troubleshooting a tremolo circuit

[BrianS]

So, I got my little Maestro GA-1RT up and running.  Rebuilt the power supply around a new, properly spec'ed Classic Tone PT.  Everything in that part of the circuit is working fine & the amp sounds great.

When I first started digging into this amp, I could see that the tremolo control (pot/switch) was very rusty and felt bad.  The tremolo did not work at all.  Thought it would be the switch/pot, but it is actually fine.  With the new PT, all my voltages are correct, but the tremolo still does not work.  When I switch it on, there's a slight thump, but other than that, there is no tonal change to the signal, no matter where the control is set.

The tube that is part of the tremolo circuit is the triode half of the 6BM8.  With the PT that was in there, this tube was subject to some high voltages and there was some redplating and nasty hum after the amp had been on for a while.  However, the pentode half of the tube still works and sounds fine, and I would image that half would have been taking the brunt of abuse from the mismatched PT, so I'm skeptical that it is the tube that is causing the problem.

The cathode bypass cap has some funky brown goo on one end, but would a bad cap cause the entire circuit to just act like it is not there?  I'd think that if one of the components of the tremolo circuit was bad or out of spec, it'd still do something to the tone/sound of the amp.  Anway, before I start unsoldering and swapping out parts, I thought I'd run this problem by you guys. 

Link to a schematic: http://www.prowessamplifiers.com/schematics/Gibson/Ga_1rt.html

Pic that shows brown goo:

[sluckey]

The tube and the cathode bypass cap should be at the top of the list of suspects. Replace both if you can. Also check the plate resistor.

Trem circuits need a lot of gain to start and sustain the oscillator. Weak tube or bad cathode bypass cap will both reduce gain, maybe below the point for the circuit to oscillate.

[SoundmasterG]

Tremolos work by having a phase shift oscillator that uses positive feedback. The triode oscillates at a low frequency and this signal is inserted in the bias circuit in your amp to modulate the bias up or down at whatever speed and intensity the trem pots are set to. It needs to maintain a certain amount of gain in order to continue oscillating, and if the cathode bypass cap has leaked out brown goo, then that cap is probably not working anymore and the gain is way down on that stage. The cap looks like it is an electrolytic cap, and it should be replaced. The schematic shows a 20uF cap, but make sure that you connect it up the same way with the negative end going to ground. The other caps and resistors used in the circuit basically set the frequency of the oscillation. If those caps or resistors have drifted in value, then that could change things so the circuit would not work anymore, since it would no longer have enough gain or the required amount of phase shift. So you may need to replace the caps and/or resistors. This circuit is not in the audio path so using ceramic caps is just fine, as is using whatever resistors that you can find that are the correct value.

Take a look at this website for some more info on phase shift oscillators and how they work.

http://www.aikenamps.com/index.php/designing-phase-shift-oscillators-for-tremolo-circuits

Also, the chemicals that were in that cathode bypass cap are corrosive and cancerous so I would suggest to be careful when cleaning up the brown goo. Wear gloves.

One other thing....when a tube red plates for long, it changes the internal characteristics of the tube and often it will not work correctly anymore. You may want to get another 6BM8 just to be sure, but you can always test it first too.

Cheers,

Greg

[eleventeen]

You're getting good advice. Phase shift oscillators (look for those three caps in series) are not reliable starters, it can take many dozens of RC constants ("period" = the mathematical inverse of "frequency") for them to start up, meaning for the output to build up sufficiently to feed back enough poop to the input so the ckt oscillates. Aged components and weak osc tube can very easily defeat the whole program. You can see this if you take a Fender trem osc ckt and replace the 12AT7 with a 12AU7. The lowered gain of the 12AU7 will in most cases not get the oscillator started. If the insides of that amp are skungy you pretty much need to replace all the series caps, at a minimum.

[BrianS]

Well...thanks...   Sounds like I've got a lot of work ahead of me.

I did read a bunch about this type of circuit last night & all your advice helps firm up my understanding.  Will check out out the Aiken link as well.

I'll be ordering a 6BM8 today & checking my local Radio shack for caps & resistors.

Thanks again.

Brian

[BrianS]

Got my bypass cap first.  Installed...nothing.  Got the new tube today. Installed...tremolo works...kind of...for about 1 second or less and then dies out...and is very fast at the lowest setting.

I now have all the parts to completely rebuild the oscillator circuit, so I guess I better get to work!

[eleventeen]

The bypass cap is probably the smaller part of it. It's almost certainly those 3 series connected R-C networks. If those caps are leaky or have drifted down in value (or both) they are likely to suck a lot of gain out of the loop from the plate back around to the grid...and if you read that interesting article from Aiken, you'll note he tried to give the circuit only enough gain to oscillate; probably because he wanted to be "proper" and because he didn't want a distorted output waveform. Again, you could see this a lot easier if the tube was (for made-up example) a 12AU7, because you could swap in a 12AT7 or 12AX7 and probably the higher gain of those tubes would overcome the gain loss in the leaky caps (and perhaps the tube itself)

I have done this: Breadboarded a phase-shift oscillator using (junky old) parts from an old organ and a 12AU7 (only got 36 of them in the organ) just to play around with it. Wouldn't start. Throw in a 12AT7 or 12AX7. Blammo, works and starts right away.

