Opamp for fx loop

[pdf64]

'Connecting the heater supply to a DC voltage above ground as far as I know doesn't mitigate hum'

See Tech Info
Q: Why do some amps connect the center tap of the filament winding, or the junction of the two resistors off the filament string, to the cathodes of the output tubes?
A: In a cathode-biased amplifier, the cathode is at a positive voltage, somewhere around 10-40V with respect to ground. If you elevate the filament "reference" above the potential of the cathode by connecting the center tap to this point, you can effectively reduce the amount of hum coupled into the tube. This is because the filament is now positive with respect to the cathode, so the cathode doesn't attract electrons (i.e. hum) from the filament. This is a very inexpensive and easy method of reducing the hum in an amplifier without having to go to a DC filament supply. In a fixed bias amplifier, the output tube cathodes are usually at ground potential, so you have to add a voltage divider from the plate supply to generate the elevated filament reference. You can experiment with the voltage level to determine the value that best minimizes the hum. Be sure to bypass the junction of the resistors to ground with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply.

[flyingdutchman]

so the cathode doesn't attract electrons (i.e. hum) from the filament

It sounds halfway plausible, but I have strong doubts about this.
There aren't supposed to be any electrons going from the filament to the cathode, or vice versa, anyway. The filament in indirectly heated valves is supposed to be isolated from the cathode, and any leakage that will show up as hum on the usually relatively low impedance of the cathode would indicate a bad valve.

[Enzo]

Why are we required to use the heater supply for this op amp? Op amps will run MUCH better on 15v rails, and if this is a Peavey amp, it probably already has such rails in place. Why screw around? Peavey uses 30v (15v x 2) for the reverb IC on just about every amp they ever made with a a reverb. That Delta Blues used a transistor for the FX loop drive, but the reverb was straight op amp.

[loudthud]

The TI data sheet for the TL072 lists the common mode input range with +/- 15V rails as +/- 11V minimum, +15V to -12V typical. This is what happends when the negative limit is exceeded and the current source for the input diff amp is cutoff. The same thing can happen to a solid state power amp if you hit it with a big enough signal, like from a tube preamp.

[frus]

I tried elevated heaters on a few amps, but never got any better (nor worse) results than just grounding the center tap. I'd say try grounding it and see if there is any difference

[pdf64]

'There aren't supposed to be any electrons going from the filament to the cathode, or vice versa'
That doesn't prevent a heap of buzzing entering the signal path if the ground ref on the heaters is compromised.
There have been previous posts on this topic (ie mitigating hum by dc elevation of heaters) on MEF, I'll have a search later if none pop up.
Pete

[flyingdutchman]

That doesn't prevent a heap of buzzing entering the signal path if the ground ref on the heaters is compromised.

Agreed, but this takes place mostly through capacitive and inductive coupling, happening outside the valve envelope.
The point here is, a free floating power transformer secondary is bound to be dirty as it's capacitively coupled to the mains. Referencing it to ground tames it a bit.

[Enzo]

Grounding a floating 6v winding will reduce the hum a floating one contributes. However, that does nothing to solve H-K currents, which are real, but not always present. Those are two separate sources of hum, and each has its own cure. On a grounded CT heater, more than half the 6v waveform the cathode is more positive than the heater of a typical triode, typical meaning sitting at a volt or two positive. There are indeed loose electrons boiling off the heater, and if the cathode is more positive than the heater, they are attracted to it, and that is an AC current through that cathode - hum. I don;t deny there are also radiated and capacitively coupled hum sources as well. ALL of those hum sources exist.

[chunkitup]

Why are we required to use the heater supply for this op amp? Op amps will run MUCH better on 15v rails, and if this is a Peavey amp, it probably already has such rails in place. Why screw around? Peavey uses 30v (15v x 2) for the reverb IC on just about every amp they ever made with a a reverb. That Delta Blues used a transistor for the FX loop drive, but the reverb was straight op amp.

Heres the scenerio Enzo:
I built a friend a sloclone with the +4db fx loop. He cant use any of his pedals with it and asked for a -10db loop. I put in a metro zero loss fx loop which colored the sound too much for his liking. He said he wished I could put in the fx loop from one of his PV amps(need to find out which one) because his effects worked perfect with it. The problem is the PT only has 720vac and 6.3vac secondaries. That is why I posted this thread to see if it was possible. I decided to try and make my own hi voltage fet loop and you can bet that I will be starting a thread here for help designing it.

[J M Fahey]

FWIW if you have around 450V DC available somewhere inside that chassis (I bet you do, think screen supply), you can easily lower them to 30V (same as +/-15V) and use a single supply Op Amp circuit.
A typical TL072 (1 should be enough) "eats" 3.5mA tops , so a 120K 2W resistor will do just fine.
Just add a 220 to 470uF x 50V cap and a 30V 1W zener in parallel with it and you are done.
Just pad the "send" signal some extra 12 to 18 dB (1/4 to 1/8 X) and add same gain to the recovery stage and you will be fine.

[MarkusBass]

Lowering 450V to 30V would be a waste of power. This is how I would solve the problem (with high voltage FETs):
Mojotone Discrete Hi-Voltage Series Effects Loop for Vacuum Tube Amplifiers - Mojotone.com


Mark

[flyingdutchman]

Quite some time ago I built an effect loop for a red-knob Fender Champ 12 using a common JFET (the BF245) cascoded with a high voltage video driver transistor (BF469). The amp's owner wanted the loop to be as transparent as possible, and was mighty pleased with the result.

[Gregg]

Considering that LND150 is readily available and you have the high voltage source I wouldn't bother with any opamps and stuff.

[J M Fahey]

Waste of power? Compared to what?
It's a tube amp, wasting power all over the place just to keep those funny metal tubes red hot so they work at all.
The less than 2 W I suggested are comparable or less han other possible solutions.
Care to share some schematic so we can calculate actual current/power consumption?
Otherwise it's all just wild speculation.

[loudthud]

Lowering 450V to 30V would be a waste of power. This is how I would solve the problem (with high voltage FETs):
Mojotone Discrete Hi-Voltage Series Effects Loop for Vacuum Tube Amplifiers - Mojotone.com

It's rather absurd to be concerned about 2 Watts in a tube amp, and then offer a $70 USD solution. That Mojo loop probably consumes between 1 and 2 Watts.