Options to replace odd cap value

[roberto]

By "bigger" you mean "stiffer"?
Yep, that could even be an SMPS and avoid the gyrator (and be lighter and cheaper as well).

Looking at the current you set for the first node I understand it is a 50W amp.
Indeed as you stated, simulation shows that specific result because of the parameters you set.
Stiffer amps always had more stable screens (think about Marshall and Soldano who are in the range of 50 to 100 uF and chokes with more inductance and less resistance), whilst other amps have lower filtration (Mesa usually has half of the capacitance, less inductance and more resistance, taken to the extreme with the Dynawatt idea).
Hammond 194F, suggested for Marshalls, has Rdc = 115 Ohm. This will make the screen node more stable.
Hammond 290KX, suggested for Marshalls 50W, has Rdc = 20 Ohm. This makes the anodes stiffer as well.

But yes, you got the point: make a stiffer PSU without ripple, then insert resistance(s) in series for the sag we want, and caps dimensioned for the RC time we want.
Like a compressor.

[R.G.]

Absolutely correct, loudthud. Real inductors store, then release their own energy. Gyrators take energy from another power supply and actively take it in and let it out to fake the operation of a real inductor. They don't store their own power like a real inductor does.

[Reeltarded]

I know a guy who swears he can FEEL the difference between 16uF and 22uF in a tweed Deluxe. And it's not Neal Young.

You owe me one noseful of coffee.

[imjonwain]

It's also worth noting the original caps used were probably spec'd at something like -20%/+50% at very best so you can factor that into picking your new caps.

[LOUDthud]

You owe me one noseful of coffee.

When a guy pays you to re-cap his amp, and supplies the parts, well you just can't say he's crazy

FWIW I ask Gerald Weber why there are no 16uF caps in his line of caps he sells, he said there isn't that much difference between 16 and 20uF. Isn't that reason enough to believe ?

[roberto]

Real inductors store, then release their own energy. Gyrators take energy from another power supply and actively take it in and let it out to fake the operation of a real inductor. They don't store their own power like a real inductor does.

If we consider just the screens, there's no need to have a "bigger" PSU just to compensate for the energy an inductor can't store. Also considering tubes in instrument amps are generally very limited in current by screen resistors (compared to hi-fi), and secondary emission starts becoming important only on high peaks, but is in any case very low compared to anode current, where the choke has practically no effect due to the impedances of the two nodes.

[Reeltarded]

FWIW I ask Gerald Weber why there are no 16uF caps in his line of caps he sells, he said there isn't that much difference between 16 and 20uF. Isn't that reason enough to believe ?

I'd rather be a conspiracy theorist. Honestly.

1991 Dallas guitar show. We were lucky to be surrounded by good people, and unlucky to be just across from Weber. This was a time when a tweed bassman with extra clarity was popular. The worst part was putting up with Ed complaining about it.

"This guy has that lick down. If he plays it one more time it will be on crutches after that..."

I miss Ed. He sure had a way with people.

[R.G.]

If we consider just the screens, there's no need to have a "bigger" PSU just to compensate for the energy an inductor can't store.

Tell me how you think that works.

Also, how much voltage compliance do you think a gyrator needs?

[roberto]

Frankly I don't understand this approach R.G.
Using words like "what do you think it is" is a very aggressive and trollish approach.
May I ask you the reasons of this rude approach?

The energy stored (so released, minus efficiency) is half the inductance multiplied the squared value of the current.
Talking about the choke of the screens, we have screens current plus preamp, let's consider LOUDthud's values: 15 and 30 mA. With a 5 H choke it becomes 560 uJ and 2.2 mJ, then it has to be deduced the efficiency due to losses.
This is why I was saying that the PSU has not to be significantly bigger.

The gyrator deltaV requirements depend on the upstream ripple.
If we use a SMPS it can be very small (or not even needed, if the SMPS is well designed).

[pompeiisneaks]

Frankly I don't understand this approach R.G.
Using words like "what do you think it is" is a very aggressive and trollish approach.
May I ask you the reasons of this rude approach?

The energy stored (so released, minus efficiency) is half the inductance multiplied the squared value of the current.
Talking about the choke of the screens, we have screens current plus preamp, let's consider LOUDthud's values: 15 and 30 mA. With a 5 H choke it becomes 560 uJ and 2.2 mJ, then it has to be deduced the efficiency due to losses.
This is why I was saying that the PSU has not to be significantly bigger.

The gyrator deltaV requirements depend on the upstream ripple.
If we use a SMPS it can be very small (or not even needed, if the SMPS is well designed).

