YUCK - tremolo installed - no tremolo - SOLVED!

[Tyrannocaster]

Okay, I got the tremolo circuitry installed in my FrankenPro but as I guessed...it doesn’t work. No lovely warbling at all.  

First step: recheck the wiring. I believe I have wired it correctly, but maybe I missed something.

Second step: Voltages

Here are the voltages for the amp:

V1 12AX7 (this is the tremolo tube and the values in parens are the Fender values from the schematic, so mine aren’t far off at all)
1 173v (170)
3 1.6v (1.7)
6 347v (340)
8 177v (175

V2 (first preamp tube, which right now is a 12AX7 and not a 12AY7 as the schem shows)
1 222v
3 1.8v
6 220v
8 1.8

V3 12AX7 (driver/PI)
1 208v
3 1.73v
6 269v
8 60 [Does this seem right?]

6L6s
3 398v
4 376v
8 30.0v

Anybody got any ideas where I should look? The amp sounds pretty good other than the no-tremolo part.

The amp:

[sluckey]

V3-8 is fine.

If your oscillator plate is a steady 170v, then the oscillator is not oscillating. You don't have a footswitch plugged in do you? That would kill the trem. Assuming it is wired corectly... Get a handful of 12AX7s and start swapping out. Increase the B+ feeding the trem circuit. The easiest way is to just jumper across that 10K dropping resistor such that the trem circuit is fed directly from the screen node. Look at the Bronco (same as Champ AB764). It's running 410v to the trem circuit.

http://www.el34world.com/charts/Schematics/files/fender/bronco_ab764_schem.pdf

[Tyrannocaster]

No, no footswitch. I can easily increase that B+, too. I thought that since the Vibro Champ called for 340 volts I was pretty much right on the money with 347. I will also try different tubes in that wiggler.

I need to change the bias of the power tubes (I actually have 560 ohm resistors in there now because I thought the PT would be a lot hotter than it apparently is - we had a conversation here about this PT a few weeks ago, it's the one out of a Hammond AO-29) but since this is a preamp tube trem that shouldn't affect this issue.

Thanks, sluckey.

[Tyrannocaster]

I bypassed that dropping resistor and tried some different tubes but no change. The speed control has no effect (of course) but I do notice that the amp's volume goes down some when I turn the depth control up to full, if that's any help. And if I spin the depth knob from all the way from one side to the other the sound drops out momentarily then returns right away. It doesn't do that if I turn it slowly.

I just rechecked all the resistor values again, this time with my meter; they are correct. I can't measure capacitance with my meter, but the caps' labels say they are the correct values. Isn't it necessary sometimes to change the value of one of the trio of caps in the oscillator cap to get the circuit going?

Or maybe I just have a bunch of funky tubes! That would be too easy.

[sluckey]

There are two vibro champs. Look at this schematic...

http://www.el34world.com/charts/Schematics/files/fender/fender_vibrochamp_ab764.pdf

Bumping the B+ up will be easy to do and will probably get it oscillating without having to select tubes. That's the first thing I'd do. And take another look at your actual circuit. It's easy to overlook something in the oscillator circuit. Concentrate on making the oscillator plate voltage bounce around. As long as it's steady, it ain't working.

[sluckey]

I do notice that the amp's volume goes down some when I turn the depth control up to full

That's probably normal. You're changing the dc voltage on the cathode of the stage you're modulating.

[HotBluePlates]

Does the oscillator oscillate? I don't recall you explicitly stating whether the voltage stays a steady 170vdc, or if it bobbles around.

[Tyrannocaster]

Steady voltage there.

I just rejiggered the B+ for that tube. I replaced the existing 10k resistor with a 750 ohm and took it straight off the first cap instead of the later one. B+ for the tube is up from 347 to 405 now. I now have 204 volts on pin one of the trem tube but no oscillation.

Judging by Willabe's recent experience it shouldn't take more than a few dozen hours of work to get it sorted out. 

