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Thread: So I have been given free reign to gut a Blues Jr and do whatever I want with it

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    So I have been given free reign to gut a Blues Jr and do whatever I want with it

    The plan is to throw out all the boards but use the PT, OT, tubes, speaker and verb tank. Reverb is a must in this situation.

    It will need to use a diode rectifier since the PT doesn't have 5v taps. Was thinking of doing something along the lines of an AC15 or Matchless Lightning, sans tube recto. Any other suggestions welcomed. It will be a backline amp for a blues jam night so I can literally do whatever I want as long as it has reverb. What would be your inclination to build in this scenario?
    ~Semi-No0b Hobbyist~

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    Supporting Member mozz's Avatar
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    I'd leave it stock and do some mods, BillM website has the info. I changed the stock OT for a buddys amp with the TO20B with some tone stack mods and he loves it, gets more play than all his other amps.
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    If its for a blues amp and you're keeping the BJ PT and OT, then I vote for something with 6V6s that keeps cleanish up to about 7/10ths on the volume dial, like a PR or DR
    Building a better world (one tube amp at a time)

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    You don't need a 5v tap to use a tube rectifier in a small amp; an indirectly heated 6.3v tube such as an EZ81 can be powered off the regular heater supply, just how Vox does it in the original AC15.

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    I was asked to add a one-tube reverb to a Blues Jr. I used the very-common 12AX7 circuit commonly found on the web. We kept everything else stock, and the customer loved the result. It too gets more play than his other amps.

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    In this case the amp has a problem that I cannot figure out (maybe the board is bad?) and he has two other Blues Jr's so having a different flavor available on stage is very much welcomed. It's also a good thing to get something with my name on it up there to be played and seen every week

    Still considering what to put in it. I do like 6v6's so a PR is a possibility.... I'm doing it practically pro bono so trying to keep from having to buy too many parts, but this guy does a ton for the local music community so I won't mind too much if I do end up spending a little money on it. Hmmm decisions decisions
    ~Semi-No0b Hobbyist~

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    Ah, yeah, if it's un-repairable, might as well gut it.

    One small amp I have always wanted to build is the Gibson GA-1RVT. It's got reverb AND tremolo in a circuit small enough to stuff into a 5F1 Champ. I have not heard any recordings of it, however, so it may not actually sound all that good. There are larger Gibson circuits you might be interested in building that seem to be well-regarded (and there are recordings of these). This route would be the "bragging rights for a rare circuit and rare tone" approach.

    If I were to build another amp from scratch, I would probably build just the clean Normal channel from a JMI-era VOX AC15 (with EF86 in pentode mode). I'd do all the tricks to reduce tube microphonics including suspension-mounting the tube socket, etc. It's not a complex circuit but sounds amazing from quiet to dimed. This would probably be the cheapest route. You might also consider building a clone of the newer AC15HW1X (but this has no magic-sounding EF86).

    An extension of this idea is to build the AC15 Normal channel and add the ability to switch between Top Boost, EF86 as pentode, EF86 as triode, AC30-style 12AX7 as first preamp stage. He would get all the JMI-era VOX sounds in one amp. You should be able to easily add a one-tube reverb to any AC15 build.

    Another from-scratch build I'd like to do is a push-pull design based on the 6BM8. That idea's been discussed at length on this forum somewhere, and the 6BM8 is known to sound amazing.

    I'm not a fan of the PR. I built one as my first build and liked it, but not enough to play it much. It was too boring--none of its features were all that good. Kind of a Jack-of-all-trades, master of none amp.
    Last edited by dchang0; 02-25-2017 at 08:04 PM.

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    Woodgrinder/Pickupwinder copperheadroads's Avatar
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    Great suggestions so far , 6v6 would be my first thing to do .& the reverb on the BJ is it tube driven ?
    "UP here in the Canada we shoot things we don't understand"

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    Stock reverb is not tube driven.

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    I recently gutted a Tweed Blues Jr and built a Princeton 6g2 in it. Turned out very nice and I like the fact that it uses a 12" speaker instead of the usual 10".

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    Quote Originally Posted by dchang0 View Post
    Ah, yeah, if it's un-repairable, might as well gut it.

    One small amp I have always wanted to build is the Gibson GA-1RVT. It's got reverb AND tremolo in a circuit small enough to stuff into a 5F1 Champ. I have not heard any recordings of it, however, so it may not actually sound all that good. There are larger Gibson circuits you might be interested in building that seem to be well-regarded (and there are recordings of these). This route would be the "bragging rights for a rare circuit and rare tone" approach.

    If I were to build another amp from scratch, I would probably build just the clean Normal channel from a JMI-era VOX AC15 (with EF86 in pentode mode). I'd do all the tricks to reduce tube microphonics including suspension-mounting the tube socket, etc. It's not a complex circuit but sounds amazing from quiet to dimed. This would probably be the cheapest route. You might also consider building a clone of the newer AC15HW1X (but this has no magic-sounding EF86).

