Cat on crack is clean, but lacking vibro

[Packerswin14]

Problem is fixed, but I get no change with the vibro-- I jumped the switchpedal jack to be sure it wasn't my pedal.


I know that it's common for an amp to squeal, but I can't find the source.  I built this vibrochamp in a salvaged cabinet.  You'll notice two extra sockets, I put those in so I can later add tube reverb.  I've eliminated the extra sockets from the circuit, that didn't fix the problem (I didn't think it would as they're basically just acting as a connecting point).  I've rechecked the connections, moved wires with a chopstick a hundred times, tried different tubes, tried re-routing wires, and I can't seem to make even a tiny difference.  If pushing on a wire or turning a pot altered the sound significantly, I'd know where to search.  When I adjust the volume, it does control the volume of the guitar signal but doesn't change the volume of the squeal-- the squeal is always deafeningly loud.  Also when I turn the vol. pot, the pitch of the squeal changes-- at no volume and at full volume, the squeal is the same, in the middle is about an octave below.  The guitar sound is pretty hideous-- thin and scratchy, distorted.

[sluckey]

RULE #1. When building a new amp that uses NFB, and it squeals, reverse the OT primary leads.

That outta be a banner on this site.   

May not be the problem, but it has to be ruled out.

[cotton]

RULE #1. When building a new amp that uses NFB, and it squeals, reverse the OT primary leads.

That outta be a banner on this site.   

May not be the problem, but it has to be ruled out.

Yeah -
The Blue and Brown rule.

[snoof]

I think that should be a sticky at EVERY amp building/repair site!  

[matta]

I once did a bunch of mods to one of these new Chinese made Fender Champion 600's, converting it to 5F1 values. I decided to replace the OT and had that lovely sound of oscillating squeal, went over everything, EXPECT the OT, only because the colours WERE right, but turns out the Chinese trafo was wired with the primary colors on opposite windings to the US made one... Doh! Again... in an NF amp, check the OT!!!

[plexi50]

I just found my problem in a 1974 champ that i put a turret board in. I reversed the OT leads with some but little change. I changed the neg feedback 2.7k to 22k and squeel was gone even with the neg feedback still connected. But that still was not the main problem. What i did was i wasnt thinking about the preamp grounding and wound up grounding the preamp cathode grounds to the PT / Heater CT ground lug. I had squeel at no volume and full volume but no squeel in the middle around 5-7

I removed the preamp ground from the PT ground lug and moved it near the input jack

Problem solved. I know better than to put a preamp ground at the PT ground lug but at the time it looked too convenient a place to solder it last week so thats where i originally soldered it to

So i learned first hand that PT currents can and will get into the preamp if you solder your preamp ground anywhere close enough to the PT

It was causing major osccillations. I was at my wits end as well and glad i took another look at my preamp grounding

It now sings and is a Champ / Pic of before and after /

[plexi50]

I know that it's common for an amp to squeal, but I can't find the source.  I built this vibrochamp in a salvaged cabinet.  You'll notice two extra sockets, I put those in so I can later add tube reverb.  I've eliminated the extra sockets from the circuit, that didn't fix the problem (I didn't think it would as they're basically just acting as a connecting point).  I've rechecked the connections, moved wires with a chopstick a hundred times, tried different tubes, tried re-routing wires, and I can't seem to make even a tiny difference.  If pushing on a wire or turning a pot altered the sound significantly, I'd know where to search.  When I adjust the volume, it does control the volume of the guitar signal but doesn't change the volume of the squeal-- the squeal is always deafeningly loud.  Also when I turn the vol. pot, the pitch of the squeal changes-- at no volume and at full volume, the squeal is the same, in the middle is about an octave below.  The guitar sound is pretty hideous-- thin and scratchy, distorted.

All of the same symptoms i just experienced

Look at pic /

[HotBluePlates]

In the event that Packerswin doesn't follow what we're talking about...

You have a yellow wire coming from the speaker jack to the board. This is for negative feedback, and connects to a resistor on the board. Unsolder this wire from its eyelet. Did the squeal go away? If so, the intended negative feedback was really positive feedback. You must swap a pair of wires at the output transformer. You have 2 choices: primary wires or secondary wires. The primary wires (usually blue and red for this amp) would be unsoldered and swapped; blue goes where red was, and red goes where blue was. Or swap secondary wires (usually black and green in this amp). Same deal, flip-flop the wires.

Do not swap both sets, or you'll be back to squeal! Once you have swapped 1 set of wires, reconnect the yellow wire from the speaker jack. There should be no squeal.

