Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on April 05, 2015, 01:26:26 am
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Could this work as an unholy pairing of good things??
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That's exactly the way I'd approach it.
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That's good enough for me!
Maybe next week buy the bits (have to wait for extra funds)
And give it a go
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I'm finally revisiting this potential build
I'm making a parts list and ooooeee there sure is a lot of components
I don't think there's a local source for a 4m ra pot or even a 3m ra pot (though I see Hoffmans has one)
Where the *&@$^$#($@$ does one get a 10m L pot????
I'm pretty curious to try this tremelo but I'm curious if those that have used it think it's worth it...I've only heard it on youtube videos and that's never a great example
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That circuit is the same as the vibrato circuit used in the revibe. Just use Hoffman's 3M and 10M pots. They work fine.
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Cool thanks
I didn't notice the 10m pots...maybe it's finally time top put in an order.
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I have a 7 henry choke off of a Baldwin 51 organ power amp chassis. The original amp used 2 6L6Gcs and a 5U4gb rectifier. the choke was hooked pretty well straight to the rectifier
I know vibrolux amps only want a 4 henry choke
Is over henry-ing okay?
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Yes, if it's up to the current.
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Cool
On the hammond site they have a vibrolux reverb choke the same as a deluxes 4L 50 ma
If I was to go with the larger one and say it had a larger current rating, say closer to 90ma , is that a big deal?
Just seems like it can handle more but one doesn't have to use it's entire capabilities, yes?
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If you had a piece of 12ga wire rated for 20A, could you only run 5A through it and be fine?
Just seems like it can handle more but one doesn't have to use it's entire capabilities, yes?
Yes.
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I'm lurking back here, looking at a data-sheet, your 90mA looks close, 80-118mA for your bias n B+, still fuzzy on henry, the bigger he is the better he filters?
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I have a 7 henry choke off of a Baldwin 51 organ power amp chassis. The original amp used 2 6L6Gcs and a 5U4gb rectifier. the choke was hooked pretty well straight to the rectifier
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Thanks
I know some of this stuff is obvious and I have been reading on it bu it is nice to have those that are in the know confirm my thinking on some of these things and I appreciate that
I'm also debating on buying one from hammond simply because the hammond unit is a nice light small one and the Baldwin organ one is about the size and weight of a deluxe reverb OT! :icon_biggrin:
Just to clarify I'm not using the the other Baldwin components(OT and PT) I'm using a Hammond M3 amp chassis and the PT that comes with it
I have a multiap vibrolux type OT ordered off Hammond a while back
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I'm also debating on buying one from hammond simply because the hammond unit is a nice light small one and the Baldwin organ one is about the size and weight of a deluxe reverb OT! :icon_biggrin:
That's the thing with salvaging sometimes. If you have room on the chassis, use it. :icon_biggrin:
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Getting going here
I was panning on using this Hammond M3 chassis and PT
Originally i was going to use the tube sockets where they are but I though a tweed bassman sort of socket and transformer orientation might work well and enable me to build a tweed type cab which I find pleasing looking.
-As you can see all down the 'new' tube socket edge there's a series of long oblong cutouts. so I could mount a long plate down that side(probably Aluminium) drill holes, and mount the sockets there. (I have new ones..yay!)
-leave the PT where it is and put the rest os the transformers and maybe caps(maybe can cap , maybe cab barn maybe inside, still deciding) on the same side I would also be covering most of that side with a aluminium plate ( have extra aluminum kicking around)
-Use the mostly blank edge for all the controls which will be facing up
Seems sensible to me. I was curious if the different transformer orientation would mess with my reverb pan, but this will be a taller cabinet (I'm probably going to use my JBL D130 in it)
Any other pitfalls I'm not seeing?
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You forgot to post a pic of the chassis.
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Crap! and I took such nice ones :cussing:
Tonight then when I get home
It's a hammond AO-29 chassis I think, From a Hammond M3 I have removed Everything except the PT
Here's a stock photo of the unit I have , mentally remove EVERYTHING except the PT :icon_biggrin:
The tubes would be mounted where the brown terminal strip is now (there's several long oblongs of space behind it)
Another question, besides space considerations and price is there any advantage to using a cap barn arrangement with individual caps over using a multi section can cap?
I'm thinking a cap barn might have less B+ leads running hither and thither
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here's the photos that go with the post...2 posts ago :icon_biggrin:
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Ummm, I think you have room for that salvaged choke. :icon_biggrin:
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yes but, yes but...my other questions!
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No takers on the use of this chassis in a tweed orientation??
seems perfectly possible to me
I was just wondering if anyone noticed any glaring problems with the idea
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So I've been slaving over a proper schematic for this amp
and here it is
I wonder if a couple people could check it out for me
Right now everything is off a AA964 Vibrolux Reverb except:
- the Harmonic Vibrato Circuit(apparently I have Ham-onic Vibrato...which could be tasty)
- the Hoffman bias steal
- Line out
- SS rectifier (might've been overkill with the 4 diodes)
My main point of concern is where the signal comes out of the Harmonic vibrato into the next stage
I'm not sure if I have that right
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Here are a few things I see on the schematic...
- Wipers of bass, treble, and speed pots must be connected to something.
- Some of the tube pin numbers are written backwards.
- 1Ω resistors on output tubes must be grounded.
- Fender used a 47Ω rather than a 100Ω resistor in the NFB divider for 8Ω speakers. The 100Ω like you have was used for 2Ω and 4Ω speaker configurations.
- 220K connected to V3-1 should be connected to V3-2. It's usually mounted directly on the reverb return (or footswitch) phono jack.
- V3A cathode resistor should be 1500Ω. You would use 820Ω only if V3A and V3B share a common cathode resistor like the original Fender circuit.
My main point of concern is where the signal comes out of the Harmonic vibrato into the next stage
I'd try it like that but be prepared to experiment once it's built if you're not happy with it. You may also want to experiment with where you pick off the reverb input signal. Another possible option would be the bottom side of the 3.3M/10pF. I'd design the board layout with these two points in mind.
An AO-29 PT will support a tube rectifier if you prefer. 4 diodes is fine too if you need a higher B+.
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Thanks for looking at it!
-the wipers on the schematics I was copying slash diagonally through the resistor part and Jschem doesn't have that symbol
I will see if I can edit the symbol a bit.
- I was flipping the tube symbols around to make it easier to copy the original schmetaic layouts and forgot to NOT flip the symbols. might fix that too
- OOOps have fixed the ground on the power tube cathodes now, forgot to do it
- Would 47K be best on the NFB divider if I have a multitap OT ? I plan- to (at least initially) use my JBL D130 so it's 8 ohms
But if I can hunt down a couple of nice 10s I might go that way too
- I thought the schematic had it going to ground? I was using the AA964 vibrolux schematic
- the aa964 has a 2200 ohm resistor on V2 and the 820 on V3A, I'll look at other circuits, maybe it's a silverface thing??
I was wondering how the harmonic vibrato would react to an extra gain stage after it, reading on the 6G12 concert seemed to suggest it didn't have piles of preamp gain
I appreciate the suggestions regarding moving things around a bit
I'm curious how it'll turn out
I have some 5U4Gbs but I think I want a little more B= or at least that was the idea. I'll probably make a plug in unit to keep my options open
A question. I have an extra dark green wire tap on my PT. It goes to a 'pedal cut' pot and produces about 27 VAC when the Pt primaries are getting 120VAC
I wonder if it could be of any use. otherwise I'll just cap it off
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Would 47K be best on the NFB divider if I have a multitap OT ? I plan- to (at least initially) use my JBL D130 so it's 8 ohms
It's 47 ohms, not 47K. Fender used 47Ω whenever the NFB was connected to an 8Ω speaker tap. 100Ω was used if NFB is connected to a 2Ω or 4Ω speaker tap.
I thought the schematic had it going to ground? I was using the AA964 vibrolux schematic
Are you talking about the 220K resistor I mentioned? If so, yes, one end connects to ground. But the other end connects to pin 2, not pin 1, just like the AA964 schematic shows.
the aa964 has a 2200 ohm resistor on V2 and the 820 on V3A, I'll look at other circuits, maybe it's a silverface thing??
