Need advice about adding HT fuse to Ampeg B25

[Joel in Texas]

Hello all.  I'm working on my Ampeg B25 again.

I want to fuse the HT supply, and I need some advice.

Here is a link to the schematic:

https://irationaudio.files.wordpress.com/2015/03/ampeg-b-25-schematic1.jpg

Where should I add it?  I was thinking it should go right before the first power supply cap (marked A on the schem). 

But in reading about adding HT fuses, some are saying it can be added to the center tap on the power transformer secondary - so maybe where the standby fuse is located on the schem? (note: I have previously removed that standby switch implementation on this amp; I current have it tied to chassis ground).  In some of the discussion about adding an HT fuse to the PT secondary CT, there were warnings NOT to do it when some bias supply implementations were present, but I didn't understand fully.

Where would you add it?  Before node A of the power supply (before the first filter cap), or at that center tap of the power supply secondary?

Thanks for any advice,

-Joel

[jjasilli]

Google Valve Wiszard fuses

[Joel in Texas]

Thank you for the response.  Sorry to say, but I need more help.

I found this Valve Wizard posting here: http://www.valvewizard.co.uk/fuses.html

For me, it is generally informative but it does not provide enough information to answer my question - at least not for someone at my level of knowledge.

I would have to fill in a lot of gaps between his explanation/examples and my specific situation.

Most of his discussion centers around solid state rectifiers, and how they tend to fail.  My Ampeg B25 uses a 5AR4 tube rectifier.  While solid state and tube rectifiers both serve to convert AC to DC, I know enough to understand that overall they can behave in really different ways.  This may or may not extend to issues of how they might tend to fail.  So I can't necessarily generalize all of what he says in his solid state examples, to my specific example.

Also, his general principle that the HT fuse should go on the AC side rather than the DC side might (?) still apply to the tube rectifier context.  Assuming it does, would I put two fuses, on both sides of the secondary AC winding before the two 5AR4 diodes?  Or can I put just one on the center tap of the secondary, where the standby switch is currently shown on the schematic? 

If it is not clear from the above, I need further advice.

Thanks again for the help so far.

[sluckey]

I would add the fuse just before node A of the power supply (before the first filter cap). Either place accomplishes the same as far as killing the B+ voltage. Makes no difference if you fuse the negative or positive side of a battery.


However, when you put the fuse in the CT of the PT if it blows you also lose the negative bias. Same with the STBY switch in this amp. I would not put a fuse or STBY switch in the CT of the PT in an amp whose negative bias circuit also depends on the CT being connected to ground.

[jjasilli]

sluckey's solution is fine.  But Merlin advises to fuse AC instead of DC. 


SS or tube rectifier makes no difference here.  Merlin's drawings re CT (Center Tap) PT are applicable.  Your tube rectifier encases 2 diodes acting just like the SS drawings.  If you want to fuse the AC portion of the circuit, follow those drawings.

[tubeswell]

See Immortal Amplifier Mods by RG Keen. He did a series of articles for Premier Guitar magazine a few years ago, and one of the articles was all about fuses on secondary winding circuits.


https://www.premierguitar.com/authors/69-r-g-keen

[Joel in Texas]

Thanks everyone for all of the good info and advice.  I appreciate it.

[trobbins]

Joel, how were you going to choose the value of the fuse?

One way is to insert a low value current sense resistor in the CT lead or in place of the fuse, and measure the rms current during normal operation.  That gives a baseline fuse current rating to keep above, but the measurement may not be too accurate depending on how well your meter reads high crest-factor waveforms.

Another way is to simulate the power supply using PSUD2.  That requires measuring transformer winding resistances, and winding voltages, and adding up the currents drawn from A and through the dropper resistors R41/2.  Simulation can also be used to get a good estimate of whether to use a fast or slow-blow fuse, but can take some effort to work through compared to just grabbing any fuse value to hand.  The linked article goes through the PSUD2 process.

https://www.dalmura.com.au/static/Valve%20amp%20fusing.pdf

[pdf64]

The issue with the bias supply and lifting of the HT winding's CT (eg fuse, standby) occurs when, as with a regular TR BF Fender, the bias is fed from a low voltage tap off the winding, then rectifier and (low voltage) reservoir cap.
If, eg a CT fuse blows and the CT loses it's 0V reference, then the bias supply cap can get a lot of voltage across it, as the winding ends get a 'non-obvious' 0V reference via their rectifier, reservoir and HT circuit, the latter tending to be much lower impedance than that of the bias supply. So a blown CT HT fuse would then result in the bias circuit being wrecked 

That's not an issue with the Ampeg B25 schematic, as the bias supply is taken from a potential divider fed by the full winding 'phase', so the Vac to it can't increase massively if the CT loses its 0V ref.

Regarding HT fusing here, whilst taking full account of the benefits of each winding end being fused, my view is that as long as the HT rectifier is trippled up (ie 3 diodes in series, each rated to full PIV) and so extremely unlikely to short, and the bias supply is ok as above, then a single HT fuse the CT return to 0V is fine. And is a lot more user friendly 

Plus, if one of the rectifers is a GZ34 and there's no silly standby, a quick blow F fuse can be used.