[Packerswin14]

Thanks to everyone, you nailed it.  One more lesson for the beginner.  The output transformer was indeed the culprit, reversed the wires, and now it sounds great!  Only problem:  My vibro champ has no vibro.  Any ideas on why the vibrato part of the circuit might not be working?  I tried my footpedal in the jack, then just to be sure, I put a jumper wire across that jack to be sure I had a good connection, and the speed and intensity knobs still don't modify the sound at all.

[tubesornothing]

If you vibrato has a roach, put an analog meter or scope on the light side and see if the meter pops up and down.  You could use a DMM on DC, but they are not so good at catching fluctuations.  If you dont have a roach, post your schemo and I'll tell you where to connect it.

[Packerswin14]

Here's the schematic:

[tubeswell]

Here's the schematic:

Well if that's the amp you built, then you can't complain about it not having any trem. There's no trem circuit in it. Are you sure you posted the right layout?

[HotBluePlates]

No roach in a VibroChamp. And that was a regular Champ schematic. Here's a VibroChamp.

Do you have a meter? If so, check on the plate of the oscillator (V2, pin 1 on Fender's schematic... whichever one you did not hook straight to the power supply on that tube). If the oscillator is working right, you should getting either needle movement when reading d.c. or a fluctuating reading on a digital meter when reading d.c. That tells you that the plate is wobbulating up and down.

You should have a wire running from the Intensity pot back to a cathode of V1 (either pin 3 or 8, depending on how you wired). Really, this wire will go to the board, with a 1.5k resistor and a bypass cap.

I'm going to make a wild guess. Do you have a 25uF bypass cap here? If so, unsolder 1 leg for a moment. Did your trem start working?

If it did, it solves one possible error, but that may not be the problem. I'll wait to see what you found.

[Packerswin14]

Here's the layout I used.  Sorry I posted the wrong one.  There's no bypass cap between the intensity control and any tube.  Also, what's the tube numbering system-- Is V1 always the preamp, etc.  The layout and schematic give them by their id (5y3gt, etc.)  Thanks!

[HotBluePlates]

Just remember... a layout and a schematic are different things. I used to work mainly from layouts when I first started, but you should get very comfortable with a schematic as well. Once you can read them with ease, you will see possible trouble spots more quickly than when using a layout.

Also, what's the tube numbering system-- Is V1 always the preamp, etc.  The layout and schematic give them by their id (5y3gt, etc.)

Unfortunately, it's never "always" anything. Most often, V1 is the tube closest to the input jack. On your layout, that is the 12AX7 farthest to the right. But unless the schematic or layout have part designators like this, along with the actual tube type, there's always the chance for confusion.

There's no bypass cap between the intensity control and any tube.

The bypass cap is there, I just didn't do a good job of pointing you to it.

Look at the layout you just posted. Above the 2nd 12AX7 (the one closest to the 6V6), there is shown a 10uF cap in parallel with a 1.5k resistor. A yellow wire is shown running from the right lug of the intensity pot to the end of the resistor/cap which also has a wire running to the input 12AX7 (the one on the right) at pin 8. In your picture, that appears to be a white wire. Unsolder 1 leg of the electrolytic cap that is at this eyelet. Your trem might start working.

Also at this same eyelet, the picture makes it look like the 0.022uF orange drop is in the eyelet for the side of the 1.5k resistor going to pin 8 of the first 12AX7, instead of to the eyelet that's 1 to the right, which has a leg of a 100k resistor. Is that right, or is it an optical illusion? If it is the case, the 0.022 cap has a leg in the wrong spot.

[Packerswin14]

Thanks for the advice about schematics.  I did take a couple years of electronics back in high school, and I do okay, but they do boggle me sometimes.  I never learned vacuum tube symbols or characteristics back then.. it was the age of solid state! 

I tried unsoldering one leg of the 10uf cap, and it didn't change anything.

The .022 is in the right place, I had it bent over to one side.

It took me a few days, when I pulled the rectifier tube out, the base sort of crumbled-- it was seriously old, though worked quite well.  Took me a few days to find the other one (right where it should be).

[HotBluePlates]

Hmm...

The idea behind checking that cap was this: the original schematic has a 2uF cap in this location, your layout diagram showed 10uF and people often see "2uF" and think it's 25uF because it's a cathode bypass cap. That cap is across a resistor where the trem is being injected, and will shunt the trem signal to ground if it's too big.

