Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobSmith on February 16, 2020, 11:17:33 am
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Thanks for input on my last post.
I decided to go with the reverb vibrato combo (aka Hoffman Revibe) which should be fun.
My kit arrived, along with transformers and chassis. I will be building a lunchbox style Revibe in a Hammond chassis (similar to my Blues Junior). That worked out pretty well, so we’ll try that again.
Obviously wanted the long reverb tank, so I needed a 17” long chassis (which they have, Hammond 1441-30BK3). I also go the matching cage cover, all powder coated black...pretty sweet. Originally I had planned to vertically mount the tank on the inside cover of the cage, with a backing plate (I ordered an extra powder coated bottom cover which I could cut down and attach to the cage.) but when the parts arrived, I started toying with putting them all on top and mounting the tank horizontal. The tank has been factory tuned for horizontal mounting, but references on the web claim vertical mounting is ideal (So why doesnt anyone sell such a tank???). Anyway, I think I got it all to fit in a way that will work, but it is a tight fit FOR SURE.
Oh, and I bought 3 different tanks for comparison...all share the same chassis form factor and mounts. (Tube amp doctor, accutronics/belton, MOD)
Layout considerations included leaving a 3/8” gap on front and back for where the cage cover rests. Keeping a small air gap between the tank and the cap can (I account for 1/16” which should be enough to keep it from banging together. Keeping the transformers far enough from the “sensitive” output side of the reverb tank (and close to V1 for recovery). Keeping enough space below tube sockets to neatly label the the tubes (will use Letraset dry transfer).
I plan to use rivnuts to mount the cap can and reverb tank as some of their fastener locations will be hidden under the circus board (it’s a big one from Hoffman).
I’m also making my own turret board for the filter caps. I didn’t want to scatter them underneath like in the Hoffman example. Way to tight for my liking. Of course keeping them under the cap can (purchased from Hoffman, but I think same as Mojotone can) will be a challenge. I’m using 1/4” standoffs to keep the height short enough and perhaps a small spacing washer under the cap can to raise the height, definitely cutting it close.
Question: what are people doing about the green wire from the wall. Hoffman build page says cap it (is this a good idea??? Seems to defeat some safety protections), but the circuit diagram seems to show it attached to the chassis. Apparently noise and ground loops were a problem in the original circuit...
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Here’s the planned layout....
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That should be a fun project. I originally connected the power cord green wire to chassis, but I think I later clipped it. You will still have a safety ground provided from the amplifier via the connecting cables all the way to the guitar. I built mine with a similar layout...
http://sluckeyamps.com/revibe/revibe.htm
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That should be a fun project. I originally connected the power cord green wire to chassis, but I think I later clipped it. You will still have a safety ground provided from the amplifier via the connecting cables all the way to the guitar. I built mine with a similar layout...
http://sluckeyamps.com/revibe/revibe.htm
How much of an improvement did clipping the ground make? I’m hoping to avoid doing that, it makes me uneasy to use an instrument cable as a safety ground (not particularly well connected or current rated).
I suppose I could also invest in a 1:1 audio isolation transformer to decouple the components or how about just lifting the ground on a cable (same as inside an amp with shielded cables.)
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How much of an improvement did clipping the ground make? I’m hoping to avoid doing that, it makes me uneasy to use an instrument cable as a safety ground (not particularly well connected or current rated).
I don't recall how much of an improvement but I assume there was some since I never reconnected the green wire. As for feeling uneasy about relying on the shield inside the instrument cable as a safety ground, consider this... You have always been relying on that shield to ground the strings! Only difference with the revibe is that you will now rely on two instrument cables to ground the strings. I don't have any issues with that. :wink:
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You can connect it to ground
or you can keep it disconnected
or ... you can give a try to this
(https://i.imgur.com/8ZQKHUB.jpg)
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Give a look to the VOX AC30CC2 Power Section Schematic
https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf (https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf)
Franco
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How much of an improvement
fwiw my revibe had unacceptable hum, once I broke the ground loop it still hummed but not a deal breaker (my "other" hum was 90% build flaws)
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You can connect it to ground
or you can keep it disconnected
or ... you can give a try to this
(https://i.imgur.com/8ZQKHUB.jpg)
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Give a look to the VOX AC30CC2 Power Section Schematic
https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf (https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf)
Franco
I briefly looked at the schematic. It’s drawn a little funny so not the easiest to read. It looks like the power section ground uses that circuit. but the preamp seems to have 2 RC networks to ground on either side of a ground bus, but the input jacks say normal grounding washer. I’m not sure I understand how that’s suppose to work.
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How much of an improvement
fwiw my revibe had unacceptable hum, once I broke the ground loop it still hummed but not a deal breaker (my "other" hum was 90% build flaws)
Anything you would have done differently? There seems to be a lot of variation in how this is built. It’s not exactly your 5e3, done a million times. Any insight would be appreciated. Thanks?
