Options to replace odd cap value

[LOUDthud]

R.G., just curious, how hard is it to get an SMPS past the FCC and possibly UL ? If a DIYer made one that had significant emissions, would there be any legal consequences ?

[pdf64]

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.

I think the issue with screen grid node ripple may not be so much idle hum as intermodulation between the signal and the ripple.
And when signal level moves the output devices into class B, hum cancellation ceases.

[bepone]

correct, but then volume is already too loud, it is supressed. and maybe is adding to "musicality" ?
it was somewhere some study on fender bassman, completely DC stiff regulated supply , vs normal- stock.. stock won according to the people there, i can believe so.. ripple looks like is on our side

[R.G.]

R.G., just curious, how hard is it to get an SMPS past the FCC and possibly UL ? If a DIYer made one that had significant emissions, would there be any legal consequences ?

Dang it, L.T. I typed in a reasonably long reply but was interrupted. I thought I had hit "submit" but apparently not. I'll try to re-create it.

[R.G.]

it was somewhere some study on fender bassman, completely DC stiff regulated supply , vs normal- stock.. stock won according to the people there, i can believe so.. ripple looks like is on our side

I would very much like to read the actual study. Got a reference to it?

Sadly, quite a lot of the internet belongs in "The Journal of Irreproduceable Results". (which is, or was, a real thing: https://en.wikipedia.org/wiki/Journal_o ... le_Results)

[martin manning]

it was somewhere some study on fender bassman, completely DC stiff regulated supply , vs normal- stock.. stock won according to the people there, i can believe so.. ripple looks like is on our side :P

I would very much like to read the actual study. Got a reference to it?

Here: https://www.ampbooks.com/mobile/classic ... AB-ripple/

[LOUDthud]

Dang it, L.T. I typed in a reasonably long reply but was interrupted. I thought I had hit "submit" but apparently not. I'll try to re-create it.

No hurry, I appreciate your thoughts on the issue.

I believe this is a summary of the study of the 5F6A that was mentioned above: https://www.ampbooks.com/mobile/classic ... AB-ripple/
At the bottom under Further Reading is a link to the entire paper, but it's in German...

[R.G.]

OK, I'll try this again.

R.G., just curious, how hard is it to get an SMPS past the FCC and possibly UL ? If a DIYer made one that had significant emissions, would there be any legal consequences ?

Let's do the legal consequences first. It's clearly illegal, and at one time the FCC could impose fines of up to $10,000 per day. I don't know if the fines have been jacked up or not. I believe that current practice is that monitoring vans are only sent out upon complaint(s), and also on who it is complaining, or volumes of complaints per area. Like so many things, the enforcement is selective. An ordinary citizen with a problem likely would get no response, I'm guessing.
It's worth interjecting here that in today's USA, it's very difficult to get through a full day without committing a felony. If you have a morbid interest, you might do a search on "overcriminalization". The flip side of over-criminalization is under- and (very!) selective prosecution.
But if you do get caught interfering with someone's baby monitor, wifi, door cam, phone, aviation radios etc., and if the FCC investigates, and if you get charged, you could be in for some stiff fines, maybe jail time in extremis, although its unlikely. The name of the game is "don't get caught", which turns out to be pretty good odds. How's that for muddy?

As for getting a badly emitting switching anything past the FCC, if you mean "approved", that certification comes from much the same places that do UL/CSA/CE/TUV/etc. as they're the only places that have the necessary equipment and the authorization. The FCC does license companies and individuals who have applied for authorization to self certify after some sort of application/testing passes; IBM, for instance, was licensed to self-certify. With today’s software defined radio modules, this may be changing, as the right equipment might be some antennas and a SWDR.

Actually getting UL (or CSA, or TUV, or ETL, or one of the others) to certify emissions, that’s going to be expensive. These places typically run to about $10k per test pass, and a one-pass certification typically requires that the owner/designer is a practicing emissions adept and has experience. I would not bet on it. As a practical matter $20k if you’re already pretty close. A badly designed thing might need complete redesign, usually in the hard part – the actual trace routing, current loop location and size, and any snubbing and filtering.

Safety certification is much the same story, about $10K per pass, and hopes of a single pass are dim. I’d say impossible without technical experience. The best I ever did was two passes. But safety (as opposed to certification) is easier to know and practice. The biggies are to (1) use three wire safety grounding (2) make sure all the user accessible metal is grounded to chassis, use UL/CSA/etc./ recognized components for AC power (3) use the fairly well known tips and tricks for wiring AC power, and (4) know and apply the creepage and clearance requirements on wiring and PCBs.

Does that help?

[roberto]

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.

I can imagine, because every time I propose a design (mainly for hi-fi I have to say) on some forums, there's someone that counts the tubes and SS and tells me it is not a tube amp because the ratio of vacuum vs SS is not above an arbitrary number they have in mind. Others say it cannot work but cannot say why. SS has opened many roads that were not possible before, improving the way tubes can work or even revutionizing their way of working to have a better sound. Whatever better means.

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.

