Selenium Bridge Rectifier On Geloso Tube Amp

[sarakisof]

Hello everyone, found a Geloso G-227A for pretty cheap  the other day. It works but definitely needs some job done. Soo, that's my current quarantine project. 

Previous owner had replaced the original bridge rectifier (Siemens 300V 200mA) with a  Philips BY123 silicon one (280V 500mA).

Scheme requires 290VAC before rectify, so before go for recapping and other dirty job, i am wondering if previous owner's decision was wise enough. Am i safe to go?

He had also probably add this blue jumbo "Fraco/Falcon" electrolytic over there, but i am not sure as i have seen some other videos and repairs on Gelosos out there with same caps used in this place and people was wondering if it was factory or repairman job. 
Previous owner's voltage selectors were the first 240V and the second -20V. In my area it's 245V. Lack of original bridge i changed the second selector to 0V. (or maybe +10V) , as with -10V & -20V i was getting pretty high B+ than 320V.

Now with 240V and 0V i'm getting around 348V on B+ and about 6.24V on EL34s filaments. With 10V voltage selector i get even closer to scheme's B+, about 336V but filaments get decreased to 6-6.1V

What do you guys think?

[danfrank]

It would be inexpensive and easy to change the rectifier to a modern 1000piv 1 amp bridge rectifier. They cost around a dollar each. Here is a link to one that will work:

https://www.allelectronics.com/item/fwb-28/2-amp-800-volt-bridge-rectifier/1.html

I would replace all electrolytic capacitors in the amp also.

[Phoenix]

Yeah, upgrade your rectifier, the 280Vrms rating is too low even before safety margin. Should be rated for the 290Vrms secondary (410Vpk) + 10% for mains variation (450Vpk) + an allowance for transformer regulation (I'd allow +20% - so 540Vpk). 1N4007 or UF4007 are rated 1000V 1A, and are only a few cents each, or you can use a bridge rectifier package, 600V rating is common.

Use the loaded filament voltage as your guide to choosing the primary taps, not B+. Selinium rectifiers have significant internal resistance, so drop more voltage than a silicon replacement, so your B+ will be higher. You can add some series resistance if you want to drop it a bit (start with 100R 5W), but it is not critical. B+ a little higher than the schematic is no big deal.

The big blue cap looks of similar vintage to the rest of the amp, no reason to suspect it's a replacement.

Good luck with your project.

[sarakisof]

Yes indeed, i had all those already in mind. But i am a bit sceptical, as i read that this selenium rectifier originally used (Siemens B300 C200) is a full wave one. I don't know if a silicon would change sonething in terms of sound, due to different wave approach. 

[amptramp]

Always change out selenium rectifiers for silicon because if a selenium rectifier goes bad, the stench is truly remarkable and toxic as well.  I had one go on a General Electric television many years ago and even though it was on a December 15 cold day that had rained in Toronto, I opened the windows wide, shut the door to the rest of the house and went out for a bicycle ride for two hours.  The stench was only slightly less when I got back.  The selenium rectifier will consist of a number of plates, each being able to withstand 20 to 45 volts of reverse voltage, so there will be a number of diodes in series for the B+, each dropping about one volt in the forward direction.  Replacement with a silicon bridge will result in somewhat higher B+ voltages.  The real issue is the low-voltage bridge generating 25 volts for the 12AX7 heaters.  A few more volts there makes a difference.  Note that the forward drop of a selenium rectifier gradually rises with time - these things are not stable.

BTW some dimwit designed in a 250 µF 25 volt electrolytic between the heater choke and the first heater, which should be running at 25.2 volts.  Expect it to have a short life - a 35 volt device is necessary and a 50 volt device would give a better derating margin.

[PRR]

...selenium rectifier originally used (Siemens B300 C200) is a full wave one. I don't know if a silicon would change sonething ...

The part danfrank pointed-to IS a Full Wave Bridge, just like the original.

The few-Volts difference in drop will not change "tone".

The voltages on that schematic do not add up from AC to DC. However I would not sweat the difference 290V to 300V, anyway today an 800V or 1000V part is pennies more (at most) than a 300V part.

[sarakisof]

The real issue is the low-voltage bridge generating 25 volts for the 12AX7 heaters.  A few more volts there makes a difference.  Note that the forward drop of a selenium rectifier gradually rises with time - these things are not stable.

As you can see in the photo below the B60 C200 selenium rectifier is still there, left intact. At 12ax7 heaters i get 22.5 Volts.
I guess i am well for now at least.

So for the moment i only go for a modern 600V or so bridge full wave replacement and look B+ if any resistance addition needed, right?

And indeed then, the usual dirty job replacing all electros and any out of specs components. All axial "polarized" poly caps were well out of specs already   ). Replaced by yummy Philips NOS HQ "gum" "box caps" i had laying around. I had bought a bunch of them from an old local retired repairman back then, like 3 big bags for 10 bucks. It's always great when you realize you have in stock the same exact values replacements and that they belong to same era. No mouser needed 

[sarakisof]

Heh guys, just a quick question on a much discussed subject. There are two 4.7nF safety caps on this Geloso amp. Hot and neutral to gnd.

