Harmonic tremolo/vibrato thumping topic

[frederikdr]

I first posted my question as a reply to an old topic in the Amp Tech Center subforum. King Fan recommended me to start a new topic here. Thank you for reading, or even better, joining the discussion .


About 7 or so years ago, I built a standalone harmonic vibrate/tremolo unit, built from scratch, not following any particular DIY kit. I did the basic two gainstage preamp that can be found in many of the brownface Fenders, and the 4 triode vibrato/tremolo design. Since there were no 10M potentiometers to be found on my side of the Atlantic, I decided to go with a 6-position rotary switch to capture a usable range of settings for the intensity control. Since then, I was living abroad for a few years and didn't play the thing anymore. Now recently I started playing it again, opened it up a few times, and started reconsidering some decisions that I made in the building process.

Back when I built it, I initially installed quite a random set of 12AX7 tubes. First time I got 'sound' from it, the thing sounded more like a drumcomputer doing a slow bassdrum sound, 'thumping', if that's the right word. Back then, I started messing around with different tubes, and managed to get the thumping to stop, once I put 7025 labelled tubes in 2 or 3 out of 3 positions. I guess I decided to go for the 7025 since that was what the Fender schematics for these brownface amps generally indicated.

Now a few days ago, I started experimenting with tubes again. Again, I took a random number of 12AX7 tubes, and tried replacing all positions, to find out which is the 'sensitive' one. The tube in the preamp position (I mean the input triode and the one after the EQ controls) doesn't affect the issue at all, which I also expected. Now, to my surprise, also the mixing stages take any of the tubes that I had available. It's specifically the LFO/oscillator stages that are picky on which 12AX7 I put in there.

Tubes that work fine in the LFO/oscillator stages:
- JJ ECC83S (the one I use, I believe was labelled V1 by the shop where I bought)
- JJ ECC803S
- TAD 7025WA
- Sovtek 7025/12AX7WA

Tubes that make the amp thump when installed in the LFO/oscillator stages:
- Bugera branded 12AX7A
- RUBY 12AX7AC5 HG
- Electro-Harmonix 12AX7EH
- Marshall branded tube (not sure about the exact type), taken from an old JCM900


Did anyone manage to discover any structural logic behind this 'thumping' issue? I have experimented with slightly higher and lower B+ voltages for the oscillator, but the issue stays the same. The capacitors around the intensity control are seen by some on online forums as a possibly relevant factor, but I haven't found any concrete indications on which values would possibly improve the issue. In the other subforum, someone said that the 2-4uf cap plays a big roll. From what I understand, this refers to the cathode bypass capacitor on the mixing stage. Since the tube in the LFO position is what makes my unit thump or not, I somehow expect the relevant factor to lie closer on that side of the circuit, but I might be thinking in ways too simplistic.


A small sound sample of the sound with a 'bad' tube (in this case the Ruby 12AX7AC5 HG tube) in oscillator position.

The frequency of the 'beat' is approximately 1/4th of the tremolo frequency.





And a sample of the unit working fine with the TAD 7025 in that LFO position:

 

[King Fan]

Good stuff. I was hoping you'd get more viewers to crowd-source ideas if you started a new thread. I'm interested in this because one of my tube-trem amps once thumped like that. The problem (and blessing) is -- it cured itself, so I don't know the cause, and hope it never comes back.

You'll likely know that 7025s were originally designed to be a more refined version of the 12AX7, but many people consider modern versions to be roughly the same. So I'm not surprised that 'decent' AX7s seem to work as well as modern-issue 7025s. You'll also know that TAD doesn't make tubes, they just rebrand them (though they say they may hand-select or specially design them).

OTOH, those Ruby/Bugera/etc. tubes that seem to thump for you might be taken to be 'cheaper' or oddball tubes. Modern 'amp' branded tubes were often made in China, and some of the those have had mixed reviews over the years.

One reason I thought it'd be smart to attract a lot of eyeballs is that these harmonic trem units are complex -- we do have some members here who've built 'em. I'm impressed you built yours from scratch, but maybe our experienced members can think of design/wiring issues that would make the LFO more susceptible to tube function or "quality."

 

[frederikdr]

What kind of tube-trem do you have, King Fan? Harmonic/bias-wiggle/..., diy/kit/Fender?

I've been reading about the history of the 7025, but as is often the case with tube descriptions, you get lost in designs claiming (or at the moment of originally being designed claiming) to be the most refined or highest grade version of something. Same with rebranded tubes. If I'd have to believe what some people on the internet say about the Ruby tube that I used in my test, it would be a fan-tas-tic tube. When talking differences in tubes and tube brands, I generally can't see the forest for the trees.

When it comes to wiring and design, I'd say the quality of my work has improved since I built this. Layout and neatness is very far from perfect, but to improve that, I should start building from scratch again. In the past I've also been underimpressed with the results of some of the 'neatness improvements' that I made to my ealier builds. Since on this unit the difference between a beautifully operating tremolo and a thumper is defined entirely by one tube in one position, I'm expecting there mùst be some good explanation, somehow relating more directly to that part of the circuit.

 

[King Fan]

Good stuff, again. BTW, I like neatness and all, but I didn’t mean your trem unit might be sloppily wired. I was thinking in terms of layout, components, wiring design.

 

[Stevo Bambino]

Do you have schematics, layout and or pictures of your build? That may help others get an idea of what may be causing it.

I bought a Revibe kit from Weber about a year ago and still have not had the time to put it together. The Revibe is a reverb and harmonic tremolo in one. So despite not having any experience with one yet, I am very interested in this thread.

 

[frederikdr]

Good stuff, again. BTW, I like neatness and all, but I didn’t mean your trem unit might be sloppily wired. I was thinking in terms of layout, components, wiring design.

