5f6a clone blowing fuses / Solid state diodes across rectifier tube

[leydenjar]

BASSMAN_5F6A.pdfGZ34.pdf
I have a bassman 5f6a copy that has just blown two fuses and I don't know why.

It doesn't blow fuses when I start it up with a limiter.

I just put new tubes in it, biased them at 35mA, they match, plate voltage is at 475vdc.

There are diodes across pins 4 and 5, and across 7 and 8 on the rectifier.

It just killed a rectifier tube as well. That time it didn't blow a fuse.

I'm getting 125 vac from the wall. It's putting 369 vac on the plates of the rectifier.

All rectifier voltages appear normal. 5.9 vac on the cathodes.

Two questions:

1. Why do you think it's blowing fuses / what am I missing?

2. Why are the solid state diodes where they don't seem to be doing anything? The HV leads go to 4 and 6 so it seems to me the diodes are just sitting there.



Thanks,
leydenjar

[booj]

the diodes would be a protection mod., with the HT wires going to the diodes first, then into the tube. the polarity of the diodes puzzles me. I would think the HT wires when using the mod would go to the anode of the diode first, then the cathode of that diode would connect to the plate of the rectifier.
If you're blowing fuses, then the rectifier fails, my first guess would be the cause was the rectifier tube. I'd check the socket for signs of arcing

[leydenjar]

Thanks Booj.

The diodes should be like this correct?



It appears to me they aren't even in the circuit the way they're soldered on the sockets now.

Is there a chance they were put there as an option? Something that could be implemented if needed later? Or did somebody put these into this Victoria and not know what they were doing?

[tubeswell]

banded ends (cathodes) of each SS diode pointing to the (respective) rectifier anode pins (4 and 6). The diodes in that photo are on the wrong pins (or the pins are labelled wrong in the photo)

GZ34: anode pins = 4 and 6, cathode pins = 2 and 8

[Enzo]

I see the remnant of a wire in pin 7?? It looks like either someone did the diode addition wrong, or the socket used to be wire for some other weird rectifier tube.


Using a slow blow fuse?

[booj]

The drawing looks right to me. It doesn't show the HT lead coming off the socket. On an AB763 Deluxe Reverb layout it shows the HT lead coming off pin 8.

[Bruce / Mission Amps]

Hmmm.... I wonder if was a mod using a DPDT switch to steer HI-V secondary voltages to the tube rectifier or the SS diodes and on to the standby switch and or main filter caps....

[R.G.]

[ATTACH=CONFIG]19765[/ATTACH][ATTACH]19766[/ATTACH]
2. Why are the solid state diodes where they don't seem to be doing anything? The HV leads go to 4 and 6 so it seems to me the diodes are just sitting there.

The mod as described in the drawing uses pins on the rectifier tube socket that are not connected as incoming wire holders for the red/HV wires from the transformer. The solid state diodes are wired in series with the incoming leads. If the GZ34 shorts, the SS diodes take over and prevent further damage to the power trans, filter caps, etc. They are doing something; they are rectifying all the time, but only kick in if the rectifier tube dies. This is one of my favorite tricks in "The Immortal Amplifier".

Does the amp match the drawing?

1. Why do you think it's blowing fuses / what am I missing?

As you note, the fuse doesn't blow when you start it on a limiter. That means that it's the inrush current when the filter caps are empty or when the transformer is de-energized. This could be going-bad-but-not-solidly-broken:
filter caps
rectifier tube
rectifier tube socket
HV wiring of any sort

or it could be an issue with the PT.

Or it could be a wrong-value fuse. Or an instant-blow instead of time-delay fuse. I'd check that last one (slow-blow fuse) first before getting into the rest.

[Enzo]

RG, look at his closeup photo in post #1. The diodes are not in series with anything, they have cathodes to pins 5 and 7 or some such.

Or i am looking at it funny.

[Bruce / Mission Amps]

They need to be in series with the High-Vac before the rectifier lugs 4 and 6 to work, and the cathodes pointing at the lugs...right? You could do that by moving the HI-Vac leads to lugs 5 and 7 and jumper over to 4 and 6 with the diodes though.


Oops never mind that is clearly shown in the drawing supplied by leydenjar.

[R.G.]

RG, look at his closeup photo in post #1. The diodes are not in series with anything, they have cathodes to pins 5 and 7 or some such.

Or i am looking at it funny.

Ah - no, I didn't look closely at his photo. It's not wired per the drawing, so it's not doing the right stuff.

I wonder if what he has is a bad implementation of the mod or a bad reversal of the mod.

[leydenjar]

Ah - no, I didn't look closely at his photo. It's not wired per the drawing, so it's not doing the right stuff.

I wonder if what he has is a bad implementation of the mod or a bad reversal of the mod.

It threw me for a loop when I saw it.

Is it a good mod to implement? i.e. Do you recommend I wire the diodes as depicted in the hand drawn layout?


I put another rectifier in and it's holding up so there's no issue with that for now.

[tubeswell]

Yes - if done properly, it can save an amp from a shorted rectifier tube.