    An extension of this idea is to build the AC15 Normal channel and add the ability to switch between Top Boost, EF86 as pentode, EF86 as triode, AC30-style 12AX7 as first preamp stage. He would get all the JMI-era VOX sounds in one amp. You should be able to easily add a one-tube reverb to any AC15 build.

    Another from-scratch build I'd like to do is a push-pull design based on the 6BM8. That idea's been discussed at length on this forum somewhere, and the 6BM8 is known to sound amazing.

    I'm not a fan of the PR. I built one as my first build and liked it, but not enough to play it much. It was too boring--none of its features were all that good. Kind of a Jack-of-all-trades, master of none amp.

    I've built a couple of copies of the early Ac15 circuit w/ ef86. I LOVED the ef86 channel and hated the bright channel. But just the single channel ef86 with reverb might be the ticket. And with a master volume it could still get all the guts of the pentode front end and not piss off the owners of the small club. Hmmmmmmmmmmmmmmm
    copperheadroads and dchang0 like this.
    ~Semi-No0b Hobbyist~

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    Question: do you think an EF86 AC15 channel would sound drastically different with a diode rectifier vs an EZ81? or will it just be a little tighter sounding?
    ~Semi-No0b Hobbyist~

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    I have not tried a diode rectifier in a JMI-era AC15 circuit myself.

    This thread may help you decide:
    https://www.thegearpage.net/board/in...-ac15s.884179/

    I too did not like the newer AC15 models' tone, but there are other factors that may be more prominent.

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    Old Timer tedmich's Avatar
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    Quote Originally Posted by mort View Post
    Question: do you think an EF86 AC15 channel would sound drastically different with a diode rectifier vs an EZ81? or will it just be a little tighter sounding?
    Korg didn't think so and used a SS rectifier in their AC15's for a very long time I believe.

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    What about omitting the choke and using a resistor in its place? Big difference or small difference?
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    In your case, I would eliminate the choke just because of its size and cost even if there was an audible improvement in having it. I also expect the audible difference to be very small, although I have not myself tried the JMI-era circuit without the choke. IMO, the downsides far outweigh the potential benefit.

    And also, I would keep the tube rectifier. Korg didn't switch to SS rectifiers for tone but to cut costs and improve reliability. I like sag, so I always build with tube rectifiers.
    But you are working within tighter constraints than I would be (budget, space, and filament current), so keeping the tube rectifier may not be an option for you.

    What you might do is start with diode rectifier and resistor in place of the choke and just leave space for the circuitry to switch over to a tube rectifier and choke if the outcome is not to his liking. It's not like you'll be out a lot of money for trying the diode and resistor option first.

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    I think I'll try the diodes first. Could add a tube later if I felt so inclined.

    This is probably all I need I'm guessing:


    I did see a 6 diode rectifier like on a Twin. Don't know what the difference is...

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    I'm not sure why they used six diodes either, but I'll bet it was a safety measure in case one of them burned out. It might also be for voltage rating.

    Also, for the choke resistor, perhaps you could use a high-wattage wirewound resistor so that you get some of the choke-like behavior.
    Last edited by dchang0; 02-26-2017 at 12:15 AM.

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    ok here's a preliminary drawing without a reverb circuit. I'm not quite sure if I got the PI just right. I copied the vox but the bottom part of the PI is where it ties into the channel that I'm omitting. I noticed in a Fender LTP there's a final 47 ohm resistor before ground. Do I need that resistor in place?


    86reverbvox.jpg

    86reverbnoverb.jpg
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    Quote Originally Posted by dchang0 View Post
    Ah, yeah, if it's un-repairable, might as well gut it.

    One small amp I have always wanted to build is the Gibson GA-1RVT. It's got reverb AND tremolo in a circuit small enough to stuff into a 5F1 Champ. I have not heard any recordings of it, however, so it may not actually sound all that good. There are larger Gibson circuits you might be interested in building that seem to be well-regarded (and there are recordings of these). This route would be the "bragging rights for a rare circuit and rare tone" approach.

    I used to have a more modern GA30RVS and that was a sweet sounding amp. Always wanted to play around with its 15w hijo
    ~Semi-No0b Hobbyist~

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    Thanks for confirming the GA30RVS sounded sweet. I am even more likely to build one of its related but smaller siblings.

    Here's Rob Robinette's excellent explanation of how the Fender AB763 works. Note that the 47 ohm resistor (along with the 820 ohm at the top) is labeled "NFB (Negative Feedback)" at the top of page 3.

    https://robrobinette.com/images/Guit..._Robinette.pdf

    Given that the AC15 was famously designed without negative feedback, I think you should leave that 47 ohm resistor out.

    I am looking for a Matchless Nighthawk Reverb schematic for you--that is going to be very, very close to what you want. Certainly it will be identical in sections to what you want--it has a master volume and tube reverb. So the only difference will be small tonal tweaks (perhaps they run the EF86 hotter or cooler than the AC15, etc.).

