Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Garrett Dawkins on April 16, 2025, 04:44:58 pm
-
I recently acquired a fried Fender Blues Jr. on the cheap. The circuit board was toast, so I gutted the amp with the intent to build something cool. I recently built a Rob Robinette JCM800 6v6 in an old Ampeg SS chassis and it came out great, so was planning to stick to the "mutt" theme and try to make this into something cool. I found Hoffman's Blues Junior build kit and thought that might be an easy way to go. But I have a few questions.
1. Anyone here done the Hoffman BJ conversion and how did it sound?
2. Is there any other amp plan out there that would be a good candidate to use the parts I have?
3. Is it worth it to upgrade the Output Transformer. I've heard the stock ones are not that great but wondered if anyone has had any experience. That's an expensive upgrade. Wonding if it's worth it.
Here's what I have to work with...
- Fender BJ III Chassis, Faceplate, and Cab
- original Fender Eminence Speaker
- original fender Output Transformer (I believe its 8000k ohm / 8 ohm)
- a Mojo780 / Heyboer replacement PT (a pleasant surprise!)
- an Accutronics reverb pan (not useful for the Hoffman build)
- Knobs / Screws / Switch / etc.
Thank for the advice!
-
here's Doug's board conversion
https://el34world.com/schematics.htm#Hoffman_Blues_Junior_Conversion
-
I modified a BJ to more AB763 specs, it turned out really good, using the circuit boards and original transformers. Still one unused triode though.
I think it would work well for a jcm-800 type build based on the existing iron and chassis.
-
I modified a BJ to more AB763 specs, it turned out really good, using the circuit boards and original transformers. Still one unused triode though.
I think it would work well for a jcm-800 type build based on the existing iron and chassis.
An AB763 spec sounds interesting - Did you stick with the EL84s or switch to 6v6/6L6? Were you able to do Reverb or Vibrato? Do you have a schematic of the build you'd be willing to share?
My first thought was also a JCM800, but I just did that build and was going for something more fenderish.
Thanks for the help!
-
I expect your easiest path is to stick with el84 power tubes since most amps with 6v6 or 6l6 tubes use a PT with higher voltage. You might could go with 6AQ5 tubes with your PT to get you into 6v6 territory.
-
I used the existing circuit board, and the existing reverb. I basically changed the tone stack and modified the coupling/bypass caps to change frequencies. It also came with attenuating signal and modifying the NFB loop.
A JCM-800 build is fairly easy to do with EL84 tubes, though you do need to attenuate the signal compared to an EL34 build. I built a similar amp, and I modified the pot values, and peaker cap values so they worked with the EL84's.
-
I wouldn't hesitate at all to do Hoffman's B-Jr mod. I've built 3 of his designs, one of them twice,,, and gonna do it again. his stuff works.
-
I wouldn't hesitate at all to do Hoffman's B-Jr mod. I've built 3 of his designs, one of them twice,,, and gonna do it again. his stuff works.
Excellent! Thanks for the feedback. That is certainly the easy route.
One more question - I have a Fender 4H/50ma Choke leftover from another project. I noticed Hoffman used a choke in his build. What are the Pros/Cons of adding the choke in the power supply? It looks like the one in the picture is a 3H, so would using a 4H be appropriate? (I'm still learning about induction LOL). I assume the choke would replace the resistor between B+ and Z (see pic) correct?
-
the choke would replace the resistor between B+ and Z (see pic) correct?
there the choke would "feel" the full current of the amp, so you need that spec also. If it can't handle the current then it could be used for G2 on the PA tubes to "stabilize" any current surges on the grids
-
wonder how a BJ build would sound/change with a 5Y3 instead of SS
Would need a 5v source of coarse, different reservoir
Would be unique
-
the choke would replace the resistor between B+ and Z (see pic) correct?
there the choke would "feel" the full current of the amp, so you need that spec also. If it can't handle the current then it could be used for G2 on the PA tubes to "stabilize" any current surges on the grids
Hmm. It looks like a push/pull pair of el84s would pull about 100-120ma. So, I'm assuming my choke is underrated for that application. Good call.
