Hoffman AB763 weak tremolo with 6L6GC's

[sam]

Any one have a fix for the weak tremolo with Hoffman AB763 when using 6L6GC's. I've built a Hoffman AB763 Deluxe Reverb and the tremolo it turned out awesome. The one I'm having the tremolo problem with my Vibrolux Reverb that I just finished last week. I found Hoffman notes on changing the speed and strength and changed a 470K for a 220K resistor and the cap from a .1mf to a .047mf, hoping to improve the term strength. Not much better. Suggestions welcome.

[shooter]

read through this n see if there's any ideas/checks

http://el34world.com/Forum/index.php?topic=24334.0

[sluckey]

See note 5...

     https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf

[sam]

I like Sluckey's Tremor-Lator idea. I'll go over my Hoffman AB763 drawings and see what changes I'll have to make to upgrade this tremolo circuit on the turret board. I think I'll insert the signal at the same point as a stock AB763. Will the stock 250KL intensity pot work, or maybe a different value?
Suggestions welcome... 

[sluckey]

250K will probably work fine. 50K-RA would be better. Very easy to do to a Hoffman 2 channel AB763 board. Just remove R30 and connect the top side of the pot to the junction of C15 and R29.

I would leave the Hoffman 250K pot in place, at least for a while. That way you can compare the two different tremolos. Just turn down the INT of the circuit you don't want to hear. BTW, when you connect the optocoupler the Hoffman bias vary trem will get stronger.

Oh yeah, here's how I modified my Bandmaster AB763...

     http://sluckeyamps.com/Bandmaster.pdf

[sam]

Looks like a good plan.. I'll get back with you after I make the changes. Oh, yeah I got to order the VTL5C1.
Thanks for the help.

[sluckey]

While you wait, why not try the red LED mod?

[sam]

fresh out of red LED's. No place in town to purchase electronic parts. Beer pubs on every other corner here in central Oregon tho    I'll wait till parts arrive.

Tell me more about the LED mod. Drawing? and how it operates.. always up to learn something new.

[sluckey]

Read this. Especially the last paragraph...

     http://www.valvewizard.co.uk/trem1.html

[Platefire]

sluckeys Tremor-Lator mod to my 1974 Pro Reverb sure did the trick. I was playing it this week marveling over how good it sounds. Previously I was getting a thumping sound and intensity was weak. Had to run the intensity full up and was still not strong enough. Now it is completely quiet and the intensity is set on 6.5----very good!

[Platefire]

Well I really didn't mean to kill this thread!!!  Life support Quick!!!

[sluckey]

Grasshopper, from hence forward you shall be known as ThreadKill.

[sam]

I'm still here... Keep the comments coming.

[glass54]

(...unkilling of the thread  )
I recently finished a One channel AB763 (Doug's Turret board), quite happy with it BUT decided I would like a little more Trem depth, so I tried the LED mod. I was quite aware of the Valves Wizard's article for some years, but Sluckey got me motivated to "must try or I haven't lived"
I tried a few different LEDs and settled on a 5mm Orange LED (Clear body) and as Platefire found, setting of 6.5 to 7 on the Intensity Pot is delightful and smooth. I did alter C26 from 22nF to 33nF (on Hoffman AB763-1 Channel schematic) to change the Speed a little ie slower.
I am impressed with such a simple mod and NO ROACH used.
Regards
Mirek

[sam]

It's alive... 

I installed Sluckey's "Tremor-Lator" and it works like a charm. I did have to swap the 250K intensity pot for the 50K. The 250K had to much resistance and no trem till it was turned up to passed 8. With the 50K at 6.5 it's good to go.

Thanks to all for helping me make a great amp better.

[2deaf]

The Tremor-Lator is a great solution, but you could also get bias-vary tremolo for 6L6's by altering Hoffman's circuit. 

 

[sam]

2deaf  Have you made this mod to a Hoffman AB763? How does it sound compared to the "stock" tremolo circuit?

I like the Tremor-Lator, however now when I switch the trem off, you get a loud POP... 

suggestions ?

[sluckey]

I don't get a pop. Look at this schematic. Is your footswitch connected like this?

     http://sluckeyamps.com/Bandmaster.pdf

[sam]

no it's not, but I can make the change and post results.

Thanks

[sam]

Change made, no more POP.. 

Amp is 100% now, Thanks Sluckey 

[2deaf]

Have you made this mod to a Hoffman AB763?

Naw.  I just drew some lines, did some calculations, and ran some experiments.  It will put out over twice the voltage of a 6G16 with a good frequency response and reasonably rounded waves.  It's a series of compromises.

At one point I got an unwanted 44Hz oscillation in the LFO that I've never managed to do before.  The .01uF cap from plate to cathode cleared it right up.

[whoops]

Have you made this mod to a Hoffman AB763?

Naw.  I just drew some lines, did some calculations, and ran some experiments.  It's a series of compromises.

I got a bit confused, so the document you posted with tremolo mods for and hoffman Ab763 using 6L6  shouldn't be take into consideration, is that it?

[2deaf]

I got a bit confused, so the document you posted with tremolo mods for and hoffman Ab763 using 6L6  shouldn't be take into consideration, is that it?

It's certainly your prerogative whether to take it into consideration or not.  I posted that document as a service to others.  I spent several hours and generated fourteen pages of graphs, calculations and data to arrive at that document.  I was fully aware when I posted the document that people at different levels were going to consider it from different points of view.  Those that understand the biasing of cathode followers along with band pass filters and their effects on clipping, frequency response, and amplitude can appreciate the values I chose.  Those who are tweaking at random can save a lot of time by starting with my values.

[sluckey]

Change made, no more POP.. 

Amp is 100% now, Thanks Sluckey

Sam, you seem to be missing a 10K resistor on the cathode of the oscillator. This 10K keeps the oscillator running all the time. Without it, there could be a delay in trem startup when you stomp the footswitch.

[whoops]

It's certainly your prerogative whether to take it into consideration or not.  I posted that document as a service to others.  I spent several hours and generated fourteen pages of graphs, calculations and data to arrive at that document.  I was fully aware when I posted the document that people at different levels were going to consider it from different points of view.  Those that understand the biasing of cathode followers along with band pass filters and their effects on clipping, frequency response, and amplitude can appreciate the values I chose.  Those who are tweaking at random can save a lot of time by starting with my values.

I'm sorry for the confusion 2deaf I didn't understand at first.

Thank you so much for your reply and for your work in working out the circuit for 6L6 tubes

Thanks

[whoops]

I always liked the Fender opto tremolo, and that was my goal when I looked for this project 10 years ago.
I realized then that the Hoffman AB763 had a Bias tremolo circuit but I decided to trust it was good.

I used 6L6 because I had parts available (power and output transformers) for an 40w/50w circuit.
The reality is I never really liked the bias tremolo with 6L6, at first, stock, is really non-existent, I tried Sluckey's mods (thank you so much Sluckey for that) and it improves the situation a bit but it's still really weak. Then I tried Note 5, a LED, and that was the mod that made the tremolo more noticeable, maybe it works for some people but I still didn't like the tremolo and started to think that I should have done what I wanted and liked in the first place and that was to have a tremolo like the Twin Reverb and other similar Fender Reverb amps.

So today I got my fix for the tremolo when using the Hoffman AB763 with 6L6 tubes,
and that is to completely remove and ditch the Bias Tremolo:

[sam]

Sluckey.. I see the 10K on your Bandmaster drawing. I missed that part.  Haven't noticed a delay on Trem startup, but I'll listen closely. And make the change if needed.

Thanks