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    Grrrrr. I am finding Matchless DC30, Spitfire, Lightning, and Chieftain Reverb schematics, but no Nighthawk Reverbs!

    There has to be one out there somewhere...

    Here's a video review of the Nighthawk Reverb that you could show him to give him an idea of what he would get:

    https://www.youtube.com/watch?v=Lf1AIAOH8Ls

    This is somebody's Spitfire clone with a modded tone stack. You can double-check your schematics against it. Note that they did not borrow the 47 ohm resistor from the Fender:

    http://www.mhumhirecords.org/DIYpage...eSchematic.bmp

    I believe this is a true-to-original Spitfire schematic:

    https://tornadoalleyfx.files.wordpre...fire_pdf_1.png

    This also does not borrow the 47 ohm NFB resistor from the Fender.

    They both DO have the master volume.
    Last edited by dchang0; 02-26-2017 at 05:36 AM.

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    Quote Originally Posted by mort View Post
    I think I'll try the diodes first. Could add a tube later if I felt so inclined.

    This is probably all I need I'm guessing:
    A bridge rectifier like that is fine with the Blues Junior PT but it won't work With a 5F1. The 5F1 needs a two diode full wave rectifier.

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    Quote Originally Posted by mort View Post
    Click image for larger version. 

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    I'm not sure that circuit will work too well. It won't have enough gain to drive the PI. The high impedance pentode is trying to drive a 56k slope resistor which could reduce its gain to something like that of a 12AX7 then there's the loss in the tone stack which could be up to 20dB (x10)

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    Quote Originally Posted by Dave H View Post
    I'm not sure that circuit will work too well. It won't have enough gain to drive the PI. The high impedance pentode is trying to drive a 56k slope resistor which could reduce its gain to something like that of a 12AX7 then there's the loss in the tone stack which could be up to 20dB (x10)

    I borrowed that tone stack from an EF86 channel I built last year. Had some help designing it but it worked very well. It did have the extra 12ax7 in there but it had way more gain than I need in this build. I'm open to suggestion though if it needs tweaked.

    schematic3add.png
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    Quote Originally Posted by mort View Post
    Click image for larger version. 

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    That one will have a lot more gain. The EF86 is driving a 500k pot not a low impedance tone stack and there's an extra 12AX7 gain stage then the tone stack is driven by a cathode follower so there's no loss due to loading.

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    How can I solve for that without adding any tubes?
    ~Semi-No0b Hobbyist~

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    Quote Originally Posted by mort View Post
    How can I solve for that without adding any tubes?
    If it doesn't have enough gain you could add an FET source follower between the EF86 and tone stack or you could just use the low loss Matchless DC30 style switched capacitor.

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    Would a higher value slope resistor lessen the load of the tone stack? This one has a 56k drawn, and I notice that Fender uses a 100k but that's still as a cathode follower like in the 2 channel Amp posted above, but what about a 220k or higher? Or would that just serve to starve the mid and bass frequencies of signal?
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    I'm playing around with Duncan's tone stack calculator. How do I know what my Zsrc and load are?
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    Quote Originally Posted by mort View Post
    I'm playing around with Duncan's tone stack calculator. How do I know what my Zsrc and load are?
    Zsrc is about 1k for a cathode follower, 35k for 12AX7 gain stage with 100k plate load and bypassed cathode resistor and about 200k for your EF86 stage.
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    ok so this is what I'm getting on the Duncan simulator.


    there are lines for all controls at 1(not zero), 5 and 10.

    I see the -20db loss with controls at 5, but I don't see the "x10" (?). Does this look like a healthy configuration?

    tonestack.jpg
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    For reference, this is the best layout I found for the widely-shared one-tube reverb circuit.

    I wish I could remember who made the layout so I could give them proper credit and thanks, but I can't find the bookmark I had for the original thread. It is designed to fit on a "daughter-card" that is easy to splice into any amp.

    Anyway, I followed this exact layout for the reverb upgrade for my customer, and he loved/loves it. Very warm and fat (as opposed to my "cold/dry room" one-tube 6BM8 reverb driver design based on Eric Barbour's example of a 6BM8 reverb recovery).

    reverb_layoutv1.0.jpg
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  34. #34
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    Quote Originally Posted by mort View Post
    I see the -20db loss with controls at 5, but I don't see the "x10" (?). Does this look like a healthy configuration?
    Sorry about that, it was poor terminology. By "x10" I meant that the signal would be attenuated by a factor of 10 i.e. -20dB.

    I see that the mid notch of your tone control has shifted down to 100Hz. Is that OK? It's usually between 500Hz and 1kHz

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    So the reverb tank in this thing is 800 ohm input impedance and all the reverb transformers I'm seeing have 8 ohm secondaries. Is there a simple way to manipulate this to make them work together or do I just need to get an 8 ohm tank? Or since I still don't have a reverb transformer, is there one that's 800 ohm secondary?
    ~Semi-No0b Hobbyist~

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