It's not a silverface thing. It's all about sharing cathode resistors. Look again at your schematic. True, there is a 820Ω resistor on the cathode of V3A. But V3A cathode is also connected to V3B cathode, so you have two cathodes sharing the same 820Ω resistor. If you want to use the 820 you should remove the 1500Ω/22µF from pin 8 and connect pins 3 and 8 together. If you don't want to share a single 820Ω cathode resistor, you should change it to 1500Ω.
A question. I have an extra dark green wire tap on my PT. It goes to a 'pedal cut' pot and produces about 27 VAC when the Pt primaries are getting 120VAC. I wonder if it could be of any use. otherwise I'll just cap it off
Not enough voltage to use as a bias tap for 6L6s, so just cap it off. This reminds me though. Since you don't have a 50VAC tap on that PT, you'll need to feed the bias circuit from the HT winding. This means you will need to change that 470Ω/3W to 100K/1W (like you would see on a Princeton Reverb).
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In your drawing the bias is wrong. It should not be connected to the B+ D node and the junction of the .1 cap/1K5 grid stopper for the 2nd power tube.
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ooops! :icon_biggrin:
-Sorry the 47K comment was a oops in my noggin..couldn't sleep last night, yes 47 ohms
- and also yes i hooked up that 220K to the wrong spot I checked it again and fixed it, thanks!
- Ans yes again, you are so right I forgot I replaced the 'E' point on V3b as it confused me
Is it fine to share the cathode on V3 as was done originally ?(saves on space and components?
- Good call on the bias, my deluxes have the 100K there ala the princeton, but i switched to to the Hoffman Ab763 setup when I did this schematic, I'll switch it back
Thanks Willabe for noticing that (re the bias), I was moving whole sections around and I think the program attched taht line to those two points. It shouldn't have been attached and I fixed it
A bit better now?
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Is it fine to share the cathode on V3 as was done originally ?(saves on space and components?
I bet you know the answer to that! :icon_biggrin:
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Right duhh
If it was done originally it was probably fine
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I've ordered my first bits from Hoffmans!
Gonna start installing tube sockets etc tomorrow!
yeeha
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You would have to test this for your self, but some say, that separating V1a from V1b, or any other 2 gain stages with a shared K R/bypass cap, gives more touch/dynamic feel. :dontknow:
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Well I was just testing the PT on this hammond(It's a replacement PT obviously the original got cooked and the new ones leads are soldered in.
Unloaded with a 5U4gb in the rectifier socket and measuring across a 22 Uf cap between pin 8 of the recitifier and the centertap (as Sluckey suggested many many mons ago)
I get about 500VDC
I wonder what that translates to when load down by a couple of 6L6Gcs 4 12ax7s and a 2 12at7s?
Anyone have a rough estimate??
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On my deluxe with a tube rectifier, plugged straight into the wall, unloaded I have 455vdc measured at point A
With the tubes in that drops to 425 and then 416 after the choke
The dluxe has 2 6v6s 212AT7s and 3 12AX7s
total drop from unloaded, measured at point B is about 40 VDC
Probably more with 6L6GCs and another 12AX7
Oh and I checked the accuracy of my variac scale and it's a little off
When I'm actually putting out 120vdc into the Hammond AO-29 PT I actually get 475 unloaded and this with a 5U4Gb or a SS rectifier. So i should probably hopefully end up nicely in the vibrolux range once it's loaded down with all those tubes!
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Anyone have a rough estimate??
Unless someone is familiar with that particular PT a rough estimate is just a wild ass guess. If you have a 5000Ω 50W resistor you can put a 100mA load on the power supply. This won't actually be a 100mA load because the voltage will drop, but it will be enough load to get some kind of idea.
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I just glanced quickly at this thread and didn't see any one else mention it (maybe I overlooked it?) but I notice that you aren't using the split plate resistor on the final triode stage before the harmonic vibrato. On the 6G12, the plate resistor is a divider with 100K / 6.8K. Without that, you're going to have WAY more gain before the vib circuit, maybe to the point of overdriving it.
hope that helps
Nathan
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On the 6G12, the plate resistor is a divider with 100K / 6.8K.
Good catch!
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Cool, Thanks!
Will the lowered gain affect the reverb circuit do you think?
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You could continue to pick off the reverb input signal just as your schematic shows but relocate the input for the vibrato to the junction of the voltage divider.
Or, and I like this better, move the reverb input to the bottom side of that 3.3M/10pF just as Fender did. The added effect will be that now the vibrato signal will pass thru the reverb. Hopefully that would be a good thing?
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Like this then?
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Yes. That's how I'd try it first. I'd also plan the layout such that if I needed to move the reverb input I could easily do so.
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I like the idea of a presence control, maybe try that instead of the 120pf bright switch arrangement. could I just steal the whole setup from the concert circuit? or would the different PI make it not as effective?
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Why not just build the 6G12 and add on the reverb?
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Well I'd thought of that but
I'm not sure if my Pt will supply a high enough voltage to the 6L6GCs
The 6G12 has 456 on the plates of the 6L6Gcs
I'm thinking I might end up more in the neighbourhood of 415 and it might be even lower with the higher negative bias voltage required , not sure if this matters other than just getting less power out of the amp
Also I dont' have experience with these amps too and I'm curious how different they sound from a AB763 type of circuit which is more of a know quantity for me
I'm game though, from what I hear the concert is a less gainy amp which tends to break up less..but that is just hearsay really
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So I initially imagined my layout would be:
preamp
reverb
harmonic tremelo
phase inverter
power amp
but I'm thinking it would be good to swap the order of the reverb and vibrato
preamp
harmonic vibrato
Reverb
phase inverter
Power amp
Will it cause a problem with the reverb tranny to be close to the PI and power amp?? , my chassis is pretty long
And can anyone forsee a problem using this chassis in a tweed sort of configuration?
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well on my way to plugging all the holes in the chassis, I love pop rivets, I've replaced many a mercedes floor with organ sheet metal, seam filler and pop rivets!
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Tube sockets in, full speed ahead
OT and choke next
This one is going to be a southpaw!
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a choke, a fancy multitap hammond OT and a power jack, reverb transformer is in the mail still
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1 thing, your PT is a lay down, so you could have all 3 pieces of iron on a different plane.
PT is shielded and has a copper band, very good, OT is shielded, very good too.
With respect to each other, orientation/position of PT is good, orientation/position of OT is good, just turn the choke 90degs. Then all 3 have their cor's on a different plain from each other, quieter. :icon_biggrin:
Edit; Or leave the choke as is and turn the OT 90degs, OT primary side facing towards the PT.
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> I love pop rivets
My experience is that the popped-off heads eventually rattle inside the rivets. You may someday want to go in with some goo to damp that.
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Better Transformer Orientation?
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Better Transformer Orientation?
Yes. It should be quieter that way.
Remember face the OT primary wires side towards the PT. OT secondary wires side towards the preamp. Just like you have it now in the pic.
Hey, where are the speaker jacks going?
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Haha
They were going to go on the same side as the transformers just above the power tubes. I was only going to use 2 of the taps (4 and 8) and have a line out as well
Bad placement? i have a similar deal on my AB763 one channel deluxes I built and it seemed to work okay
I could also put one between the power tube sockets (on the back edge) and one between the power tube socket and the PI tube)
I have a few options
I really should've offset my tube sockets a 1/4" or so to have a little room
I'm probably going to put my footswitch jacks, and maybe even my line out on the faceplate
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What are the dimensions of that chassis?
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20.5 long 5" wide by 2.5" deep
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Well because I'm totally crazy :icon_biggrin: ( and because that side of the amp was mostly aluminium faceplate and steel with long holes in it)
I shifted the preamp tubes 3/8" in one direction and the power tubes and rectifier 1/4" in the other os now I have about 3/4" more between the PI and power tubes (about 2" clear space so I can mount a couple of speaker jacks front to back between them.
My new aluminium plate looks better than the old one anyway.
Onward
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Well because I'm totally crazy :icon_biggrin:
No, not crazy, just didn't think the layout through. We've all done that.
I shifted the preamp tubes 3/8" in one direction and the power tubes and rectifier 1/4" in the other os now I have about 3/4" more between the PI and power tubes (about 2" clear space so I can mount a couple of speaker jacks front to back between them.
My new aluminium plate looks better than the old one anyway.
Good deal. :icon_biggrin:
But it could/would be better to mount the speaker jacks on the same plain as the tubes.
