6G11 bias-vari trem

[worth]

You know... I don't get it. This sought -after 6L6 trem only sounds good when the amp is over biased at like 20ma... then the amp sounds horribly thin. Did Fender send these amps out over biased like this as a compromise between good trem, and passable tone ?!!

[phsyconoodler]

Are you sure everything else is working properly in the amp,like the oscillator tube,all the voltages are correct and your bias supply and pots are all in top condition?

[worth]

Yes.. I've experienced this many times with 6L6 trem. I bias the amp up to 45-50 ma. , and the trem just about dies away .. then I'll drop the bias to 20-25 ma... SHAZAAM !! .. great trem, horrible tone. And yes.. I'm aware of all the trem mods.

[HotBluePlates]

You know... I don't get it. This sought -after 6L6 trem only sounds good when the amp is over biased at like 20ma... then the amp sounds horribly thin. Did Fender send these amps out over biased like this as a compromise between good trem, and passable tone ?!!

When "overbiased" is your bias voltage near -45v, like the schematic?

What comes to mind is to get a good trem effect, you need to be able to turn the tube off and on to get a good impression of depth.

At first, I used to think, "well, obviously we don't have enough output from the trem circuit" and hence the mods to try to increase the size of the trem signal. If you look close enough, there's a voltage divider in addition to the Intensity pot (hint: it uses resistor and caps).

Continuing that thought, you'd think that the trem wouldn't work as good with the bias voltage large (tube current low) because the trem signal is working against the bias voltage, and a big bias voltage seems like it would swamp the trem signal, right? So shouldn't the trem work better with the tube running with a small bias voltage (and more tube current)?

So I'm thinking there's some interaction with the size of the bias voltage and size of the trem signal. I don't know if the trem pushes the tube grid up near 0v and the tube is unable to respond to all of its swing, or that the trem works better when the tube is down near cutoff.

It would be helpful to have a 6L6 bias-vary amp on hand and measure the bias voltage, size of the trem signal, instantaneous voltage on the tube grid and instantaneous tube current (with a scope via a 1Ω resistor). Something is going on that we're not picking up, and if you figure that out, you'll know what factors to balance to get sweet trem with your ideal bias.

[tubeswell]

What HBP said, you have to have sufficient voltage swing in the LFO to force the output tube into cutoff. The hotter the output tube is biased, the more that its tube current 'fights' the trem wiggle, so the bigger the LFO swing at the grid needs to be to get the slammy effect.

[phsyconoodler]

I have that issue with Princeton Reverb circuits as well,so I mod them to Traynor bias vary systems and they work at any bias setting.
  Fender made that amp in the 50's so don't come down too hard on him.Just find out what's happening and fix it or mod it.
  I have built a few 63 vibroverb's and they respond nicely at high current's.Maybe a look at that circuit might spartk up something to help you?

 You do have to remember that when using the bias-vary trem the upper current limit needs to be monitored so you don't have a huge current swing,or the tubes will suffer and possibly red-plate.

[HotBluePlates]

Look at the 6G16 Vibroverb schematic.

The trem circuit has 2 stages: the oscillator and cathode follower. Between the cathode follower output and the Intensity pot, there is a 470k resistor and 0.1uF cap to gorund, then another 0.1uF cap to the Intensity pot.

The 470k resistor and 0.1uF cap to ground form a voltage divider, before you even get to the Intensity pot. In the 6G11 Vibrolux, these parts are 220k and a 0.05uF from Intensity pot wiper to ground. These items are not present in the Traynor bias-vary tremolo circuit (to be fair, the YSR-1 I looked at also uses EL34's, which would be easier to trem than 6L6's).

To be fair, the voltage divider serves a purpose. In the AA1164 Princeton Reverb, there's a series 1M, then a 0.02uF to ground, then a series 0.1uF then the Intensity pot. 1MΩ and 0.02uF are -3dB at 7.9Hz, which dumps any noise that's a higher frequency than the actual trem signal. 1M and 0.1uF are -3dB at 1.6Hz, so there is some loss of trem signal, but not too much. If you're really concerned, you bump up that 0.1uF cap.

