Setting up a bias type tremolo circuit

[WimWalther]

Thought this might deserve its own thread.

What are the parameters / goals when setting up bias modulation tremolo circuit? What is the ideal voltage range?

One thought would be to find the delta-V for cutoff (maybe it's -15V) and then set the osc output to 2X of that figure P-P (so 30V P-P in this example). Of course, this might also be enough range for grid current.. grids driven positive.

Is there a RoT for this?

OT: While we're here, is anyone competent with Photoshop / Illustrator / GIMP or even recent versions of MS Paint? Well I'm not, but I need a small little task done.. should take all of 15 min.

[Jonas]

I might be able to help.... what you need?

[sluckey]

Is this related to your 6G2 with EL84s and busted trem?

[WimWalther]

Is this related to your 6G2 with EL84s and busted trem?

Sure, good working example. The trem oscillator is set up for 6V6 grid voltages, but I'm running EL84s. The latter runs with g1 at ca. -15V, where the former expects.. not sure, -35V?

Do we happen to know the amplitude of the stock trem osc? I don't have access to my scope atm, nor do I own a peak-hold meter.

Oh, and the trem's not dead. A disused ground wire had worked underneath the board and was shorting the osc to gnd. But now the trem signal is showing up as driver throw even with intensity at zero.

[Bieworm]

You might want to tweak the intensity pot value. On a build where I used EL90 substituting 6V6GT I had to put in a 50k pot instead of the 250k.

[sluckey]

Do we happen to know the amplitude of the stock trem osc? I don't have access to my scope atm, nor do I own a peak-hold meter.

That circuit will typically have 150V_pp @5Hz on the plate. That's 53V_rms. I can use my Fluke 87V to measure frequency and AC volts. Which meter do you have?

As you have found the trem signal is too high for EL84s. All you need to do is pad it down. Replace the circled 220K with a bigger resistor. I'd experiment with 1M and go up/down until satisfied.

[WimWalther]

I have a handful of different meters, no Fluke 87V though. But come to think, I may have one capable of doing AC volts & freq down into the low Hz.

Have to see. It's not the sort of thing I normally ever need to measure.

[WimWalther]

So I'm taking another look at the 6g2 tremolo circuit, and I realized that unless the foot switch is connected, there's no way to completely shut off the tremolo. Even with Depth set to minimum, there will always be some oscillator signal present in the output.

Why did they do this? Adding an integral switch to the Depth pot would fix it..

[sluckey]

So I'm taking another look at the 6g2 tremolo circuit, and I realized that unless the foot switch is connected, there's no way to completely shut off the tremolo. Even with Depth set to minimum, there will always be some oscillator signal present in the output.

No, that's not how it works. Point "Y" is at AC ground due to the 25µF bias filter cap. When the pot wiper is rotated to the right (max CCW) the wiper is connected to AC ground. No trem signal can get to the grids as long as that filter cap is working properly.

Did you even try my resistor suggestion.

[tdvt]

Following this & it has raised some questions for me, relevant but not exactly on-topic. Sorry for the hijack.


I was thinking about adding a LarMar MV to a Hoffman AB763 I built (6G16 bias trem), but that seems like a lot of bias manipulation & wonder if that MV circuit is even compatible with bias trem..?

The second thought was has anyone experimented with or adapted the Fender LFO to manipulate a pre-amp tube cathode on something bigger than a Champ? 

[WimWalther]

No, that's not how it works.  (...)

Ok, I guess I see that now. Sooo.. now I need a new explanation for how the tremolo signal is appearing at the output when the Intensity control is zeroed.

First step will be fixing the amplitude issue, and go from there.

Did you even try my resistor suggestion.

Haven't done anything with it in a couple of days. Health issues, just not feeling it.

[sluckey]

Ok, I guess I see that now. Sooo.. now I need a new explanation for how the tremolo signal is appearing at the output when the Intensity control is zeroed.

First step will be fixing the amplitude issue, and go from there.