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I mounted the speaker jacks on the same edge (the 'bottom' edge) the tubes are mounted on. that's why I moved the sockets...i believe that's what you mean right?
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Yes. :icon_biggrin:
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I urge you to rip your board to 2.5" wide and plan your layout accordingly. This may avoid a struggle when putting the board in that 5" wide chassis.
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I have a couple choices on hand
The original cone boards 2 - 7.5" x 2" = 15" x 2"
and some turret boards from a baldwin organ preamp
one of which is 2.5" x 13"
with a couple of 5" ones
all of which would need more turrets!
I might check out the local electronics store too
what do you think
preamp
reverb
harmonic vibrato
PI
POwer amp
or
Preamp
harmonic vibrato
reverb
PI
power amp
Either way there'll be a little going back and forth
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No answers on the layout question?? :dontknow: :help:
okay what about this one
On a 30-40 watt amp is a standby necessary?? good idea??
THanks
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standby necessary
I've only used 1 standby and from all I've read, unless you're gonna leave it unused, *cooking* for long periods of time there probably isn't any earth shaking benefit.
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On a 30-40 watt amp is a standby necessary?? good idea??
I always used my stand by switch at gigs. All the guys did. When the boss/band hit the stage, your ready to go at the flip of a switch. Doesn't look good to be standing there waiting for the tubes to warm up, the band leader/boss won't tolerate it, dollar waiting on a dime and the bar owner won't like it either, if he/she's used to pro bands.
There's 2 views on having a stand by to prevent cathode (K) stripping. That's where applying the B+ dcv to the tubes plate before the K is warmed up fully by the heater cause the K coating to boil and the coating flakes off, shortening the tubes life, prematurely killing it.
Some say this only happens with B+ dcv's at 1K and above, power triodes and transmitting tubes.
And rectifier tubes that have indirectly heated K's (GZ34/5AR4, 5V4) are slow turn on and are easier on the tubes. But you still have to wait for them to warm up fully before you can play.
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OKay, apologies if I'm being a PITA
This is my third and final desperate time at asking this question
I have my chassis mostly mounted up, heaters and power supply wired so I was going to start filling up my board
I did the preamp and now I have to make a decision
This amp is a bit of an experiment
I have the preamp feeding into the harmonic vibrato and then coming out of vibrato into the reverb and the last 12AX7 stage prior to the PI. This could change if things start interacting poorly with each other
Classic Fender layout is:
Preamp
Reverb
tremelo(harmonic vibrato)
PI
Power amp
My schematic would suggest that I reverse the positions of the reverb and tremelo(Harmonic Vibrato)
like this:
Preamp
Harmonic Vibrato
Reveb
PI
Power Amp
As I've only built one reverb amp, I'm just a little nervous that the reverb circuit won't like being closer to the PI , power amp and the TRansformers down at that end.
Also if the harmonic vibrato circuit doesn't work out, (for whatever reason) I can always go to a bias vary which would be easier to implement if I used the traditional layout
Any ideas? Concerns? Things I should consider?? Recommendations?? :help: :help: :help:
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typically layout follows signal path. if necessary, redraw your schematic should you need clarification to determine signal flow.
sometimes, subsystems have to cross one another, so use discretion on what would introduce the least amount of noise/hum/feedback components. e.g. what layout keeps the signal path the shortest overall and keep the most sensitive subsystems away from noise sources such as power transformers, high current power supply rails, other signal sources, and power amps.
--pete
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+1 on what DL just wrote. :icon_biggrin:
Classic Fender layout is:
Preamp
Reverb
tremelo(harmonic vibrato)
PI
Power amp
No it's not. Leo never built an amp with harmonic vibrato and reverb. Leo changed from the harmonic vibrato (4 1/2 triodes) to bias trem (2 triodes) when he added verb to his amps.
As I've only built one reverb amp, I'm just a little nervous that the reverb circuit won't like being closer to the PI, power amp and the TRansformers down at that end.
This is the thing that your going to have find out for yourself. I don't think I would be to worried about putting the verb recovery tube close to the PI. And I would also try the harmonic vibrato before the verb like you are.
The weakest signal in this build will be coming back out of the verb tank to the grid of the verb recovery triodes grid. Less than the amps input signal from a guitar. Watch that carefully how/where you put that.
But this is where good layout and lead dress skills come in. And the right decoupling in the B+ power supply including it's grounding, which you found out about when you added verb to a build.
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Okey Dokey
Onwards! :icon_biggrin:
Just for the record
I know Fender never had an amp with Harmonic Vibrato and reverb
and I'm aware of the signal path, I just have limited experience regarding design in general
Thanks very much!
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another orientation/layout question if I may regarding the reverb driver transformer and associated jacks
Here's a picture of the chassis with a similar sized reverb transformer on it..this is not the actual transformer though it does play one on TV (it's actually a reverb driver from a garnet unit but it's 600ohm pri and 1.7 ohm sec.)
I have it laid out above the reverb circuit tubes on an angle similiar to the way fender has it laid out.
to the left of it is a hole which I plan to put a grommet in and run the transformer leads through and then to the left of that I will put the two RCAs jacks for the tank.(one of which is shown with green tape)
To the right of the transformer will be the 1/4" jacks for the reverb and vibrato footswitches
Okay??
The other shots show the heaters, faceplate. and most of the pots installed and AC power supply and PT wired up(Waiting on my 1st exciting Hoffman order, It's making it's way from Florida over to Texas I hope it doesn't hit every state! :icon_biggrin: )
The Boards are mostly stuffed, just have to do the bias circuit and maybe the first filter right on the end
mounted them to check placement etc, might still do a little adjusting to straighten them out
One other question...one of my boards has a burnt spot!! I was testing it with my meter and couldn't get any continuity anywhere on it, totally open, but still I think it's not the best idea that I went right through the burnt area...we'll see. Might have to move everything and cut out a section if it becomes a problem. Admittedly I'm kind of impetuous
As you can see I had to mark the boards with marker indicating which sides the tubes would be on as this build is backwards to the conventional fender layout. Very confusing at first!
;note the extra speaker jack hole , moved to accommodate the heater wires...this is a hallmark of my layout 'style' :icon_biggrin:
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NO takers on my reverb tranny layour question? :dontknow:
One update is my electrical technician brother says the burnt circuit board likely won't be a problem
I'm curious what others think
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NO takers on my reverb tranny layour question?
Mount the RT so the green and black wires can solder directly to a RCA jack. Hopefully the blue wire will reach the driver tube socket and the red will reach the board.
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Thanks!
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Hey Sluckey
You said a few posts back
- "Fender used a 47Ω rather than a 100Ω resistor in the NFB divider for 8Ω speakers. The 100Ω like you have was used for 2Ω and 4Ω speaker configurations."
Does this apply if you're using the 8 ohm tap of a multitap OT ?(apologies if this is a really dumb question)
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No.
Let's say you have an OT with a 2, 4, 8 ohm taps. If you hook up the -FB loop to the 8 ohm tap, it will always only see that taps voltage. It doesn't care what the tap is hooked up to the speakers.
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If you connect your feedback circuit to the 8Ω tap, use a 47Ω resistor like Fender did. Doesn't matter which tap your speaker is connected to.
This is not a critical issue. I have a 100Ω resistor in my TDR. It uses an 8Ω tap. I realized the mistake long after building the amp and plan to correct it if I ever open the amp up again. It's been 9 years now. :icon_biggrin:
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Well I got my amp wired up and fired up
I haven't received my trem pots but I did a bojang with a 2M pot for the speed and a couple of 4.7M resistors for the intensity to try it out (the pots will probably arrive in the next day or so)
Right now the amp is kind of noisy (the bojang trem control) hanging out the side can't help and various other bits to adjust
And I can't seem to get the bass control to work properly (have to check that out more)
However it currently passes signal (not huge amounts as there is almost NO bass) and in my experience that's a big part of the volume
The Harmonic vibrato is working though I think I need to tweak the voltages via the dropping resistors
The reverb works too but is currently a little quiet
All the other plate voltages seem to be in the ballpark so that's good
Obviously much trouble shooting and finishing up needs to be done but so far something has been accomplished!
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ooops had the 10k mid accidntally swapped with the 250K bass
better, bottom end is there
but the amp is too quiet
time to start going over things
Build amp, then fix it! :laugh:
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but the amp is too quiet
Can you define that a little better?