[HotBluePlates]

Are you certain the caps from the plate to grid all have good solder joints, and all of the resistors in between have good connections to ground? Or that the "ground" for these resistors has a good connection to ground?

[Tyrannocaster]

I think so. I am thinking that I must have made some really dumb error in layout but can't see it. I'm going to take some photos and post them, although it will be difficult to see much beyond the basic circuit board and the tube socket. There is some wiring (uninterrupted) that is routed below the main board to get to the pots and the pots themselves will be very hard to photograph because of the chassis overhang.

[Tyrannocaster]

Here are a thousand words:


Maybe somebody can see where I plugged the output into the output or anything else equally lame. And yeah, I know my wiring job is messy but I just never do it very well, I'm afraid.

[HotBluePlates]

I see a presumably 1M resistor from the red wire connected to pin 2, which runs to ground.

The red cap that connects to it runs to another red cap. The junction of those red caps should have a 1M resistor running to pin 3 of the oscillator.

Is that resistor missing? If so, the oscillator won't oscillate.

I assume the yellow wire connected to the white 0.022uF cap runs under the board to the 3M reverse-audio Speed control.

In a nutshell, each series cap and resistor to ground is a phase-shift stage. Each stage shifts the phase something less than 90 degrees, therefore it takes three such stages to ensure the loop will shift the phase 180 degrees and oscillate.

[sluckey]

That oughta do it.

[Tyrannocaster]

Damned if you're not right! It's an error in my layout, which I followed faithfully and then managed not to see when I traced the signal while trying to troubleshoot. Heck, I should have just posted the picture first. Well I was right about one thing: that was worth more than a thousand words.

Thanks! I'll get soldering later this morning and maybe my neighbors will hear some Link Wray chords vibrating in the air.

[Tyrannocaster]

It works; yippee tie one on! So it was a simple error in drawing the layout and leaving one resistor out. I never would have found it on my own though, as I proved to be totally blind to it.

It works really well, too; the speed range is perfect for my taste and the depth is fine at both fast and slow speeds. There is some mild motorboating evident at the fastest speed, but it doesn't get louder as you turn the amp up, which means that it's effectively absent unless you are playing very quietly. I guess that makes sense, since the volume control is in front of the tremolo and the power amp is on full all the time since I took out the master volume. I may try another master circuit at some point but the amp doesn't cry out for one.

There is one problem, though; the switch makes a major pop when you turn the trem on or off. It does it with both the footswitch and the switch on the amp so I think it's the circuit and nothing to do with the switch itself. Is this a case where a judiciously applied cap might clean this up? And I know that sometimes you can use a big resistor, too.

Thank you guys for your help in debugging this. I have been wanting to do a tremolo that doesn't work on the power tubes' bias for a long time and now I have one. The big advantage to this one is that it should be more portable to other designs than the more circuit specific power tube wigglers.

[sluckey]

There is one problem, though; the switch makes a major pop when you turn the trem on or off.

I don't see a wire for the FS on your pic. Where did you connect it? Needs to be connected as shown in this pic...

[Tyrannocaster]

Jeeze, you're right - it was in the wrong place! That red wire on the left is actually the footswitch connection. Amazing that it worked, I guess. I'll fix that as soon as I get offline; hopefully, that will fix the pop issue. That's another error in my layout and I followed the layout faithfully. This is why I like working from other people's layouts.

I did want to post a very quick clip I just made of how the trem sounds. Keep in mind that it was 6AM on Sunday morning and I had it turned down to a whisper volume; this emphasizes the motorboating that occurs at fast speed, but it also shows you just how quiet this amp is in terms of noise - there just isn't any hum at all. The low frequency motorboating that is quite apparent here does not get louder as you turn the amp up to normal levels.

