Revibe test point values

[Skip Hagey]

Just finished my Revibe build and am getting no output, yet, but am wondering what the target values are that I should find at the tube pins. Schematic doesn't show any and I just need a place to start trouble shooting...Thanks in advance, Skip

[sluckey]

My schematic has all voltages listed...

     http://sluckeyamps.com/revibe/revibe.pdf

[Skip Hagey]

Thanks

[PRR]

> a place to start trouble shooting

In ignorance:

First B+ filter cap should be 250V to 400V, exact value not critical to get it to "work".

Other filter caps should be some lower.

Small-tube plates will be a fraction of B+.... 100V or 200V, not 1V or full B+.

Small tube cathodes at 1V-2V. Big tube cathodes 10V-25V.

Grids at zero +/-0.1V.

There are exceptions. Cathode followers have plate at full B+ and cathode at some fraction of B+. Grid voltage reading here is probably bogus. Revibe V4 is biased a bit funny with cathode and plates higher than a typical amplifier stage, but still in a ballpark. Blackface 12AT7 is biased "like a big tube" because it does a semi-big job (smacks the springs). Trem oscillators (where the trem pedal connects) often read funny when trem is defeated.

Those rules should quick-check about 95% of guitar amp stages. Anything doesn't look right, look harder for a mis-connection, then post here.

[Skip Hagey]

OK, it's been a long week but finally got back to this project... I've documented the test points and my tube chart is here:  http://el34world.com/charts/valve/ValveData.php?e=view&f=19441

I also have the rail voltages and they are; A 205, B+240, B 237.7, C 236, D 188.

Do I need to have foot switches in the two RCA jacks for this to work?   

[sluckey]

V4 pins 3 and 8 should be tied together, yet you show two different voltages. Recheck.

V5 pin 2 should be zero volts, not 120. Recheck.

You don't need footswitches for this to work. Which layout/schematic did you use? Did you make your own board?

Click the link in my signature line of this message.

[Skip Hagey]

I used Doug's Layout, Most current, and his board.
V4 Pins 3 and 8 show 4 V, V5 Pin 2 shows 120 mV.. and is pulsing like the vibrato should...
And I just plugged it up again with guitar and amp and got quite the shock.  Guess I need to lift the green wire from the Power cord...?

[PRR]

> Guess I need to lift the green wire from the Power cord...?

NO!

[Skip Hagey]

ok...I'll keep digging...thanks.  Eliminated the "shock" issue but still have a ground loop,large hum thru amp with slight thru-put of signal.  Slowly but surely...

[basschops1528]

PRR, Doug's instructions say to not use the ground wire from the power cord because it creates a loop and to just heat shrink it/ iso terminal. How can that safety measure be implemented without introducing a loop for hum?

[SILVERGUN]

If you read all of Doug's notes he explains that the ground path gets shared through the signal cable once you hook the revibe up to a companion amp.
It is not meant to be a standalone unit.
 
PRR was stopping Skip from making a HUGE mistake because Skip was trying to fix his problem of "got quite the shock" by disconnecting the chassis ground.
He must figure out why he got shocked in the first place.
 
If you ever get shocked, the last thing you would want to do is eliminate the only other path to ground for the piece of equipment that just shocked you, especially as a troubleshooting technique for the initial shock....the thought process needs to be repaired before the builder gets electrocuted.
 
 

[basschops1528]

So after Skip solved the problem, theoretically he should not use the ground wire as Doug mentioned to reduce the chance of hum, given there are no other issues?

[Willabe]

Eliminated the "shock" issue......

How? 

[sluckey]

Yes, that's the question. What did you do to eliminate your shock issue?

[Skip Hagey]

Yep, It was Doug's instructions that led me to the idea i might disconnect the ground from the power cord, but the STOP order made me go back and review all connections and finally  figured out the output plug was wired improperly and fixing that stopped the shocking issue. So Thanks PRR, u saved me from a possibly worse issue!  I still have a ground issue but now it is only a hum. both my amp and revibe are grounded through their AC cords, could that possibly be the source of this hum, because of the grounding through the guitar cable?

[shooter]

could that possibly be the source of this hum

It could, you might try a *common* bus strip for both, then into the wall.  You might try Isolation jacks(in/out) on one unit.

[tubeswell]

PRR, Doug's instructions say to not use the ground wire from the power cord because it creates a loop and to just heat shrink it/ iso terminal. How can that safety measure be implemented without introducing a loop for hum?

