Bias vary trem & Fixed bias - what optimum combination?

[tubeswell]

My latest build is a 5F6A with a 6G16 trem circuit (and a reverb) - schematic attached.

I've got most of the 'trouble' out of the build, now I just want to 'shoot' it .

Anyway I decided to start a new thread cos the one on eth debugging BB was getting too long to follow, and this focusses on a new aspect so...

(Reverb aside for the moment) the trem is not very intense, and I am wondering what to do about getting a more hypnotic intensity out of it. (I love the way my 5G9 sounds - granted its a different amp with different tubes and different trem circuit, but I figured that the 6G16 trem circuit would've been about right for a 5F6A running 6L6GCs to get a real shimmery intense trem - correct me if I'm wrong, something is obviously not as I expected)

The output tubes are JJ 6L6GCs with plate voltage of about 444, screen voltage of 441, and tube current of 34mA. (470R screen grid resistors)

The trem circuit LFO plate voltage is 210 (cathode voltage 2.28) and the CF stage has a plate of 440, grid of 210, and cathode of 217.

1) So can I bias the 6L6s in such a way as to increase the intensity of the trem?

2) Or do I have to start pulling bits out of the trem circuit? The 6G16 circuit has a 22nF cap from the plate to the cathode of the LFO stage, and I built this in to my amp. (I guess this is some kind of intensity bleed cap?) Would this cap be better out? or reduced in capacity?

[txstrat]

The Deluxe Reverb II circuit has a 500pF cap between plate and cathode to prevent the circuit from oscillating. Don't know if the .02 cap in the 6G16 circuit is there for the same reason. The Vibrolux 6G11 has no cap at all at that place, a different value resistor before the intensity pot and no cap (.1) to ground. You might have a look at the schematic. All other values seem to be equal at a quick glance. I'd try to change the specs for 6G11 values and see what happens.

[tubeswell]

Okay I'll try a range of cap values in there. Thanks txstrat

What bout the bias? 50% too cool? (and if they were hotter, would that in theory make the trem wiggle more effective? - due to the grid voltage swing being closer to zero, in order to drive the tubes into cut-off?)

[txstrat]

You remember my 6g3 clone?
I biased the tubes in that amp at 70%. I think with the bias vary trem ON, the tubes get into hotter areas but I wouldn't bother, since the tubes can stand 80% as permanent setting as well (OK at least the JJ 6V6s I chose). And 6L6 are sturdier than 6V6 from their specs alone.

[tubeswell]

Okay I experimented progressively going from the 22nF to a .01uF, then a .0047uF, then finally a 500pF cap between the plate and the cathode of the LFO, but I couldn't tell much difference. Then I rebiased the output tubes to 17W and I think that has increased the intensity a wee bit. Still not the same as my 5G9 tho'. So what else can I try?

[txstrat]

Following the signal path from the cathode of the 12AX7 to the depth pot. Right after the 470k resistor is a .1 cap to ground (another .1 leading to the pot). I would lift it on one side to see what happens, since it might shunts some of the signal to ground.

[Tubenit]

increasing trem

You might find some useful information here:

Hoffman AB763 Tremolo 6L6 mods?

I would experiment with lowering the 470k resistor to 220k. I'm refering to the 470k between the 470k cathode resistor & the .1uf/.1uf juncture.

With respect, Tubenit

[MWJB]

Regarding bias, often running the bias a shade cooler will affect the intensity of the vibrato, see what happens around 28mA per tube.

[tubeswell]

You might find some useful information here:

Hoffman AB763 Tremolo 6L6 mods?

I would experiment with lowering the 470k resistor to 220k. I'm refering to the 470k between the 470k cathode resistor & the .1uf/.1uf juncture.

With respect, Tubenit

I lowered the 470k in question, and it is more intense, however when the speed pot is on any setting under about "4" a clicking sound (in time with the Tremolo) starts up (and goes away when I speed it up again). is that something to do with the LFO waveform and the output tube grid bias?