The totally unmusical clip also gives a bare hint of how much tonal range that combination of the interactive controls gives. I will do a much more comprehensive set of clips when the amp is a bit more finished but for now, check out the groovy vibe: https://www.box.com/s/bc85675c07cc7c3d8768

EDIT: yes, that fixed the pop issue. We need a new smiley icon that means "egg on my face", LOL. One thing I like about this tremolo which the modelling amp designers never incorporate is that the tremolo, when engaged, gives you a louder signal than the straight one. Maybe this is caused by the load it places on the circuit even when it's not in use (well, that's my guess) but whatever, when you turn the tremolo on you get an effect that seems like a flower blooming as it kicks in. This particular amp has way more gain than it needs so any loss attributable to the tremolo is no problem. Or maybe the tremolo actually increases the signal level on the peaks to a level higher than the straight normal peaks. It's pretty nice, in any case.

[sluckey]

Nice strong sounding trem. You may have to live with the choppy sound at high speed. Maybe tubegeek will ring in and tell us if his vibrochamp does that.

Now on to the NFB. I bet it'll work if you put the 47Ω resistor under the cathode resistor just like on the VC. Be prepared to swap OT plate leads if it squeals or just doesn't sound right.

[Tyrannocaster]

I'm not really concerned with the motorboat effect since it's only a problem at extremely quiet levels, such as in that clip. Remember, I stuck a pretty good mike 8 inches in front of the speaker and the amp was turned down very nearly all the way to keep from waking anybody in the house up - that means the bass response got emphasized, too, thanks to the close miking and linear frequency response (well, it is linear compared to the cranked amp, I mean). When you turn the amp up to any normal level of playing volume (not to mention really turning it up) it's not so obvious.

[tubeswell]

To alleviate the high speed 'chop' you could try a 56k 'load' (instead of 68k) on the CF. I'm not saying it will work, but it might be worth the experiment to lower the CF output signal swing.

[Tyrannocaster]

There seems to be another issue now, though. When I turn off the trem with the switch (no pop now) and have the Depth setting at full (which is where you'd usually have it) the amp's volume is too low. If I turn the depth pot down while the trem is still switched off the volume comes back up to full.

If I don't use the switch at all and just control things with the Depth pot everything seems fine.

??

[sluckey]

Try this... Disconnect the wire from the ungrounded side of the depth pot. Connect a .1µF cap to that terminal. Connect the wire to the other end of the cap. IOW, put a cap between the pot and the wire going to the tube.

I don't know if this will lower the depth too much but it should prevent the depth pot from changing the gain (volume) as it is turned.

[Tyrannocaster]

Try this... Disconnect the wire from the ungrounded side of the depth pot. Connect a .1µF cap to that terminal. Connect the wire to the other end of the cap. IOW, put a cap between the pot and the wire going to the tube.

I don't know if this will lower the depth too much but it should prevent the depth pot from changing the gain (volume) as it is turned.

I tried it; it doesn't drop the volume but the tremolo is so weak as to be nearly unnoticeable. I'm back to the way it was.

[tubeswell]

Are you sure the depth pot is wired as per the schematic? The pot's resistance as seen by the driver stage's cathode resistor, should be constant.

[Tyrannocaster]

I would say "yes" except that I have made so many wiring errors on this tremolo (and the rest of the amp worked fine out of the box!) that I just looked again. Here is what I have, viewing the depth pot from behind with the lugs on top:

1. Ground.
2. To the 68k resistor which then goes on to pin 8 of the oscillator.
3. To the cathode bypass cap/resistor of the wiggled tube. Note: I have a 4.7 uF cap on this rather than the 10 uF that the Vibro Champ uses, but I don't think that would cause this.

When I look at the AA764 Vibro Champ layout that I copied this from it seems like I wired it right. I can't post a photo because the chassis makes it hard to get the lens in there to see it, but how hard can it be to see if a pot is wired right, especially when one lug goes straight to ground?

But I think you're probably right - it's probably some dumb error of mine. I find it interesting that with the switch wired to the wrong place it worked right (no volume drop) but popped. Now it doesn't pop, but I get the volume drop when the tremolo isn't on and the depth pot is at max. Either way, the tremolo works fine without the switch but has problems with it.