So, in terms of safety and hum cancel, is it better to go for two Y2 caps or just replace 2 prong to 3 prong cable?

What if i do both?

[Phoenix]

Heh guys, just a quick question on a much discussed subject. There are two 4.7nF safety caps on this Geloso amp. Hot and neutral to gnd.

So, in terms of safety and hum cancel, is it better to go for two Y2 caps or just replace 2 prong to 3 prong cable?

What if i do both?

Leave them out and wire the chassis to protective earth (3 prong cable). They were used in the original to AC ground the chassis, with a protective earth connection they no longer serve this function. They're also not very effective as an RFI filter without a proper CLC (capacitor/common-mode choke/capacitor) filter (power transformer takes care of most of this anyway), and pose a small shock hazard if the fuse blows and you touch the unplugged power lead because there's no discharge resistor (remember most injury involving electricity is from recoiling from a shock and knocking your head or tripping).

[willienillie]

and pose a small shock hazard if you touch the unplugged power lead because there's no discharge resistor

Or a large shock hazard if one of the caps shorts.

[Phoenix]

Or a large shock hazard if one of the caps shorts.

Well yes, but Sarakisof was talking about replacing them with Class Y capacitors, which are specifically intended for just this purpose, and are short-circuit proof. These are not the same as death capacitors like in the original.

[sarakisof]

A bit confused here. Aren't Y safety caps supposed to be connected between line and ground? Like death caps but they fail open, that's why we use them. And X caps across the lines. Am i wrong here?
If i was to replace them, wouldn't i use Y's?

[Phoenix]

A bit confused here. Aren't Y safety caps supposed to be connected between line and ground? Like death caps but they fail open, that's why we use them. And X caps across the lines. Am i wrong here?
If i was to replace them, wouldn't i use Y's?

Yes, class X are for cross-line, class Y are for line to ground. Class Y can substitute for class X, but NOT the other way around, as class Y standards are more stringent than class X.
However they are not required here, the function of the caps in the original ungrounded chassis was to AC ground it. With a proper protective earth connection they are redundant.

Class X and Y capacitors are commonly seen on switchmode power supplies to reduce conducted interference exiting the appliance and making its way to other appliances, they also help with immunity from external conducted interference, though this is really only a secondary function. Class Y capacitors are also used on isolated switchmode supplies to shunt primary-to-secondary transformer capacitance and reference secondary voltages to primary, preventing the secondary from floating to a high voltage.

You no longer require these capacitors to AC ground the chassis as it should be earth-bonded, this is not a switchmode suppy so you do not need to filter spurious emissions, they are not very effective as an RFI filter as they are not part of a CLC filter, and the line-frequency power transformer serves to filter external RFI anyway. Including them will only increase earth leakage current (under 400uA on 240V 50Hz for a 4n7 class Y cap, but something to be avoided unless it is necessary).

[willienillie]

I gotta admit, I've never heard of "X caps" or "Y caps" before.  I've heard of "self-healing" caps, where supposedly the metallized bit vaporizes or otherwise burns off in the case of an internal failure (something like that), thus preventing shorts.  Would that be related to Y caps?

I know there are some caps I trust more than others.  When I do tube amp repairs for people, I remove ground caps and install grounded power cords.  There are some rare owners that don't want that, they just have to have the external parts look all original or whatever.  In those cases I'll change the ground cap to a 600V or 630V orange drop, I've never seen one of those fail (when used within it's ratings of course).  I think those are considered self-healing caps, and I think that means they aren't likely to short, but really I don't know.

[Phoenix]

I gotta admit, I've never heard of "X caps" or "Y caps" before.  I've heard of "self-healing" caps, where supposedly the metallized bit vaporizes or otherwise burns off in the case of an internal failure (something like that), thus preventing shorts.  Would that be related to Y caps?

I know there are some caps I trust more than others.  When I do tube amp repairs for people, I remove ground caps and install grounded power cords.  There are some rare owners that don't want that, they just have to have the external parts look all original or whatever.  In those cases I'll change the ground cap to a 600V or 630V orange drop, I've never seen one of those fail (when used within it's ratings of course).  I think those are considered self-healing caps, and I think that means they aren't likely to short, but really I don't know.

You should really check your local laws, but I'd be surprised if what you're doing is not illegal (it certainly would be in most jurisdictions).
It's definitely not safe.
Not all series of orange drops are self healing btw.
Class Y caps exist exactly for this purpose, and orange drops are not they.

EDIT: My policy is to decline any customer who refuses to make their equipment safe. In practice no-one has ever refused in over a decade when the safety implications have been explained to them. My liability insurance would not cover me if I did not adhere to modern safety standards, and if you're thinking a waiver would cover you for such customers, note that even when properly drafted by a lawyer, such documents rarely hold water when put to the test - the onus is on you to take all reasonable safety measures, and if you flout one, someone cannot simply sign away their - or others who may use their equipment - safety.