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Here's another revibe you may enjoy...
https://el34world.com/projects/ReVibeRack1.htm
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I’m not sure I understand how that’s suppose to work.
Connect all your circuit ground on one point on your board
Connect the green wire to the chassis (here in Italy green/yellow wire)
Join the ground point on your board, where the green wire is connected to the chassis, using the circuit I posted
Small noise signal don't cross the small circuit, higher voltages (due to circuit problem) go across the circuit and are directed to the wall ground, protecting you
I hope I was able to explain me in a comprensible way
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BobSmith
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Franco
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I’m having some trouble with the download. I didn’t want to make an account. But I appreciate the effort. Does it matter if the jacks (instrument and reverb tanks) are grounded to the chassis? It would be a great pain to isolate these as well. Yikes!
All along my entire chain, wall ground is used everywhere (pedal board, multiple amps on splitters, Etc). Im surprised I don’t get much hum already. I’m not aware of any hidden isolation transformers (I even have a germanium fuzz with funky polarity in the chain) although i do use a strymon Zuma (isolating dc power supply for pedals). I’m thinking the worst case is I need to place an isolation transformer in the chain somewhere, which is probably a good tool to have around.
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Stupid question...
What’s with the weird order of knobs and jacks on these? Is there a reason fender did this, and we all copy?
Dwell-OUTPUT-Mix-Tone-IN. (Original 6g15 pattern)
The location of that output jack bugs me!
I’m marking out my cutouts and was going to go:
OUTPUT-Speed-?blinking LED-Intensity Dwell-Mix-Tone-IN
Unless I’m missing something, this shouldn’t matter.
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You don't need to make an account
just
push on Download this File
then
copy the characters you see
then
push on Download Now
Franco
p.s.: You have one other Private Message
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Unless I’m missing something, this shouldn’t matter.
The only consideration, and it's for any builds;
lead lengths, crossing low signal wires with big signal wires, keeping low signals away from noisy circuitry.......
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Who can say why Leo laid his out the way he did. I didn't like it either.
Didn't like Doug's layout either. His board layout would have worked better (IMO) if he put the input in the output hole, the dwell in the input hole, and the output in the dwell hole. Interconnecting wires between the front panel and the board would have been much neater. ***BUT*** Doug's revibe was designed to fit into the stock 6G15 chassis which he was selling at the time. It was already silk-screened so that is probably the reason for his pot layout. You only had to drill two holes for the vib pots and one hole on the back for the footswitch. Done. Easy peasy. :icon_biggrin:
And I'm a rogue! My revibe looks nothing like the original 6G15. I used good layout practices and laid my board and front panel out together. My layout follows the schematic signal flow very closely. All signal wires are short and neat. I think everybody should copy me! :l2:
Since you are using Doug's board you probably should not stray too far from his pot layout. May I suggest (L-R on Hoffman's layout) in, tone, mix, dwell, out... skip a bit then int, speed, blinky. This keeps all the sensitive signal carrying wires grouped together and the non signal carrying vibrato wires grouped together but away from the signal stuff. Just an idea.
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I think everybody should copy me! :l2:
:laugh:
been trying for years, but you have a knack that is HARD to replicate!!
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Since you are using Doug's board you probably should not stray too far from his pot layout. May I suggest (L-R on Hoffman's layout) in, tone, mix, dwell, out... skip a bit then int, speed, blinky.
The layout Hoffman uses is an underside/upside down look so from the front it would be reversed. Moving my output does make sense after looking at the board. It needs to be near the mix knob too.
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There is no one perfect way to do this. It's OK to change the layout around to suit. Just keep in mind good building practices. You've seen several examples of successful builds. Maybe yours will just be another.
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From the covid bunker...
Here are some updates on the build, finally getting some traction on the project.
I decided to add some switches, one for the lift circuit and one for the blinking LED to bring the cap/resistor back in(in case I don’t like the LED cathode voltages).
I got the chassis done with labeling and ground pads stripped. Everything fits as planned. Now just wiring things up.
Here are the pics so far.
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And some more...
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Looks great! :thumbsup:
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2 questions for the forum:
1. Use of shielded cables
I got plenty of the stuff. Schematic/layout calls for 3 locations: 2 from input, and 1 to output. Any other places this would be beneficial? I'm thinking maybe use one from the Reverb tank output to V1. I'm guessing that is a small signal, so shielding may be good here.
Also as for routing, I'd like to pass them below my board rather than above. It gets them out of the way a little better, and hopefully keeps them from flopping around too much.
2. Order of grounds
As you may see, I'm using a lift circuit, so I'm collecting all circuit grounds at my copper bus bar, then 1 wire to the lift circuit then to ground (has a bypass switch too. I'm wondering if the order of attachment may be helpful in reducing noise. In other words, should certain grounds be closer to the chassis ground than others?Theorectically it should be all at the same potential but eddy currents, transients, and other unpredictables, blab blab...maybe this makes a difference.
