changing bias vari to the roach

[proaudioguy]

I am finally getting around to modifying my Hoffman Twin Reverb to use the BUG since the Tremolo just will not work consistently.  I can get it sounding great with a cold bias, but when you dime the amp, its gone, no tremolo.  The depth of the tremolo is directly related to both the bias, and how hard you drive the amp.  I want the Tremolo to just work no matter how loud I play it.


To this end, I am looking for a DIY LC file so I can work out how to layout the AB763 Tremolo parts on the existing turrets.  I got the boards 20 years ago and have all the documentation, but that was all PAPER back then. I have DIY Layout Creator.  It sure would be a lot easier to just open the file and delete the componants off it, check for turrets that may have moved compared to when I got mine, and then start fitting in the new componants, than to draw it all from scratch.  I looked all over the site and could only find PDFs but not the file itself.


Anyone have it?


proaudioguy at tigeraudioinc dot com

[sluckey]

Who ever dimes a TR?   

There is no DIY LC file. All of Hoffman's drawings were done with Visio and printed to pdf files that you see on the site. When Doug changed over to using Visio he may have slightly changed his original board layout. I would contact him and ask, otherwise just compare your actual board to his Visio format board to see if they are the same.

You can replace the 4.7K/25µF on the oscillator cathode to greatly increase the tremolo signal and that will probably give you good trem effect at all bias settings. Easy to do and worth a try before doing a more extensive mod.

I have a very slightly modified Hoffman AB763 layout/schematic that replaces his 6G16 bias vary trem with my Trem-O-Nator circuit. IMO the TON is much better than the bias vary trem of the Fender roach trem. Easy to convert ***IF*** your board matches Doug's Visio board. I can post that file if you are interested.

[acheld]

Anyone have it?

Doug has his DIY files here:   https://el34world.com/charts/DiyLayoutCreator/files/Hoffman_Diy_Files/Hoffman_Diy_Files.htm

Easy to modify.   And Sluckey's TON circuit works great.

Don't forget the earplugs when dimed!   

[kagliostro]

Thanks for sharing

I didn't know of the existence of those files

(despite I use Visio is interesting to know)

Franco

[sluckey]

Doug has his DIY files here:   https://el34world.com/charts/DiyLayoutCreator/files/Hoffman_Diy_Files/Hoffman_Diy_Files.htm

All I see are boards with turrets/eyelets. No components, tube sockets, wiring, etc. Great if you want to drill a board. Otherwise, not very useful.

[proaudioguy]

Who ever dimes a TR?   

There is no DIY LC file. All of Hoffman's drawings were done with Visio and printed to pdf files that you see on the site. When Doug changed over to using Visio he may have slightly changed his original board layout. I would contact him and ask, otherwise just compare your actual board to his Visio format board to see if they are the same.

You can replace the 4.7K/25µF on the oscillator cathode to greatly increase the tremolo signal and that will probably give you good trem effect at all bias settings. Easy to do and worth a try before doing a more extensive mod.

I have a very slightly modified Hoffman AB763 layout/schematic that replaces his 6G16 bias vary trem with my Trem-O-Nator circuit. IMO the TON is much better than the bias vary trem of the Fender roach trem. Easy to convert ***IF*** your board matches Doug's Visio board. I can post that file if you are interested.

Hey Sluckey, you and I and Tubenit and the others have been working on this Tremolo for more than 10 years.  Its not happening.  I’m changing it to the bug.  I experimented with the BUG on a cut piece of a fiber board above it at once point and it worked, btu the amp didn’t sound as good.  My compromise is just to put the switched intensity pot on there to switch the bug out of circuit.

[proaudioguy]

Its similar.  It would be awesome if the componants were there.  No worries.

[acheld]

It would be awesome if the componants were there.

Yeah, it would be helpful I suppose.  I use these boards as the beginnings of my own builds, which are usually heavily modified from Doug's standard builds.

Once I have a schematic, I start "building" the board in DIYLC, adding components along the way.  Having the components (including the tube sockets, pots, and other off-board bits) in place allows me to visualize the build as I go.   

