Cathode bias BF amp

[firemedic]

I'd like to convert my AB763 vibroverb (doug's version, power tube bias vary trem) to cathode bias.
Is it just a matter of disconnecting the fixed bias circuit from the power tube grid leak resistors, and cathode biasing as per usual?
That is, is the VV tremolo circuit compatible with a cathode bias setup? Or would it need modification?
 

[sluckey]

Probably. Don't forget to ground the intensity pot.

[firemedic]

Where the bias voltage was connected, right?

These may seem like simpleminded questions but the truth is I don't understand how an oscillator circuit works.

In case it wasn't real obvious.

[Fresh_Start]

I don't understand exactly how the oscillating tube circuit works either, but the output is a small AC voltage.  Superimposing that AC voltage on your DC bias voltage diddles the power tubes' bias so that their output goes up and down.  The capacitor network between the plate and the grid of the oscillator creates a phase shift, if I'm not mistaken, and the "Speed" pot varies the frequency resulting from the phase shift.

Anyway, if you ground the 220K grid return resistors for the power tubes (where the bias voltage used to be applied), connecting the wiper of the Intensity pot to that same point would simply ground the output of the oscillator circuit... I think.  However, sluckey probably was suggesting grounding the end of the Intensity pot which used to be connected to the bias supply, leaving the 220K grid return resistors connected to the wiper of the Intensity pot.  That way, with the wiper at the grounded end you have no tremolo signal on the power tube grids.  As you turn the Intensity pot up, the DC voltage on the power tube grids will jiggle around "zero volts" (a.k.a. "ground) and give you tremolo.

I'm not 100% confident that this is correct but hope it helps.

Chip

[firemedic]

That sounds about right.
I haven't even tried warming up the fixed bias yet, I'm gonna at least try that before doing anything more drastic. My drummer is LOUD & I need all the wattage with headroom as I can get. tubenit & sluckey have suggested a couple of tone mods as well. Will post with results......

[sluckey]

However, sluckey probably was suggesting grounding the end of the Intensity pot which used to be connected to the bias supply, leaving the 220K grid return resistors connected to the wiper of the Intensity pot.

Yes, that's what I meant. Just too lazy to type it all.   

My drummer is LOUD & I need all the wattage with headroom as I can get.

Then you'll probably squeeze out more clean power by leaving the fixed bias in place, ie, don't convert to cathode bias.

[firemedic]

Yeah, but tone comes first. I wound up doing most of the PA mods tubenit suggested & it helped but not enough.
So I converted to cathode bias & that's the sound I was looking for!
Wooooooooooooooooooooo!

The trem is not very dramatic, it may be something to revisit soon.
I'll post a schem ASAP, I may return to some of the original PA values since there is less bass now also. But it's a better starting point.

[topbrent]

Convert the trem over to the vibrochamp style and you will have plenty of trem on tap.  It wiggles the cathode bias of your preamp tube, not power amp and it sounds great.

The VibroChamp trem is similar in design, but it uses fewer parts on the second half of the circuit, first half of the circuit is identical.

Pull up the schem/layout for both the 6g16 and the vibrochamp and you should probably be able to convert over without too much hassle.

[sluckey]

The trem is not very dramatic, it may be something to revisit soon.

If you built this strictly by Doug's AB763 layout, you're missing a 22uF cap on the cathode of V5A (pin 3). The cap should be parallel with that 4.7K that connects to V5-3. Putting the cap there will really pump up the tremolo strength.

Look at this pdf. There are some more notes about trem included on page 2.

http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

[Fresh_Start]

sluckey's notes were a HUGE help when I built a Super Reverb with the bias-vary tremolo.  I changed the 470K resistor to 220K (note 3) and the .1uf cap going to ground to .047uf (note 4) to get a strong enough signal for 6L6s.  Although that amp was fixed bias, I'm sure making those changes will boost the tremolo signal.

HTH

Chip

[firemedic]

When I built the VV I used the Hoffman values, including all the changes to deepen & slow the trem, so something else is amiss. Basically I disconnected the bias wire from the intensity pot & left it taped off, then grounded the pot.
Perhaps I will change to a preamp trem just to simplify the power section; I want to do a fixed/cathode bias switch but I'm not sure how to do it with the bias trem arrangement. Any ideas?

[PRR]

> I don't understand how an oscillator circuit works.

Nothing-much to do with the oscillator.

> Where the bias voltage was connected, right?

Exactly. With "fixed" bias the grids must connect (via resistors) to a negative 25 (or so) volt supply, cathodes grounded. With "self" bias, the grids must go to a zero volt (ground) "supply", and the cathodes stand-up on 25V (or so) of resistance.