I'm curious to hear your thoughts. (What are the sounds of thoughts anyway...)
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is a small signal
I'd agree, and under the board has worked for me well.
Grounds, I like any big n noisey stuff on the "main chassis ground", typically near PT. The small signal stuff I take to a common "signal" ground near the input That way the chassis can resolve the blah blah :icon_biggrin:
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Build updates. Basically all wired up. A few odds and ends on the chassis (grounding hole and pads) need to be done.
I found some cool Switchcraft RCA plugs in black!
Here are some pictures before moving on to testing:
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Mo pics.
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How did the testing go?
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Where did you find those RCA plugs?
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I bought them at Markertek awhile back. About 5 bucks apiece...ouch
https://www.markertek.com/product/sw3502rabau/switchcraft-3502rabau-right-angle-cable-mount-rca-plug-gold-pin-black-body
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How did the testing go?
I powered it up, reverb and vibrato work, and sound really good! Way better than my Strymon Flint! But it’s not perfect. Under certain settings, there are some strange noises. I need some time to characterize the problem better.
3 issues so far:
I don’t see a difference between ground bypass and the lift circuit. In fact, weird thing is when the unit is unplugged I get the expected 10ohm difference but that disappears when plugged in. I don’t understand why that is. I have all the chassis grounds attached to the copper bus bar. All jacks (input/output and the 4 rca jacks have isolating washers. One wire then goes to the lift circuit.
The LED (red) between ground and cathode on vibrato is a bit weird particularly at higher speed. Makes a rhythmic popping like noise. At slower speeds, the noise is gone. I don’t think it’s there when I’m using the original cap+resistor. The Led has some unusual behavior as well, it takes about 5 secs after hitting the button to get the vibrato goin.
Finally, the reverb channel makes a hum when the reverb side of the foot pedal is plugged in. It’s gone when unplugged. I think the pedal is very susceptible to picking up noise. I wonder if I need to lift some of the
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The Led has some unusual behavior as well, it takes about 5 secs after hitting the button to get the vibrato goin.
That's a quirk of the oscillator circuit, not caused by the LED. Usually a strong 12AX7 will fix this, but not always.
Look at my version of the revibe. Note 2 on the schematic fixed this for me. My fix is to let the oscillator run all the time. I moved the footswitch to the top of the INT pot. The footswitch simply kills the LFO signal after the oscillator circuit, without actually shutting down the oscillator. Very quick/easy mod. Try it. You'll like it...
http://sluckeyamps.com/revibe/revibe.pdf
PS... Some of the other mods I did may apply to you. Read the whole list.
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when the unit is unplugged I get the expected 10ohm difference
I would expect under power, it would be a "voltage drop" case, not sure a meter will measure R well with current flowing. If your meter is good, try measuring the Vdrop :dontknow:
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Finally, the reverb channel makes a hum when the reverb side of the foot pedal is plugged in. It’s gone when unplugged.
You must use a shielded cable for the reverb footswitch. Are you?
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when the unit is unplugged I get the expected 10ohm difference
I would expect under power, it would be a "voltage drop" case, not sure a meter will measure R well with current flowing. If your meter is good, try measuring the Vdrop :dontknow:
This one I figured out I think. I think it’s getting to earth ground thru instrument cable and my amp. I have to go back to Merlin to make sure that’s what I want to do.
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Finally, the reverb channel makes a hum when the reverb side of the foot pedal is plugged in. It’s gone when unplugged.
You must use a shielded cable for the reverb footswitch. Are you?
I was afraid of this... who sells this. Or must I make my own?
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Mojo has what you need...
https://www.mojotone.com/amp-parts/Wire_1/Fender-Double-and-Single-Footswitch-Wire
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Mojo has what you need...
https://www.mojotone.com/amp-parts/Wire_1/Fender-Double-and-Single-Footswitch-Wire
Thank you! I may need to do another Mojo buy...
Its hard to tell from the description, but what exactly is the composition of this wire? Are there 2 shielded wires inside? ( I see a shielded cable and a white cable). I think I've seen this long ago in my fender twin reverb footswitch that broke.
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Same stuff, better pic...
(https://hoffmanamps.com/MyStore/catalog/images/IMG_6405.jpg)
This is what Fender has used for 60 years.
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How did the testing go?