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Okay. compared to me Ab763 deluxes I would say it's significantly quieter, doesn't seem to start breaking up till around 9 on the volume dial, this with bass almost cranked mids at around 3-4 and same with treble
The deluxe (in my apartment) would be nervous making loud and starting to break up at around 4.5 on the volume
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You know the drill. We need voltages for all tube pins (even if the pin reads zero) and all power supply nodes. And repost your final schematic.
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here are some voltages:
they're all for the most part LOW!
V1 pre V2 vibrato V3 vibrato V4 reverb(12AT7) V5 Reverb12ax V6 PI 12aT V7 6L6
1- 147 1 - 118 1 - 272 1 +6 - 385 1 - 158 1 - 158 plate - 393
2 - 2 - 2 - 2 2 2 input grid - -40
3 - 1.2 3 - 1.18 3+8 - 2.48 3+8 9.04 3+8 - 1.22 3 +8 - 67 cathode - 25ma
6 - 148 6 - 158 6 - 269 4 - 148 6 - 152
7 7
8 - 1.14 8 - 1.22
I changed the schematic except I forgot to change the fact that currently(haha) I'm using a 5U4Gb rectifier as I hadn't made a plug in unit yet
annd as you can see I perhaps unnecessarily added another filtering stage for the reverb and put in a 2watt 1K resitor prior to the preamp...I'm not sure if that's totally unnecessary and or perhaps harmful..it seemed to make sense somehow
I had used an old bumblebee cap in the reverb recovery stage because I forgot to pick up a .003...tuned out it was LEAKY! surprise, so I popped in a new .0047 and it solved the problem with the voltage on the preamp stage of V5 (the voltage was really LOW! 96vdc)
The reverb is going whoppa whoppa whoppa through the speaker
THe tremelo works
the amp is quiet
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oh a little adendum. I just tried it with an SS recfier (I borrowed one from another amp that's just using 2 diodes) and most of my voltages jumped up around 20-30 vdc (except for the pre which just came up around 10-15)
the bias current came up to 36 ma and the amp is waaay louder
It makes some big pops on the power and standby (I added the standby for kicks) as well it was making quite a pop when I plugged the guitar cable in.
Still some stuff to check
Annoyingly my hoffmans order came while I was off doing elections training so I have to wait till tomorrow..no doubt some duty to pay!
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Did some checking of solder joints and I have all the voltages pretty close, pre a little lowish at 156, power amp, Demonic vibrato which sound great though I only have it 1/2 intensity
the amp is pretty quiet (noise-wise) plenty loud (volume wise)
The reverb however makes a very scary fast oscillating low(sub low) like noise through the speaker(the sort of think that's REALLY LOUd and sounds like it's going to pop your speakers!).so I currently have it disconnected.
Any ideas on that, maybe it doesn't like the tremelo?
I'm going to go over the circuit
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How horribly embarrassing. I'd forgotten to attach a ground wire to the 100K reverb pot!..hmm I wonder why it made that scary noise?? :icon_biggrin:
Ah well
Seems to all work pretty good though I might swap out the .22 decoupling pot coming out of the vibrato for something a little bigger and see if I can squeeze a little more bottom out of this unit, That and a bright switch.
Then maybe rearrange some grounds a little
So I'm confused about one(well many) grounding things
I was under the impression that it's best to ground things at ground for the B+ node they are using
But some fothese diagrams show everything but the power amp(and maybe the PI) grounded with the preamp at the input jack
1/2 my reverb and all of the harmonic vibrato use B+ point B(same as the power tubes) so my inclination was to ground it at that point...but then I have wires running everywhere!
which is supposed to be bad
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and all of the harmonic vibrato use B+ point B(same as the power tubes) so my inclination was to ground it at that point...
Well, point B feeds into the vibrato circuit, but V4 and V5 each have their own B+ node. So ground those circuits where their filter caps are grounded, NOT WHERE NODE B CAP IS GROUNDED.
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Thanks
I have a little terminal strip beside the vibrato circuit on the board containing the filters and the dropping resistors for that circuit.
i guess this begs the question, where do I ground that node to. I'm assuming the preamp ground bus?
I have two main points right now, a small terminal strip on a PT lug with the center tap, earth ground and power amp, I'm thinking the PI should be there too?
and a bus bar on the preamp vibrato board that grounds to the input jack
The reverb doesn't have any filters but it uses the power tube B+ 12at7 and the preamp B+ 12AX7
It confuses me! :w2:
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i guess this begs the question, where do I ground that node to. I'm assuming the preamp ground bus?
We're talking about two filter caps (two nodes), one for the oscillator and one for the modulator. Where are those caps grounded now? I would probably have both grounded with the preamp grounds.
The reverb doesn't have any filters but it uses the power tube B+ 12at7 and the preamp B+ 12AX7
It confuses me!
Ground the driver with the power amp ground. Ground the recovery with the preamp ground.
And remember, all grounds must have zero ohms resistance with the PT high voltage center tap.
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Cool! I appreciate that .time to start shortening some wires!
Got my bright switch installed and Now for the lineout and footswitch jacks which I know I should've done beforehand but I was putting off making the decision till I had a better idea of the topography. THis is my relative lack of experience/layout showing.
Besides how else am I going to drive a drill bit through my wiring and shower the inside of my chassis with metal shavings??
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I have a couple of cabinets kicking around that it might be good to utilize, or I can start anew (I have a sheet of 1/2" baltic birch(the good stuff)
-The one on the left was originally mounting 2 12s and the converted sherwood S-1000 amp. It's a little deep and a little wide, but I could cut it down front to back pretty easily and side to side with a bit of patience. I would also lengthen the faceplate cutout. It's Baltic Birch
-The one on the right I banged together from an antique armoire my building manager demolished (grrr), though the finish is beat, the ply is really good quality with many plys(like baltic birch) It has a birds-eye maple veneer on the outside and a mahogany veneer on the inside. It currently is mounting my JBL D130 and could be made to fit the harmonilux or visa versa(I would have to bend /cut off0 the original horizontal mounting tabs on the amp chassis).
A vertical mount cutout could be made in the cabinet easily and I would swap the handle to the long side.
They both sound really nice.
I have a 15 and some very usable 12's
Any opinions? I like both and generally don't want anything to be larger than it needs to be (25 years of using a super reverb)
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Sorry bumping this back to the amp, I got too ahead of myself
I have a weird problem
My preamp voltages seem to keep getting lower
for a bit I had them at a decent level 160-170 and now they are different every time i fire up the amp and get lower as it sits there on..like water dripping from a bucket. this morning they were at 98?? I spent some time reheating and checking all the power rail connections and grounds and it seemed to go up to 140 and then slowly start sinkink.
THis is affecting the recovery and preamp stage of the reverb(same B+ node) and maybe the PI to a lesser degree, but not the Power amp B+ node. I've been checking the point that the B+ feeds into the PI and preamp and that seems good
In 10 min of playing with a meter on pin 1 ov V1 it went from 155 vdc to 99 and was still going down
I have a hole in my bucket!
Any other ideas about what to check?
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and then slowly start sinkink.
guesses would be you have a slowly dyeing cap or tube.
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I swapped out the PI reverb recovery and preamp tube , same deal
Tried taking out the preamp tube and testing at the reverb recovery 12ax7
B+ on reverb recovery tube still dropping
Took out the reverb recovery tube and tested at the Pi 12at7
And it was still dropping
Al the caps are new. I'm going to pick up a couple more filter supply caps and decoupling caps and swap a few things out(I planned to do the power amp decoupling caps anyway)
Any other ideas. I can watch the meter dropping by tenths of a volt , goes down pretty quickly
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My first bet is on a filter cap. The one farthest up stream (ie, closest to rectifier) that is dropping volts is most likely.
Second bet is look for something hot that should not be hot. Then find out why it's hot.
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Once again the mighty seers have spoken and all is resolved...at least with that problem!
My 'extra' filter stage on the reverb recovery stage seemed to be the problem
I decided to clip out those filter caps, and the 1K resistor I had seperating it from the preamp stage and BOOM
I had 200 flipping steady volts on V1 and V5 and 199 on my PI !!!! yikes!!
and 420 on my 6L6s at 34 ma -42 volts
Should I bother bringing those numbers down at all?