Best method that I have come across for these types of stand alone units (that get plugged in in front of a guitar amp) with their own 3-wire (earthed chassis) mains cords, is to elevate the signal-ground return path from the chassis ground with a hum-loop blocker. See figure 13.17 in the attached article (written by Merlin Blencowe. (You will want to electrically isolate all plug jack sleeve connections from the chassis, and run all these isolated grounds to the elevated ground return. The attached article has good suggestions for galactic grounding schemes, which if implemented in combination with the hum loop block network, will dramatically reduce ground loop hum.) I've tried this in several units and it works every time. For the HLB network, I use a 5W 15R wire-wound resistor || a 600V 0.1uF film cap || 2 x anti-parallel 6A diodes.


It's also a good idea to keep the reverb pans' sensitive output transducer at the opposite end of the chassis from the Power Transformer (to avoid unwanted mains-frequency EMF coupling into the transducer)

[PRR]

Many shared-common "loops" are vastly reduced if both boxes are on the same power strip. Even better, plug revibe into the power outlet on the back of the amp. That's what is is for! Sadly few amps have such an outlet.

The small-diff ground breaker documented by Merlin has been around a while. It's not solid and not floating.

If all is well it may break hum.

My fear (unsupported!) is that When Things Go Wrong, it may not blow house fuse and save your life like a safety ground is supposed to do. It is a truism that $10 transistors will die to protect $1 fuses. I don't see how diode versus circuit breaker is any different. Especially a relatively big breaker, 13A-20A. <6 Ohms may break hum but won't blow a 20A breaker in good time. 2 Ohms will blow a breaker semi-quick but the 7,200 Watt peak energy may blow most common resistors.

(* Merlin's 10 Ohm value is scaled for UK 230V 13A system. In the US's 120V system, 10 Ohms will not blow a breaker.)

[HotBluePlates]

...  Guess I need to lift the green wire from the Power cord...?

... still have a ground loop,large hum thru amp with slight thru-put of signal. ...

If you don't have obviously-working signal, you probably also don't know "for sure" what is causing hum. It could be ground loop, or maybe loop plus some other cause.

Anyway, there are ways to build-in a safe ground-lift feature to an amp or effect while still maintaining the safety ground (3rd wire) bonded to the chassis. They're not terribly hard to incorporate if you plan for it in advance, but they're often painful to add to an existing build.

The easiest solution is using a transformer to isolate the 2 circuits which each have their own 3rd wire. Ebtech makes a product which does the isolating at the power outlet, and GigRig (plus probably many others) makes a product which isolates at the interconnecting cables. Those are the right, safe, ways to break a ground loop while still passing signal.

To envision the GigRig Humdinger variant, think about your guitar cable: it is simply 2 wires, a hot & a ground. The ground, or return path, is typically chassis-connected at each device. But you have an amp with a 3rd wire, and now an effect with a 3rd wire, so the guitar cable completes the ground loop. And it's physically fairly big, and big loops are most likely to be a noise source.

If you placed a 1:1 transformer between the effect & amp interconnect, you'd break the loop. There are only 2 wires each on the primary & secondary, one for the hot & one for "return". Except the transformer doesn't care whether "return" is actually connected to ground, it will pass signal regardless while isolating one side from the other at d.c.

You could find & buy your own transformer, jacks, box and phase reversal switch, but I'm not sure you will find things substantially cheaper than commercially-available products once you pay for a high-quality transformer.

[Skip Hagey]

Gosh (You can tell how old I am by using that intro...) thanks to all for the info, it's a lot to digest and I have a few questions about it all, but I think I will continue to find the offending issue the old fashioned way...diligence.

I have tried plugging them both into a properly wired and grounded circuit in my 100+ yr old home and no change.

Am I correct in assuming that from the B+ tag from the bridge is to be connected to the choke and then to the electrolytics? I am still a little confused by the schem's and layout, but this is how one learns is it not?  Thanks again all and please continue to chime in, I love it!!!

[sluckey]

Am I correct in assuming that from the B+ tag from the bridge is to be connected to the choke and then to the electrolytics?

That's not correct. The bridge + connects to the choke AND to a cap. Then the other wire from the choke connects to another cap.

[Skip Hagey]

OK, I thought that looked funny, glad I asked. I'll order one today!

[sluckey]

The bridge + terminal also connects to a 4.7K/3W resistor on board.

How many filter caps do you have? Look at page 2 of Hoffman's layout. There are 5 big electrolytic caps. Page 2 shows how all of them should be connected to the board.

[HotBluePlates]

... If you don't have obviously-working signal, you probably also don't know "for sure" what is causing hum. ...

I thought I'd note that the reason for my statement above is recent personal experience.

I built a small amp yesterday and at first power-on was greeted with no guitar signal but some kind of weird pulsating hum/buzz. Hoffman Law: "If it was wired right, it would be working now."

