Sanity-Check a Schematic / Build Idea For Me.

[Dr Tony Balls]

Hi All,

I'm working on designing a build for a freind and before I actually break ground on it I was hoping to get a sanity check from the experience base to identify any potential pitfalls in my approach.  The details are:

• AB165 Bassman preamp.  Normal channel only.
• ECL84 push-pull power amp.  I'm taking a lot of inspiration from the SoLow Watt project here, among others.  I've built a few amps with this power section and find them to sound great and be perfect for home/apartment amp usage.
• Added Tremolo.  Specifically bias wiggle tremolo on the preamp.  This part I basically lifted directly from an AA764 Vibro-Champ
• Added Reverb.  This is a pretty standard Fender reverb circuit.  I borrowed from the AB763 Vibroverb here but its essentially the same in many Fender amps.

Schematic is below.  Hi-Res version here.




Questions are:

• In general are there any red-flags that i'm missing?
• This tremolo would normally be operating on a higher voltage supply.  Should I expect any issues using ~100V lower supply?
• Same question on the reverb.  Is operating that circuit on lower voltage going to cause issues?

Thanks in advance for any feedback!

[Willabe]

Questions are:

   This tremolo would normally be operating on a higher voltage supply.  Should I expect any issues using ~100V lower supply?

Yes, the trem oscillator might not start. It has to have enough gain to get started.

[Latole]

Questions are:

   This tremolo would normally be operating on a higher voltage supply.  Should I expect any issues using ~100V lower supply?

Yes, the trem oscillator might not start. It has to have enough gain to get started.

I agree . Dr Tony Balls,See Vibroverb 6G16  circuit

[Dr Tony Balls]

Thanks for the input!

I did look at the 6G16 tremolo initially, but the request shifted to having the tremolo before the reverb, thus I looked to the Vibro-Champ for a preamp-based bias-shifting tremolo.  The oscillator circuit is the same in both Vibro-Champ and Vibroverb, though, just run off 340VDC and 440VDC, respectively (as labeled on schematics).

I did a little hunting and found this thread (below) where a user added Vibro-Champ tremolo to an AC-4 style build which apparently runs fine off of 231VDC supply (final schematic in last post of thread).  So it appears that the tremolo oscillator *will* start on a voltage that low.  So that makes me feel more confident about the tremolo.

https://el34world.com/Forum/index.php?topic=27253.0

I'm still unsure about reverb function off of lower supply voltage, though.  Any issues there?

[acheld]

I've had good luck with Sluckey's Trem-O-Nator circuit.  Check out https://www.sluckeyamps.com/trem/trem.htm.   This is not a bias wiggle, but sounds great.   

I've built a couple amps with supply voltage in the low 300s, IIRC.

[Merlin]

Yes ECC83 will oscillate no problem down to 200V at least, especially if you use LED bias. Any reason you don't want to go for opto trem though? (e.g. LDR after the vol pot) The quality of tremolo will be better.

Reverb will work fine, just bias the ECC81 up to 5mA.
 

[tubenit]

I would suggest .047, or .033, or .02 coupling cap after the LTPI before the ECL84's.


With respect, Tubenit

[stratomaster]

Since you aren't using NFB nor the 2nd PI input you can keep the traditional arrangement of reverb and tremolo, but just blend in the reverb return to the 2nd PI input.  Just use a resistor to ground after the 3.3M resistor to mimic the attenuation of the reverb return.  Then you can use the optical trem as Merlin recommended as the reverb is essentially in parallel with the tremolo. May get you what the client is after.

Sorry. Misunderstood the goal.  Thought client wanted verb after trem or similar.

[Dr Tony Balls]

Yes ECC83 will oscillate no problem down to 200V at least, especially if you use LED bias. Any reason you don't want to go for opto trem though? (e.g. LDR after the vol pot) The quality of tremolo will be better.

Reverb will work fine, just bias the ECC81 up to 5mA.

Thanks for the confirmation!  The real reason to not do opto is that a bias wiggle tremolo was requested, presumably because they want that sound with whatever quality limits it has.  Some folks just like the bias wiggle sound.