-
the choke would replace the resistor between B+ and Z (see pic) correct?
there the choke would "feel" the full current of the amp, so you need that spec also. If it can't handle the current then it could be used for G2 on the PA tubes to "stabilize" any current surges on the grids
Hmm. It looks like a push/pull pair of el84s would pull about 100-120ma. So, I'm assuming my choke is underrated for that application. Good call.
Your 50mA choke is fine. Output tubes plate supply comes from node "B+". The choke only has to supply current for the screens and other little tubes.
-
wonder how a BJ build would sound/change with a 5Y3 instead of SS
Would need a 5v source of coarse, different reservoir
Would be unique
Cool idea, but my PT lacks a 5v feed and I'm not confident I could build a correct voltage divider. I'm going to stick with the KISS principal.
-
Output tubes plate supply comes from node "B+"
that's what a quick look gets ya!
always nice get'n pulled outta the deep end, again :icon_biggrin:
-
I've built Doug's BJ conversion a number of times, and they sound great.
Best not to try to make this into something it can never be -- the iron is designed for EL84's, so unless you want to make the investment in bigger trannies, stick with what you have.
SS rectifier works well in this circuit.
It is true that the stock OT is a bit anemic, but having said that I would build the amp and see if you feel it needs more oomph. In my experience with this amp, the difference is minimal. Certainly your speaker choice will have more effect on the tone.
I have also built this circuit with cathode biased EL84's, similar to the current BJ design. I don't think cathode biasing make's much difference in tone, at least with this circuit. I believe this was done due to decrease servicing requirements of the prior models, which were biassed very hot and ran through EL84's like crazy.
Comment: For some reason, the BJ has always been a magnet for modders, trying this, that and the other. Bill Machrone (BillM) made a cottage industry of it years ago, but there are many others who sold mods (Fromel being the most prominent). In my opinion, the basic mods do NOT change the basic characteristics of this sweet little amp.
Go with Doug's conversion, you won't be sorry.
-
Thanks for all the great input. I believe I'll go with what I have, add the choke and build to Doug's specs. Looks like a fun project.
Comment: For some reason, the BJ has always been a magnet for modders, trying this, that and the other. Bill Machrone (BillM) made a cottage industry of it years ago, but there are many others who sold mods (Fromel being the most prominent). In my opinion, the basic mods do NOT change the basic characteristics of this sweet little amp.
I have to wonder if some poorly executed mods led to the demise of this amp. The prior owner(s) made a mess of the circuit board and burned up a lot of parts.
-
The Blues Jr I gutted was heavily modified, so I having to make a few adjustments to Doug's standard Blues Junior build plan and could use some advice.
The power switch installed in the amp was a 3 position DPST instead of the typical 2 position switch, so I figured it's best to hook it up with an off/standby/on configuration. I want to make sure I connect it up correctly.
Here's what I'm working with...
My PT has 3 secondaries - 1. HT, 2. Bias, 3. Heaters
HT is running to a solid-state bridge rectifier
Doug's plan is Fixed bias - So the secondary has a bias tap
Can you confirm this is the correct way to wire the switch and transformer
Primary Side
AC Power Black (line) -> to fuse-> to PWR switch -> to PT Primary Black -Question: Is the fuse best on the Black or White side?
AC Power White (neutral) -> directly to transformer Primary white -Question: is it ok that the switch is not breaking the power circuit both sides. I know this is common in older amps, but not typical in new amps.
Power earth -> to dedicated ground screw on chassis
Secondary Side
HT Secondary #1 -> to positive input of the Rectifier -> to PWR switch -> Question: Should the switch break the circuit after the rectifier between the rectifier output and the first capacitor?