Maybe it's just me, but I don't see any big differences in the bias-vary trem used on the 6G16 Vibroverb and the 6G11 Vibrolux, outside of placement of the cap to ground (the different values result in a -3dB point of ~14Hz, which should give a stronger trem than the Vibroverb). You can try axing those parts, a la Traynor and see if you get excessive noise from the oscillator.

Also, I've never personally had a problem with too little trem in any Princeton or Princeton Reverb. I should probably caveat that by saying the only ones I messed with were black- and silver-face models rather than new builds.

[tubeswell]

You'll also notice that the fender schematic for the 6G11 shows the 6L6GC at a low-ish 360 on the plates - quite low for a typical 6L6GC voltage. What voltage are your ones running at worth?

[worth]

My 6G11 plate voltage is around 400v. The weak 6L6 trem is also present in my Vibroverb.. I like to bias a little under 70% plate dissapation.

[tubeswell]

What is the current rating on your 5V winding? I'm wondering if you tried a 5U4G in there, maybe it might lower the voltages a little? (or not?)

[HotBluePlates]

I like to bias a little under 70% plate dissapation.

What's the voltage at pin 5 of the 6L6's in your amps?

[worth]

Pin 5 voltage , after the 1.5K grid resistor, is 37.9V.

[tubeswell]

Higher output tube current does result in weaker trem with bias-vary trem.

Is there a need to bias at 70%? or would (say) 60% (or 62%?) suffice?

[worth]

It seems to take much less than 60% plate dissapation to get the trem working at it's optimum.. so I can either have an OK trem mixed with an OK tone , ( cold bias ) or a GREAT tone, ( 70% plate dissapation ) with bad trem.. So.. I choose to ditch the trem. I want my car to GO when I press the throttle.. I'll give up air-conditioning , power windows , and emission controls for that feeling. Ill use another amp that's set-up just for great trem when I need it. --thanks for the replies !

[Willabe]

I built a 5G9 tweed tremolux and have the 6V6's bias set at 35mA each. Amp and trem sound great.

These amps do need a strong LFO/CF tube for the trem to work.


                 
                  Brad     

[HotBluePlates]

Maybe then measure the size of the a.c. output from the oscillator when the trem is on.

Your amp is not set wheer Fender designed it 2 ways: higher plate voltage and lower bias voltage, which suggests a double-whammy making the trem seem weaker.

I say that cause I remember playing a reissue 6G16 Vibroverb (back when Fender first did reissue amps; the 6G16 and standalone reverb were among the first reissues) and had no problems with weak trem.

so I can either have an OK trem mixed with an OK tone , ( cold bias ) or a GREAT tone, ( 70% plate dissapation ) with bad trem..

I must be the only guy that never heard the difference between good and great tone due to outputt ube bias. I mean, when biased hotter I hear a bit more midrangey sound (which I interpret to be more low-order distortion coloring the sound) and quicker breakup, but I can't say I've ever heard an otherwise good sounding amp turn into a dog due to cold bias.

And the cooler bias in Fender's original schematic was there to help hit the target output power goal; you'd probably get less output power (maybe not enough to matter) by biasing hotter.

[worth]

If you are a guitar player.. always listening to small circuit changes, then , you will hear the change , for the better, between an amp generically biased at , say 35ma. , or the same amp biased near 70% plate dissapation according to the outtubes installed in the amp in question. It's the difference between liking your amp, or deciding , to buy a different one.

[Willabe]

If you are a guitar player.. always listening to small circuit changes, then , you will hear the change , for the better, between an amp generically biased at , say 35ma. , or the same amp biased near 70% plate dissipation according to the out tubes installed in the amp in question. It's the difference between liking your amp, or deciding , to buy a different one.

Maybe for some, maybe even most, but believe it or not some do like their amp biased a little cooler.