[BrianS]

Well, I replaced all the components with brand new stuff, same values...still doesn't work.

I read the Aiken article, but it hurt my brain this late at night...still, the circuit should be designed right. 

I know the plate voltage is high when compared to the voltages listed on the schematic.  I get about 200V on the plate where it "should" be 150V.  I'll have to check the rest of the voltages tomorrow & see if I can figure out how to calculate some stuff...maybe that will tell me something.

I suppose this kind of issue is what separates the men from the boys...

[HotBluePlates]

There is a switch for the trem circuit. Is this on the back of one of the pots?

You say the pot is good. Are you certain the switch is good? Can you bridge it with a jumper to keep it always on for testing?

Do you measure any voltage variation at the oscillator plate?

[BrianS]

^Yep, switch works.  I actually replaced the switch/pot with a brand new unit of the same value.  I may put the old one, which also works, back in.

I had dreams about that darn circuit all night.  Unfortunately, I did not dream of a solution...

[HotBluePlates]

Got a spare 1-1.5kΩ resistor laying around? Try tacking it across the existing 4.7kΩ resistor at the end of your board.

I'm wondering if the tube is just biased too cool to get enough gain to get going.

[Willabe]

I'm wondering if the tube is just biased too cool to get enough gain to get going.

Yep.

Phsyconoodler had a build that was giving him fits, turned out the fix was to up the gain on the LFO.

Here's the link. The link starts at reply 81 and lists what he did to fix it. 

http://el34world.com/Forum/index.php?topic=16282.msg160098#msg160098

             Brad      


                

[sluckey]

I'm thinking the gain of the triode (70) in the 6BM8 is not quite enough for reliable oscillation. Bet if it was a 12AX7 it would be working now.

Here's something very easy to try...
Replace the 4.7K and 20µF cathode resistor/cap with a 5mm red LED (available at Radio Shack). LED cathode connects to ground. Surely it will oscillate with the LED. In fact, it will probably be too strong and you will likely need to pad the trem signal down some.

Did you replace the 270K plate resistor?

[BrianS]

^Just tried it...nothing.  Although I thought I might have heard some wobble when I first turned the switch on.

My plate voltage with the 1k resistor tacked across the 4.7k on the cathode is 150V...which is what the schematic says it should be.

However, the voltage at pin 8 (cathode) is only .5V.  Schematic says it should be 2.2V.

Edit...it's been about an hour since I typed the above.  Lot's on my mind today.  I will try some of the suggestions mentioned.  If the above voltages tell you anything, please post.

Thanks.

[tubeswell]

My plate voltage with the 1k resistor tacked across the 4.7k on the cathode is 150V...which is what the schematic says it should be.

However, the voltage at pin 8 (cathode) is only .5V.  Schematic says it should be 2.2V.

So you effectively have ~820R at the cathode and the tube current is 0.6mA. Something is wrong with that triode. What does the heater voltage look like across the filament (out of curiosity)?

[BrianS]

So you effectively have ~820R at the cathode and the tube current is 0.6mA. Something is wrong with that triode. What does the heater voltage look like across the filament (out of curiosity)?

Holy mother of mercy...last night after I replaced all the components, I put the OLD tube back in instead of the new one!!  After reading your post tubeswell, I thought, well I better try the old tube.  So I popped the tube out and went to get the old one, only to find that is the one I had just pulled out! 

In order to work on the amp last night, I had to pull the 6BM8 out so I could set the chassis on my bench.  I put the new tube back into its box and set it aside.  When I went to try the new components, I inadvertently installed the old tube...which was sitting in an open box next to the amp!

So now I'm going to pull the 1k resistor off the cathode and see if it works that way.  If so, it's done.  If not, I'll get the proper resistor value put in there.

FYI, the tremolo is pretty fast, even at its lowest speed settings.  I have to believe it was designed that way for whatever reason.  I'm certainly not going to mess with it to try and slow it down!

Lots of lessons learned...and re-learned...for me.  Thanks a bunch for your help fellas!   I'll post some soundclips & pics of the amp when she's all together at last.

[Willabe]

Alright you got it going!

It's easy to slow it down, all you have to do is make any 1 or 2 of the 3 LFO caps larger. If they are 0.01 put in a 0.02 and see if that's slow enough. If not change a 2nd 0.01 to 0.02. The larger cap takes a little longer to charge up and that slows the LFO down.


           Brad     

[HotBluePlates]

This thread and the other should teach a bunch of lessons about trem circuits:

- For the most part they're not complex; know how they work before diving in.
- The oscillator needs a solid path from output to input
     - This is where replacing 3 caps will fix some non-working trems
     - This is where some will not have a good ground for one of the 3 resistor may come into play
- The oscillator must have enough gain at lows to overcome losses to oscillate
     - Weak tube?
     - Leaky bypass cap?
     - Need an LED instead of resistor/bypass cap?
- Know how the footswitch works, and whether the circuit is always-on or always-off (some big amps are off until the footswitch turns them on)
- Know how the trem is coupled into the rest of the amp
     - Where/how the trem is injected should tell you something about the size of the signal needed, and possible faults to disrupt trem injection.

