Hoffman AB763 Tremolo 6L6 mods?

[bnwitt]

I was reading an old post on mods to the AB763 Tremolo components when running 6l6s and I'm a little confused as to what the final suggestions were.  Grounhog Ken, did we ever come up with the final component changes suggested to increase the depth of the Tremolo with 6L6s?

[tubenit]

I think this may be from Ken out of the FAQ from the archives ....... however the thread it references may have fallen off the back side?  Maybe Ken can verify if these are his comments?

"10)  How can I increase the vibrato on my Fender amp (or slow it down)?
 
The Tremolux 6G9-B circuit has a beefier tremolo drive than the brown Vibroverb circuit--and definitely more intense than the Blackface Princeton.  

http://www.el34world.com/charts/Schematics/files/fender/tremolux_6g9b_schem.pdf
  
Reference the Hoffman AB763 layout.  
Locate the 4.7K cathode resistor on on V5 pin 3.  
Install a 22uF 50v electrolytic cap in parallel with the 4.7K resistor.  (positive end to pin 3, negative to ground)  
This should help even the wimpiest 12AX7s to oscillate.  
  
Locate the two 470K resistors that connect to pin 8 of V5.  
Change the one that is horizontal on the layout to 220K.  
There are two .1 caps that attach to the right end of this 470K (now 220K) resistor.  
Change the .1uF cap that goes to ground (the left one, not the one that goes to the intensity pot) to .047uf.  
  
  
The original Hoffman AB763 is probably fine for 6v6 amps. "
  
To slow down the vibrato change the caps in that section from .02/.01/.01 to .02/.01/.02
 "

With respect, Tubenit

[bnwitt]

thanks tubenit.  I found the posts.  It would be good to get this in the archives.  From what I read, Doug says change the horizontal 470k resistor off of V5 Pin 8 to 220k.  The other thread goes on to say:

1. Change the horizontal 470k resistor off of V5 pin 8 to 220K, 180K or 150K
2. Add a 22uf/50v bypass cap on the V5 oscillator's 4.7k cathode resistor (V5 pin 3).
3. Change the left .1uf cap under the horizontal 470k (now changed to 220k, 180k or 150k) to .047uf
4. Change the first trem cap from .01uf to .022uf so you have .022/.01/.022  (this for a slower tremolo if desired)

I've attached a little drawing

Also, it was mentioned to try a 12AT7 in the V5 position.

[kriswel]

I have played around quite a bit with this stuff but the thing I found that works best is to bias it a bit cold.  About 17ma on 6l6's has been the magic number for me.  I have thought about using a switch for cold/hot bias for this reason.

kris

[bnwitt]

I have played around quite a bit with this stuff but the thing I found that works best is to bias it a bit cold.  About 17ma on 6l6's has been the magic number for me.  I have thought about using a switch for cold/hot bias for this reason.

kris

Kris,
 thanks, I'll start off with a cool bias point when I crank up this amp.

[Geezer]

I have played around quite a bit with this stuff but the thing I found that works best is to bias it a bit cold

Now that's something I never would have thought of!
Thanks for the useful tip.... 8-)

[phsyconoodler]

I find the opposite to be true for my vibro-verb.If I bias cold it has a weak trem.Biased hot it sings like a canary.
  And it hoffman's layout is like a vibro-verb then it's not an AB763 anymore is it?The layouts are very similar for the AB763 and the 6G16 until it gets to the trem and reverb sections.So when a guy says AB763 it means it has the roach.
  The only drawback to bias vary trems is it works on both channels which can be a problem if you use one channel to mix in delay.
  I am switching mine to the early bandmaster trem circuit so I can use the normal channel for effects mixing.

[bnwitt]

 The only drawback to bias vary trems is it works on both channels which can be a problem if you use one channel to mix in delay.

why not just turn it off when using the normal channel?

[kriswel]

I find if you play your amp at high volumes whith a warm bias the trem can tend to be week.  Although you may like the sound better with a warmer bias,  I still maintain it has more "depth" when it is colder.  If you use a bias checker and play around with it I think you will see what I am talking about.  Try it right up around 35ma and down around 17ma.  The trick is to find a nice balance between the tone you like and a good trem.  I like a colder tone with trem anyway.  

kris

[HotBluePlates]

And of course, all that depends on the actual volume level you're playing at. With that in mind, you'll want to make your bias adjustments for trem level at the volume you'll most often be playing at. You might notice one setting works best at low volume and another works best at higher volume.

I'm sure this is why Fender eventually went to the optoisolator trem circuit with the bigger amps. Those big amps had a relatively large negative bias voltage for the trem circuit to overcome and they probably noticed that the trem level was more consistent in those amps if they just grounded and ungrounded the signal.

[Tiny_Daddy]

Thanks guys, I will put those mods in. Once again those who don't know history are doomed to repeat it. As stated above, please archive this thread.


Found this website that reports some mods for tremolo. Don't know if they work but thought I'd provide the link.

http://www.lynx.bc.ca/~jc/tremoloCircuits.html

Tubenit

[bruno]

Not sure if this is the right place to ask... I recently built an ab763 amp, with the hoffman layout, I changed the horizontal resistor to 150k, added a 22uf cap across the 4.7k resistor, changed the left 0.1uf cap to a 0.047uf, I have around 470 volts on the plates of the 6L6s and the amp is biased around 45ma. Still the tremolo is barely noticeable, the effect can only be heard when the intensity pot is past 3/4 of the taper. Any ideas on what I could try next?

thanks

[bnwitt]

Bruno,
I noticed you didn't mention changing the 0.01uf cap to a 0.022uf.  Also, have you tried different tubes in the oscillator.  Some tubes are stronger than others.