Okay, that didn't do it. In that case, and assuming new parts, reflow the solder joints to all the caps and resistors around that oscillator, and make sure that any underboard wires are verified to be in the right location (check 3 times... we all booger that one) by making continuity checks with the power off. If you meter does do continuity, then use ohms and look for close to 0 ohms between 2 points that are connected with a wire.

If that cap was not too big to allow the trem to happen, then I'm banking on simple builder error: cold solder joint or an wire 1 eyelet to the left or right.

[Packerswin14]

I reflowed the joints, double-checked the connections, looked under the circuit board to see if anything was out of place, or if there were any solder drips arcing to ground, nothing.  I cleaned up a few lengthy bits of wire, and bypassed the vibrato footswitch jack, and all I did was add some hum (darn, now I have to find that, too). 

If it were a bad component, where should I start?  Resistors are easy enough to test.  I have an ESR meter, but those low-farad capacitors don't register on it. 

Anyone else have any ideas?

[sluckey]

Hoffman's Law states "If it were wired right, it would be working." (or something like that) I think you should keep looking for a wiring error or wrong part value. Print the schematic and layout. Start at the input jack on the schematic and trace/verify the wiring for each component by comparing the schematic and layout to the actual amp. Highlight as you go. When you have all three in agreement, it will work.

Take some hi rez pics (not 640x480) that clearly show wiring and component values. These pics will most likely be too big to post on the forum, so put them somewhere on the net and post links to them. Having more eyes to examine the amp often reveals overlooked errors.

[Packerswin14]

I don't have anywhere to post higher-res pictures, but I'm still hopeful someone will see something.  I've compared the layout that I've been using to the original Fender layout and original Fender Schem.  I found a wire missing on the layout diagram I've been using, from pin 6 on the 12ax7 to pin 4 on the 6v6.  That didn't fix anything, and I'm surprised a wire marked "340v" on the schematic didn't prevent the amp from working.  There was one change:  The vibro still doesn't work, but now when you turn up the intensity knob, the overall volume of the amp decreases slightly.  Anyone have any ideas?  Does anyone know which components actually create the Vibrato?

[sluckey]

I don't have anywhere to post higher-res pictures, but I'm still hopeful someone will see something.

Many people use photobucket with good results.

I've compared the layout that I've been using to the original Fender layout and original Fender Schem.  I found a wire missing on the layout diagram I've been using, from pin 6 on the 12ax7 to pin 4 on the 6v6.  That didn't fix anything, and I'm surprised a wire marked "340v" on the schematic didn't prevent the amp from working.

That wire supplies B+ voltage to the oscillator/buffer circuit. The tremolo could not possibly work without that B+ connection. Can you show us the layout you are using?

There was one change:  The vibro still doesn't work, but now when you turn up the intensity knob, the overall volume of the amp decreases slightly.  Anyone have any ideas?  Does anyone know which components actually create the Vibrato?

You're getting closer, but I think there are still more wiring errors or incorrect components. The attached pic shows all the components used to create the tremolo signal.

[Packerswin14]

Thanks for the diagram.  I still can't find the problem, but it's helpful to know where to look.  The layout I'm using is above, and seems to be identical to the original Fender layout, except for the missing wire.  FYI, don't buy a kit from Triode Electronics-- This is the second error in the layout diagram I've found, they missed sending me one of the resistors, and the Power Transformer was dead and had to be returned for a replacement.  I'm wondering with that poor luck so far with their kit if I should start assuming components might be faulty.  Should I start replacing caps?

I had put in two resistors in parallel to fill in for the resistor that was missing in the kit.  Just in case I did the math wrong, I pulled those out and put in a 2.2k where the schematic calls for a 2.7k, and figured I was pretty close.

I signed up for photobucket and posted my pictures in higher resolution. Note that since I took the pics, I bridged the vibrato footswitch jack and power switch (the footjack to eliminate my pedal from the circuit, and the power switch because it didn't fit the chassis well, so I'm going to find a better one), put in the missing wire, and replaced the 2 parallel resistors with a 2.2k.  I also shortened a bunch of the wires and cleaned up some messing-looking areas, reflowed all the solder joints, pulled the board out to check the underside, rechecked all my connections and tested for continuity, tested all the resistors, and have now re-examined every connection about 11 times.  I'm thinking I'm going to look really stupid when I find the cause.
http://s875.photobucket.com/albums/ab312/Packerswin14/VibroChampbuild/

Attached is the layout I used.  The only other thing I can find that's different between the Fender layout and this one is that the Fender has the heaters running to ground on one side, and this layout (and my build) have them all wired with two wires in parallel, not connected to ground.
Thanks for your help, guys!