HT Secondary #2 -> negative side input of the Rectifier
Heater Secondary 1 & 2 -> to Pilot Light -> artificial tap -> Filaments
Bias Secondary #1 -> to -> Bias Diode Question: This would get power on standby at switch position 2. I assume this is ok?
Bias Secondary #2 -> to Ground
Thanks for the help and let me know if I need to clarify my explanation.
-
Thanks for the help and let me know if I need to clarify my explanation.
Can you please post a schematic of the proposed switch wiring?
-
Thanks for the help and let me know if I need to clarify my explanation.
Can you please post a schematic of the proposed switch wiring?
Sure - is there a good web tool or software you would recommend? I tried Circuitlab but it doesn't support tubes.
-
Sure - is there a good web tool or software you would recommend? I tried Circuitlab but it doesn't support tubes.
Here;
https://el34world.com/Forum/index.php?board=26.0
The above is the topic page on this and in it you can download all of Doug's parts. Saves a LOT of time.
And an older more simple one;
https://el34world.com/Forum/index.php?topic=590.0
With this link, it's the topic page for PC Express/JSchem and you can take parts from anybodies schematic in there. Or take an already drawn schematic and download it, then modify it for your build.
https://el34world.com/Forum/index.php?board=12.0
-
Perfect - I'll get to work on learning those programs, so I can do a better job in the future. I think i figure out the wiring question by looking at some other fender schematics. I still have two generic questions. (Assuming standard US 120v AC power)
Should the main AC fuse be on the black or white side? I've seen it both ways.
Should the standby switch cut the HT circuit before or after the solid-state rectifier? Does it matter?
Is there any safety concern if the main AC pwr supply switch only cuts the Black lead vs. both the Black and White? (old Fenders only cut one side, new amps always cut both)
-
The internet is your friend.. Also if you notice Doug's circuit interrupts both AC and Neutral with his power switch and there is no standby, which is really not needed unless you just have to have one. Although I haven't researched it I would lay odds down that interrupting both hot and neutral with a power on/off is irequired by the current NEC.
IMO the hot lead should be fused i.e. hot lead - fuse - switch in that order., Neutral - switch - PT in that order. How was the BJ wired?
-
How was the BJ wired?
The amp had been modified before I got it. It has a 3-position power switch, hence the idea to add a standby. So, I'd need a new switch to do the way Doug designed it.
Thanks for the help!
-
in the USA, we always switch the "line" side of power.... black. neutral (white) is optional
how many poles and throws does the switch have? is there a neutral/off position?
-
in the USA, we always switch the "line" side of power.... black. neutral (white) is optional
how many poles and throws does the switch have? is there a neutral/off position?
3 position DPST - Like this one (Which includes a schematic from Doug that answers my question.)
https://el34world.com/charts/SwitchDPST.htm
-
An AB763 spec sounds interesting - Did you stick with the EL84s or switch to 6v6/6L6? Were you able to do Reverb or Vibrato? Do you have a schematic of the build you'd be willing to share?
This is an EL84 amp I built last year, loosely based on the 60s Marshall 18W amp they called a 1974 when it had a 12" speaker. (Gotta love the logic Marshall uses to name their amps.) I added a one-tube reverb suggested by tubenit and I'm more than happy with it. Also added a Master Volume which gives a lot of control over the sound and mix of any distortion. It can play clean or dirty--whatever you want.
Should be a nice project. Good luck with it.
-
I finished up the amp building exactly to Doug's Blue Jr Conversion spec except for adding the standby switch. Everything works ok, but the amp just doesn't sound all that great to me.
The volume is good (It's loud!) and all the controls appear to be working fine. However, it has too way much treble for my taste. It sounds like a fender with the bright stuck in the "on" position and it lacks Fender-ish sustain and warmth. The overdrive tone is overly fuzzy when the preamp breaks up and seems to jump right past the bluesy "growl" position. The fat switch and meat controls work but make it even brighter.
