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

  1. #36
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    Ah yes, I forgot to mention that I ended up buying a new tank for the tube reverb conversion I did. It was only $15 or $19 for a MOD tank at the time.

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    Yeah, no big deal to buy one. Just didn't want to get one unnecessarily.
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  3. #38
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    Before you build the reverb circuit, you could always go with a capacitor-coupled reverb driver. This would save you having to buy a reverb transformer and a new reverb tank as well as save space inside the amp.
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  4. #39
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    Quote Originally Posted by dchang0 View Post
    Before you build the reverb circuit, you could always go with a capacitor-coupled reverb driver. This would save you having to buy a reverb transformer and a new reverb tank as well as save space inside the amp.
    I'm all ears! Got a link to a drawing?
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    I found this one but it uses a different tube type.

    Channel Road Amplification: Transformerless Reverb Driver
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  6. #41
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    Here's a pretty good analysis:

    Doug Circuits

    It shows several examples from real-world amp designs.

    IIRC, Ampeg and Marshall had capacitor-coupled reverb drivers (Ampeg Gemini).

    I also designed and built a 6BM8 one-tube capacitor-coupled reverb. It's pretty powerful and can overdrive the tank. Here is the thread--look at post 55 for the schematic:

    Eric Barbour's 6BM8 One-Tube Reverb Imagined...

    You should probably change the 470nF coupling cap to something like a 0.022uF. I kinda overkilled it with the 470nF.
    Last edited by dchang0; 02-28-2017 at 10:43 PM.
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  7. #42
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    Quote Originally Posted by mort View Post
    I'm all ears! Got a link to a drawing?
    Have a look at a schemo for mid60's Ampeg, like a Gemini I or Reverberocket. Instead of driving a transformer, they have a sturdy 5W 10K resistor between paralleled tube plates and hi voltage supply, a 12AU7 or 6CG7/6FQ7, pick the signal off thru a film cap, 0.47 uF in most cases but you can use as little as 0.1 uF. I'd go 600V on that cap. Send that signal to your tank input. Not the most efficient circuit in the world, but it does work and people love the sound of those old Ampeg reverbs. One refinement I'd suggest, put a load resistor say 100K to 470K on the reverb side of that cap so that it doesn't charge up. That will save you 2 things, 1) getting a shock if you touch the plug intended to drive the tank, and B) wrecking the tank transducer if the cap is charged, then you plug it into the tank and it discharges thru the transducer. Ampeg left this out because they assumed the tank would always be plugged in (duh) also it saved a couple pennies during construction, always good, for them.
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  8. #43
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    YYou guys think that a single 12ax7 triode will do the work of the whole 6cg7 in this schematic?


    ampeg-reverb.jpg
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  9. #44
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by mort View Post
    YYou guys think that a single 12ax7 triode will do the work of the whole 6cg7 in this schematic?
    Like will a Porsche 911 do the job of a Ford 350 pickup truck? Hm lemme think about that...... . . . . . . .

    NO.

    Ampeg is using one triode for a little gain, the second as a power delivery system. Power, you'll need it. Please consider, if not 6CG7, how about a 12AU7? If you simply sub in a 12AX7 here, you may find yourself subbing that 12AX7 a lot, as it will be way overstressed in this application. Yes I know, Mesa Boogie use an AX to drive their reverb, 400V on the plate OW! - good for them - I don't approve, that's my position & I'm gonna stick to it.

    FWIW I have a similar project to yours to complete in March. I'm considering using a JJ ECC832, equiv.to 12DW7/7247. Dual triode, usual footprint. You find them used in Ampeg SVT and some other amps. One triode is like a 12AX7, hi gain low power, the other 12AU7 high power low gain. You may find you do not need the extra gain stage as shown in your Ampeg schemo. Instead of a whole extra gain stage, how about a bypass cap across the cathode resistor in the drive amp? That'll give us some cheap gain, almost for free, bypass cap might cost a couple of dimes . I'm planning to use the hi power triode to drive the tank, and hi gain to recover the signal.
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  10. #45
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    Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...

    Justin
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  11. #46
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    I do have some 12au7's. I'm trying to keep the reverb to only two triodes if possible. Also trying to order the least $$ amount of parts possible. But I'd rather add a tube than have to buy a transformer and different verb tank.
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  12. #47
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    After some thought, I think I just better add a tube. That will give me the 3rd triode for the transformerless reverb and a triode to use as a cathode follower after the ef86 to drive the tone stack (it's been suggested this will be better than just the ef86 trying to drive the stack...)