[sluckey]

Note that since I took the pics, I bridged the vibrato footswitch jack and power switch (the footjack to eliminate my pedal from the circuit

Are you saying you have a shorting jumper across the footswitch (FS) jack terminals? If so, remove it. That trem oscillator requires an open circuit at the FS jack to operate. You would short the FS to kill the tremolo.

[PRR]

Where is this at? The amp plays well (no fault), except it won't tremolate?

You have not answered HBP's "Do you have a meter?"

Can you measure all six voltages on vibrato tube pins?

Measure with pedal jack shorted. You should get ballpark the voltages in this picture. The pin 2 voltage should be Zero.

Measure with pedal jack UN-shorted. If your analog meter wobbbles wildly around the right voltages, or your DVM freaks out and shows all possible voltages, the wobulator is wobbling and the trouble is in how that gets to the preamp cathode.

If DC voltages are correct but no wobble, check the resistance at the pedal switch, should be 1Meg to ground. Or heck, just cut the wire to be SURE no jack-fault is your problem.

Replace the tube with a different brand or era. The oscillator needs "enough" gain. Some 12AX7s just won't work here, even if they work fine as amplifier stages.

Get some high-quality caps (such as from Doug) to replace the 01-01-02 string and the 22u-47u cathode cap (observe polarity). The first 01 cap must be 400V rated. The other timing caps could be 200V, the cathode cap only needs to be 2V, but 400V and 25V-50V parts are fine.

If the oscillator wobbles itself but not the preamp, poke the Intensity control. As you turn the knob you should have variable wobble and DC on the center lug, zero at one end, and a variable small wobble at the other end.

> put in a 2.2k where the schematic calls for a 2.7k

Fine. Perhaps "too clean/tame" for a 21st century tube amp, but -later- you can experiment with 4.7K or 10K where Leo used 2.7K and you used 2.2K.

[Packerswin14]

Here are the tube voltages, 1st with the jack unshorted, then shorted (I used a jumper wire to connect the two sides of the jack).
1  192, 196
2 0, 0
3 1.8, 1.7
4/5 .01, .001
6  382, 382
7  192, 197
8  197, 199
9  .002, 0

Shouldn't I be getting something at pins 4/5 and 9?  The heaters should be getting 1.7 from the PT, right?  The amp is functioning, so I'm confused.  Does it matter if my two heater wires reverse ( heater wire a is at pin 9 on the first tube, then reverses position before the second tube)?

I did check continuity for the whole heater circuit, and there's no break in the chain.

If anyone has an idea, let me know.  Otherwise, I'll order some new caps.

Mike

[simonallaway]

When I was measuring the same pins on my Marshall's preamp tubes I wasn't getting anything on 4/5 and 9....until I learned it was AC voltage, not DC.

[HotBluePlates]

Yep, ac on the heater pins which explains no reading when your meter is set for dc.

Did your meter reading bobble around with the jack unshorted? Or was it a steady reading?

If steady, then there's no wobble, the oscillator's not working. Check/replace the 3 caps from plate to grid and verify all resistors from these caps to ground are not open and are actually grounded. Once those are right, it should oscillate.

[Packerswin14]

Whoops, that's the ticket with the heater voltage-- knew that made no sense.  I knew better. 

I measured the pin voltages with a jumper wire across the two vibrato footswitch jack terminals that are connected to the circuit, and the voltages were all steady and unwavering.  I'll pick up some new caps, and see if that does the trick.  Thanks for all the help, everyone, I'll post the result in a couple weeks (I'll be out of town for a while).

[HotBluePlates]

I measured the pin voltages with a jumper wire across the two vibrato footswitch jack terminals that are connected to the circuit, and the voltages were all steady and unwavering.

That much is *normal*. Now tell us what happens when the jack is unshorted. That's when the oscillator should be working and plate voltage should no longer be stable.

[sluckey]

Unsolder that yellow wire from the footswitch jack and just let it hang in the air. Does it work now?

[HotBluePlates]

Unsolder that yellow wire from the footswitch jack and just let it hang in the air. Does it work now?

Genius Steve... I couldn't tell from the pictures if it was a shorting jack or not. If he used the same jacks as the input, it will never work.

[sluckey]

We've all been beating around that point for several days. Maybe we can finally eliminate that as a possible problem. Since his voltage readings were the same with shorted jack as open jack, just maybe he has the jack wired wrong. If it ain't, at least we can move on to something else.

[Packerswin14]

Finally got some free time to get back to this project, and lo and behold, I did have the Vibro jack wired wrong.  Thanks for nailing it again.