Maybe increasing the dropping resistance just before the PI or just after
That would certainly make for more voltage up stream at the Power amp giving the amp a little more juice
What say you? and Thanks very much!!
P.S Got my pots from Doug today (Thanks Doug, quick shipping!!) Holy auto-wah Batman what a sound with the vibrato!
I do get a little thump through the speaker at the highest intensity settings
Also it'd be nice if it went a liiiiittle slower
Any way to accomplish that?
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My revibe uses the same trem circuit. Look at the fast/slow switch.
http://home.comcast.net/~seluckey/amps/revibe/revibe.pdf (http://home.comcast.net/~seluckey/amps/revibe/revibe.pdf)
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Cool
i'll give that a try I have a 4.7meg resistor that should be close enough and the appropriate pot and a little switch kicking around. So nice when others have laid the ground work
I might add another 2.2 or 4.7 K to the first dropping resistor as I was trying it with some different power tubes and after or maybe 2.2 to both 10K resistors
I'm trying an older pair of 6L6s and I seem to be getting 422 on the plates of teh power tubes...245 on the phase inverter and around 225 on V1 and V5..amp sounds good though. I popped a 12ax7 in the phase inverter spot and it seems to brighten it up a bit, just goofin' around.
from reading people claim the brownface amps are midway between a tweed and a Blackface amp
Less scooped and bright than Bf but more hifi than a tweed. I've never really compared them
I wonder if this is in part a result of the vibrato circuit as I need to push the bass and treble more on this amp than I do the deluxe to achieve a similar sound, also the bright switch is quite a bit less effective (in a good way) than my deluxe builds(which don't have the bright cap)
How hard would it be to add a presence control to this hybrid circuit?
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i'll give that a try I have a 4.7meg resistor that should be close enough and the appropriate pot and a little switch kicking around. So nice when others have laid the ground work
I meant ONLY look at the switch. It's putting a .01 cap in parallel with the original .01 cap. That slows the oscillator down. You can use a switch like I did or you can simply change the original .01 cap to a .02.
All that other stuff is not important toward changing the speed. You may also be interested in the LED since it gives a little eye candy. Red or yellow work best.
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How hard would it be to add a presence control to this hybrid circuit?
Very easy. Just change a couple resistors in the NFB circuit and add a pot/cap. Copy the 5F6A presence/NFB circuit.
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ooh that is easy and easy to try with alligator clips to see if I like it before commiting to it(drilling holes)
Is there a way to adjust the vibrato, mine seems to whoop whoop around 7 on the intensity dial and sooner if the speed is turned up higher
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Wow that bassman 5F6A presence control has way more effect than the concert 6G-12 one!
i tried both and I really enjoy the tactile sound..with a space echo in front it sure accentuates the hiss though
I guess if I find the max volume of the control a bit much I could just drop the value of the NFB resistor..i think
I'm going to try it tomorrow when I can play it more at volume
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a couple questions if i may:
- I was going to install my line out (see schematic)
Would it be okay to mount it on the faceplate between the white reverb knob and the pilot light?
I'd have to run a line over the board from the speaker output
Too potentially noisy? I just know when it's all boxed up it's be harder to get at the tube side as it's going to have a back panel ala a tweed amp
- Harmonic Vibrato whoop whoop after about 7 on the dial (sooner when you turn up the speed) anything I might adjust? idears? (don't know if this is the problem but my voltage on the 2nd 12AX7 in this circuit is about 35 volts too low, 285 rather than 320)
excuse my kooky lead dress..it really is a very quiet amp, even dimed (with no guitar, my giutars have single coils...they're always noisy)
Also a picture of my scary looking SS rectifier, perfectly safe but it should dissuade the unwary
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Found a solution to where I should put my footswitch and line out jacks, I built a little panel above the tube sockets(the jacks alternate with them). this way I can have a little cutout in my back panel and Have all these cables on the back of the amp instead of the faceplate, Also the wires to them are a lot shorter.
IN order of appearance ,trem, reverb, line out
The picture shows the jacks, the alligator clip spaghetti is me sampling presence controls
I have a question:
I've been doing some comparing and this amp is loud and clean up till about 8 on the volume.
However to achieve the same max. clean volume as my deluxe reverb build (around 4.5 on the dial) I have to turn it to about 6 or 7! and even cranked it's only distorting a little which seems kind of wrong for a blackface as they seem to break up around 4.5 to 5
Also I seem to need ALOT more bass and treble out of the tone controls to achieve a nice sciooped tone and the more the amp is turned up the more prominent the mids get, and not in a good way.
to eliminate a possibility I bypassed the harmonic vibrato(making it essentially a normal AA964 vibrolux and it actually becomes quieter and thinner so it's not vibrato circuit
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and even cranked it's only distorting a little
what I've found when that happens is usually I don't have enough drive to my PA section, hence cleanish, or I I took to much gain outta my pre section so there is no preamp distortion, which usually doesn't give me enough drive either.
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V1B has a split plate load resistor (100K and 6.8K) and the output is taken across the 6.8K which means that the output is a small fraction of the total signal that V1B has available directly on the plate. You can experiment with those two resistors. Try a 47K + 47K to see if that is more to your liking. Try to keep the total resistance about 100K. Even better, replace those two resistors with a 100K pot (mil spec or PEC) and adjust for whatever gain you want.
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THanks!
So my impression was, that split plate resistors provided more resistance to the B+ going to the plate..but not a lot more with the difference between the 2 plates being about 6 volts
So I must be missing something on how this actually functions, there's some sort of interaction between the resistors and the signal output of the pre, because if I bypass the 6.8K the signal drops hugely (I know I wasn't supposed to do that, I was doing it by way of experimentation)
IS there a simple explanation someone can give me to explain what is happening with the split plate resistors?
I'd like to understand
And would the differences I've noticed in the effectiveness of the tone stack and the amplifiers tone when turned up loud?
(past 7 or 8 at anything below 2-3 I can still hear the acoustic tone of my plywood archtop it's that quiet)
The amp seems very mid heavy when turned up loud
ON a side note while meticulously going over my schematic from start to finish I found 2 mistaken cap values on my board in the vibrato circuit! I had a .05 where I should've had a .005 and I had a .022 where I should've had a .22!!
the result was.....less intensity, so now it whop whops at around 9 on the intensity dial instead of 7
i also notice my power tube bias goes up a couple mv and wiggles a bit (1 volt up and down)when the vibrato is on
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Normally the ac signal is taken from before the 100K plate R, red arrow. taken from there you have 100K between the signal and ac ground, which is through the B+ filter cap.
In your amp, signal is taken after the 100K but before the 6.8K R, blue arrow. So 1st the signal has to go through the 100K plate R which is in series with the plate signal, then there's only 6.8K between the signal and ac ground, green arrow.
The plate still see's the total of the 2 series plate R's but the ac signal is knocked down.
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duhh :icon_biggrin: thanks for that!, I was bit blind to the fact that the signal was being taken off AFTER the 100K resistor
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Well i think that's what I need. I did 82k/22k instead of 100K/6.8Kand it makes a huge difference.
A lot more gain and it seems to make the over all sound of the amp a little more bright and well 'spanky'!
Perhaps it just the preamp contributing more
The vibrato still seems to work fine too
I was opting for a minimal change so it wouldn't impact the vibrato effect
THanks! The help I get here from everyone is invaluable to my builds/fun and I really appreciate it!
An unrelated vibrato question..I annoyingly just noticed that more modern electronics shop(which I assumed wouldn't have high value pots) has both 10M and 5M linear pots. (though I was happy to finally give Doug some business)
I know the circuit calls for 4MRA but could I try a 5M linear in it's place and maybe have the full range of the speed control? rather than add a switch(which is a perfectly good other option that I might implement, A new pot is less invasive). I find I have too much fast (it seems to almost disappear) and not enough slow
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Just replace that .01 with a .02. That's the least invasive of all. You don't have to have the switch. Using a bigger cap will slow the entire speed range. Using a switch allows you to make it slow or make it fast.
The vib circuit will work just fine with linear pots. Your control of the circuit may not be spread out across the pot rotation the way you like. Try and see.
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Do you mean the .01 you use with your switch? and eliminate the switch? just making sure
And I was noticing these changes in the revibe circuit.