The explanation is that 25µF filter cap is not a perfect zero ohm AC ground. At 5Hz that cap looks like a 1.2K resistor. Put that 1.2K in series with the 250K pot and 220K resistor, do a little voltage divider math, and you will see that a wee bit of the strong trem signal always gets through, even with the pot set to zero. That wee bit has no audible effect on 6V6s but it does on the sensitive EL84s.

The solution is simple. You know what to do. Oh, one other thing... Add a 0.1µF cap between ground and the junction of the 220K and 0.1µF, like the 6G16 circuit. This will smooth out the LFO signal and also reduce the amplitude, both good things for your EL84s. Reducing the LFO signal is your path to happiness.   

[WimWalther]

The solution is simple. You know what to do. Oh, one other thing... Add a 0.1µF cap between ground and the junction of the 220K and 0.1µF, like the 6G16 circuit. This will smooth out the LFO signal and also reduce the amplitude, both good things for your EL84s. Reducing the LFO signal is your path to happiness.   

I took a look at the 6G16. Are you referring to the 0.1 to gnd at the junction of the 470K & 0.1 feeding the 250K Int. control?

[WimWalther]

Spent some time on this last night.

Subbing 1M for the 220K feeding the Int. pot cut the osc amplitude down to ~10VAC (rms). Osc range reads between 5-10Hz.

Its worth noting that these low freqs. are outside the published specs for my meter, so accuracy is not assured.

Trem signal is getting into the front end somehow. Not sure if it's being picked up by the wiring or if the PS is being modulated. Need to investigate this.

There's also some popping with Int set high.

[sluckey]

Post some hi-rez pics of your amp. Also post voltages for all filter caps and every tube pin, even the pins that read zero volts.

Are you using all new filter caps?

[dwinstonwood]

There's also some popping with Int set high.

I'm almost certain that is lead dress. My AC4 did the same thing with Intensity dimed. Except mine sounded more like ticking, not popping. I pushed some wires around until it was completely silent.

[WimWalther]

There's also some popping with Int set high.

I'm almost certain that is lead dress. My AC4 did the same thing with Intensity dimed. Except mine sounded more like ticking, not popping. I pushed some wires around until it was completely silent.

Interesting, thanks for that. Once I solve how the trem is getting into the front end, it may fix the popping as well.

Guess we'll see. ;-)

[WimWalther]

Post some hi-rez pics of your amp. Also post voltages for all filter caps and every tube pin, even the pins that read zero volts.

Are you using all new filter caps?

I'll see what I can do, thanks.

PSU caps are all original, though they did test just fine. There are 4x 50uF sections available, 3 of which are in use. There is also a 64R 10W from pin 8 of the rectifier to the 1st cap section (serves EL84 plates & Pi).

[sluckey]

There is also a 64R 10W from pin 8 of the rectifier to the 1st cap section (serves EL84 plates & Pi).

That first cap should feed the EL84 plates and the trem osc. plate, not the PI.

This brings up something I was going to suggest back when I drew up the layout. The trem osc should really get it's B+ from node B which feeds the output tube screens. Node B is much better filtered than Node A. Node A has very high ripple and should only be used on a push/pull output circuit because of the common mode rejection cancellation of ripple. The trem osc has no such ripple cancellation. Additionally, having the trem osc on the plate node "may" be part of the trem issue you are having. All the other Fender tremolo amps use the screen node to supply B+ to the trem osc.

Look at the layout and find the red wire marked A that connects to a 220K on the board. I suggest connecting this wire to the Node B filter cap.

[WimWalther]

@sluckey

I see your post above and will respond when I have time to really absorb it. For now, I did go back and re-test the filters. Here's what I found:

All (4) 50uF sections, of which (3) are currently in use, measure between 48.9-52.4uF. ESRs range from 0R8-1R2. All fine, totally serviceable.

ETA: I misspoke. The PI runs of node C along with both gain stages, per the Fender schematic. The EL84 plates & screens run off of nodes A & B, respectively.

Do you still suggest moving the PI to node B?

[sluckey]

Do you still suggest moving the PI to node B?

