Fusing and diodes to protect the PT

[pnadora]

Hallo,
I am making good progress on my EF86 amp build (thanks to you all).
I have now come to the point of wiring the power supply. I want to protect the power transformer and I am debating my options.
Merlin suggests secondary fusing. I have also read that putting diodes in series with the tube rectifier is useful. Would the diodes make the fuses obsolete?
Merlin also suggests that the mains fuse should be one of the ceramic HFC fuses, but in most amps I know the fuse in the panel is a normal glass fuse.
What are your opinions on fusing? Do you fuse the heaters as well?
Best regards!

[HotBluePlates]

... Merlin suggests secondary fusing. ... What are your opinions on fusing? Do you fuse the heaters as well? ...

Merlin is not the only one to recommend secondary fusing (or even a fuse for each leg of the primary winding). It's a great idea to protect one of the most expensive components in the amp.

I keep meaning to try it, but haven't. But my complacency is likely because I also have never had a damaged transformer in any amp I've had.

... I have also read that putting diodes in series with the tube rectifier is useful. Would the diodes make the fuses obsolete? ...

No; both are good ideas to have, even at the same time.

The diodes protect the filter caps from damage if the rectifier tube shorts plate-to-cathode. The fuses protect the PT from excessive current draw. Now, the excessive current draw might happen because the tube rectifier shorted (and you had no diodes). However, anything else which causes excessive current draw could still kill the power transformer because the diodes will cheerfully pass the current.

[Tone Junkie]

Ive only had one transformer problem. That was before I built the light bulb limiter for start up. Man those things really smell bad when they go. LOL
Bill

[Mike_J]

Here are the extra fuses and diodes I installed in my one-channel Deluxe Reverb build. Didn't add a fuse between the standby switch and the first filter cap. Sluckey said he didn't think they would be necessary until you get to at least a 50 watt build. The diodes going to the rectifier tube sockets from the HT secondary windings are UF. Supposedly keeps the amp quieter at shut off if I remember correctly. Any way the amp is pretty quiet when it shuts down. Haven't had any trouble with fuses blowing so far so good.

[HotBluePlates]

... The diodes going to the rectifier tube sockets from the HT secondary windings are UF. Supposedly keeps the amp quieter at shut off if I remember correctly. Any way the amp is pretty quiet when it shuts down. ...

Ultra-fast diodes prevents some noise when the diode shuts offs, or switches off, 60 times per second when the a.c. feeding it swings negative.

[Paul1453]

The diodes before the tube rectifier make sense to me.
This is a very inexpensive prevention measure.

Fusing the 6.3 and 5 VAC windings doesn't seem worthwhile to me.
These are high current windings with thick wire that can withstand a moderate amount of abuse.
If one of these windings shorts, you are likely to blow the primary AC input fuse before you melt these windings.
As long as that primary fuse is the correct value and not way over fused.

Fusing the HV secondary with it's thin wire windings and low current capacity might be worthwhile.
You might damage these windings before the primary AC fuse blows, even if the primary is correctly fused.

It seems to me that if manufacturers were having PT warranty repairs because the HV secondary kept blowing,
they would have already added another fuse on those windings.      

[trobbins]

It can be very disheartening when a blown power transformer, or output transformer, or set of NOS KTxx, or all of the above are found to be dead, and some detective work identifies a fault was caused by another part (eg. a leaking coupling capacitor costing less than a $) and the AC mains side fuse didn't save the expensive bits in the amp.

With a bit of care and awareness and incentive to add some extra parts, an amp can be made more bulletproof and hopefully survive a few more decades.  Some technical discussion and a process to determine what fuse to use is given in the link:
http://dalmura.com.au/projects/Valve%20amp%20fusing.pdf

Also note that there is no advantage to using fast diodes in series with a valve diode - standard 1N4007 is fine for that application - as the valve controls the diode turn-off characteristic (ie. making turn-off and turn-on a nice soft transition with no reverse or forward recovery effects).

Ciao, Tim

[pnadora]

Thanks for the answers! I think I will go for the better safe than sorry method and fuse the heaters the HT winding and put the 1n4007 diodes in series with the rectifier.

[eleventeen]

The idea of the diodes in front of the rectifier plates is an excellent one but it comes into even sharper focus if you are using a 5AR4/GZ34. These tubes in US made NOS, not to mention Mullard or Amperex, are now extreme fetish items and cost ridiculous.


As an alternative I recommend finding and possibly stockpiling NOS US-made 5V4s which are also indirectly-heated-cathode rectifiers---meaning, unlike a 5Y3 or 5U4, there is a separate element in the tube, a cathode separate from the filament itself, that is the item that is heated up and which boils off the electrons the tube uses to do its voodoo. A 5Y3 will drop your output B+ by 35-50 volts, relative to a GZ34, a 5U4 a little less. A 5V4 will drop only about 10-15 volts relative to a GZ34, hardly noticeable. US made NOS 5V4 are readily available for $15 or so....for a while....until folks catch on. Perfectly compatible with a socket wired for GZ34. Plug in and go.


So to avoid paying $75-200 for a silly rectifier tube, of course one goes for a new $15 GZ34. But as some have noted, brand new GZ34 are made in Russia and the quality and integrity of some of those, depending upon exactly when they were made...is not the greatest. Let's just say inconsistent. When such a tube fails, a cathode-to-plate short occurs and this can definitely be hazardous to your PT. It is a different story with 5Y3/5U4 because the filament to plate distance is a lot greater than in a GZ34. It does not hurt with those tubes either, the series diodes cost you 1 volt which is nothing in tubeland. But it is a great idea for almost zero cost if your plan is to use GZ34 and I would heartily recommend it/them.

[Paul1453]

The cost of old GZ34's floored me!
I could not believe what people are consistently paying for them on Ebay. 

For a penny pincher like me, if I have to pay >$20 for a rectifier tube, I'm going SS instead. 

[trobbins]

It's well worth the effort to check if an alternative tube rectifier can do the job.  The link below is an excellent listing of 5V rectifier tubes with ratings and advice.


Certainly adding a few 1N4007 in series in front of each plate in a tube rectifier with a lowish PIV rating is an excellent way to use that tube in place of a more costly tube sporting a high PIV rating.  Of course meeting a PIV rating is not the only concern.


http://www.fourwater.com/files/fullrect.txt