"C27 changed to .047μF and C18 changed to .01μF because they were attenuating the higher LFO frequencies too much. "
Worth doing in this context?
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Oops just figured this out it's basically adding a cap in parallel . I figure this out when I alligator clipped a .022 over the .01(making .032) and I liked the result, maybe a little slow on the fast end, perhaps I'll pick up a .027
I also tried clipping a resistor in paralell to the 4.7 meg resistor to increse intensity..I liked that too. might monkey with it
So I put AB763 deluxe reverb build back to back with this build, both going through the same JBLD130 and I have to say at similar preamp and volume setting the deluxe with the bright switch OFF sounds like this build with it on!
They're roughly the same volume
The deluxe is much brighter
The deluxe is also quite a bit more naturally scoooped in the mids
I think this is a result of less preamp gain in this amp??
Or is there a lot of high end attenuation in the vibrato circuit?
The bright switch seems a lot less effective in this amp
Fun comparing them
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Hey S.Luckey would the mods you did to your stand-alone revibe be practical in my application?
I'm just curious about them
You change the .03 to .01 and you change a .1 to .047 because they attenuate the LFO high frequencies
Does this help with the high end of the entire amp or is it particular to the sound of the vibrato?
And you put a 5.M cap in parallel with the speed pot to "limit the low end of the frequency range"
Does this help with whop whop or helicoptering as they call it?
I get a little of this at higher intensity settings at faster speeds
or is it more similar in result to the previous mod except affecting the low end
I'm going to move my footswitch too i like being able to turn it on instantaneously
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Hey S.Luckey would the mods you did to your stand-alone revibe be practical in my application?
The mods I did will work in your circuit too. But if you are happy with the circuit as is there is no need. I had two objectives for modifying the LFO circuit.
First, I wanted an LED indicator that blinks at the rate of the LFO. This is mainly a visual mod, although it does increase the trem signal and it makes the amplitude of the trem signal more constant from slow speed to fast speed. But, while scoping the trem signal at my footswitch jack I noticed that the amplitude still dropped off noticeably at the faster speeds. Changing the .03 to .01 and the .1 to .047 increased the trem signal amplitude at the faster speeds, keeping the amplitude more constant across the entire speed range. Net result, I didn't have to readjust Intensity just because I changed speeds.
Second, I wanted the fast/slow speed capability because I like a slower trem. Switching a .01 cap in parallel with the original .01 cap causes the oscillator to slow down for a swampy Delta trem. But it also causes the fast speed to slow down too and sometimes I want the faster speed. Using a switch gives both speed ranges. Having an extended speed range is great, but the oscillator cannot work well over a wider range without a lot of other circuit changes. I found that below about 3Hz, the oscillator would sometimes drop out, just die. So, I put a 5.1M in parallel with the 3M speed pot to prevent the speed from going slow enough to cause drop out.
None of these mods affect the guitar sound. They deal strictly with the LFO circuit.
About your whop whop... That's likely because the two 180° out of phase trem signals are not totally cancelling in the modulator tube. For perfect cancellation the two trem signals must be exactly the same amplitude at the modulator grids (pins 2 and 7) ***AND*** the two triodes must be well matched ***AND*** the two 100K and two 470K resistors on the plates (pins 1 and 6) must be matched. A mismatch of any of these three parameters will result in some uncancelled trem signal appearing at the output of the modulator.
Changing both tubes in the trem circuit can affect the whop whop. That's where I'd start. Checking those four resistors I mentioned would be the second thing I'd do. Then I'd put a scope on the modulator grids to ensure the two trem signals are equal. If not, look back to the phase inverter to correct. If they are equal, move the scope to the modulator plates. If the signals are not equal then start rotating tubes. When everything is matched the trem signal will be completely cancelled and should not appear at the output of the two 470K mixing resistors. Since whop whop is not a mission critical parameter, don't expect perfect cancellation. Even if you get perfect cancellation, that's likely to change as the tubes age.
You can probably get the whop whop down to acceptable levels by simply turning the intensity down a bit.
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THanks for taking the time to explain, It really helps. I appreciate it no end
My tremelo works well, despite the whop whop at higher intensity and speed settings
with a .02 cap (fast slow switch simplified mod) I get a nice slow speed that doesn't drop out
and I can use that slow speed at max. intensity no problem, very cool phasey sound, no whop whop!
The faster I put the speed, the less the intensity, but perversely the sooner the whop whop comes in
At fastest speed I can't get the intensity past 4
At about 4 on the speed I can't get intensity past 5.5
I currently have the two caps mods implemented ( to keep the intensity equal through the speed range)
I might swap that back as it seems I have a whop whop limit regardless
I tried swapping out the tubes with some effect on the intensity and whop whop. the only way i can test the balance of them is measuring the B+ on their plates
The resistors are all brand new flameproof types that generally measure dead on!
Oh and I think a fair bit of the whop whop comes from the efficiency of the JBL D130 as I tried it with a pair of 10s and a lot less whop whop
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Here's what you do if you're a complete nutbar and you want to try your amp in rehearsal with 2 x 12's before you decide to cut down the cabinet to make a smaller 2 x 10 or 10 and 12 cab
The cutout for the controls is tight because i'm leaving room to shift it to one side if I decide to make the cab less wide
The really funny thing about these pictures is that if you look closely (to the left and right of the amp in the cab) you can see the old hardcover books I used to prop the amp up on as I mounted it...in a hurry prior to rehearsal....and then I forgot they were in there!!!!
There are no back panels mounted of course and I used the original bottom for the hammond amp for the tryout
The speakers are a JBL D123 and a 70s Rola driver
The amp sounded really good! I received compliments from my bandmates
Huzzah for high heat furnace tape!
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I like it! I've still got a 12" speaker in a "cab" that I made out of scrap plywood (sheathing, not the nice stuff) about 18 months ago. I've got some chestnut boards scavenged from our home re-model that I keep meaning to use to make a pretty cabinet, but just don't get around to it.
Speaker sounds just fine in the plywood box. :icon_biggrin:
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Toxophilite, I think you have done a pretty cool thing with this build. Thanks for sharing your success!
I've got some chestnut boards scavenged from our home re-model that I keep meaning to use to make a pretty cabinet, but just don't get around to it.
John, chestnut can be a magnificently beautiful wood. IF you do something with it, please consider sharing the photos. I would love to see what you did to honor that wood. Many chestnut trees have suffered a blight and the wood has become increasingly difficult to find.
With respect, Tubenit
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Oh man, way to put the pressure on! :laugh: I am not even 10% the craftsman you are. One reason I haven't gotten to it yet is I am waiting till I have time to concentrate just on that. I plan on cutting off a little scrap from the board end, and sanding and cleaning it up and see how it looks. Anything I put on will be clear; just my personal preference but I always like the natural color of any wood as much as possible.
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I'm with you on the natural colour. I make traditional one piece longbows and recurves out of a variety of beautiful woods and some people ask me what colour I'm going to paint them????!!!! :dontknow:
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Dang, you build bows? If not too much trouble, post a couple pics in Other Topics. I had shot some archery a few years back (using a creaky old Bear compound) and really liked it. Bare bow, nothing hanging on it but the arrow rest. It was sort of amazing to me how quickly I could "feel" the arrow's spot of impact.
"spot" being a 15" circle. :laugh:
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Doesn't take much to get me to show and tell a few bows..i love 'em!. I put up some pictures on other hobbies, maybe I'll put my bands up too for kicks
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Here is a finished schematic if anyone is interested
Seems to work really good
The major changes in this one are the voltage divider plate resistors coming out of the second gain stage being adjusted for more gain which makes the amp also more pleasing sounding to my ears, breaks up around 5
Also some S Luckey inspired inspired changes in the vibrato and only 2 diodes in the SS rectifier
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A question for Doug or S.Luckey if I may
regarding the entrance to the PI
There's a .047 cap then a 220K resistir then on Sluckeys AB763 lite there's a 270K resistor to ground
and on Hoffmans AB763 one channel there's 47k to ground at this point
Does this reflect the fact that Sluckey Ab763 lite doesn't have reverb and tremelo?
Also with this build, which is essentially a vibrolux with the 6G12 harmonic vibrato shoe-horned in I have a similar setup except I have a 220K resistor to ground as i couldn't find a 270k at that point( I forgot to buy some!)
Does the 220k make it quieter?
Would this build better suit the 47K to ground or the 270K?