[Dr Tony Balls]

Hi all,

Happy to report that this has been a pretty decent success so far.  My only issue is the tremolo.  There's a very noticeable THUMP sound that varies with the trem speed, and i'd like to find a way to minimize it.  It lessens and goes away completely with the intensity control, which confirms that its related to the direct injection of voltage at the cathode of V1B and not a lead address issue creeping in somewhere else.  I've tried changing R35 to a larger value (replaced it with a 1M pot) and all the way up at 1M the thump is mostly gone and the tremolo sounds decently intense.  Anyone have any other tricks to tame that or is that basically it?   I've heard a lot of people say that with this type of tremolo the thump is just a fact of life that must be lived with.

Updated schematic with voltages below.  Full size version here.

[Latole]

I ask to Google and found an answer from an expert who was in this forum; Slucky .

Diode across the intensity pot

https://www.tdpri.com/threads/filtering-out-tremolo-thump-question.1026286/

[Lectroid]

sluckey is brilliant.  Such a simple solution and focused right at the point of the problem.  So typical.

[Latole]

sluckey is brilliant.  Such a simple solution and focused right at the point of the problem.  So typical.

Yes, Slucky was an exceptionally competent member.
It's a shame he had to say goodbye.

[Dr Tony Balls]

Thanks everyone.  I was aware of Sluckey's genius mod however that implies to a standard Princeton type of bias wiggler circuit that varies the negative bias voltage applied to the power tubes.  In this case (Vibrochamp) an oscillating positive voltage is applied directly to the cathode of a preamp stage.  Adding the diode mod just drops all of the positive voltage to ground, thus losing all tremolo effect altogether.

Anyone else got any tips?

[pdf64]

To remove thump, some serious ie multi stage HPF of the signal path is required, as per AC30 / Dominator.
https://el34world.com/charts/Schematics/files/Vox/Vox_ac3093pr.pdf
https://el34world.com/charts/Schematics/files/Watkins/Watkins_dominator.pdf

As it is, the low end is pretty much unrestricted down to earthquake frequencies, which the speaker etc is doing its best to reproduce, hence 'thump' 

[Dr Tony Balls]

To remove thump, some serious ie multi stage HPF of the signal path is required, as per AC30 / Dominator.
https://el34world.com/charts/Schematics/files/Vox/Vox_ac3093pr.pdf
https://el34world.com/charts/Schematics/files/Watkins/Watkins_dominator.pdf

As it is, the low end is pretty much unrestricted down to earthquake frequencies, which the speaker etc is doing its best to reproduce, hence 'thump' 

I tried this by looking toward the AC30.  I added the following RC filter b/w C5 and R9, but all it did was make the thumping brighter. 



I guess when I get down to it my question is why does this work in the Vibro Champ, and not here?  It may be that the added gain stages are accentuating the thump, or that the Vibro Champ is never that UN-thumpy to begin with.  It may be as good as it gets like it is.

[Willabe]

Look at this from Sluckey;

https://www.sluckeyamps.com/trem/trem.htm

[Latole]

Look at this from Sluckey;

https://www.sluckeyamps.com/trem/trem.htm

This is extraordinary information, thank you for sharing.

[tubeswell]

Anyone else got any tips?

That you played around with the intensity pot gives you a clue. You can try ‘optimising’ the strength of the maximum LFO signal being fed into the signal chain by playing around with voltage dividers.

[Dr Tony Balls]

Resurrecting this thread with my eventual results and an additional question.

THE RESULTS

I wound up scraping the bias wiggle trem on the preamp and implementing it on the power tubes, as it was just too much to remove the initial thumps I was getting.  Tremolo solved.  The reverb was way too intense as I initially had it, but with some adjustment we got it under control.  And finally, there was some bad distortion originally.  A sizzle like sound that took a lot to track down but ultimately it seemed that the signal going from phase inverter to the power tubes was too much and it was tamed by some series resistance before the master volume.  Final schematic below:




THE QUESTION

IS the triode in an ECL84 sufficient to be used for a LTPI in this application?  I've seen others use them (SoLowWatt, for example), and i've heard theyre a meduim gain triode akin to a 12AT7.  But I've built an ECL84 amp before where ive used an actual 12AT7 for the LTPI instead of the triodes in the ECL84s and it didnt seem to have this distortion issue.  Does anyone have input on that?

[tubeswell]

was some bad distortion originally.  A sizzle like sound that took a lot to track down but ultimately it seemed that the signal going from phase inverter ... Does anyone have input on that?

Why did you run the NFB to the grid of the inverting LTP triode? (More common to put it at the LTP tail) Have you tried disconnecting the NFB to see?