(Also, in deference to MWJB - I don't see why increasing the negative bias voltage would make it more intense. Can you explain that please?)

Despite this, it is still not as intense as the 5G9 trem, and it must be able to get more intense

[MWJB]

"(Also, in deference to MWJB - I don't see why increasing the negative bias voltage would make it more intense. Can you explain that please?)" - a more noticable swing from low to high current (being a bias trem, this is how it works, if it made no difference, you wouldn't get any trem).

[pdf64]

The 470k plate resistor on the oscillator section might be limiting the voltage swing, ie amplitude.
Most of the old designs show it as a 100k or 220k.
And the cathode resistor of that stage usually smaller than 4k7, try 3k3 - 1k5.
And the cathode resistor of the following buffer as less than your 470k, try 220k.
The aim is to get the biggest signal out of that oscillator / buffer to the intensity pot, so check out on the scope, or with meter, the effect of each change and combination of changes.
So as above, reducing that series resistor to 220k will help, but also the DC blocking cap could be increased in value to let more oscillator signal pass - a 1uF film cap would help, or just parallel up some 0.1uFs.
A low pass filter somewhere in the oscillator signal path would help get rid of the click - maybe a cap between the plate and ground of the oscillator, and / or re-instate a cap to ground on the hot end of the intensity pot. The exact value wil be a compromise between the amplitude of the oscillator signal and the click.
The click might be an artifact of the oscillator or buffer going into cut off, so biasing them a bit hotter, as suggested above, might help anyway.
Peter.

[tubeswell]

Okay I took out the SS rectifier and put the GZ34 back in. This brought the B+ down and the 6L6 plate voltages are now about 420 with the tubes biased to 35mA. This has opened up the LFO a bit more. And the amp has a nicer tone. (Only thing is now I'm getting some unpleasant kind of intermittent static buzzing on the bass notes and as I turn the vol up - is that crossover distortion?)

I changed the 470k series resistor to 220k and that opened up the LFO a bit more too.

I'm just looking at the Tremolux 6G9B circuit and I see it doesn't have a 0.1uF to ground before the blocking cap to the depth pot, but has a 0.05uF cap from the depth pot wiper to ground. Would that also open it up more? It also doesn't have a cap between the plate and the cathode of the LFO. Why is that 0.02uF cap there in the Vibroverb 6G16 circuit?

[MWJB]

"(Only thing is now I'm getting some unpleasant kind of intermittent static buzzing on the bass notes and as I turn the vol up - is that crossover distortion?)" Unlikely at 35mA, you could even go lower (28-29mA) without crossover disortion becoming a problem at moderate volume settings.

[Merlinb]

It also doesn't have a cap between the plate and the cathode of the LFO. Why is that 0.02uF cap there in the Vibroverb 6G16 circuit?

The various caps to ground (before or after the depth pot, or between anode and cathode) are all there to make the trem sound 'smoother' by filtering out higher harmonics from the trem wave, making it more like a proper sine wave. They're not essential, and as you have found out, they usually don't make a lot of difference to the trem sound. You can build the circuit without them all, and then try adding one before or after the depth pot to see if it makes any useful difference.

[tubeswell]

"(Only thing is now I'm getting some unpleasant kind of intermittent static buzzing on the bass notes and as I turn the vol up - is that crossover distortion?)" Unlikely at 35mA, you could even go lower (28-29mA) without crossover disortion becoming a problem at moderate volume settings.

Thanks MWJB

Actually after having read Chapter 2 in Merlin's book, I wonder whether maybe its blocking distortion. I don't have a scope to see for sure, but now I am armed with equations , so I'll take new voltages again and try and figure out mathematically where its coming from. Come to think of it, I have a 220uF bypass cap on the reverb recovery stage - maybe putting a 22uF one there will help it (but that stage is somewhat loaded down with the reverb tone control, and I even get the distortion with the reverb recovery entirely mixed out). Maybe 1k5 grid stoppers on the output tubes would help too.