Is it merely a matter of taste?
Just curious
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There's a .047 cap then a 220K resistir then on Sluckeys AB763 lite there's a 270K resistor to ground
and on Hoffmans AB763 one channel there's 47k to ground at this point
Does this reflect the fact that Sluckey Ab763 lite doesn't have reverb and tremelo?
This 47K is all about simulating the 50K Intensity pot of the original AB763 amp. Doug's amp used a different tremolo circuit so he removed that 50K pot and just put a 47K resistor in circuit to keep the gain the same as the original AB763. I did the same in my single channel AB763, but I also combined the 47K (omitted INT pot) and the 220K mixing resistor (omitted second channel). 47K + 220K = 268K, so I just used a commonly available 270K to replace both resistances.
Your circuit is just fine with a 220K and 220K voltage divider going into the PI. This will cut the input signal seen by the PI in half. You may want to experiment with removing the 220K that goes to ground to increase this signal. You may or may not like it any better. This 'may' also compensate for the changes you made to the split plate load resistors in the second preamp (don't really know).
So, are you pleased with the reverb connected per your drawing? Is it as strong as the reverb on your AB763 amps?
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I noticed a few small errors in your schematic that I think are just draftsman errors.
1. Look at the PI input. The 820Ω NFB resistor does not connect to the .001µF PI input cap. That line should just cross over with no dot.
2. Look at the VIBRATO oscillator. You have a vertical 1MΩ resistor, top end connected to the junction of two caps, bottom end connected to pin 2 of the tube. The bottom end of that 1MΩ should be connected to pin 3 instead of pin 2.
3. Look at the power supply. The 100K that feeds the bias diode should be connected to the end of the secondary winding, not a bias tap as shown. Your PT doesn't have a bias tap, does it?
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Thank goodness for your sharp eyes
and thank goodness I usually follow the layout when building!!(confirming anything confusing wit the schematic)
fixed all those things I believe
I added the faux heater centertaps as well
The reverb seems to be exactly the same as my AB763 deluxe build (and I'm using an identical tank) Plenty of it when needed!
As usual anything past 2-4 enters character/effect/Pipeline mode
I know how the rectifier/bias is supposed to hook up if I have an amp in front of me. it's the transformer diagram I find #$&45 confusing
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The 100K in the bias supply is still wrong. It should connect to the top of the winding with the center tap. You have it connected to the 5VAC winding and the voltage at that point is BIG positive DC. I don't even know why you have that 5VAC winding connected to anything???
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... About your whop whop... That's likely because the two 180° out of phase trem signals are not totally cancelling in the modulator tube. For perfect cancellation the two trem signals must be exactly the same ...
Sluckey: what do you think about replacing the plate loads of V5 with a single 250kΩ linear pot, with outer lugs to either V5 plate and wiper to B+? He could adjust that (probably internally) to balance the signals and see if he can eliminate the helicoptering.
I'm guessing the pot will need to be an RV4-type to have sufficient voltage rating from the track to the pot body.
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That would probably work to balance the trem signals so they would cancel better. It will also affect the guitar signal. Another way to perfectly balance the trem signal would be to put a pot in the plate of V4B and adjust for cancellation of the trem signal by monitoring the recombined outputs of V5 with no guitar signal present. Might need a scope or a DMM such as the Fluke 87 that is capable of measuring the trem signal. I may experiment with this a bit tomorrow. If I make any interesting discoveries I'll post back.
This is all assuming the whop whop is caused by unbalanced trem signals to begin with. :dontknow: I don't remember having this issue with my revibe but it's been awhile since I plugged into it.
This whop whop may just be a JBL thing. IIRC Toxophilite had a similar issue with the bias modulating tremolo in another amp but the issue disappeared when he used another speaker.
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This whop whop may just be a JBL thing. IIRC Toxophilite had a similar issue with the bias modulating tremolo in another amp but the issue disappeared when he used another speaker.
Maybe just hiss + trem modulation + JBL extended treble (vs regular guitar speakers)?
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I don't remember having this issue with my revibe
FWIW I got my revibe out this spring and it did *whoop* at around 7, but it was WAY to much for me anyway so I just didn't go there, was happy with 4-5 sound.
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Well my revibe whop whops more on the faster speeds and a lot more with the JBL D130
With non-JBL speakers it still whoops, just a lot higher up on the dial, so usable,
In general the higher speeds are less intense, adjusting the response of the dial (ala the Sluckey mods) makes it so the whop whop comes earlier
I suspect it's an efficiency thing with the speakers
And yes my bias vary trem does whop whop less with non-JBL speakers.(though it still distorts)That particular D130 (not a D130F so more efficient) I have is frighteningly efficient , more so than any other speaker I have(an old rola 12 comes close) and I've had many different JBLs over the past 10 years.
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S.Luckey, or Doug, whomever has a moment to spare me
I have a last dim question regarding this amp and the resistors going into the PI
(I'm currently monkeying with it for fun and experiments sake)
Slucky explained this to me when I was asking about my 220K resistor to ground:
This 47K is all about simulating the 50K Intensity pot of the original AB763 amp. Doug's amp used a different tremolo circuit so he removed that 50K pot and just put a 47K resistor in circuit to keep the gain the same as the original AB763. I did the same in my single channel AB763, but I also combined the 47K (omitted INT pot) and the 220K mixing resistor (omitted second channel). 47K + 220K = 268K, so I just used a commonly available 270K to replace both resistances.
I wanted to ask about the differences between the two
In Doug's one channel it uses the reverb but no tremelo thus the 47k to ground to keep the gain the same as an AB763, but there are also no mixing resistors(being a one channel amp)
On Sluckey's (which is also one channel but more like a AA763 deluxe with no tremelo) there is the 220K mixing resistor and a 270K to ground
Is this just 2 ways of achieving the same result?? or is there a difference in gain?(besides the extra post reverb gain stage in Doug's)
by the way thanks once again to Doug for having this forum and Sluckey and everyone else for the help .
I wouldn't be successfully building amps I've alway wanted (or imagined I wanted ) without the assistance and encouragement I've received here
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Is this a difficult question to answer? or an overly simple one that I should know
I've tried both combinations.
At best i could tell just the 47k to ground with no mixing resitor was a little louder and brighter maybe
Can anyone chip in with a theoretical difference if there is one :dontknow:
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We figured you already figured it out: "just 2 ways of achieving the same result"
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Hoffman's circuit loads (decreases gain) the previous stage more but still provides more signal to the PI than my circuit. My circuit has a lighter load on the previous stage but the voltage divider knocks the signal down by almost 50%. That's why Hoffman's circuit is louder. Since you have done it both ways you are in a better position to compare the two slightly different circuits.
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Thanks
I was curious as to the why of it and I appreciate the explanation
I was guessing about the two different ways to achieve the same things though the numbers made sense that way
I think I need to do some more reading and get past copying and learn what's going on a little more
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Hello
I've been enjoying this Amp
I did a recording session with it a couple weeks back
I was wondering about something
I like the harmonic vibrato but it would be cool to be able to bypass that whole circuit and get a straight AB763 vibrolux sound sometimes
I feel running the signal through that circuit somehow alters it's EQ
Would be at all possible to install some sort of bypass switch?
or is it impossible with the split plate resistors of the gain stage prior?
here's the schematic again for ease;
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You can do it with a DPDT switch. One half of the switch will connect the .047 either to the junction of the 82K and 22K (as shown on your schematic) or directly to the plate. The other half of the switch would connect the bottom end of the 3.3M/10pf either to the .1 (as shown on the schematic) or the bottom end of the .047. The 500pf feeding into the reverb driver would always stay connected to the bottom of the 3.3M/10pf.
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Thanks!
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This is what I had in mind. The switch is shown in the normal position. Flip the other way for bypass.
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Cool Thanks for the bit o' schematic!
I tried it out and..
I actually hooked it up differently originally(I had been planning it in the car :icon_biggrin: prior to consulting the schematic again) and it worked properly but made an unholy loud pop through the speakers when switching
I thought it was my hookup so i hooked up like the schematic and it did the same. Worked but 'POP!!!!'
I am using a relatively inexpensive mini switch that is rated for 3 amps at 250 VAC (6A at 125)
Would it be better with a bigger more expensinve switch?
or do I need a big cap across the switch??