    So all day that's 4 preamp tubes. The ef86 front end, a PI, 3 triodes for reverb, and a triode to drive the tone stack. Sound like a good plan?
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  13. #48
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    Quote Originally Posted by Justin Thomas View Post
    Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...

    Justin
    I'm actually also building a GU-12 right now. But I don't have to think about design considerations because it's all getting pulled straight from the stock circuit.
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  14. #49
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    Quote Originally Posted by Justin Thomas View Post
    Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...
    I sure wouldn't build anything new with triple triode tubes. Tough enough to find 'em for the older amps, the ones we need for repairs & maintenance now practically nonobtainium.

    Yes Mort it looks like you'll have to add another tube socket if you're going to use an EF86 preamp. To spare one triode in my project, I'm planning on a concertina type output drive stage, as in a Princeton.
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  15. #50
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    Just because I'm in a sharing mood, here's teh GU-12 I'm working on. And yes the cap has been moved to the side of the 10w sand resistor

    This one is actually for another amp builder. He's busy building his own stuff, had been thinking about wanting this circuit so he can get his Exile On Main Street on, and asked me to build it for him.



    16938559_813321192155628_2592584124969332810_n.jpg17021632_813321195488961_1025011519616013555_n.jpg15400943_813321185488962_5877268877964690486_n.jpg
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  16. #51
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    Quote Originally Posted by Leo_Gnardo View Post
    ...... To spare one triode in my project, I'm planning on a concertina type output drive stage, as in a Princeton.
    Funny,......that was my first thought when I read the question. I even pulled up a Princeton schematic to refresh my memory.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    What would be the audible difference between a concertina split load and a LTP?
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  18. #53
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    Quote Originally Posted by mort View Post
    What would be the audible difference between a concertina split load and a LTP?
    My ears tell me, not much until you start pushing past the clean zone. Amp pushed into distortion, they'll act and sound differently. The old Orange amps OR120 & 80 also used concertina drive. In lower power amps it's a cool sound, think George Thorogood playing thru his dimed Princeton. Allen amps has essentially a double power Princeton (woops I forget which one), also a Cape Cod amp builder Gries, and I have one of his sitting right here, it sounds terrific to me and its owner. That's more or less what I'm going to try & stuff into a Blues Jr. box. In high power amps, the Oranges, to me the concertina sounds brittle when overdriven, and as we see in another thread here, it can stress the heck out of one side of the output push-pull pair, again when pushed very hard. I'm hoping that won't be too much of a problem as long as we stay out of the 500V + power supply zone.

  19. #54
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    That's odd to me, Leo, I've always thought the concertina sound was less brittle or rather "warmer" because of the asymmetrical nature of the circuit. But then, I don't like a ton of swimming reverb either, which I think can add to the brittle sound. Saying that, I guess a consideration here would be, are we going for a "surf guitar box" or something with what I would call a "reasonable" amount of reverb?

    Edit: Sorry to confuse the concertina circuit with reverb discussion, but IMO, there is a tonal interaction.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  20. #55
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    In my case it just needs to have some reverb, so the err would be toward "reasonable".
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  21. #56
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    Here's one of several demos on YouTube that might help.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  22. #57
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    Quote Originally Posted by The Dude View Post
    That's odd to me, Leo, I've always thought the concertina sound was less brittle or rather "warmer" because of the asymmetrical nature of the circuit.
    Just my opinion, brittleness when pushing those old Oranges well into clip. Could have something to do with EL34 breakup too, also asymmetric by that point, being thrashed by 500V and more. These Blues Jr conversions won't be going there, I expect we'll be more in the Thorogood Princeton zone and that's a good place by me. IOW the brittleness of Oranges may have more to do with extra-hi voltage and crushed pentodes.

    Thanx for the video, yes different tones but different speakers, EL84 vs 6V6, and who knows what all else, even where's the mic pointed. Princeton wins the warmer tone prize in this vid.
    Last edited by Leo_Gnardo; 03-01-2017 at 02:19 PM.
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  23. #58
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    I agree with that, Leo. Too many other variables to say it's the concertina circuit alone that makes the tonal difference. And, more likely it's not. I guess the point is that it's certainly possible to make a good sounding "warm" amp with that circuit and free up a triode.
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  24. #59
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    If you want a spitfire-like circuit, but with a pentode-like pre-amp tube in V1, you could use something like a 12DW7 as a cascode, with the AU7 side as the input triode and the AX7 side as the output triode. This ought to produce good gain with pentode-like characteristics, but with more reliability than an actual EF86 pentode.

    Or otherwise do a more conventional setup with a dual triode wired either as a parallel V1 stage with single and parallel - 'normal' and 'hot' - switching at the input jacks - see attachment. This also gives versatility for parallel sets of input effects.

