection, and would that have an affect on the power?  What would the steps be to test that theory?

The 500k value will not load down your signal as much as the 50k will, so you are lowering overall gain by using 50k value and increasing gain by using 100k & 500k....BUT to be getting distortion? Hmmm. As long as you are down low on the pot it shouldn't be distorting, maybe only when it's turned up more.
 
To answer your shielded cable question, yes test your inner conductor wire's continuity to ground. You should show infinity ohms but might have to disconnect it or pull a tube so nothing is in parallel.
When I make my shielded cables I rarely ever have a problem using the normal RG-174 or equal. I have a variable adjustment to control my iron's heat/power - but I'll have it pretty hot anyway since I like to get on and off quickly while soldering anything. I rarely ever use heat sinks on anything for decades and can't remember the last time I ever burned anything? Maybe it's all in technique & experience but I don't think so. I'm not that different than anyone else, it just takes common sense, solder paste when needed, and normal kester solder.
 
Making shielded cables:
Strip back plenty off the insulation, and separate the ground shielding and twist together. Strip inner insulation down and twist inner conductor wire. Pretty basic so far...
Place a drop of solder paste on both exposed wires and pre-tin them. This step is important and maybe where others go wrong? Then before I go forward and use it I will simply check the continuity and you will know right then if you over heated anything or not? Next, pre-tin the pot or socket lugs you are attaching to. Then it's a very simple matter of getting on & off the actual final soldering of the connections. This works every time...for me.

[airmech50]

Just wanted to comment on the 50k master volume issue. I changed mine to 1M and it works much better. Amp has much more volume and sounds better overall. I will try 500k as at setting over 9 on the dial I get some hum. Signal too hot? Also for info I'm using 5881 power tubes with the proper transformers. I just wanted to let anyone reading in the future know that it does indeed work better. Thanks JOJOKEO for your observations.