Myself I have found that I can hear a difference in output tubes but had a hard time with the preamp tubes. Well at the time anyway. I should try it again and see if I hear a difference.

I tried 3 different matched sets of power tubes in my BF SR years ago. NOS GE 6L6GC sovtek 5881's and NOS Philips JAN 5881's. All set at 35mA.

I'll say that it was easier to hear the difference when I played through the amp than when my friend did. Don't know why. ( It was also easier to hear the difference in 3 sets of 4x10" speakers when I played through them as opposed to listening to my friend play through them, way easier.)

The sovteks sounded thin and flat, the GE's sounded clean but lacking harmonics, to me any way, my friend liked them best. I loved the Philips Jan 5881's best, by far. Not too, clean not too much breakup, lot more harmonics.

I have a 5E3 (cathode bias) and 5G9 (grid bias, 35mA) that I built. I have tried NOS RCA's and NOS Philips JAN 6V6's in them both. I don't hear much difference at all if any. Haven't tried any new production 6V6's in them yet.

Really didn't (at the time) hear much difference with different 12 _ _ 7.


               Brad                        

[sluckey]

I suspect that the tradeoffs between hot bias and good trem is what made Leo drop that trem circuit with the blackface era. Too many complaints about weak trem.

[Willabe]

I suspect that the tradeoffs between hot bias and good trem is what made Leo drop that trem circuit with the blackface era. Too many complaints about weak trem.

Trem ticking issues too? Or even PA problems, blow ups because it plays with the bias?

Maybe all 3? Lot of amps going out the door that could come back. The less problems the better.


                     Brad      

[HotBluePlates]

so I can either have an OK trem mixed with an OK tone , ( cold bias ) or a GREAT tone, ( 70% plate dissapation ) with bad trem..

I must be the only guy that never heard the difference between good and great tone due to output tube bias.

Bolded for emphasis. I didn't say I don't hear any tome change with bias, but rather that a good amp doesn't become unusable only with a bias change. I also firmly believe a lot of the tone change resulting from a bias change can be had by making other circuit changes.

I guess the difference is I don't place a lot of importance on bias beyond proper operation of the tubes. I generally check idle current to ensure the tubes aren't in danger of melting on one hand, or not passing any current on the other.

I also don't ascribe the to "35mA rule". I have a pair of 807's in my latest build which idle around 60mA or so. Whatever's right for the amp is right; I don't get worked up about bias.

But rather than go down the road of subjectivity (which doesn't fix your situation), I still think it would be educational to measure the output a.c. of the trem oscillator in light of your present bias voltage and figure how to give you the idle current/tone you want and the trem you wanted.

... Unless you've removed the trem circuit, in which case there's nothing we can fix for you.

[smackoj]

I owned a BF Sup Rev for thirty yrs. Back when I knew 0 about amps. I decided to sell it when I found out there was high demand, and the guy who bought it asked me, "does the tremolo work"?  I never even knew it HAD tremolo! I always just used the reverb. I told him I didn't know if it worked?  He changed some settings on it, played a few bars and said, yep, it works....I remember sorta hearing a trem effect but it was certainly nothing like the volume and presence of the reverb.

This anecdote is just what I know of the Leo trem circuit from those yrs.

[worth]

I've had guys say to me," I used to love the tone of my amp, but somehow the tone changed, and it just doesn't sound that good anymore. " Turns out, they get the idea that it's time for new output tubes ...put them in without re-biasing. I check the bias.. sure enough it's biased cold. After I reset the bias, they're in love with their amp again. I think some people CAN be a little "over the top " about subtle changes in tone.. I guess I'm one of them. I realise , though , that when your playing in a band situation, as opposed to playing by yourself at home, your probably not going to hear tone changes due to biasing anyway. HBP.. I haven't removed the trem circuit yet , just disconnected some connections... so I may give it another try.

[HotBluePlates]

Cool.. If you do choose to give it another shot, be sure to measure the a.c. output of the trem circuit, and let's compare it to the bias voltage on the output tubes. Maybe we can figure a method to get both good trem and good tone.