I would like to get the switch issue solved and move on, though.

EDIT: For the hell of it, I replaced V2 (that is the first preamp tube in the amp - V1 is the tremolo oscillator) with various other tubes, including a 12AU7. I'm starting to think that this volume issue with the switch may be tube related and will spend some time looking at that.

[HotBluePlates]

... a very quick clip I just made of how the trem sounds. ... this emphasizes the motorboating that occurs at fast speed. ...

That's not motorboating, or at least doesn't sound the way that low-frequency oscillation through the power supply (which is termed "motorboating" due to the putt-putt-putt) sounds.

What you have is a byproduct of the VibroChamp-style trem, and a speaker with some bass response.

The reason this particular trem circuit is not used on bigger amps is that the way it's injected at a preamp cathode will cause you to hear pulsing through the speaker, even without you playing. Much of the development of tremolo circuits was about getting rid of this pulsing, including the circuit in the tweed Tremolux, which Fender patented. The tweed Vibrolux also had a circuit to cancel the pulsing, by applying the trem to modulate the bias of a push-pull output stage.

But the VibroChamp was a student-level amp, with an 8" speaker in a tiny cabinet with relatively little bass response. The relative lack of low end helped mitigate the problem, while the "student-level" status set the bar lower. It would appear Fender "devolved" their trem circuit to add this feature to a low-cost amp in their line.

All of the VibroChamps I owned had some amount of pulsing background noise, mostly from hiss in the amp. I think your case is more noticeable because of the added bass response of your speaker.

One thing I like about this tremolo which the modelling amp designers never incorporate is that the tremolo, when engaged, gives you a louder signal than the straight one. Maybe this is caused by the load it places on the circuit even when it's not in use ...

Not a load in this case.

Rather, the trem signal both subtracts from and adds to the bias of the stage it's injected at. So you're right to say it makes the amp louder when turned on; softer too, but you notice the louder part. Of course, that's how the trem works by turning the volume up and down. The simpler optoisolator trem doesn't turn the volume up because it can only shunt the signal to ground.

There seems to be another issue now, though. When I turn off the trem with the switch (no pop now) and have the Depth setting at full (which is where you'd usually have it) the amp's volume is too low. If I turn the depth pot down while the trem is still switched off the volume comes back up to full.

If I don't use the switch at all and just control things with the Depth pot everything seems fine.

I've long since sold my VibroChamps. Maybe a current owner can see if the same thing happens with theirs. I never had that issue, and don't see a ready mechanism for a properly-wired amp to do that, but I also never used the footswitch with those amps. I just turned the Intensity pot up and down with no footswitch.

Try this... Disconnect the wire from the ungrounded side of the depth pot. Connect a .1µF cap to that terminal. Connect the wire to the other end of the cap. IOW, put a cap between the pot and the wire going to the tube.

He can't do this without removing his 5uF bypass cap. The 0.1uF cap will form an a.c. voltage divider with the 5uF cap and reduce the trem signal at the V2a cathode to almost nil.

So, try lifting one leg of the 5uF cap temporarily to see if the trem issues are fixed, and you can live with the loss of dry gain.

If the trem winds up working, but you have to have the extra gain, you could probably use a 50uF (maybe 25v or so) cap, with the positive end towards the Depth pot and the negative towards the tube cathode you're injecting the trem at.

[Tyrannocaster]

You're right - it is not correct to use the term "motorboating" for this because that refers to oscillation and that's not what happens here. As you noted, it's a low frequency spike that becomes more apparent as the trem speed goes up (if you were able to set it at, say, 20hz you'd actually have a tone just from the number of pulses - that's not counting effect of the pulse itself!) and I believe you are also right to say it is a byproduct of the circuit. As I have said before, it isn't that bad when the amp is turned up to a reasonable level, and I just spent about 45 minutes playing it with different tubes in the V2 position and also with a dummy load through the board vs. through a speaker, just to eliminate that variable.