I'd like to step in there and note that it does seem possible that your previous replies to him seemed a bit aggressive although maybe unintentionally. His simple pointed request seems to be asking how to justify your previous disagreement with his point? In any case I agree please don't try to get this into flame war territory. You're directly countering his statements. both of you could back them up with factual, non emotional responses and benefit everyone in the process...

My 2c anyway.

~Phil

[roberto]

Hi Phil,

it is absolutely NOT my intention to start any flaming.
If I seemed “strong” in my posts, it was not my intention to do it, and re-reading me frankly I don’t see it, but could be me.

PS
I'm not in disagreement with what he wrote, I just proposed a possible solution to the ripple vs sag... saga.

[Aurora]

I've spent my whole working life in electronics, ranging from audio to radar, with some electrooptics in between...
Gyrators has nothing to do in power supplys. Get caps at or slightly above the old value that needs to be replaced, with a suitable margin for voltage.

[roberto]

Hi Aurora, can I ask you to please elaborate more the "Gyrators has nothing to do in power supplys"?
As you know they are widely used on many SE designs of valuable designers to replace chokes with good results, lower costs and weight.
It isn't the only solution, of course, not the total panacea for sure, but it is not even the absolute worst.

Back on topic, for this specific application I agree a single cap is the cleanest solution: https://ampgarage.com/forum/viewtopic.p ... 32#p446332

[R.G.]

Frankly I don't understand this approach R.G.
Using words like "what do you think it is" is a very aggressive and trollish approach.
May I ask you the reasons of this rude approach?

It wasn't intended to be argumentative. It was an honest technical question. I wish it hadn't come across as anything else.

The reason there's an inductor in guitar amps at all is that it was the only reasonable way to get enough filtering of power supply ripple for the screens and preamp tubes in an era where tubes were all they had. With 50V or more of ripple on the main B+, a tube amp really needs to have that ripple scrubbed off before sending it to the screens - which are another input to the power tubes. And the power to screens is not insubstantial; they can amount for as much as 8-10% of the power tube's cathode current. Simply put, inductors were the cheapest way to reject a lot of ripple quickly.

The reason they're still there is that the guitar amp industry still loves to copy the Golden Age. I can't tell you how much blow back I have received over the years for proposing SS diode replacment/updates in tube amp power supplies, using MOSFETs in tube amps and so on. I still have a file collecting some of the most frantic ones somewhere.

The gyrator deltaV requirements depend on the upstream ripple.

Correct. That translates to "the gyrator has to be able to provide an output swing as wide as any voltage the inductor would have done, whether above or below the power supply line with ripple on it." This is one problem with power gyrators that do any filtering. The power to the gyrator needs to extend above the highest voltage it will filter, and it needs enough current capability to supply the load under those voltage swings. A gyrator replacement for the choke in a guitar amp needs to keep the average voltage on its output pin at that voltage when the input voltage it's filtering drops below the average. That difference, from the average output voltage to the lowest point of the input ripple has to come from somewhere else, as does the current the gyrator passes to the output. Unless you have a gyrator output that is always within the ripple of the incoming gyrator voltage, you do need another power supply, somehow.

If we use a SMPS it can be very small (or not even needed, if the SMPS is well designed).

Theoretically, yes. Practice gets messy. Using an SMPS in a guitar amp is going to be a highly controversial item. My choice would be to skip partial solutions and just use an SMPS for the whole amp, skipping all the heavy metal. If you can make an SMPS quiet enough to run the screens, you can make one quiet enough to run the whole amp. Of course, that introduces the cries of "I want my sag back!" Yes, you would have to find a way to modulate the plate, screen and other supplies in a sag-like way.

Power gyrators are tricky. As Aurora notes, I haven't seen one in a power supply application. That's not to say they don't exist, somewhere, but I've been looking at power supply designs since the 1970s and not noticed one.

Edit:
Forgot to mention this. Think about linear regulators. If you feed one a moving reference voltage to follow, it does its best to follow the input. If you can generate a voltage you want the screens to follow, apply this to a simple regulator (maybe a MOSFET source follower) and feed that to the screens. Once you have active circuitry there, you can do other stuff, like tube-saving if the screen current tries to go too high, things like that.

[bepone]

The reason there's an inductor in guitar amps at all is that it was the only reasonable way to get enough filtering of power supply ripple for the screens and preamp tubes in an era where tubes were all they had.

Not really. In PP amp you dont need too much filtering at all to avoid hum at the output. It is enough resistor 300-500ohm between Ub+ and Ug2 node. If the tubes are matched, hum is cancelled out. Actually in those days was even easier to avoid hum because mostly tubes where built to standard with not so huge variations like today.