Tremolo insertion on a single channel AC30 TB style build

[Jennings]

Hi folks...seeking a little guidance on fettling where I add my one triode trem into the attached schematic, and how I implement that if possible please. 


Just for clarity, the circuit built is as drawn except:


- where I've added red "X" marks with black lines through the build has omitted those components and replaced with jumpers / direct links.
- where I have added a red "X" from depth to ground I have cut/removed that link now in the build.


To begin with I cut the trem in where it's pictured, but I found that even when I cut the depth control to ground, increased the depth pot value (up as high as a Meg) and added even a 220k series resistor between it and the audit path the trem comes through but the audio signal is almost wiped out.  I'm struggling to balance getting the trem in but retaining audio signal volume level without having to massively up the channel volume to compensate for turning the trem on.  I've also tried cutting the trem in after the volume pot (where I've added the yellowish circle on the drawing) and not had any better luck with curing the massive volume drop in audio signal when I turn on the trem.


Am I simply needing to continue the fettle until I eventually settle on the right values round the depth pot, any series resistance and so on, or am I not approaching this correctly?

[SEL49]

Try a larger depth pot and maybe a resistor between the pot and the insertion point. Look at the Trem-O-Nator page and try some of the different adaptations by various people. A little experimenting is usually required.

[Jennings]

Try a larger depth pot and maybe a resistor between the pot and the insertion point. Look at the Trem-O-Nator page and try some of the different adaptations by various people. A little experimenting is usually required.

It's those folks that used it on Fender and Marshall type builds etc which inspired me to use my spare triode for a trem in my AC30 build.  Especially as I wasn't that interested in using it for a switchable gain stage.  So far I've gone up to a 1M with 220k series resistor and it still doesn't seem to play nicely yet...inserted after the tone stack it seemed to interact too much with the stack controls as well as killing the signal.  I'll keep fiddling and see where that goes though.

[pdf64]

The 22uF cathode bypass at V2 pin8 looks wrong?

[ac427v]

I had good success on a plexi style amp using the original Trem-o-Nator oscillator.
Used a 250k RA pot wired as a variable resistor. A 250k linear worked almost as well as the 250k RA.
Connected the input of the intensity pot (VR5 on your schematic) to the input of the volume control (VR2)
I suspect my version would work with your Supro oscillator. I used a 470k ohm plate resistor to up the gain to my oscillator. Keep us posted!

[Jennings]

The 22uF cathode bypass at V2 pin8 looks wrong?

Good catch…drawing error there. I’ve got back to my PC today and updated the schematic properly 

I had good success on a plexi style amp using the original Trem-o-Nator oscillator.
Used a 250k RA pot wired as a variable resistor. A 250k linear worked almost as well as the 250k RA.
Connected the input of the intensity pot (VR5 on your schematic) to the input of the volume control (VR2)
I suspect my version would work with your Supro oscillator. I used a 470k ohm plate resistor to up the gain to my oscillator. Keep us posted!

Thanks for the advice and encouragement…much appreciated. I’m sure I’m missing something or just need a fettle somewhere…or perhaps it’s just this amp and my ears with the inherent volume drop when using the tremolo.


I’ve tried 50k, 250k, 500k and 1M pots, wired currently as a variable resistor with wiper and one and to the LDR, and other pot outer lug to the volume….I get the same result inserting the circuit after the tone stack (except with some interplay with the tone controls), and with any/all of those pit values. Basically as you slowly rotate the depth/intensity you get a slight gain/volume loss just before the trem starts to cut in, and as soon as the trem comes through you get a big drop in gain/volume. It’s as if the LDR suddenly is sapping all audio signal volume and gain regardless of whether the trem is on or off the beat. If that description makes sense. Click the footswitch and stop the trem, and the audio comes back to “normal” volume level.  It sort of sounds like the audio circuit gets loaded down somehow. Could it be I just have unrealistic expectations on the balance I’ll be able to achieve between non-trem and trem volume levels on a single channel amp where I can’t blend preamp channels?

[ac427v]

The amp should only lose volume when the trem oscillation cycle decreases the resistance to ground. When the oscillator is at the high part of the cycle, the amp volume should be unchanged.
Is the Vactrol damaged or not wired properly? I recall the resistor in the Vactrol has a value over 10meg ohms when the oscillator is off. Verify that yours is correct. Verify that it is wired correctly in the circuit. If the LED leads of the Vactrol were reversed, it may be damaged???
There is an internet thread about a Vactrol from Amazon not working properly. The one used by Steve Luckey is no longer available. Some have reported that Amplified Parts Xvive VTL5C1 is an effective substitute.
You have tried all the possible variations of insertion points and pot values. Great trouble shooting!
The only other explanation I can think of is that the untested Supro oscillator does not work with with this trem design. This is a long shot because if Sluckey thinks it would work, it should work  
You may need to use the tried-and-true Trem-o-Nator oscillator. It sounds great and has several advantages over other trem circuits.
I enjoy watching you and others trying new ideas because I can steal them without doing all the trial-and error work   Good luck.

[Jennings]

That's exactly what's been bugging me...it's almost like a loading / blocking effect.  I've been suspecting the vactrol for a while too since there's no change with position or pot values etc.  It is wired correctly, but I can't remember where I got it from, and it's simply marked as SR and VTL.  It was bought as a VTL5C1 though, but I have no idea how closely it conforms to spec.  I do have a spare (I obviously bought two at the same time), so will try subbing that in.  Trial and error on the fettle...I'll get there in the end, and then there'll be another potential circuit that's been built/documented 

[pdf64]

Sorry if I missed it, but what resistance do you measure between the X of the intensity pot (X isolated from the insertion point)and ground, amp off?
Then amp on, trem switched off.

[Jennings]

Sorry if I missed it, but what resistance do you measure between the X of the intensity pot (X isolated from the insertion point)and ground, amp off?
Then amp on, trem switched off.

No apology required, as I've only just measured to investigate now...


Amp off = 200M plus resistance.
Amp on = 60M plus with the footswitch open (no continuity) versus just the pot resistance when closed (continuity).  Pot measures 246k.


However, I did discover something...with the amp off my measurement was about 80M and slowly falling, but using my hand to further darken the area the vactrol is mounted made the reading rapidly rise to 200M plus.  So we can conclude that these plastic packages don't fully shield out light ingress in and of themselves.  I know the chassis is usually enclosed, but with the pilot neon and things like speaker jacks not too far away it's possible there's some stray light impact on the package.  I did also find I do have a load of loose, un-labelled LDRs in my parts draws which I'd forgotten about.  So I can also build my own LED/LDR packages. 

[pdf64]

How strange, I'm struggling to envisage how the 60-80M dark resistance is crippling the audio signal 
Maybe it's a capacitance thing, what capacitance does it measure?

[Jennings]

It is rather baffling…I’m away with work until Thursday evening, so I’ll pick it up again then. I’ll measure capacitance and also better seal the vactrol. I have also noticed that my LDR grounds to the preamp ground bus at the PI point, so I shall try moving that to the ground for the insertion point area instead.