As far as I can tell the basics check out. I've checked the resistor values. The voltages match the schematic. I set the bias at ~70% dissipation. (I've tried the bias a bit lower, but it doesn't help.)
Does anyone have experience with this build? Is that normal? Any ideas on how to tame the high end (maybe increasing the grid stop resistor from 10k to 68k?) or is there a build mistakes I might be missing?
-
Hmm, that doesn't sound right.
Photos of your build would be helpful.
-
Here's a picture. I did add a ground to the preamp side right after I took this one.
-
one more...
-
I'm not a fan of the tone of a blues junior. You could open up the 1st coupling cap to a .022 or .047 instead of a .0022.
You could also change the tone stack to more emulate a twin or super reverb.
These changes could come with signal attenuation.
-
Hi Garrett, great work! I also did the BJ conversion exactly according to Doug's plan. I used the Heyboer TO20 for the OT. I got the tone capacitors from TAD Sprague/Orange Drops (715 P). After the conversion, I used "normal" tubes (JJ/EH). The BJ's tone was lifeless and very mid-range. So I also gave the BJ new tubes (TAD tube set for BJ). And "wow" the sun came up. However, the master should already be set to at least 6 and the mids around 3. So I play the BJ through a Tone King Ironman Attunator, and I'm really happy. Greetings from Germany, Uli.
-
First off, nice looking work!
Tough for me to diagnose at a distance. Your description sounds like there's lots of distortion, or even possibly oscillation.
Couple of simple things to try:
1. Check the bias of your output tubes. I normally run EL84's cool at 65%, but most folks run them at 70%. Just make sure they aren't way out of spec (60-70% max plate dissipation is good).
2. Reverse the blue and brown OT leads to your output tubes. Your sound will either be better or worse.
I'm assuming your voltages are as expected, right?
If the above does not help, my personal next step would be to pull out my 'scope and dive into the signal path to see where the problem might be.
-
Thanks for all the suggestions! I'll try a few of those ideas and get back to you. I believe the amp is operating correctly. It's more that I want to tweak the character of the tone to cut some gain and high end.
Neil1956 - Great point about adjusting the tubes first. I had a JJ/Ruby 12AX7 "HG+" in V1. I swapped out V1 for an unlabeled vintage 12a[?]7 I had on my bench and that helped warm the tone up tremendously and smoothed out the overdrive tone. I'm also figuring out the tone controls, as they interact with each other quite a bit. Rolling off the guitar volume or using lower gain pickups seems to be improve it even more.
AlNewman - I'll try some different values in the V1 coupling cap and see what happens. That seems like a great next step.
acheld - Yep, I'm biased right at 70% now, so I think that's ok. I don't have a scope but getting rid of the high gain-tube in V1 seemed to solve that problem.
Much appreciated!
-
Ok. I took a break on this project for a while and decided it was a great time to learn to use an oscilloscope and confirm I didn't have a problem before I start making any changes. So, now that I have a scope and I'm getting the hang of using it, I can confirm there's nothing funky with the signal path like blocking distortion or other unwanted noises or spikes. The voltages are correct, part values are correct, no cold soldier - essentially the amp is working as designed. It appears that this is a personal problem; I just don't like the way it sounds.
A few things I learned about it with the scope,
1. The preamp has a ton of gain even in the early stages (too much) and starts to distort the waveform with the pre-amp around 5.
2. The mid control has a HUGE impact on the amplitude of the signal. It really cuts out a lot of the signal vs shaping the tone.
3. The fat switch seems to add a huge amount of volume (amplitude boost) but just pushes the distortion into icky/nasty territory instead of smooth, harmonic, ear pleasing gain.
3. The jumpered 2nd and 3rd triodes create a huge signal boost going into the phase inverter that makes the signal MUCH louder and results in fairly harsh distortion down the line. I guess it does push the EL-84s to break up sooner. That part might be helpful.