Thanks again
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I wuz gonna say: S's plan will POP.
(Nano-switch or frankenstein switch don't matter.)
Consider this:
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Excellent
I think I have a couple of 3.3m resistors and a .047 cap too
Thanks!
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3Meg, 10Meg, 8.79Meg... whatever. Much larger than 100K, much smaller than 100Meg.
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Damn. now I'll have to flip a coin! :icon_biggrin:
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Do you understand why the R to stop the POP and why the wide value choice? :icon_biggrin:
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I'd be more than happy if you'd explain, truly I would!
I don't know the theory, though I know there's similiar setup with a 10 meg R on the bright switch on my super reverb
I would guess that it's large enough not to let audio signal through and lessen the signal but high enough to stop the pop
Unfortunately I tried PRR's setup with a couple of 4.7 meg resistors( I had them available) and it still made a huge POP!!!!
I have to wait till morning to tray anything else or even try that again as I live in apartment and that pop noise is pretty startling
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Any more ideas as to how to make this switchable without a giant Pop ? :help:
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Install a mute switch and always flip it first.
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> it still made a huge POP!!!!
I'll speculate that it is not wired as drawn.
Have you pondered Willabe (http://el34world.com/Forum/index.php?topic=18529.msg203347#msg203347)'s question?
The one plan switches between points at +200V and +280V DC. The 80V jump over-over-loads the amp.
The second plan SHOULD have all three switch terminals at *zero* VDC at all times. Does it?
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You could be right, however I did check it a few times to make sure, but I'm quite capable of making mistakes
At the time it was late and I couldn't keep experimenting with it further due to noise constraints
Perhaps I'll wire it up again this eve and measure some voltages
thanks
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Awoke in the middle of the night, so what else to do but tinker
Just to make sure i understand how to wire it. I will write it out.
-Prr's version of the switch has me breaking the circuit between the .1 cap after the harmonic vibrato and the 10pf/3.3meg prior to the last 12ax7 gain stage.
-At the junction of the .1 cap and the 500pf cap that comes from the reverb circuit I add a 4.7m resistor to ground .
The junction of these 3 components goes to the one side of the switch
- the middle of the switch is attached at the 10pf/3.3m resistor going to the last 12ax7 gain stage
-A .047 cap is soldered to the junction of the 2nd gain stage plate and it's plate resistor (in this case a 82k)
A 4.7m resistor to ground is attached to the other leg of this cap. the junction of the cap and 4.7m resistor goes to other side of the switch.
No large voltages could be read at any on the switch points, though when switch to the harmonic vibrato there was a small fluxating voltage on the last gain stage side of the switch(middle)and the harmonic vibrato side.
The switch still made a pop
Though as SLuckey suggested (or something similar) when I flip the standby I of course don't get the pop...humourously I generally don't use my standby as it 'pops too
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-Prr's version of the switch has me breaking the circuit between the .1 cap after the harmonic vibrato and the 10pf/3.3meg prior to the last 12ax7 gain stage.
correct
-At the junction of the .1 cap and the 500pf cap that comes from the reverb circuit I add a 4.7m resistor to ground. The junction of these 3 components goes to the one side of the switch
wrong. The 500pf going to the reverb input stays connected to the bottom end of the 10pf/3.3M. The 500pf will no longer be connected to the .1µF.
- the middle of the switch is attached at the 10pf/3.3m resistor going to the last 12ax7 gain stage
half right. Don't forget, the 500pf will also be connected to the middle of the switch.
-A .047 cap is soldered to the junction of the 2nd gain stage plate and it's plate resistor (in this case a 82k)
A 4.7m resistor to ground is attached to the other leg of this cap. the junction of the cap and 4.7m resistor goes to other side of the switch.
correct
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Thanks
I wondered about that and should've asked!
That helps some
It pops less consistantly and primarily on the switch to bypass
it is a quieter pop
When the vibrato is bypassed the amps gain jumps by a HUGE amount
2 on bypass is equivalent to about 4-5 on vibrato
maybe that's why some people claim that the brown amps are often clean most of the way up the volume control?
Mine was certainly like that till I adjusted the voltage divider on the 2nd gain stage's plate
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> When the vibrato is bypassed the amps gain jumps by a HUGE amount
By my thumb-count the vibe path has gain 0.2*25*0.5= 2.5, relative to the shortcut path.
The vibe channel has 12AX7 gain, a LOT. So the input is tapped-down 82K:22K, there's some loss in the filter, then gain near 50, then some loss in the mix network.
If vibe is weaker than direct, something is wrong. Weak vibe section? 2.2K where you think 22K?
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hmm
Well I just went over the vibrato circuit and I found a mistake
I'm pretty sure it's the only one
On the second 12ax7's joint cathodes I have a 25/22 bypass cap instead of a 2/25
I swapped a 2.2/100 in there that I had kicking around
The vibrato channel is a little louder now but still noticeably quieter than when bypassed, and that's even with the vibrato intensity turned right down(essentially off)
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Voltages?
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V2
1 - 125
3 - 1.25
6 - 168
8 - 1.26
V3
1 - 271
3+8 - 3.03
6 273
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The original concert circuit has the normal channel joining the vibrato channel via 1m resistor just prior to the PI
Is that a load I should be simulating somehow?
Like one can do when building a one channel AB763 circuit
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no
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Well I discovered something unusual that has me perplexed
Since I built this amp there has been times when it has sounded great and other times when it seems to lose it's bottom end, significant volume(via the bottom end) and be unpleasantly middy.
As I can only use it infrequently at practice it's been hard to tell if I was fooling myself
I've gone around and resoldered things etc, the usual stuff
I just noticed when messing with my tagged in bypass switch that if I just touch a metal tool to any of metal leads of the plate power supply components for V1 V2 V3 V4 or V5 (pre PI). I could make the bottom come back and the amps sounds good, then I can reverse it back to bad sounding by touching it again.
It makes no sense, i's like there's something electromagnetic happening in the preamp and tremelo power supply
I can almost turn it on and off just by touching my pliers to any exposed B+ lead
:dontknow: :dontknow: :dontknow:
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I can almost turn it on and off just by touching my pliers to any exposed B+ lead
That MIGHT cause a life shorting event :icon_biggrin:
Off the top, it *seems* you have a static buildup?, maybe some sort of *floating* capacitance?
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Rubber handled pliers! :icon_biggrin:
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Rubber handled
I thought that also with an "insulated" tweaker, adjusting 3kvdc, F :cussing: was I wrong :icon_biggrin:
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huh?? what?..unnnnh ..sorry I just regained consciousness....yuck burned hair smells gross!
Seriously though
Anybody has an idea on this one?? :dontknow: :help: It's pretty confusing to me
I've been poking at all the joints and components with a wooden stick with no luck
It's the damnedest thing!
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Seriously though
Yes, very serious, you need to be ABSOLUTELY SURE that the isolation on the tool will handle the voltage.
AND even then, keep your other hand in back away from the chassis to any path to ground through your body ACROSS YOUR HEART!
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Right
Thanks very much
I understand the dangers and appreciate the concern.
Please, please, excuse my levity
I was really hoping to divert attention back to my original request for insight.
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keep your other hand
That's why I'm still around to build , in the pinky, out the palm,
and levity is good; "what doesn't kill you...." brings out the dark humor :icon_biggrin:
I have *seen* your symptoms before, my *scrambled* memory keeps coming back to some kind of *floating* condition for the circuit in question, resistive ground/B+, parallel capacitance (that shouldn't be there), static buildup.
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Hmm
Are you referring to parasitic capcitance? or the miller affect?
like here:http://www.valvewizard.co.uk/gridstopper.html
It kind of sounds like it from the reading I've done
I wonder if it's:
A - my lead dress (though the amp is nice and quiet)
or B- a result of using the harmonic vibrato circuit in the AB763 circuit
or something else
Interestingly enough it doesn't seem to do the tonal and volume change thing wen the harmonic vibrato is bypassed
Frankly ,though it's a cool and unique effect, it is a lot more of an effect(like a phaser or leslie) I like the tremelo to just be an amplitube thing so I might be swapping in a 6G16 bias vary. ( I have a few of those cone boards, I might just make a new one and swap it in, cleaner)
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Out with the old, in with the new
Straight up vibrolux with 6G11-A bias vary tremelo now, Working great so far