I powered it up, reverb and vibrato work, and sound really good! Way better than my Strymon Flint! But it’s not perfect. Under certain settings, there are some strange noises. I need some time to characterize the problem better.3 issues so far:I don’t see a difference between ground bypass and the lift circuit. In fact, weird thing is when the unit is unplugged I get the expected 10ohm difference but that disappears when plugged in. I don’t understand why that is. I have all the chassis grounds attached to the copper bus bar. All jacks (input/output and the 4 rca jacks have isolating washers. One wire then goes to the lift circuit.The LED (red) between ground and cathode on vibrato is a bit weird particularly at higher speed. Makes a rhythmic popping like noise. At slower speeds, the noise is gone. I don’t think it’s there when I’m using the original cap+resistor. The Led has some unusual behavior as well, it takes about 5 secs after hitting the button to get the vibrato goin. Finally, the reverb channel makes a hum when the reverb side of the foot pedal is plugged in. It’s gone when unplugged. I think the pedal is very susceptible to picking up noise. I wonder if I need to lift some of the
It's not just the lift circuit that matters, it's the way in which you group the ground returns on the ground buss, and eliminating other potential (unwanted) sources of electromagnetic coupling.
As a rule of thumb, the high-current return parts of the circuit need to be grouped together in a way that minimises interaction with the low-current return parts of the circuit. Otherwise, micro-rises in ground potential (A/C 'micro-wobble' that matches the signal in the high current parts of the circuit) within the ground return circuit will reverse-shunt into the power supply rail supplying the low-current return parts of the circuit.
I've found that a 'galactic' ground return grouping method (on the signal ground buss) in combination with a hum-loop blocker between the signal ground buss and chassis ground, works well for stand alone reverb units. Merlin Blencowe sets this out in his grounding article on the Valve Wizard website, but I've attached a schematic of this for a 6G15 that I use, which works very well at eliminating unwanted ground loop noise. (Red = high-current returns and buss. Green = low-current returns). This principle is easily adapted to the Revibe.
But apart from that;
1) minimise unwanted EM coupling by shielding sensitive grid wires (or keeping them physically short and observing good lead dress practices), and keep the reverb pan's noise-sensitive output transducer away from the PT end of the amp chassis, and use shield cable for the reverb foot switch, and
2) try elevating the heaters by 30V or so to minimise S:N ratio in the first input stage and in the reverb recovery stage, or using a hum-dinger; and
3) (this may be too late for you and it may not make a difference, or it may) if you haven't already, choose a PT with a Faraday shield; and
4) since it's a reverb unit (and reverb recovery stages are particularly prone to high S:N), try minimising power supply transients (that could be caused by rectifier diode switching) by either using UF rectifier diodes, or snubber caps on the diodes (1 cap in parallel with each rectifier diode - minimum 1kV and 0.01 to 0.1 uF).
YMMV
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As I recall this had a really poor insulating jacket (very heat sensitive) when I worked with it before. $2.39 a foot is shockingly expensive for that poor cable.
I'm thinking of finding an adapter for a modern TRS type plug to vintage type RCA reverb. HOSA makes one for about $5.
https://www.amazon.com/Hosa-YPR-257-Stereo-Breakout-Cable/dp/B000068O5C/ref=sr_1_4?dchild=1&keywords=1%2F4+TRSF+to+RCA&qid=1586212255&s=electronics&sr=1-4
If I had known this, I probably would have wired it for TRS from the beginning. :(
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I've found that a 'galactic' ground return grouping method (on the signal ground buss) in combination with a hum-loop blocker between the signal ground buss and chassis ground, works well for stand alone reverb units. Merlin Blencowe sets this out in his grounding article on the Valve Wizard website, but I've attached a schematic of this for a 6G15 that I use, which works very well at eliminating unwanted ground loop noise. (Red = high-current returns and buss. Green = low-current returns). This principle is easily adapted to the Revibe.
But apart from that;
1) minimise unwanted EM coupling by shielding sensitive grid wires (or keeping them physically short and observing good lead dress practices), and keep the reverb pan's noise-sensitive output transducer away from the PT end of the amp chassis, and use shield cable for the reverb foot switch, and
2) try elevating the heaters by 30V or so to minimise S:N ratio in the first input stage and in the reverb recovery stage, or using a hum-dinger; and
3) (this may be too late for you and it may not make a difference, or it may) if you haven't already, choose a PT with a Faraday shield; and
4) since it's a reverb unit (and reverb recovery stages are particularly prone to high S:N), try minimising power supply transients (that could be caused by rectifier diode switching) by either using UF rectifier diodes, or snubber caps on the diodes (1 cap in parallel with each rectifier diode - minimum 1kV and 0.01 to 0.1 uF).
So you ground the input jack shorting tab to your circuit's local ground bus? What do you do with the ground tab? Leave it open? I think you'd have to, otherwise you will be connecting your circuit ground bus to the chassis ground in the amp via the instrument cable. (Merlin doesn't comment on this, but I think this is my problem. Plugged in, I'm measuring the resistance of my Instrument cable and ground ciruit of the room (about 0.9 Ohm).