Now I understand our misunderstanding. I haven't proposed to replace the choke with a gyrator, but to have a first "rippleless" PSU node using a gyrator. Then, from it, derivate the power supply of the amp through a low filtered nodes to have the sag as we like. Something like: PT > rectifier > 220u > gyrator > 220u > 10R (to simulate PT's secondary winding resistance) > 22u to B+ > 1k > 10u for screens > etc...

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.

I will check myself as well, because I've seen some in SE designs, of course for the whole amp. I've used them, limiting the dissipated power below 10W and using TO247 mosfets on an heatsink or directly on the chassis. I've found an old simulation I did, and results weren't that far: with 450Vdc + 40Vpp ripple upstream I got 413Vdc + 100mVpp downstream considering 400mA.

Gyrator.jpg

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.

I simulated something similar (never built!) when I was trying to apply UL to GU50 without using a dedicated winding on the OT. The gate was DC referenced around 250V and then plate signal was scaled down and AC coupled to the gate to make screens swing accordingly.

[roberto]

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.

Noisewyse I agree, but then the amp will become very sensitive to the volume you play at. I've done some tests and (also depending on the loadline you have chosen) the resistor can be set to be good at low volumes (higher value, lower screen current) or high volumes (the opposite), but the sweet spot is always controversial. A choke simplifies it alot.

[roberto]

R.G., just curious, how hard is it to get an SMPS past the FCC and possibly UL ? If a DIYer made one that had significant emissions, would there be any legal consequences ?

I clearly remember a commercial guitar tube amp with SMPS but I cannot recall which one it was. Sold worldwide, so CE and UL certificated for sure. I've seen some of the earliest after the production was moved to China, that had issues (same happened to Orange with small plate resistors: same rating, but smaller).

Just for experiments, you can find online DC-DC converters that can go up to 450V and are rated 70W.
I've used them for preamp supply (they need an RC before the preamp and better to shield them internally), never for small amps (they could fit a pair of EL84 around 320V and 8k loadline).
For that preamp I just used an SMPS for 230Vac to 12Vdc, then the DC-DC converter to reach around 400V, an RC filtration and then the preamp.
Nice thing is that you can set the voltage you prefer, and whatever the wall voltage is, you'll always get that sound from your preamp.

[roberto]

Maybe the thread can be split? We are going OT.

In any case, to test a SMPS with SE amps, these can be used:
https://it.aliexpress.com/item/1005003572385033.html
https://it.aliexpress.com/item/1005003138732085.html

[LOUDthud]

I clearly remember a commercial guitar tube amp with SMPS but I cannot recall which one it was. Sold worldwide, so CE and UL certificated for sure. I've seen some of the earliest after the production was moved to China, that had issues (same happened to Orange with small plate resistors: same rating, but smaller).

Just for experiments, you can find online DC-DC converters that can go up to 450V and are rated 70W.
I've used them for preamp supply (they need an RC before the preamp and better to shield them internally), never for small amps (they could fit a pair of EL84 around 320V and 8k loadline).
For that preamp I just used an SMPS for 230Vac to 12Vdc, then the DC-DC converter to reach around 400V, an RC filtration and then the preamp.
Nice thing is that you can set the voltage you prefer, and whatever the wall voltage is, you'll always get that sound from your preamp.

In the USA, we have an agency of the Federal Government known as the FCC which stands for Federal Communications Commission. One of their tasks is to insure that Radio Communications are not interfered with. They have set limits on the amount of interference an appliance can radiate in a residential setting. Limits for a commercial setting are generally twice as much. Just about any electronic device must be tested and the company that creates it is subject to fines and other measures if limits are not met. Analog products generally do not create or use Radio Frequencies so they are generally exempt from testing requirements. If a stomp box has one of those DC to DC converters, it falls under the rules and must be tested. In the beginning, Dell Computer did not have their products tested. The FCC showed up one day and (I've heard) put chains on the doors until products were brought into compliance. I don't know if fines were imposed.

In the USA, UL and CE testing is not mandated by government. UL stands for Underwriters Laboratory. They test the safety of products as far as; can the user be harmed (electrocuted) and; can the product catch on fire. Testing is not required, but if your product kills somebody or burns down a building, action can be brought against you in a Court of Law. Monetary judgements are almost without limits up to the amount of loss. If your product has been tested and passed by UL, the plaintiff must prove that UL's requirements did not protect them. If your product is sold in a country that requires CE testing, then it is subject to laws in that country.

[pdf64]

R.G., just curious, how hard is it to get an SMPS past the FCC and possibly UL ? If a DIYer made one that had significant emissions, would there be any legal consequences ?

I clearly remember a commercial guitar tube amp with SMPS but I cannot recall which one it was. Sold worldwide, so CE and UL certificated for sure.

Maybe Crate V33? https://el34world.com/charts/Schematics ... 33_212.pdf

[roberto]

Good catch pdf64!
The one in my mind was european, but then it isn’t the only one!

LOUDthud,
thanks for the FCC explanation.I thought UL was covering the emissions too. We have something similar in Italy and EU as well, but I don’t recall the name.