[sarakisof]

A bit confused here. Aren't Y safety caps supposed to be connected between line and ground? Like death caps but they fail open, that's why we use them. And X caps across the lines. Am i wrong here?
If i was to replace them, wouldn't i use Y's?

Yes, class X are for cross-line, class Y are for line to ground. Class Y can substitute for class X, but NOT the other way around, as class Y standards are more stringent than class X.
However they are not required here, the function of the caps in the original ungrounded chassis was to AC ground it. With a proper protective earth connection they are redundant.

Class X and Y capacitors are commonly seen on switchmode power supplies to reduce conducted interference exiting the appliance and making its way to other appliances, they also help with immunity from external conducted interference, though this is really only a secondary function. Class Y capacitors are also used on isolated switchmode supplies to shunt primary-to-secondary transformer capacitance and reference secondary voltages to primary, preventing the secondary from floating to a high voltage.

You no longer require these capacitors to AC ground the chassis as it should be earth-bonded, this is not a switchmode suppy so you do not need to filter spurious emissions, they are not very effective as an RFI filter as they are not part of a CLC filter, and the line-frequency power transformer serves to filter external RFI anyway. Including them will only increase earth leakage current (under 400uA on 240V 50Hz for a 4n7 class Y cap, but something to be avoided unless it is necessary).

Perfect explanation, well done Phoenix 

[sarakisof]

I gotta admit, I've never heard of "X caps" or "Y caps" before.  I've heard of "self-healing" caps, where supposedly the metallized bit vaporizes or otherwise burns off in the case of an internal failure (something like that), thus preventing shorts.  Would that be related to Y caps?

I know there are some caps I trust more than others.  When I do tube amp repairs for people, I remove ground caps and install grounded power cords.  There are some rare owners that don't want that, they just have to have the external parts look all original or whatever.  In those cases I'll change the ground cap to a 600V or 630V orange drop, I've never seen one of those fail (when used within it's ratings of course).  I think those are considered self-healing caps, and I think that means they aren't likely to short, but really I don't know.

You should really check your local laws, but I'd be surprised if what you're doing is not illegal (it certainly would be in most jurisdictions).
It's definitely not safe.
Not all series of orange drops are self healing btw.
Class Y caps exist exactly for this purpose, and orange drops are not they.

EDIT: My policy is to decline any customer who refuses to make their equipment safe. In practice no-one has ever refused in over a decade when the safety implications have been explained to them. My liability insurance would not cover me if I did not adhere to modern safety standards, and if you're thinking a waiver would cover you for such customers, note that even when properly drafted by a lawyer, such documents rarely hold water when put to the test - the onus is on you to take all reasonable safety measures, and if you flout one, someone cannot simply sign away their - or others who may use their equipment - safety.

Yeap i think you should decline and use a 3 prong cable in anyway or replace with appropriate X or Y class rated safety caps at least.
I will use a 3 prong cable and ground chassis at tranny's chassis gnd screw as i do on all my high voltage repairs, but just wanted to be extra sure, in terms of hum cancelling. But Phoenix said everything.

Peace.

[sarakisof]

Me too, i know many keybordists who wanted their tube organ/combo or amp be repaired "all original wise" and didnt even want to replace the cable.
In some tube organs of that era (Farfisa Foyer for example) i have also seen a 3 prong cable be installed from factory , but yellow-green gnd wire was cut inside the power connector, ending up a common 2 prong of that era. In the other end inside chassis it was indeed factory connected to chassis, so all you had to do was just using a new 3 prong power socket connector 

[willienillie]

You should really check your local laws, but I'd be surprised if what you're doing is not illegal (it certainly would be in most jurisdictions).
It's definitely not safe.
Not all series of orange drops are self healing btw.
Class Y caps exist exactly for this purpose, and orange drops are not they.

It's an extremely rare situation, I can't even remember the last time it happened, but it's been many years.  Yes I do everything I can to inform people of the safety situation.  Thanks for the clarification on the orange drops.  If not self-healing, they're still a damn sight safer than a 400V paper and wax cap from the 1950s.  I'll look into Y caps if this situation ever comes up again, or more likely just insist on the grounded cord.

Laws?  Illegal?  I think those concepts are considered "racist" now, lol.

[amptramp]

Check here:

https://www.justradios.com/X1Y2capacitors.html
https://www.justradios.com/Y2capacitors.html
https://www.justradios.com/X2capacitors.html

for all you ever wanted to know about ordering in X and Y capacitors.  I have a box of regular caps from justradios and proprietor Dave Cantelon is also a member of the London Vintage Radio Club which is where I got mine and the antique radio community recommends his stuff.  Note that these carry worldwide ratings so you can use them on 240 VAC systems.  BTW he carries a number of interesting parts you may not be able to get anywhere else.