All wires short as I can, shielded where signal low, running at 90 deg to other wires, away from magnetic sources. Heater CT is attached to cathode of 6v6.
Sluckey pointed out that I need a shielded reverb cable to the footswitch. I'm pretty sure I don't have one.
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So you ground the input jack shorting tab to your circuit's local ground bus? What do you do with the ground tab? Leave it open?
The input jack sleeve switch contact has to be connected to the input jack sleeve contact, and yes, these are connected to the 'low-current return' end of the signal ground buss (along with all the other low-current ground returns and the various isolated jack sleeve returns). The only connection for the signal ground is via the hum-loop blocker. (Otherwise the hum loop blocker won't do its job)
The safety ground (on the other hand) is connected directly to the chassis - and this is essential.
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You can get a proper Fender style footswitch with shielded cable pretty cheap...
https://www.tubesandmore.com/products/footswitch-box-fender-two-button-vintage-rca-plugs
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Testing and tweaking...
So I checked out my footswitch. Inside it seems to be 2 shielded cables (fender only shielded the reverb). I also found some excess flux found it’s way into the connector. After using some MG chem flux remover and some deoxit, working the plug, it seemed to have minimized the noise to practically nothing. (There is a bit if you REALLY listen), but I’m thinking this is probably as good as it’s gonna be unless the single shield cable from fender possesses some kind of hum sucking magic. (Maybe the vibrato shield picks up some noise and passes it on to a friend?). Anyway, I have a special adapter coming so I can use the footswitch from my twin reverb which is laying around because I use a shorting “always on” plug with that.
I found an error in my implementation of the switch at the V5a oscillating tube. I had the 1 Meg R28 only connected to the R29/C17 bypass cap leg on the cathode. When switching to the LED, the 1 Meg R28 was not connected to the LED, but rather stayed with the bypass cap/resistor. That was causing the weird charge up time for the vibrato circuit on the LED. Once I fixed that, the startup time for. The oscillator with the LED switches on went to practically zero. No need to move the location of the footswitch.
Last problem:
The original R29/C17 works great, no problem, sounds fantastic thru the entire range of speed and intensity (frankly, I don’t here any “weakness” in tremolo strength as others have observed). Nice ‘n swampy. (I suspect the values of the 3 preceding RC networks are tuned for this.). On the other hand when I switch to LED, I make 2 observations:
1. I noticed that when switching to the LED, it seems to slow down a bit, not a whole lot, but perceptible. Strange... why would the oscillation frequency change....does R28 need a dedicated path to ground instead of connecting to the LED?
2. (Bigger of the 2 issues). Turning the speed pot clockwise, the tremolo speeds up as expected, but when it gets to a fast tremolo (a frequency that the original bypass cap can definitely handle), the intensity seems to weakens (not enough time for the turn on/turn off of the LED to keep up with the rate?). When the speed pot gets near full on, a LOUD pulsing buzzing is made. I thought it was maybe the pot being fouled, but it’s not there with the original circuit. So I think the pot is fine.
I’m wondering if in full on (R35 pot is shorted) the frequency control as provided by the fixed 100k resistor R36 is just too fast. Perhaps adding a greater resistor may prevent this? Any thoughts for other possibilities or how I could implement this?
In my observation, it definitely seems that the LED has a less usable frequency range than the original bypass cap/resistor, at both the super fast tremolo (loud buzz) and basically all the pot range from 12 o’clock down. It’s as if a 500k-1Meg range (rather than 100k to 3Meg range) is more suitable for the LED. 500k may eliminate the buzz, and 1 Meg would make a better more usable low end. In the Sluckey example he added a large resistor in parallel with the 3Meg pot which effectively did this.
Thoughts?
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Picture
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Still having weirdness...
I discovered that what I thought was a 10meg RA pot for intensity was a 3 meg RA pot. (Neither was well marked and so I guess I picked up the wrong one.). I changed it out for the correct one this evening (while having my doubts it would fix my problem) and not surprisingly, it did not.
Still the non-LED bypass cap/resistor circuit works great (although I think I liked the sweep of the 3 meg RA pot slightly better!).
On the other hand, the LED circuit selection giving problems when on max speed and max intensity. At this setting (or very close to it) makes a rhythmic loud buzzing with the tremolo. First I swapped out the speed pot for another one (in case it was bad) but no change. I doubled the post pot resistor from 100k to 200k (I hoped there was some weird interference susceptibility at higher frequencies of the oscillator with the LED), didn’t help. I also tried moving the path of the post speed pot to ground (alligator clipped a resistor and tried various paths), no improvement. As I mentioned above, I already tried the “correct” pot value.
Do I need to consider adding some shielded signal wire? If so which ones would be good?
Anything else I should try?
Also, any problem with going back to the 3Meg pot on the intensity? The intensity was off at minimum setting (zero on the knob), which is why I didn’t notice it was wrong until I started poking around and the sweep had better control in my opinion.