What's driving me nuts -
The clean sound is OK but starts to break up super early. Decent with a stock Strat, old PAFs, but can't deal with hot pickups.
The amp only has decent sustain ONLY if you lower the preamp volume and crank the power amp into clipping. That seems to negate the value of the three prior gain stages.
The preamp distortion is harsh, lacks sustain, and I can’t get a pleasing harmonic quality.
There is no "warm" anything. No crunchy distortion, no clearly defined lead tones, nothing that sounds remotely like a Santana like "singy" lead tone.
I’m wondering if I should just start over and rethink the entire pre-amp section. Here’s what I’m thinking – please tell me if this is crazy.
- Change the first grid stop resistor to 64k to cut off some of the initial highs.
- lower the bypass cap values on the first two stages. (maybe 4K to 25K range)
- Remove the jumpers between the V2a and V2b triodes. I think the extra gain is overkill and sucking tone.
- And MAYBE, use the extra V2a triode to add a cold clipper between the tone stack and the final gain stage (V2b) to get a smoother sounding overdrive without all the distortion. (I’d love some thoughts on this one. Would it work? I’d have to squeeze the parts in off the board, so it might look a little point-to-point and risk triggering my OCD. LOL)
- I’ve also read up on the LTP “Smoothing Cap” concept. That might be a good add too. But more of a final addition after I fix the rest.
For reference - https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf
Thanks for the help!
-
Change V1 to an AU7, if that gets you closer to "yep", then look at the data sheet and re-bias it. I typically wind up with a cathode R ~2.2k and plate R ~~ 56k
that should keep you "clean" through the TS with enough left over to give the next stage some OD room.
-
I just don't like the way it sounds.
…
1. The preamp has a ton of gain even in the early stages (too much) and starts to distort the waveform with the pre-amp around 5.
2. The mid control has a HUGE impact on the amplitude of the signal. It really cuts out a lot of the signal vs shaping the tone.
3. The fat switch seems to add a huge amount of volume (amplitude boost) but just pushes the distortion into icky/nasty territory instead of smooth, harmonic, ear pleasing gain.
3. The jumpered 2nd and 3rd triodes create a huge signal boost going into the phase inverter that makes the signal MUCH louder and results in fairly harsh distortion down the line. I guess it does push the EL-84s to break up sooner. That part might be helpful.
What's driving me nuts -
The clean sound is OK but starts to break up super early. Decent with a stock Strat, old PAFs, but can't deal with hot pickups.
The amp only has decent sustain ONLY if you lower the preamp volume and crank the power amp into clipping. That seems to negate the value of the three prior gain stages.
The preamp distortion is harsh, lacks sustain, and I can’t get a pleasing harmonic quality.
There is no "warm" anything. No crunchy distortion, no clearly defined lead tones, nothing that sounds remotely like a Santana like "singy" lead tone.
I’m wondering if I should just start over and rethink the entire pre-amp section.
EL84s only use a tiny amount of bias (grid-to-cathode) voltage, so it only takes 2 or 3 12AX7 triodes to push EL84 grids into clipping. Short of going to octal output tubes (6V6s typically need twice the bias voltage that EL84s need), then some tweaks to the preamp may help. The easiest way of losing gain is to remove one or more cathode bypass caps. In a high gain preamp, you can improve sustain/overdrive by alternating hot* biased stages with cold** biased stages ending with a hard cold clipping last stage*** (before the PI). You might also attenuate preamp gain with hard wired voltage dividers between stages or putting split load resistors on one or more preamp tubes.
* on a 12ax7 make Ck= 820R
** Ck =10k
*** Ck = 39k
-
I just don't like the way it sounds.
…
1. The preamp has a ton of gain even in the early stages (too much) and starts to distort the waveform with the pre-amp around 5.
2. The mid control has a HUGE impact on the amplitude of the signal. It really cuts out a lot of the signal vs shaping the tone.