[dwinstonwood]

One thing you can do in DiyLC is use the insert image tool under the Misc tab. Convert Hoffman's PDF layout to a JPG or PNG and insert (actual size). You can lower the opacity if you want, and simply position all of the components directly over the JPG. Then, you can delete the image and you're left with your own layout!

[proaudioguy]

I spent Sat night working on the layout, Sun night I went for it.  The amp sounds no where near as good as before.  Now it sounds like every other Twin Reverb you ever heard.  I am looking forward to getting the character of my amp back when not using the Tremolo, which is most of the time, by installing a 50KRA with a switch.  I wish I could do this with the footswitch but it would most certainly cause a big jump in volume.  I had to order the Switched 50KRA pot from Germany.  Its ridiculous that not a single US amp parts supplier has Alpha or CTS make these for them since they already make the 50KRA, and the SPST switched pot for the Champs and it should just be a matter of mixing existing parts.I’ve been looking for the part for years.  The part was cheap enough but ended up at $40ish with the shipping.  The Tremolo works great now.  Not quite as pleasant, but consistent as various volumes, and now I can bias the amp hotter.  I haven’t had a chance to dime the amp yet, but I’m sure it has a lot less drive going into the PI, but I’ll get that back when I install the switch..


This amp was my first ever tube amp, and I’ve been fiddling with it since the 1990s.  I learned most of what I know (which isn’t much) on this amp.  Forgive my mess.  For this modification, I used colors of wires I had on hand.  It was definitely prettier to look at before.  I used carbon comp resisters because I had them.  The orange drops are what I had on hand.
The only issue I had was a tiny bit of ticking.  The ticking was there even with the intensity off.  I moved 2 wires (after the photo was taken).  They are the yellow and brown that go down to the left side of the V5.  I moved them to the right away from the Normal/ Vibrato mixing resisters.  That appears to have solved it.

[kagliostro]

.... One thing you can do in DiyLC is use the insert image tool under the Misc tab ....

An image can also be imported in Visio, so, starting from a .diy file you can obtain an image and use the image to perform a Visio .vsd file

Franco

[EL34]

The DIYLC library has files you can edit and make your ownThere is AB763 with parts in the eyelet board section
There are components with sizes you can import
There's all kinds of useful stuff there you can use with DIYLC

[proaudioguy]

so I rebased the amp since I change the tremolo to one that will work at a hotter bias and it sounds a whole lot more like my amp now. In fact, it sounds so much like my amp that I feel stupid for spending $40 on a switched intensity pot that I may never install. I guess I’m gonna have to try lifting that leg just to see what the difference is. So the tubes were drawing 27 mA before I change the bias and that’s what sounded good and worked with the bias vari trem.  Now they are at about 46 mA. 70% is 49ma according to the RobRob calculator.


I have V1 out and only 2 old Sovteck 5881s and 1 JBLK120 in there.  Baffle has 1 speaker hole.  I can dime yhe amp without having to leave the room.  For clean, comfortable slightly compressed sound I have it at 6-7.  With 2 JBLs (or EVs) and all 4 tubes I could not be in the room with it.


MY QUESTION:


Why does the amp sound basically the same as it did before (slightly less OD tones) with the bias hot and the roach compared to the bias vari and a cold bias?


I had bought a quartet of blackplate RCAs for the amp for about $400 7-8 years ago and they just didn’t work for me “cold” so I resold them because these Sovtechs sounded better cold, which is where I had to be for the trem to function.  Will the blackplates sound even better now?  (Lol)

[Bieworm]

I think you won’t get your bang for the HUGE buck when swapping the Soviets for Yankees. There are a gazillion of stomps out there that will push the amp to that sweet spot you’re yearning for. I’m a lower watt amp guy, and 10 watts will get me to that spot with my Brutus amp. When I need to push it some more I engage my Mucho Boosto (Durham Electronics)
I will never opt for the roach trem. It’s not delivering my whooshy swampy oomph in any way. I go mosfet nowadays, just like the Tone King Imperial. Saves $$$ and sounds great!