Thanks for the help!
Here’s the latest circuit picture. (Diagram is in above post). I have the 10meg intensity pot only tack soldered in, incase I wanted to change it out again. It may be hard to see, but I have a SPDT mini toggle next to the LED between the pots.
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As I understand, the revibe works great when the oscillator is using the R/C on the cathode of the oscillator. Right? If so, the solution seems obvious to me... Dump the LED and switch then connect the R/C directly to the tube socket like Hoffman did.
There should be a solution for using the LED also. I know my unit works fine with the LED, but I did several mods that deal with tweaking the LFO. If I were to replace the LED with the R/C, I'd probably have to re-tweak the LFO. It may be that trying to switch between R/C and LED is futile.
At first I though that your extra wide chassis may be a factor. It's still a concern for me because it requires your wires to the tube sockets to be very long. I see this as a playground for gremlins.
Adding the LED is mainly for eye candy with the revibe, but something else happens when you use the LED. Mainly, the LFO output will be considerably larger (and as you've noticed, the frequency range shifts slightly). I think this increase in LFO signal is at the root of your "weirdness". I'd like for you to look at that LFO signal. Connect your scope to the hot side of the INT pot. Set the scope to view a 5Hz sine wave. I also like to set the speed exactly for 5Hz as measured on the scope just for a reference. You should have a clean sine wave about 18Vpp when using the LED. Now switch to R/C. What is the amplitude and freq of the LFO now? The freq shift is probably slight, maybe only a fraction of 1Hz.
You also liked a 3M pot better than the 10M pot. There's a 4.7M resistor in series with the INT pot. This forms a voltage divider. The 10M pot will give you a much bigger LFO signal to send to the modulator. The 3M pot will give a considerably smaller LFO signal.
I think all this means you can probably tweak the LFO for good results with either LED or the R/C but probably not both. As John Sebastian once said, "Say yes to one and let the other one ride."
Still very curious about the LFO signal amplitude/freq changes when you flip the switch.
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Still having weirdness...
I discovered that what I thought was a 10meg RA pot for intensity was a 3 meg RA pot. (Neither was well marked and so I guess I picked up the wrong one.). I changed it out for the correct one this evening (while having my doubts it would fix my problem) and not surprisingly, it did not.
Still the non-LED bypass cap/resistor circuit works great (although I think I liked the sweep of the 3 meg RA pot slightly better!).
On the other hand, the LED circuit selection giving problems when on max speed and max intensity. At this setting (or very close to it) makes a rhythmic loud buzzing with the tremolo. First I swapped out the speed pot for another one (in case it was bad) but no change. I doubled the post pot resistor from 100k to 200k (I hoped there was some weird interference susceptibility at higher frequencies of the oscillator with the LED), didn’t help. I also tried moving the path of the post speed pot to ground (alligator clipped a resistor and tried various paths), no improvement. As I mentioned above, I already tried the “correct” pot value.
Do I need to consider adding some shielded signal wire? If so which ones would be good?
Anything else I should try?
Also, any problem with going back to the 3Meg pot on the intensity? The intensity was off at minimum setting (zero on the knob), which is why I didn’t notice it was wrong until I started poking around and the sweep had better control in my opinion.
Thanks for the help!
Here’s the latest circuit picture. (Diagram is in above post). I have the 10meg intensity pot only tack soldered in, incase I wanted to change it out again. It may be hard to see, but I have a SPDT mini toggle next to the LED between the pots.
I don't know if this will be helpful or not, but your description of the sound rang a bell.
When I was tweaking my Weber ReVibe, I got a tip from Jeff Gehring who was the original designer*. He said that you can lower the value of R37 (referring to this schematic https://www.tedweber.com/media/kits/5h15_schem.jpg) and get more reverb intensity, but if you go too low you will get "a very unpleasant sound synchronous with the speed setting."
I experimented with a trim pot on R37 and when I went really low it made a bizarre oscilloscope pattern and it did sound very strange.
* he designed the Weber version. I don't know whether he or the denizens of EL34world were "first" in absolute time
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He said that you can lower the value of R37 (referring to this schematic https://www.tedweber.com/media/kits/5h15_schem.jpg) and get more reverb intensity,
Surely you meant to say Vibrato intensity?
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He said that you can lower the value of R37 (referring to this schematic https://www.tedweber.com/media/kits/5h15_schem.jpg) and get more reverb intensity,
Surely you meant to say Vibrato intensity?
Unless I wired it backwards and got reverb intensity and vibrato dwell ;-)
No, you're correct. Miscommunication between brain and fingers....
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......I see this as a playground for gremlins. ..............
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As I understand, the revibe works great when the oscillator is using the R/C on the cathode of the oscillator. Right? If so, the solution seems obvious to me... Dump the LED and switch then connect the R/C directly to the tube socket like Hoffman did.