So where the grid network (in this case including a Depth pot) used to connect to -25V, you change that to connect to 0V.

[firemedic]

I think, intuitively & not based on any concrete facts, calculations, or know-how, that the reason the trem is not as strong with the cathode bias is that it must be self-correcting as grid voltage wiggles around.
Being as I'm installing a switch for the bias, I'll keep the trem as is and if I really need heavy tremolo I'll use the fixed bias arrangement.

[Fresh_Start]

I think, intuitively & not based on any concrete facts, calculations, or know-how, that the reason the trem is not as strong with the cathode bias is that it must be self-correcting as grid voltage wiggles around.
Being as I'm installing a switch for the bias, I'll keep the trem as is and if I really need heavy tremolo I'll use the fixed bias arrangement.

I was thinking the same thing! 

Do you have the 25uf cathode bypass cap on V5a?

If changing from 470K to 220K increases the tremolo signal, why not try dropping that resistor to 100K?  It's forming the "top" of a voltage divider with the Intensity pot.  Heck, jumper that resistor entirely and see what happens. (if you're installing a fixed/cathode bias switch, you may need to switch the value of this resistor)

The other thought I have is that decreasing the value of the cathode resistor of V5a should boost its output signal.  However, I don't know if that would have any OTHER effect on the tremolo signal.

Cheers,

Chip

[firemedic]

Here are the preamp & poweramp schematics. As you read this consider that it's just me & my drummer, no bass player. So I need more bass in my sound than most guitar players.
It differs from the BF circuit in several ways:

-The "normal" channel input tube is a whole 12AX7, paralleled triodes. 1/2 of the reverb send tube is hijacked for tonestack recovery, and fed to the V4b grid just like the reverb channel. It's pretty fat. Same # of stages = ability to bridge the channels at the input jacks. Very fat when I want it to be.

-Initially I lowered the bypass cap values a lot on the "reverb" channel but eventually compromised with selected 10uFs, also the reverb bypass caps are significantly smaller. Standard FMV stack.

-I tried a 0.0033 coupling cap to feed the PI but it got pretty muddy. Another compromise: 0.0022uF (instead of the original 0.001), makes it just very slightly muddy w/ the bass dimed, but otherwise the bass is very effective & deep but thins out nicely when turned down. In fact all the tone controls seem to have a better sound in a larger range of settings.

-I like 12AY7s. This amp sounds pretty tweedy at times, especially with cathode bias. Some folks don't much notice the difference betweed fixed & cathode but I sure do. The fixed bias still sounds real good though.

I'm sure I will continue to tweak this amp but here it is for now.

[Fresh_Start]

I think you want to re-wire that bias switch so that the 47uf filter cap for the bias supply is grounded on both ends when you select cathode bias.  IOW you don't need to break the connection between the bias supply and the Intensity pot for cathode bias - just ground it.  See attached.

Have you already tried a jumper across the 100k/220k/470k resistor leading to the Intensity pot with cathode bias?  That might be what you want to do with the second pole of your DP/DT switch (assuming you already have it).

BTW file names are reversed.  Also, you can have multiple "sheets" in a single SCH file, one for preamp & another for poweramp.

Cheers,

Chip

[tubenit]

I am not suggesting that you do this, but the nicest vibrato I ever got in an amp was a DanElectro Centurian which used a 6AU6 pentode.

Silvertone also used this with a cathode biased 6L6 amp.

http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1474.pdf

I built it into one of my amps and felt like it worked quite well with 5881's. Did not try it specifically with 6L6's.

With respect, Tubenit

[firemedic]

"I think you want to re-wire that bias switch so that the 47uf filter cap for the bias supply is grounded on both ends when you select cathode bias.  IOW you don't need to break the connection between the bias supply and the Intensity pot for cathode bias - just ground it.  See attached."

I thought I was avoiding any excess current draw from the bias circuit, by just hanging it in the air there.

Tubenit, do ya think a 5879 will work in the tremolo circuit? 

[tubenit]

Tubenit, do ya think a 5879 will work in the tremolo circuit? 

That's a great question!  Answer is that I don't know?  I bet Sluckey could answer that one. I don't remember any circuitry where I have seen a 5879 used for vibrato, but it might be out there?  My guess is it would work, but I don't know if it would for 6L6's?

On one of the amps I built I used a 5879 paralleled with a 6Au6 where I could pull either one. My impression is the gain was about the same. But my impression might not be even close to the tube spec sheets (which I really don't know how to read).

With respect, Tubenit

[firemedic]

It's kind of moot, I don't intend to change the trem really. I'd have to shuffle the power supply nodes & stuff to get the voltage down for a pentode; I have plenty of sacrificial chinese 12AX7s I don't mind putting 400+ volts on.