    Or a DC-coupled pair with a CF driving the tone stack

    Or a BF style pre-amp with the 2nd triode as a tone stack recovery stage driving the LTP.

    (The 47R resistor under the tail of the LTP in BF fender amps is part of the NFB loop, with something like a 2k7 resistor in the upper leg of the NFB voltage divider)
    Attached Thumbnails Attached Thumbnails switchable-parallel-single-v1.png  
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  25. #60
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    OK so I've ordered and recieved some parts for this build, and I went ahead and got a reverb transformer and tank so all my option would be open. Here's where I'm at on the drawing, just added in the cathode follower between the EF86 and tone stack. Does that look about right? If so, I just need to settle on what type of reverb circuit to use and where to tie it into the signal path.

    Attachment 42916
    Attached Thumbnails Attached Thumbnails 86reverb.jpg  
    Last edited by mort; 03-27-2017 at 03:06 AM.
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  26. #61
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    So if I use a Fender reverb circuit like from a deluxe reverb, can it go between the the treble pot and the PI? Just stick it in there with a 10pF cap and 3.3M resistor like on the Fender?

    fdr_del_rev_ab763.jpeg
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  27. #62
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    Quote Originally Posted by mort View Post
    So if I use a Fender reverb circuit like from a deluxe reverb, can it go between the the treble pot and the PI? Just stick it in there with a 10pF cap and 3.3M resistor like on the Fender?

    Click image for larger version. 

Name:	fdr_del_rev_ab763.jpeg 
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    Referring to your schemo in #60, if you do it like that you won't have the reverb volume "follow" that of the volume control. Have another look at the Fender schemo, the reverb drive comes after the volume control. Some early Gibson amps didn't do like that, their reverb isn't proportional to the volume control setting, you have to adjust volume & reverb independently. OTOH you can leave the volume off, have reverb only and sound like you're at the other end of the tunnel, if that's your thing.

  28. #63
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    So how about inserting the reverb between the volume control and PI?
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  29. #64
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    Quote Originally Posted by mort View Post
    So how about inserting the reverb between the volume control and PI?
    That's more like it. Again look at what Fender did, you're going to have to add a couple of tubes, and doubtless the noise floor's going to increase in your amp as you will have to follow the lossy mixer with a gain stage. You might be able to squeak away with a single tube, say 12DW7 or ECC832 (one triode like a 12AX7, the other like a 12AU7) at the cost of having less drive signal to the reverb tank, sending a signal right off the volume control wiper, thru a small cap to the current amp triode (AU side), and from the same wiper to your 3M3 mix resistor. It's possible to vary the mixer resistors to balance dry vs reverb too. Gonna take some experimentation if you don't follow the Fender formula.

  30. #65
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    My thought was to add a whole 12at7 to drive the reverb tank and use the other half of the 12ax7 cathode follower as the reverb recovery. That sums to EF86>1/2 12ax7>12at7>1/2 12ax7>12ax7 PI(or 12at7)>EL84 x 2

    Sound about right?

    Or am I still going to need that triode following the mixer?
    Last edited by mort; 03-27-2017 at 03:30 PM.
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  31. #66
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    Quote Originally Posted by mort View Post
    My thought was to add a whole 12at7 to drive the reverb tank and use the other half of the 12ax7 cathode follower as the reverb recovery. That sums to EF86>1/2 12ax7>12at7>1/2 12ax7>12ax7 PI(or 12at7)>EL84 x 2

    Sound about right?
    That may well work. Again without that extra gain stage following volume control, you may not be "spanking the tank" the way Fender does. Maybe just as well, it usually sounds too much for me besides how many Fender players say "I can only turn the reverb control up to 2, 2 and a half, then it's too much." Just about all of 'em. One other thing's a little bit of a worry: crosstalk between the triodes in your 12AX7 might result in some squeeky feedback. If you encounter that, you might need to reduce hi frequency bandwidth with a disc cap or two in parallel with its plate resistors.

  32. #67
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    Ok so here's the complete circuit rough draft. See any glaring problems?
    Attached Thumbnails Attached Thumbnails 86reverb.jpg  
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  33. #68
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    Made some changes. Added the gain stage just before the PI and dropped a triode out of the reverb driver.

    Thoughts?
    Attached Thumbnails Attached Thumbnails 86reverb2.jpg  
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  34. #69
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    If you have to use a single triode to drive the tank, then it would be a good idea to at least use a 12DW7 (like Leo spoke of in post #64) for better drive.
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    I just might do that
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