There should be a solution for using the LED also. I know my unit works fine with the LED, but I did several mods that deal with tweaking the LFO. If I were to replace the LED with the R/C, I'd probably have to re-tweak the LFO. It may be that trying to switch between R/C and LED is futile.
At first I though that your extra wide chassis may be a factor. It's still a concern for me because it requires your wires to the tube sockets to be very long. I see this as a playground for gremlins.
Adding the LED is mainly for eye candy with the revibe, but something else happens when you use the LED. Mainly, the LFO output will be considerably larger (and as you've noticed, the frequency range shifts slightly). I think this increase in LFO signal is at the root of your "weirdness". I'd like for you to look at that LFO signal. Connect your scope to the hot side of the INT pot. Set the scope to view a 5Hz sine wave. I also like to set the speed exactly for 5Hz as measured on the scope just for a reference. You should have a clean sine wave about 18Vpp when using the LED. Now switch to R/C. What is the amplitude and freq of the LFO now? The freq shift is probably slight, maybe only a fraction of 1Hz.
You also liked a 3M pot better than the 10M pot. There's a 4.7M resistor in series with the INT pot. This forms a voltage divider. The 10M pot will give you a much bigger LFO signal to send to the modulator. The 3M pot will give a considerably smaller LFO signal.
I think all this means you can probably tweak the LFO for good results with either LED or the R/C but probably not both. As John Sebastian once said, "Say yes to one and let the other one ride."
Still very curious about the LFO signal amplitude/freq changes when you flip the switch.
Indeed R/C sounds good, but the LED is really cool. I think if I can get the LED in a happy place, the R/C will probably work too. (I just have that feeling...)
So I did the test you suggested. I dialed in a speed of 5hz on the R/C (about noon on the pot) monitoring the wiper and hot side of intensity. I had the intensity down all the way (because as noticed that as I bring the intensity up on the dial, the intensity voltage dropped...not sure if that’s normal). I didn’t get 18v peak to peak, but rather only 7.2v (?problem?). See picture 1.
Then I switched to the LED. Settings on knobs same (noon on the speed, off intensity). The speed dropped down a bit to 4.3hz. Intensity went up to 7.3v (up about 0.1v). Still a nice sine wave. See picture 2.
I made a video of this test and some other tests. I demonstrate the intensity voltage drop when turning up (?problem?) and also the noise I hear (and see on scope) with high speed and intensity on LED switch. I’m not sure what the noise is coming from. Is it maybe a tube? It’s like something is discharging voltage to ground. Even monitoring the reverb side seems to pickup some noise when this happens?
I’m a little puzzled about where to go. I’m happy to try other test cases you may propose. I’m hoping to keep the LED and R/C circuits together. The Flashing light is very cool.
I’ll post a link to YouTube when it uploads...
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9+ minutes of me narrating the test above.
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Enjoyed the video. Wish I could do that. Nice test equipment too. I have the grand daddy of your function generator, 3310B, knobs rather than push buttons. BTW, what is your day job and electronics background. I suspect we may have similar backgrounds.
I made some measurements on my revibe this morning for you to compare.
My reference was made at the INT pot high side with Speed set to 5Hz and INT pot max CCW (zero). My amplitude was 16.8Vpp. With INT set to max CW the amplitude dropped to 8.8Vpp. This is normal operation because now there is 2.47M (470K + 1M +1M) resistance in parallel with the 10M INT pot. This changes the voltage divider ratio set up by the 4.7M resistor and 10M pot such as to reduce the signal available at the high side of the pot. Nothing unusual about that.
Next, I checked the frequency/amplitude range with INT set to max CCW. Low freq = 3.09Hz, amplitude = 14.6Vpp. High freq = 10.17Hz, amplitude = 11.2Vpp.
Finally, I checked the frequency/amplitude range with INT set to max CW. Low freq = 3.09Hz, amplitude = 8.16Vpp. High freq = 10.17Hz, amplitude = 5.76Vpp.
As you can see, the LFO amplitude drops off at higher frequencies. This is normal but I changed C27 and C18 to allow higher amplitude at higher frequency. This will increase your max speed. (NOTE... the oscillator will stop oscillating if the amplitude drops too much.) You may want to change these caps also. Refer to the notes on my schematic.
I never had any of those hash spikes on my waveform, regardless of the SPEED and/or INT settings. Clean all the way. I believe your hash spikes and low amplitude are related and hopefully will be an easy fix. Start by replacing the tube. Roll several 12AX7s and keep the one that gives the highest amplitude. If still low amplitude and/or hash spikes, replace the LED. I am using a 5mm red LED. If still no joy, check DC voltages on the tube pins 1,2,3 and 6,7,8. Disable the oscillator with your footswitch so the voltages will be steady. Also measure the B+ node voltage that feeds the oscillator tube.