So I took a few hours off without messing with the amp. When I came back to it, I notice the output tube had a crazy light-purple glow between the plate & cathode, and one of the preamp tubes seemed much darker than the others. Knowing the output tube had to be bad (gassy), I tried installing a different output tube and the glow went away. So the gassy tube went in the trash.

I opened the amp focused on finding my wiring error. Just with a visual inspection, I noticed a wire from an output tube pin wrong to the wrong turret, and my preamp pentode had no connection for the suppressor grid (I forgot when I drew up a layout that this one tube did not internally connect the suppressor to the cathode). The preamp pentode also had the screen connected to the wrong turret.

I fixed those issues, and fired up the amp again focused only on seeing if the pentode channel worked (the amp also has a triode preamp channel). With the new output tube and corrected connections, the strange noises were gone and the pentode channel worked as it should.

The triode channel was still non-functional. I notice one of the triodes lit up brightly (maybe too bright red?) while the other was dark. To find out if the tube or the socket was bad, I swapped the positions of the two triode tubes. The dark socket stayed dark, etc. So now I checked continuity of the heater wiring with the power off. All was good except for 1 pin of the dark socket. After unsoldering, I found the wire fit through the hole in the socket lug along with a bit of the Teflon insulation. Solder held it in place but it was not enough to ensure a good electrical connection. So unsolder, strip a little, resolder.

Afterwards, the triode channel worked. The amp overall has extremely low noise, though there is a tiny bit on the triode channel if you put your ear right on the grill cloth. But if my a.c. is on I can't even hear the noise over that. Complete troubleshooting & repair time was about 20 minutes, mostly because the heater wiring repair required removal/replacement of a number of parts to gain access (which was also a good reminder that along with the PT primary wiring function check I made as soon as that was installed, I should have followed up with a heater wiring function check before assembling the rest of the amp).

All of the above is to say that while plugging a mains-powered effect like this into an amp is a guaranteed ground loop (and you should get/use an isolation device), I'd hold off on any other hum chasing until you're 1000% sure the wiring/soldering is absolutely correct.

In my case, I had very strange noise symptoms and I'm not going to attempt to figure out how they happened or the mechanism which caused them. I knew it was a fresh build and except for the definite bum output tube the fault(s) had to be wiring error. I wouldn't have connected the weird sounds I heard with a missing suppressor connection or the other wiring errors, but a close visual inspection after taking some time away from the amp found them.

[Skip Hagey]

I'm running only four (4) electrolytics so I obviously am one short. One is ordered. In the meantime I am hunting for other bad wiring or connection problems et al. Hot Blue Plates, your experience is pretty much what I've been through  on all my builds and I'm getting used to it.  Taking a few steps back is definitely the most important step though I think.

[Skip Hagey]

Well gang, I finally (!) got my revibe project "finished" and it sounds great!  Thanks for all your help, but i do have one more question, for now...
While playing i'm getting a lot of sag or compression not present without the revibe in the circuit.  While trouble shooting I found about five resistors that were not consistent with the  shematic and luckily I had most of the ones I needed as spares.  All but one; V4-B cathode resistor Is supposed to be 4.7k and 1/2 watt. I had to replace it with two 2.4k in series but they are probably 2watts  Could the higher wattage value contribute to the extra squish? Thanks again, Skip

[HotBluePlates]

... Could the higher wattage value contribute to the extra squish? ...

Nope, no impact.

[Skip Hagey]

Any Ideas as to the source of so much squish? Is it normal?

[PRR]

> Could the higher wattage value contribute

That's just to avoid burn-up. That resistor actually dissipates 0.003 Watts. They don't make resistors that small; and we would not want to handle such dinky parts. A half-Watt is ample and traditionally the cheapest size (though 1/4W is now cheaper).

Your 4W replacement for a 0.5W spec in a 0.003W job is like: say my gas fireplace is rated 2 inch clearance from wood (150 degree char). But I have some Titanium plate (1,500 deg melt). You "spent too much" for the over-kill parts. But 2W resistors aint Titanium plate, and shipping cost for one 1/2W part would be far more than the value of two 2W parts.

> a lot of sag or compression

My schematic has all voltages listed...
     http://sluckeyamps.com/revibe/revibe.pdf 

Get clip-leads. Set up to measure key points while you play. See which voltages sag. I suggest plate voltages. 6V6 cathode may be better for that stage.

My suspicion is that a grid resistor is not really holding its grid to zero. You can't measure grids and get good signal, but plate or cathode voltages will change and that leads to the right neighborhood.

> I found about five resistors that were not

From personal experience-- there's always more errors (values or joints) than you think.

[Skip Hagey]

The 3-4 watt resistors were just some i had on hand, I'll eventually replace them with correct units when I have enough other items to order.  As for running your tests, I'll get on that right away, Thanks