I wanted a preamp injected tremolo for this because I really like cathode biased amps with dual bypass resistors on the output tubes; it seems to let me use poorly matched tubes better, of which I have quite a number. I was also hoping the the circuit would be portable to other amps more easily than the output tube bias types, but with all these issues I don't know if that's true. Of course, most people wouldn't be trying to use this in an amp with two power tube bias resistors and two interactive channels, LOL!

One thing I can say for sure: this thing is really touchy about what tube is in V2, the first preamp tube. The trem is dramatically different depending on the tube - not the actual tremolo sound itself, but whether or not I get that odd volume drop when the switch is used. Additionally, the choice of tube really impacts the interactivity of the two channels. 12AX7s bring it out gloriously but it's too much gain for the amp and I don't like the quality of the resulting distortion at all. A 12AU7 gets pretty clean (this is a good Rockabilly amp that way, as is my '55) and makes a nice spanky tone but when I use a 12AU7 and turn the "unused" channel down all the way it oscillates - can't have that. It's not a whistling sort of oscillation, more of a buzz-crackle type. I only have 1 12AY7 here and it doesn't sound very good in the amp at all, although it sounds great in my old '55 Tweed Deluxe; go figure. I'm going to play with the amp (musically play, I mean) some more and see what I think works and what doesn't - luckily I am a much better player and listener than amp builder, and I'm also willing to go with whatever the amp seems to do best rather than try to force it into my ill-conceived notion of how it is supposed to sound. I just don't want any obvious problems such as the strange volume drop or oscillation,  etc.

And thanks for that detailed response!

[Tyrannocaster]

Is there perhaps some other way to switch the trem off? Like switching to the equivalent of the depth control turned to zero? I can't deal with the switch and the odd behavior of the depth pot when the trem is turned off. I would swear it was working correctly earlier but now it sure isn't, not  with the 12AU7. The trem works fine overall, I just want it to sound like the trem is turned all the way down when I hit the switch. Instead, when the trem gets grounded I get a big volume drop and some distortion, too. I looked at the depth pot's wiring again and I'm convinced it's correct. It isn't the switch because it does it with different switches.

And just to see if maybe the strange interactive preamp channels were par tof the problem I disconnected the one that is shorted to ground. No effect at all on the switch/volume drop behavior, but the amp lost all its quirky character. The interactivity does make it nearly impossible to get exactly the sound you want (there are some amazing possibilities there) AND then just turn it up or down - change either volume control and you also change the tone. It wouldn't be a good amp for live playing. Well, unless you added some sort of master volume to use for that; I'll have to mull that over. I've never tried using one that way - in the past I've always tried to use them to get dirtier tones at lower volumes and it hasn't worked well with any of them.

[sluckey]

I can't deal with the switch and the odd behavior of the depth pot when the trem is turned off.

The footswitch is not causing the loss of volume. The way you had the FS wired when it was causing a big pop was masking the volume drop.

Remember the cap trick (that didn't work) on the depth pot? Try putting a resistor (1K to about 10K) where that cap was and see if that helps.

If this doesn't help, you could always convert to power tube trem. It works with cathode bias circuits too.

[Tyrannocaster]

Remember the cap trick (that didn't work) on the depth pot? Try putting a resistor (1K to about 10K) where that cap was and see if that helps.

If this doesn't help, you could always convert to power tube trem. It works with cathode bias circuits too.

I'll try the resistor later today. Can you do a power tube bias tremolo circuit on an amp that uses separate bias resistors for each tube? I also wanted to try this Vibro Champ trem out because in the past I have had so much difficulty with bias tube trems; looks like they can all have their share of bugs.

[Tyrannocaster]

It looks like I have to choose between a switch and a deep tremolo. The 10k resistor helps with the volume drop when the trem is off, but it decreases the depth of the trem when it is on. Lower values increase the off volume drop and the tremolo depth. I imagine that going even higher would solve the volume drop off completely but then I would have little or no tremolo left.