I never suggested moving the PI. I SUGGESTED MOVING THE TREM OSC TO NODE B.

[WimWalther]

I never suggested moving the PI. I SUGGESTED MOVING THE TREM OSC TO NODE B.

My mistake, it was very late.

[WimWalther]

Photo as requested. This is still a WIP, a few items are just tacked and lead dress / routing is tentative. Need to replace the input jack arrangement as well as install a shielded cable.

[WimWalther]

Full set of voltages, by request. Apologies for the delay, I'm just not getting much done these days.. =/

Tubes are numbered L to R, the 6E2 indicator (V1) is being skipped for these purposes.

V2 - 7025 / 12AX7
1) 174 VDC
2) ~0 VDC
3) 1.26 VDC
6)157 VDC
7) ~3  mVDC
8) 1.33 VDC

V3 - 12AX7
1, 2, 3) Unstable (LFO)
6) 206VDC
7) 18.5 VDC
8) 58 VDC

V4 - EL84
1) -15 VDC
2) -15.5 VDC
3) 57mV
6) ~150mV
7) 327 VDC
8) 280mV
9) 324 VDC

V5 - EL84
1) -15 VDC
2) -15 VDC
3) 55 mVDC
6) 230 mVDC
7) 328 VDC
8) 50 mVDC
9) 325 VDC.

V6 - 5Y3
1) Unstable mV
2) 338 VDC
3) -34.8 VDC *
4) 317 VAC
5) -36 VDC *
6) 317 VAC
7) Unstable mV
8) 340 VDC
2 & 8) 5.4 VAC
(* = Bias supply connections)

Power supply nodes:
A) 332 VDC
B) 322 VDC
C) 263 VDC
Bias) -15 5 VDC
6.3V fils) 6.67 VAC

EL84 Ik (pair) 52.5 mADC

AC Line) 118 2 VAC

The 6.3V fils are running a touch higher than I'd prefer.. not crazy, but I like to see them in the 6.3 to 6.5V range.

V3 pins 6 & 7 seem a bit off as well, need to take another look at that area. I ran into issues with that tube in the V2 position, so I swapped it to V3 as a test. Seemed to work ok there, so it was left. But it may have a problem..

[sluckey]

V3 voltages look good to me. Most meters cannot measure the grid voltage of that hi-z bootstrapped bias cathodyne. Better to measure the voltage at the bottom of the 1M resistor because that will be the real voltage on the grid (pin 7). BTW, any bootstrapped biased circuit such as this cathodyne, or cathode follower, or LTP PI will give false grid voltage readings when using a 10M input Z meter.

Do you have the tremolo issues under control?

[WimWalther]

V3 voltages look good to me. Most meters cannot measure the grid voltage of that hi-z bootstrapped bias cathodyne. Better to measure the voltage at the bottom of the 1M resistor because that will be the real voltage on the grid (pin 7). (...) will give false grid voltage readings when using a 10M input Z meter.

Right, I see that now. But I can't quite explain the 18.5V reading. With the 1M resistor + 10M meter, doesn't it form a 10:1 divider, which introduces a 10% error? Or is the presence of the meter also disturbing the grid voltage?

Guess I'd expect to read ~50V where it's actually 55V.  not 18.5V

Do you have the tremolo issues under control?

Not yet. That's all still where I left off, and needs a fresh look. Like I said, just not getting much done. =/

[HotBluePlates]

... I can't quite explain the 18.5V reading. With the 1M resistor + 10M meter, doesn't it form a 10:1 divider ...

No, not a divider.  But if the circuit normally has a 9MΩ input impedance (due to "bootstrapping" in the "Biasing" section here and in the "Biasing" section here), then adding your 10MΩ meter loads the input circuit down to 4.7MΩ.

[PRR]

> doesn't it form a 10:1 divider, which introduces a 10% error?

Which the tube then amplifies.

[WimWalther]

@PRR

And R6 is standing in for the Zin of the meter?

[HotBluePlates]

And R6 is standing in for the Zin of the meter?

Yes.