3. The fat switch seems to add a huge amount of volume (amplitude boost) but just pushes the distortion into icky/nasty territory instead of smooth, harmonic, ear pleasing gain.
3. The jumpered 2nd and 3rd triodes create a huge signal boost going into the phase inverter that makes the signal MUCH louder and results in fairly harsh distortion down the line. I guess it does push the EL-84s to break up sooner. That part might be helpful.
What's driving me nuts -
The clean sound is OK but starts to break up super early. Decent with a stock Strat, old PAFs, but can't deal with hot pickups.
The amp only has decent sustain ONLY if you lower the preamp volume and crank the power amp into clipping. That seems to negate the value of the three prior gain stages.
The preamp distortion is harsh, lacks sustain, and I can’t get a pleasing harmonic quality.
There is no "warm" anything. No crunchy distortion, no clearly defined lead tones, nothing that sounds remotely like a Santana like "singy" lead tone.
I’m wondering if I should just start over and rethink the entire pre-amp section.
EL84s only use a tiny amount of bias (grid-to-cathode) voltage, so it only takes 2 or 3 12AX7 triodes to push EL84 grids into clipping. Short of going to octal output tubes (6V6s typically need twice the bias voltage that EL84s need), then some tweaks to the preamp may help. The easiest way of losing gain is to remove one or more cathode bypass caps. In a high gain preamp, you can improve sustain/overdrive by alternating hot* biased stages with cold** biased stages ending with a hard cold clipping last stage*** (before the PI). You might also attenuate preamp gain with hard wired voltage dividers between stages or putting split load resistors on one or more preamp tubes.
* on a 12ax7 make Ck= 820R
** Ck =10k
*** Ck = 39k
I like this idea. And keeping it to 3 stages is much easier with the current configuration. Just to make sure I understand your suggestion - is the description below correct?
---V1a=Hot Bias w/820 Cathode Resistor and remove or lower bypass cap value. (Or connect bypass cap to the Fat switch?)
---V1b=Cold Bias 10K Cathode Resistor and remove bypass cap
(TMB tone stack)
---V2a=Very Cold Bias. 29k Cathode Resistor
---V2b - Unused
LTP Phase Inverter (V3)
I will try the above and see where it lands before digging into the voltage dividers and attenuation.
Great suggestions - thanks!
-
You might want to have a normal bias in V1a, then cold bias, then hot, then very cold then PI.
Or if only three stages, normal, hot, then cold
-
Ok, will give this a shot when I get some time to work on in and report back. Thanks everyone for the advice and suggestions - even the ones I didn't take are much appreciated!
I have a ton to learn about amp building. This has been a highly educational project for me.
-
You might try others suggestions but if you find YMMV you gotta experiment til you get what you want.
-
You might want to have a normal bias in V1a, then cold bias, then hot, then very cold then PI.
Or if only three stages, normal, hot, then cold
Wow! That was the ticket. Like a whole new amp.
Here's what I changed:
Stage one (V1a) - increased the grid stopper from 10k to 64k
Stage two (V1b) - Changed the cathode resistor from 1.5K to 820
Stage three -(V2a) - Changed the cathode resistor from 1.5 to 10K (cold clipper)
Removed the V2a - V2b jumpers - V2b is no longer used
I also tweaked the bias on the power tubes. My original measurements must have been off. The EL-84s were running a little cold. Pushed it back to 65-70% range. That helped eliminate the fizz.
It's now much warmer, lots of crunchy distortion and sustain. It sounds great with the power section pushed into clipping. It can't quite reach the "high-gain" territory, but a Tube Driver or ProCo Rat easily push it into that range if needed. The preamp creates nice flat tops on the waves when viewed on a scope.
I'm a happy camper now. Thanks for the guidance!
-
You can try using V2b as a bypassable cathode follower to drive the tone stack. This will put more signal into the cold clipper and will give you more clipping. The triode is just sitting there... Why not put it to work?