I am about ready to simply remove the switch jack and have a knob-only tremolo. It's not ideal, but it's better than the alternative. I am pretty sure now that I've wired this correctly; I wonder how it works in the Vibro Champ. I've played those and I don't remember any volume drop when you turn off the tremolo - Fender would never have let it out of the factory if it did that. So it has to be possible to get it to work.

[sluckey]

I am about ready to simply remove the switch jack and have a knob-only tremolo. It's not ideal, but it's better than the alternative. I am pretty sure now that I've wired this correctly; I wonder how it works in the Vibro Champ. I've played those and I don't remember any volume drop when you turn off the tremolo - Fender would never have let it out of the factory if it did that. So it has to be possible to get it to work.

I believe it's wired correctly too. But what does that mean? You have a cross breed between a 5E5, 5D3, and VC. Fender never built this crossbreed so it's unfair to say they would have never let it outta the factory. Just because that trem works for a VC doesn't mean it will work properly with your amp.

I still believe we can make it work satisfactorly for you with a little tweaking, so hang in there. I have two more ideas. Maybe one will work.

1. Again, remember the cap trick that didn't work? Well combine my idea with HBP's idea. Use a .1uF cap and remove the 5uf cathode bypass cap from the gain stage. Does that work? If this works but the overall gain is now too low, reconnect the 5uf bypass cap and replace the .1uf cap with a 50uF cap (positive lead to the pot. Does this work?

2. Put a diode (1N4007 is fine) between the depth pot and the gain stage cathode Just like the cap trick) The diode's cathode connects to the pot. Does this work?

[sluckey]

Ding! I just had another idea. Put the 10K B+ dropping resistor back in to lower the supply voltage to the trem circuit.

[Tyrannocaster]

Thanks for the suggestions! The diode didn't do anything except prevent the trem from working at all. Putting the old B+ back resistor didn't seem to have any effect, positive or negative. I can try the other idea later today.

EDIT: Removing the cathode bypass cap and adding the .1 cap to the pot killed the trem, although I could hear a ticking sound.

[Tyrannocaster]

Since sending the oscillator to ground via the footswitch is causing so many problems I can't help wondering if there isn't another way to stop the tremolo via a switch without the side effects I'm getting now (or any others, like the POP I got when I had the thing wired wrong. Although IIRC that way didn't seem to plunge the volume down when the trem was off, it just gave me a monumental, deal-breaking pop when I hit the switch). I suppose I could put a relay in there somehow but it seems like a lot of work for a tremolo switch and I'm even less familiar with relays than I am now with tremolos. 

[Willabe]

I found this on J.C. Maillet's site, don't know if this will help?

Note 3 Using a Matched set of power tubes helps eliminate power supply feedthrough that can potentially turn into a form of LFO pumping in your speakers ... it starts from the Tremolo circuit, drives the power tubes vertically at first, manifests through the output circuit through widely varying Plate currents, loading the power supply on half-cycles, feeding back into the preamp through its supply feed, and with preamp bass control maxed then feds back to the output thus driving the power tubes into "Asymmetric" LFO mode and pumping the speakers (if it starts to happen it will seem very weird at first) ... using reasonably matched power tubes and employing the same power supply feeds as noted in the above schematics typically guarantees problem-free operation following proper power tube biasing ...

Here's the whole page;

http://www.lynx.bc.ca/~jc/tremoloCircuits.html


                  Brad    

[Tyrannocaster]

Although I have separate power tube bypass resistors and caps so I can use less than ideal pairs, the pair I have in there is a decent match. I suspect that the power supply feedthrough (I've never heard of that with trems - something new all the time) happens with tremolos that wiggle the power tubes, as most of them do. There aren't that many circuits that work on preamp tubes and I bet those are there mostly for low-end single ended amps.