It ìs wired sloppily. My phone camera is in terrible condition, so pictures might turn out terribly, but I will try when I have the unit opened in the next days.

When it comes to schematic, I followed quite closely the circuit of, for example, the 6g12, where I left out the non-tremolo channel and obviously the poweramp. I didn't use a choke, but power supply filtering should be plenty.

 

[King Fan]

This is where it can get harder. Smart folks -- smarter than me -- can often gain insight by viewing a schematic. Voltages and photos often help them, too. Share a well-lit pic of the whole layout, and maybe one of the pots and one of the sockets, and folks can decide if they need more photo details. Then, can you share voltages? Finally, the challenge is to draw up a schematic. Again, I'm not usually smart enough to use this information, but I'm hoping others may be.

If all that doesn't work, I was hoping that a wider viewership might include someone who goes, "Aha, loud trem thump with certain tubes -- I know this." But I worry the odds aren't in our favor.

 

[frederikdr]

This morning, I took some pictures, which made me reflect on some of the uglier parts of the unit's guts :S. (See attachments)

During the day, I decided on a few parts that I would try to improve on this evening:

- path to ground from speed potentiometer shouldn't be grounded at the output jack

- the large loop made by the brown wire around the intensity pot/rotary-switch shouldn't be there

- one of the black wires, from the power supply board to the LFO's plate resistors should run more straight

I improved on all three points (but didn't make a picture of 'the after' yet). Now, while working on it tonight, I was looking at the Revibe schematic that I printed out recently. There, I noticed that the designer clearly states C22 should be formed by a 220n in parallel with a 47n capacitor, after which I checked one of the Fender schematics, where this capacitor is a 250n. Looking at my own 'creation', I noticed that in the building process, years ago, I chose 220n to be the 'closest option'. After inspecting the caps I had lying around, I now decided to try adding a 33n in parallel.

After dinner it was time to try out the changes. First with the 'good' TAD 7025 in the LFO position: everything worked as before.

Then came 'le moment suprème', where I installed the Ruby 'bad' tube. Powered up... I played for a little while: the thump is gone! Different intensity levels, different speed settings, and so far, the thump left the unit!

Could it be the C22 making the difference? Who knows...

 

[King Fan]

Great work. Figuring it out yourself is always rewarding. I’m not sure which fix made it better, but if it stays fixed, who cares?

 

[frederikdr]

I cheered too early. The unit is still thumping with particular tubes in the LFO position. The Ruby tube works fine somehow, and yesterday I think I also managed to get the EHX to work well. The Marshall and Bugera ones still gets thumpin' in that LFO position. What is remarkable however, is that with some tubes there's no thumping at maximum intensity, only thumps when turning intensity down. At the highest speed setting, the thump also tends to stop now!

 

[wsscott1930]

I'm looking at a buddy's original 1965 BF Deluxe Reverb that's got the thumping issue. It started out with a symptom of decreasing output volume level when the Vibrato Intensity pot was increased, and found it was a bad tube, after I replaced the roach with no success in fixing the issue. No thumping before the tube was replaced or afterwards however. Then my buddy picked it up and took it home and turned it on a few days later, and the volume issue was solved but the "thumping" appeared! What could have happened? Any thoughts on that? Is replacing the 25uf cap and maybe the 10M resistor a fix. Thanks.

 

[frederikdr]

I'm looking at a buddy's original 1965 BF Deluxe Reverb that's got the thumping issue. It started out with a symptom of decreasing output volume level when the Vibrato Intensity pot was increased, and found it was a bad tube, after I replaced the roach with no success in fixing the issue. No thumping before the tube was replaced or afterwards however. Then my buddy picked it up and took it home and turned it on a few days later, and the volume issue was solved but the "thumping" appeared! What could have happened? Any thoughts on that? Is replacing the 25uf cap and maybe the 10M resistor a fix. Thanks.

Which circuit are we talking about? Powertube bias wiggle tremolo? Which tube was replaced, the one with the LFO triodes?

 

[wsscott1930]

Sorry, the V5-12AX7 tube that is powering the roach was replaced.

 

[frederikdr]

You say that the amp worked well after your service, but started thumping after the owner took it back home. Could some wires have moved during transport? LFO signal carrying wires might induce some unwanted signal in the audio signalpath, I believe.

 

[Bendyha]

I'm looking at a buddy's original 1965 BF Deluxe Reverb that's got the thumping issue.

Which circuit are we talking about?

The amp in question uses a optocoupler/optoisolator, (that is being referred to here with the quaint Americanism - roach), to shunt the input to the phase inverter to signal ground, with a variable pulsation. It is not a harmonic tremolo, and doesn't really belong in this thread, perhaps wsscott1930 will start his own on that topic. I also believe that the thumping found there, is being induced and injected in a totally different manner than what is being discussed otherwise in this thread.
I wish I had insight into the cause of your thump frederikr, but other than gremlins, I have not thought of a plausible explanation, but will be watching the thread with interest.

 

[wsscott1930]

Thanks for the input. Ah yes, I didn't realize this was an international forum, and true I guess the "roach" is an Americanism, but it does kind of look like one! I also thought maybe something got shaken loose on the ride back home. I expect my friend to bring it over in the next week or so, and I'll let you all know what I found out. But I won't clog up this topic with a "sub-topic". Best!

 

[frederikdr]

Nothing wrong with Americanisms! Fender being an icon of American'ness, it makes a lot of sense to use this word! I was confused because somehow in my mind, the Deluxe has a bias wiggle tremolo, not sure where I got this from. Other point of confusion was the 10M resistor, which I associate with the discussion we were having about the harmonic tremolo .