[Willabe]

HBP, what you just wrote there should go in the archives of favorite topics IMO. Would be easy to find for future reference.


             Brad     

[HotBluePlates]

I don't know about that...

A troubleshooting tree for trem circuits could go in the archives. Sluckey knows what I'm talking about, with a quick theory of operation, then a step-by-step chart of what circuit element to check, a go/no-go reading for that circuit element, and what to do based on your reading. You see those a lot in military/government electronics maintenance facilities.

[Willabe]

Well, even better then.

Those circuits seem to trip guys up quit often including ticking and noise problems.


             Brad     

[tubeswell]

I've been waiting for several years for Merlin Blencowe to write a book on 'valve' based effects for tube amplifiers. Maybe its time for us to do this thread instead - like HPB says

Off the top of my head here's some possible subtopics on vibrato and tremolo effects

• How LF Oscillators work (3 and 4 phase RC networks, varying LFO speed, bootstrapping for 'kick starting', gain, buffering, depth controls and trem-wiggle attenuation, trem-wiggle insertion methods, pre-amp vibro-champ method, bias-vary output stage, fender harmonic vibrato, fender blackface trem, magnatone vibrato, vox vibrato,
  Tips for troubleshooting LF oscillators (RC network, gain, LFB, buffering why and wherefores, hot/cold biasing in trem insertion points, roaches and opto-isolators, foot-switches...)
  Ways of inserting bias-vary trem for a cathode-biased output stage (suggest looking at the Fender Excelsior insertion method)
  Bias wiggle trem in pre-amp stages (there's a whole heap of possibilities, including 'vibro-champ' method)
  Bias wiggle trem for paraphase inverters (look at Fender 5E9A)
  Weird and wonderful vibratos (Warblers using SR units, Merlin's 'vibratron', noise-cancelling vibrato from the Jack Darr book etc)

...

[Willabe]

I've been waiting for several years for Merlin Blencowe to write a book on 'valve' based effects for tube amplifiers. Maybe its time for us to do this thread instead - like HPB says

Off the top of my head here's some possible subtopics on vibrato and tremolo effects.......

Great idea and great list of topic points!

Bias wiggle trem in pre-amp stages (there's a whole heap of possibilities, including 'vibro-champ' method)

Gibson has a few that are supposed to sound great. Some with an octal and some with a 6EU7, 7199. These triodes could be re-biased
12AX7's.

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-40%20Les%20Paul%20with%205879.pdf

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-40_55_Later.pdf

http://el34world.com/charts/Schematics/files/gibson/Gibson_ga2rvt.pdf

Here's a Gibson K bias-vary trem.

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-16T.pdf
              

                           Brad    

[HotBluePlates]

Gibson has a few that are supposed to sound great. Some with an octal and some with a 6EU7, 7199. These triodes could be re-biased
12AX7's.

The oscillators of the first 2 you linked use 6SQ7's, which is essentially a single-triode 12AX7 in an octal package. The 3rd is a 6EU7, which is a 12AX7 with a different pinout, and the 4th is a 12AX7. So as far as the oscillators, they'll all be 100% plug & play with a 12AX7.

[Willabe]

OK, I brought up possibly changing the LFO/driver/buffer to a 12AX7 because it's easer to get a hold of new 1's.

I don't think NOS 12AX7's are needed for a good trem/vibrato LFO/driver/buffer as long as it's strong enough to oscillate. That way you could save any NOS 12AX7's for a better purpose like the input pre.


             Brad     

[g-man]

Deleted

[sluckey]

Regarding slowing the speed in a Fender amp. As Willabe mentions in reply #16 this can be done by increasing the cap size. The speed pot on most Fender amps has a 100K resistor to ground on the slowest setting. Would reducing that resistor also slow the speed, or is it needed for some other purpose?

That 100K resistor sets the fast speed limit not the slow speed. You can slow it down more by increasing the size of the pot (maybe to 5M) or increasing the size of one or more of the three caps in the feedback path from plate to grid.

[eleventeen]

"Would reducing the size of that resistor (actually...pot) slow the speed..."


In general, we'd want to INCREASE a resistor in an RC circuit to slow the speed. A higher resistance implies a cap will take longer to charge, just like a larger cap implies a cap will take longer to charge. Longer time = lower frequency.

[g-man]

Deleted

[HotBluePlates]

The speed pot on most Fender amps has a 100K resistor to ground on the slowest setting. Would reducing that resistor also slow the speed, or is it needed for some other purpose?

Also think about what happens if you set the pot for minimum resistance: if that resistor were 0Ω, the oscillator signal would be grounded, killing the trem. So it should not be made too small.

[sluckey]

So it should not be made too small.

I just ran into that situation a couple days ago. Had to increase to 220K to prevent oscillator drop out.