I'm pleased with the sound of this tremolo despite the annoying issue with the switch. I am making this amp as a birthday present for a friend who doesn't have a tube amp, wants one, and can't afford anything decent. He plays in a Beatles copy band and I thought this would be a great amp for old Beatles tunes (from the first albums). It's an excellent Rockabilly amp with a 12AU7 in the first preamp slot. I'm not so happy with the quality of the distortion when you use higher gain tubes but for playing She Loves You with a Casino this is a good amp. And let's face it, anything would work to recreate that crappy dirty sound Lennon got on Revolution.  But since my buddy plays live it would be nice if his footswitch worked.

[Willabe]

I know HBP said it's not PSU LFO, but at this point I'd be tempted to see if giving the term tube it's own ground star node would help?

I know you had it in at first but the 10K B+ dropping R was too much, so you took it out.

Since you've tried about everything else, if it were me, I'd try it again but this time with a 1K or 470R + 20uF/450 cap and run all the grounds from the term tube to a single ground star and then run a single wire to your ground buss. Might not fix it but it couldn't hurt?

Just a thought, but it will put the idea to rest one way or the other if you try it again.

                  Brad      

[Willabe]

Did you ever add the FB loop? If so I wonder if that's the problem now? FB loops that have the OT primary leads wired backwards don't always squeal, sometimes they do other odd things.

You have another thread on this but the 2 could be related? And they both are being injected into the same place. The guys might have missed this as it's in 2 different threads?

Here's your other thread on it;

http://www.el34world.com/Forum/index.php?topic=14166.0
              

                                 Brad      

[Tyrannocaster]

No, I never added the NFB; I tried a couple of versions but with no luck and then this whole thing came up. As far as the B+ resistor goes, both of them seem to work fine for the oscillator - once I got the wiring errors fixed (duh) the circuit seemed pretty tolerant of voltage differences. 750 ohms or 10k - it still provides oscillation for the tremolo.

Grounding. Hmm, have to think about that. I tried a new approach with this amp, using the ideas from the valve Wizard site. In the past I used Doug's scheme right out of the box and it worked well, except that I HATE that grounding buss across the backs of the pots. since I was going to build this amp in stages that wasn't going to work anyway so I added a buss along the edge of the board and then tried to ground each stage together in multiple stars as Merlin describes. I must say that in terms of hum this is the quietest amp I have ever built; I love that part of it and I will definitely try that approach again. As to whether the tremolo switch loading the circuit when the pot is turned up but switched off could be caused by a grounding node issue...wow, I just have no idea.

[sluckey]

I think you should add the NFB circuit exactly as implemented in the VC. It 'could' be a factor although I doubt it.

The trem circuit is behaving exactly as I think it should. The way I see it.... When the trem is on the dc voltage on the osc plate is about 170v (as on the schematic). But when you disable the oscillator with the footswitch, the voltage at the osc plate will rise considerably (tube drawing less current). This rise in plate voltage is direct coupled to the cathode follower's grid and causes the voltage on it's cathode to rise also. The increased dc voltage is then passed on to the gain stage and will reduce the gain of that stage. The fact that you get the gain back simply by turning the depth control down to zero is normal because you are no longer passing that dc voltage on to the gain stage.

There may be a way around the above situation. Let's try cap coupling between the osc plate and the cathode follower. To do this, remove the jumper between pin 1 and 7. Also move the red wire (the one that goes to the white cap) from pin 7 to pin 1. Now put a .1uF cap between pins 1 and 7. Finally, put a 1M resistor between pins 7 and 8. Now the trem osc is cap coupled to the cathode follower and the dc voltage on the osc plate will be blocked from the cathode follower and will not affect the gain of the gain stage. There will still be a dc voltage on the cathode of the cathode follower just due to it's operation, but the change in dc voltage from the plate of the osc tube will no longer be a factor.

It's a quick and easy mod and can be done (or undone) right on the tube socket. I offer no guarantees but it sounds like it just may work. Definetly worth a try.