[HotBluePlates]

Why does the amp sound basically the same ... with the bias hot and the roach compared to the bias vari and a cold bias?

The bias-vary trem circuit doesn't "load" the preamp signal; take this as our "baseline."

The opto-trem adds a 50kΩ Intensity pot wired to ground.  It is a heavy "load" that causes the ramp stage right before to deliver less output.  So signal-level going into the phase inverter is reduced compared to the baseline.

Now you've taken the scenario above & biased the output tubes hotter.  This reduces the negative-voltage at Pin 5 of the 6L6s, causing them to need less-signal before distortion.  While the 50kΩ Intensity pot effectively reduced-signal right before the phase inverter, the hotter-biased 6L6s effectively "increased-signal" at the output tube grids (by making same-signal have more effect).

... the Tremolo just will not work consistently.  I can get it sounding great with a cold bias, but when you dime the amp, it's gone, no tremolo.  The depth of the tremolo is directly related to both the bias, and how hard you drive the amp.  I want the Tremolo to just work no matter how loud I play it. ...

Didya know that some pedal-emulations of amp trem not only copy this "flaw" but emphasize how players can use it expressively?

eature=shared&t=504

eature=shared&t=201

eature=shared&t=242

[proaudioguy]

Why does the amp sound basically the same ... with the bias hot and the roach compared to the bias vari and a cold bias?

The bias-vary trem circuit doesn't "load" the preamp signal; take this as our "baseline."

The opto-trem adds a 50kΩ Intensity pot wired to ground.  It is a heavy "load" that causes the ramp stage right before to deliver less output.  So signal-level going into the phase inverter is reduced compared to the baseline.

Now you've taken the scenario above & biased the output tubes hotter.  This reduces the negative-voltage at Pin 5 of the 6L6s, causing them to need less-signal before distortion.  While the 50kΩ Intensity pot effectively reduced-signal right before the phase inverter, the hotter-biased 6L6s effectively "increased-signal" at the output tube grids (by making same-signal have more effect).

Hey there HBP,...This is what I thought but, there is a 47kOhm resister to ground in the bias vari trem to emulate the loading down of the roach intensity.The load is in pretty mucht eh same place in the circuit so there has to be more than just that.  Also, its not just that the amp is louder.  It may be, but I didn’t really have a chance to notice that.  Its a totally differetn sound. 


I built a Princeton, and that has a bias vari trem and it sounds fantastic.  It does not appear to suffer from being cranked.  I do keep the bias on that kind of cold as well, but it sounds good.
I have a buddy that says he disconnects the tremolo and just uses a pedal for that.  He prefers the sound of the amp with it disconnected.  I’m definitely going to try it.  Once this is all settled, I am considering the “cool mod” to the Normal channel, and then adding the ability to INSERT the normal channel between the Vib channel and the Reverb driver.

[HotBluePlates]

... there is a 47Ohm resister to ground in the bias vari trem to emulate the loading down of the roach intensity. ...

No, that 47Ω resistor is part of the feedback loop.  It does not do what you suggest.

I built a Princeton, and that has a bias vari trem and it sounds fantastic.  It does not appear to suffer from being cranked.  ...

That's a 2x 6V6 amp, and 6V6s use smaller bias voltages.  As a result, the same trem circuit slaps the 6V6 silly, where it needs a larger signal for 6L6.

Nobody ever complained about "weak bias trem" until they copied the brown Vibroverb's circuit of having bias-vary tremolo for 6L6 output tubes (notice how Fender didn't retain that for any 6L6 models when moving to the blackface era).

[proaudioguy]

Yes sir.  I get it.  I was on this forum 20 years ago when I discovered this circuit and started sourcing the parts for this particular amp.  I’ve been struggling with that tremolo ever since I got through the other issues.  Its a thing I d not touch for months and someitmes years at a time, so its taken a long time to get here.


I would love an explanation of signal flow to get my head wrapped around that.  I really love the one on the robrob site for the AB763.

