Weber 5G15/Fender 6G15 reverb grounding

[Greg_L]

1k/0.5W or 1.5k/1W. Screen resistor will somewhat reduce tube gain. With the 1750AX it is not required.

Cool thanks. I'll check my stash and get on that.

[Enzo]

The tube will always be more likely at risk, as the transformer is just a piece of wire wrapped around a hunk of iron.

[g1]

I know Helmholtz mentioned that the tank input inductance is not a simple constant 8 ohm, but I'll just pretend it is for a minute.
The power tube would like to see 6K impedance for optimum operation (again not simple, that 6K is only under specific conditions).
The xfrmr is 22K with 8ohm on the secondary. Put 4ohm on the secondary and it is more like 11K primary. 2 ohm on the secondary and now you are close with around 5K equivalent primary.
So now I'll rephrase the question and it's like you are asking, what happens if my amp wants a 2ohm load and I hook it up to an 8 ohm cab?
Does thinking about it that way help at all? That will be a very familiar question that you will probably find a fair bit of info about. Probably lots of misinformation too, but such is the nature of the internet.

[Enzo]

Tank input is not simple and varies with freq. But so does a loudspeaker. Impedances in these things are just nominal. Your 8 ohm speaker is only 8 ohms at one point on the response curve.

[Greg_L]

I know Helmholtz mentioned that the tank input inductance is not a simple constant 8 ohm, but I'll just pretend it is for a minute.
The power tube would like to see 6K impedance for optimum operation (again not simple, that 6K is only under specific conditions).
The xfrmr is 22K with 8ohm on the secondary. Put 4ohm on the secondary and it is more like 11K primary. 2 ohm on the secondary and now you are close with around 5K equivalent primary.
So now I'll rephrase the question and it's like you are asking, what happens if my amp wants a 2ohm load and I hook it up to an 8 ohm cab?
Does thinking about it that way help at all? That will be a very familiar question that you will probably find a fair bit of info about. Probably lots of misinformation too, but such is the nature of the internet.

So it's like a "reflected load" kind of thing? So which side is more important to "get right"? Primary or secondary?

In real-world applications, I've mismatched amps and speakers a bunch of times. Sometimes on purpose. I've never had a problem except the times that there has been no load at all and my cool ass cranks the amps without realizing it. That was a fun one. But usually with some kind of load the amp just does it's thing. I know they aren't as fragile as folklore would have one believe.

[Greg_L]

Tank input is not simple and varies with freq. But so does a loudspeaker. Impedances in these things are just nominal. Your 8 ohm speaker is only 8 ohms at one point on the response curve.

Agreed, I understand that. But how far off is too far?

[Helmholtz]

Agreed, I understand that. But how far off is too far?

I'd say more than a factor of 2. But depends on plate and screen voltages.
Too low impedance increases plate dissipation, too high impedance increases screen dissipation. There should be no redplating or "redscreening" at full drive.

[Greg_L]

I'd say more than a factor of 2. But depends on plate and screen voltages.
Too low impedance increases plate dissipation, too high impedance increases screen dissipation. Theres should be no redplating or "redscreening" at full drive.

I haven't noticed any redplating on my modest little single ended 6V6. My bias is relatively cool as well. The secondary of my incorrect transformer matches the reverb tanks impedance, but the primary is off for the tube.


Another question....just another curiosity. There are no grid stoppers on the inputs on this reverb build. The guitar signal splits at the input and runs straight to the grids. Why were these omitted in the design? Should I add them, like you'd find in "normal" amps? Are they missing because they probably will be present in the actual amp that this thing will be feeding?

[Helmholtz]

I haven't noticed any redplating on my modest little single ended 6V6. My bias is relatively cool as well. The secondary of my incorrect transformer matches the reverb tanks impedance, but the primary is off for the tube.

As said, plate dissipation reduces with high impedance, so no risk for redplating. It's the tiny screen wire you should look at.
Bias has little influence on screen dissipation at full power.

[Helmholtz]

So it's like a "reflected load" kind of thing? So which side is more important to "get right"? Primary or secondary?

Yes, it's always reflected impedance. Your reverb transformer does not "have" a primary (or secondary) impedance. Transformer just have turns ratios, no impedances. If it's specified as 22K/8R it means that a secondary load of 8 Ohm wiil be reflected as 22k at the primary. Primary impedance will change linearly with the load impedance.

Edit: Actually it is not correct to say that a transformer has "no" impedance, as no impedance means zero impedance = short. In fact a non-terminated transformer has a very high impedance, high enough to not interfere with circuit impedances.

[Greg_L]

As said, plate dissipation reduces with high impedance, so no risk for redplating. It's the tiny screen wire you should look at.
Bias has little influence on screen dissipation at full power.

Right, thanks. I'll be addressing the screen grid resistor this weekend.

Yes it's always reflected impedance. Your reverb transformer does not "have" a primary (or secondary) impedance. Transformer just have turns ratios, no impedances. If it's specified as 22K/8R it means that a secondary load of 8 Ohm wiil be reflected as 22k at the primary. Primary impedance will change linearly with the load impedance.

I'm starting to get it, thanks. And what enzo said...it works so don't mess with it. Lol. I'm just trying to wrap my head around this stuff.

[Helmholtz]

Tank input is not simple and varies with freq. But so does a loudspeaker. Impedances in these things are just nominal. Your 8 ohm speaker is only 8 ohms at one point on the response curve.

True, but a reverb transducer is much more inductive than a speaker. At 1kHz the inductive part of a speaker's impedance is typically around 20%. A reverb transducer is almost 100% inductive, meaning that its impedance varies linearly with frequency. So if a reverb transducer is specified with 8 Ohm @ 1kHz, its impedance will be just above 4 Ohm @ 500Hz and 16 Ohm @ 2kHz.

[Helmholtz]

I'll be addressing the screen grid resistor this weekend.

I reconsidered and think you should better use 1k/1W or 1.5k/2W as screen resistor with your mismatched load.

[Greg_L]

I reconsidered and think you should better use 1k/1W or 1.5k/2W as screen resistor with your mismatched load.

Will do. Thanks. I was gonna overdo it anyway.

[Enzo]

Whenever I claim a transformer has "no impedance", I make every effort to ALWAYS include "inherent". it has no inherent impedance. In other words you cannot measure transformer impedance with a meter. The impedance of one side ALWAYS depends upon the impedance on the other side. Steve Connor once made a case for transformers are designed with a particular impedance in mind, so the wire sizes and insulation and actual numbers of turns and so on were considered. But I think that took things too far from the discussion. SO a 100k to 50k transformer and an 8 ohm to 4 ohm transformer would have the same turns ratios, but each would not likely work well in the other's place. But yet the no inherent impedance idea still holds.

I think also context is everything. We are talking here about a super low power driver circuit for a tiny reverb coil. No significant power is being handled, so it would seem to me any potential red plating would be due to DC conditions, not demands of the signal path.