If this doesn't work well, then I'm about out of ideas. You may have to accept it for what it is. And remember, this trem circuit came from the cheapest, bottom of the line, trem tube amp that Fender ever built. There may be a reason this circuit never made it into Fender's upper class models.

Hopefully one of the heavy theory guys can confirm or reject my idea about cap coupling between the osc and the CF. Give it a try and let us know.

BTW, I've looked for alternate ways to switch the trem on/off and just don't see one that works without popping using a simple SPST switch that will have one leg connected to ground.

[Tyrannocaster]

Thanks for putting in the time to figure that out, but unfortunately it doesn't help. The volume still drops when switched off and now the tremolo has a really bad LF spike with each throb. I'm putting it back to stock and taking out the switch. I've put it back to stock and taken out the switch entirely. It's just going to have to be like one of those low end amps that don't have a switch and it's too bad, but I'm not very good at re-wiring amps and this one is getting really ugly from all the soldering and re-jiggering I've been doing on the tremolo. I wish I knew how you guys are able to apparently rework circuitry so much and still have it look good because it's a skill I just don't have and my stuff starts out looking lousy to begin with; it doesn't need help.  

I'll have to make some sound clips of the amp because its clean tones are just great with that interactive pot setup from the 5D3 and there is a lot of variety that you couldn't get without it. The overdriven tones are disappointing to me - they are harsh and jagged sounding, not much like my 5D3 at all. (But the 5E5 Pro doesn't have the same PI as the 5D3 - it's similar to the 5E3 Deluxe. Plus, it uses 6L6s where the Deluxe uses 6V6s.) I believe the problem is before the power amp but I can't figure out where exactly. I don't have a scope and wouldn't know what to do with one if I did. Since the first gain stage and volume controls are exactly the same as my great sounding old 5D3 I'm thinking that's not where the problem is so that just leaves V3, which is the second tube in the preamp chain. If I could get it sounding better with gain I could maybe try higher gain tubes in the first slot but they sound pretty bad and a 12AU7 sounds very good and for its intended purpose (playing early Beatles tunes live) it would do very well right now. It's bright and very jangly and the early breakup is quite nice; it's the later stuff that sounds like blocking distortion or whatever it is - a gritty, grainy kind of sound that just doesn't belong there to my ears. It may sound like I'm unhappy with the amp but I'm not. I won't be playing it, the guy who will is an acoustic player who doesn't really "get" distortion anyway, and there are some excellent sounds to be had here. And the tremolo sounds great, which is neat.

I have some JJ 6V6s in it now and they are handling the 400 volts fine but it's too early in the morning to crank it up to hear how they really sound. I do know that the old Fender branded 6L6s I was using to test it with are not the best tubes - that why I put them in at first so as not to risk my good ones.

[Tyrannocaster]

I cranked the amp up with 6V6s in it and it sounds much better when breaking up than it did with the 6L6s; much more like what I am used to, although my 5D3 still sounds better. But this is acceptable distortion. When I measured the voltages and computed the dissipation I got 16 watts on the tubes, which is higher than the usual stated 14 for 6V6; no red plating and the amp sounds much better. The 6L6s were coming in at about 22 watts, which is lower than the stated max for that tube (30), and I didn't seem to be able to change that much with the bias resistors as detailed above. Since I know the JJs are robust tubes and the amp sounds better this way I'll leave the bias alone. When playing clean the 6L6s are louder, although not by a ton - just as you'd expect from the published specs for the two tubes. Cathode bias always seems to favor the little guys like 6V6s and EL84s, while the others perform much better with fixed bias. It does make me wonder - if I could get the 6L6s up to 30 watts would they sound better?

[Tyrannocaster]

Amp is coming along with the new output tubes; they make an enormous difference. I am able to use tubes other than a 12AU7 now.

Here's a test I did with the amp on the bench running straight into the board (there is EQ and delay on it, obviously) just to have some fun:

https://www.box.com/s/a5b4ded616921d465e6f

That's with the JJ 6V6s and a 5751 in the first preamp slot.