[tubenit]

From Sluckey:

Simply replace the cathode resistor/capacitor for the oscillator tube with a LED. That's all! I did and my tremolo signal voltage increased significantly with the lower frequency benefiting the most. Here are my numbers...

    Cathode R/C... 5Hz = 65Vpp    Cathode LED... 5Hz = 178Vpp

    Cathode R/C... 8Hz = 166Vpp    Cathode LED... 8Hz = 226Vpp

And you have a visual indicator that can be mounted on the control panel too! I put this mod in a simple cathode biased 6V6 PP amp that used bias modulated trem and the trem strength is now way more than I'll ever need. Should work just fine in a fixed bias amp too.Here's my schematic...

    http://home.comcast.net/~seluckey/amps/RCA/Ampeg_J12B.pdf

This is not my idea. You can read about it on Valve Wizard's site...   http://www.valvewizard.co.uk/trem1.html

I had seen this mod before but always thought it was just about a visual indicator. I had overlooked Merlin's statement "By replacing the cathode resistor with an LED we eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop."

I believe this mod is for you if you've been dissatisfied with having to compromise between hot bias guitar sound and strong tremolo on a bias modulated trem amp such as a Princeton Reverb or Hoffman's AB763.
 

[jufa82]

Hi and thank you for the awesome information!

I was wondering does it matter which colour the led is you put in there??


Greetings!!

[Geezer]

I don't know the answer, but I do know that each color has different diode characteristics, so it is possible that there could be some differences in response between, say a green and a red LED in this application.

As cheap as LED's are, I say experiment with different colors......if there is a difference, you may find one that fits your tonal requirements/expectations better than a red.

G

Edit: a quote from Merlin....."A red LED will be suitable in most cases, providing about 1.6V bias, although other colours can be experimented with of course."

At the bottom of this page>>>> http://www.valvewizard.co.uk/trem1.html

[jufa82]

Thank you for your answer!

I was just wondering if any of the colours would affect on the amount of the deepness in the intensity even more. At the moment I have a white led there because the voltage drop is bigger on white leds than the others but I do not notice much of a difference from the red one in the sound but I don't even know if it really matters anyway so I guess I should try the green one next  I'm trying to achieve the chunkiest cut off I can ever get.

Best regards!!

[sluckey]

At the moment I have a white led there because the voltage drop is bigger on white leds than the others but I do not notice much of a difference from the red one

I played with Radioshack's bag of assorted LEDs. The one's that have a voltage drop of 1.6 to 2 volts gave the most LFO output signal, but they all worked fairly well. I rejected the green (about 3.5v if I remember correctly) because it had less light output and would not work properly at LFO frequencies below 4Hz.

I now have a variety of different colors that are not the typical diffused colored plastic lens. The plastic lens is actually clear and the color of the light is really red, green, blue, white, yellow, etc. I like the looks of these better because they look more like an incandescent bulb. Just a personal choice.

[jufa82]

First of all thanks again Steve!!

How can I measure the voltage drop myself to be exact? Should I just put the DMM between ground and the pin 3 of the oscillator tube and measure the DC voltage there? (I made the 6G16 Vibroverb tremolo circuit to my '73 silverface vibrolux reverb)

I've read about these led voltage drops in here:
http://en.wikipedia.org/wiki/Light-emitting_diode

If you scroll down the page one quarter there comes this diagram about the voltage drop and etc. According to this diagram the red one should be the best choice am I right? And the white one I have there is the one with the strongest voltage drop. Uups.. 
By the way one other concern:  I increased the negative feedback loop resistor from 820ohm to 1500ohm and noticed a louder hiss from the amp at idle. Is this typical or have I done something else which I haven't notice? If you know the answer please enlighten me? I've spend two months now trying to make this amp a litlle less noisy at idle, especially with the output tube bias vary tremolo but I just can't get it absolutely silent without that thumping but I think that's not the case anyway with these oldies goldies.

Thank you very much!

[sluckey]

How can I measure the voltage drop myself to be exact?

One meter lead on the cathode, the other lead on the anode. IOW, directly across the diode.

I increased the negative feedback loop resistor from 820ohm to 1500ohm and noticed a louder hiss from the amp at idle.

You decreased the amount of NFB which increases the gain of the amp. As gain increases, noise will also increase.

I just can't get it absolutely silent without that thumping

Put a diode across the INTENSITY pot. There's more info about this in the thread "Is your tremolo too weak?"

[jufa82]

Thank you so much Mr. Sluckey!
Sorry my bad yes I mean't decrease of course.
I measured the led(diode) and I got 2.6V across it. When tremolo is on, the readings of course vary (mine from 2.06V to 2.6V) and depending on the speed setting too. I got a little confused with the voltage drop term meaning, that if the voltage across the diode is 2.6V, is that directly the same thing as the voltage drop? Sorry for the stupid questions here. I',m getting so close here but still far away 

Best regards!!
Jussi