Good luck and report back. I've attached a scope pic but it just visually states some of what I said above. I think your sine wave looks better than mine. Mine seems a little bent on the negative half cycle. Oh yeah, all I did was scope my revibe this morning. I did not play guitar through it.
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Nice test equipment too
+1
I got the ugly second cousin to his scope 1012B :laugh:
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Once upon a time I was an electrical engineer in the semiconductor industry, but I retired from that and now doing something in a totally different industry. Let’s just say I fix things for a living... ;)
So, I started getting desperate and cast a wide net on causes. I rolled the tubes, (all three 12ax7 performed the same), tried another reverb tank (I thought I was hearing the springs rattling, and I thought maybe I was too rough with it when removing all the glue from the small foam block above the spring). All the same. I had previously chopsticked everything pretty hard, but I tried again, and think I found the problem.
The wire going to the grid of 5B crosses over the turret lacing that has the 5B plate voltage, when lifted out of the way, the noise ceases. That wire is very sensitive. After playing with it enough, it has to be pretty far from the line to remove all traces of the noise from all combinations of sweeps of speed and intensity. See picture below.
Although Hoffman wire could probably hold itself like that, I can’t leave it that way. I could run it the other direction and then under the board and hope to avoid the trouble spot, or (and I’m thinking this is the right way to go) use shielded cable with the ground going to my bus bar.
I made another brief video showing how the wire creates the problem on the scope. I don’t know know why I’m getting the lower voltage than the Sluckey Revibe, but the performance is good otherwise and not related to the noise solution it seems. I’m thinking that our differences in C18 and C27 may be attenuating more of the signal.
I’m also posting my DC voltages on the tubes. (I’m using all Mercury transformers, same as for the Reverb unit).
V1
1-182
3-1.48
6-181
8-1.46
V2
1-141
3-1.90
6-147
8-1.99
V3
1-23
3-318
4-327
8-23
V4
1-288
3-3.28
6-293
8-3.28
V5
1-128 (R/C), 147 (LED)
3-1.31 (R/C), 1.73 (LED)
6-170
8-1.36
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Yay! I would use shielded wire grounded on one end. The buss should be fine. (long wires, gremlins' playground :icon_biggrin: )
I'm pretty sure that if you change those two caps our voltages would be about the same. Not really a reason though since you are happy with it as is. I changed them because they were attenuating the higher freq range (I wanted to go up to 10HZ) so much that the oscillator would occasionally drop out. Of course they also attenuate the low freq range )not as much though) but I was not having problems with drop out at 3Hz. Anyhow, once I boosted the signal amplitude I could run from 3Hz to 10Hz with no drop out. I don't recall what the amplitude was before I changed the caps. The 5.1M across the speed pot was all part of this too.
This has been a good refresher for me. I see a couple of my notes no longer make sense to me so I may explore that a bit more and correct the notes to be more meaningful. As a bonus, it gave me a real world exercise for my new chinese scope. Can only look at a function gen before getting bored!
BTW, I've been working radar systems since about '68. My last 40 years were with FAA. Retired since 2012. Enjoy the revibe. IMO it's one of those projects that any hobbyist should build, much more useful than just one more amp! :icon_biggrin:
PS... I lied about my LED. It's actually orange. Still gives 1.7v drop.
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Let’s just say I fix things for a living... ;)
:laugh:
sounds like the guys in the 'lil house above my Radar when I was In.
I did most of the board repair for the Ship, they would hand me a scrap of paper for a schematic n tell me "that's ALL you get" :icon_biggrin:
moved on to Ct and MR scanners in civie cloths :laugh:
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I did a combination of solutions for the noisy grid wire. I exchanged it with a shielded cable and ran it the other way so it wouldn’t cross over the high voltage line. Very good results! No noise!
So the insides have been buttoned up. I only need to attach a ground wire from chassis to cage top and we’re done!
A very satisfying build, and fabulous sound to listen to! I like the switch to the LED, because it does give a slightly broader range of speed (lower frequency with LED, higher with R/C). there is some small tonal difference between the 2 when accounting for speed, but it’s really subtle and I couldn’t say really what’s “better”. Both swamply good!
Here are some build shots...
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Very nice job. Lot's of layout planning and it shows. :thumbsup:
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I was thinking of building a second one of these.
I’m very happy with how this one turned out. But I think for my next one I will simplify a few areas. I will lose the doghouse and put the caps inside. The filter cap board should fit on the opposite side of the chassis. That will leave more room on the top, shorter wires and less holes to drill (mounts for doghouse). I will remove the lift switch, it’s always in the lift position, so why bother. I’m still not sure about the LED switch. LED always seems to be on. Perhaps the “Sluckey” slow switch is a more useful feature. Again, simpler.