[sluckey]

Hey there HBP,...This is what I thought but, there is a 47Ohm resister to ground in the bias vari trem to emulate the loading down of the roach intensity.

That resistor is 47K, not 47Ω.

[HotBluePlates]

That resistor is 47K, not 47Ω.

Thanks for that correction!  R30 here.

[proaudioguy]

Didya know that some pedal-emulations of amp trem not only copy this "flaw" but emphasize how players can use it expressively?

eature=shared&t=504

If ONLY my Twin Reverb sounded like the opening of this video.  I could not get it anywhere near that distorted before the tremolo was totally gone.  It was still clean.  I very much like how THIS works, but it just doesn’t work with the 6L6s.  Perhaps the oscillator has to be a lot stronger.  As stated many times over the years, there is a reason Fender went to the more expensive bug design on the bigger amps.


I wonder if it would have worked any better with the same PI design as the Princeton.


Btw, I don’t use any pedals.  I own a few, but they largely stay in the box.  I pull them out from time to time which equates to probably 2% of my playing.  Maybe that will change some day.  My nexxt project is to go back to the Deluxe Reverb II and get that thing to sound dead to nuts like the Deluxe Reverb.  I am awaiting a 100Ohm and 200Ohm 100W transistors to play with the SAG before I give up on that and drill a hole for a 6U4GB, and buy a power transformer..  While I wait, I am going to split the V1 in half and see if that thins out the strident low mids that seem to be a signature of this and the Concert of the same era (which I also own).

[proaudioguy]

That resistor is 47K, not 47Ω.

Thanks for that correction!  R30 here.

RIGHT, and I don’t se how that is part of the oscilator curcuit since it doesn’t even touch it.  I cannot find it anywhere in the 6G16.  It looks to me like a “sit in” for the 50KRA pot that is missing by modifying the AB763.  If not, I’d love an explanation of the signal flow where this. is not sending signal on the Vib channel to ground.  Calling it the Vib channel in this case is confusing since the Trem works on both channels in this design.

[astronomicum]

FWIW, if you feel like some adventure, you could try this. Can't say it is a rigorously tested design because I am unaware if anyone else tried it other than me, and if so, what their results may have been. If anything, may give you some ideas.

https://el34world.com/Forum/index.php?topic=30042.msg331016#msg331016

[tdvt]

FWIW, if you feel like some adventure, you could try this. Can't say it is a rigorously tested design because I am unaware if anyone else tried it other than me, and if so, what their results may have been. If anything, may give you some ideas.

I had saved your drawing when you posted, but not tried it.

I go mosfet nowadays, just like the Tone King Imperial. Saves $$$ and sounds great!

That said, though I lean hard to the tube-side, I would be open to trying a SS trem circuit.

Wondering if anyone has built a SS pre-amp tube bias trem, á la, Champ, Valco, etc. ?


Partially  answered my own question with a quick search:  https://el34world.com/Forum/index.php?topic=23811.0
Will have to look further, though at first quick glance these look like power-tube manipulation circuits.

[sluckey]

I built one back in the '70s but it ties into the signal chain just like an AB763. Could easily be adapted to vary the cathode of a preamp tube though.

     https://sluckeyamps.com/misc/76_Amp_Project.pdf

[d95err]

When the poweramp is pushed into clipping, the tremolo will effectively modulate the amount of distortion, rather than the output volume.

That can result in the tremolo effect disappearing when you hit the strings hard, and then fade back in when you let a chord ring out.

To me, that’s what makes tremolo such a cool effect.

[proaudioguy]

When the poweramp is pushed into clipping, the tremolo will effectively modulate the amount of distortion, rather than the output volume.

That can result in the tremolo effect disappearing when you hit the strings hard, and then fade back in when you let a chord ring out.

To me, that’s what makes tremolo such a cool effect.

I agree 100% but it didn’t work.  Get to 4 and it was almost gone, 6 it was gone.  So on 10 is never came back.  My Bias current was 27ma just to get it working.  I’m up to 44 now with the bug.