VHT Lead 40 Blowing Internal Fuses

[mikeboone]

Have you tried with everything connected except the red winding of the PT?
If it still blows, then bias and power tube issues are not suspects.

I haven't tried everything disconnected yet. The initial indication is the fuses are blowing on the violet taps for that secondary power supply. I really don't understand how pulling the power tubes had an affect on that since the heaters are AC off of a separate tap that isn't blowing fuses.

I will give this a look... At times I get too focused on an idea. I need to reboot.

Thanks.

[mikeboone]

What is the actual voltage on the output plates in this amp? The schematic for the 30 may not be the same as the 40.

When the fuse blows, is it after a warm up or does it blow quickly? Are you using the correct fuses?

The actual plate voltage will have to be measured unloaded. The limiter has an affect on that. I can't measure it plugged in standard power, it blows those fuses.

I am using 1A fast acting fuses just like the blown factory fuses.

[mikeboone]

Set the bias for most negative voltage you can get and see if that helps.
I don't think the -13V bias is that far out though, seems ball park for EL84's at that plate voltage.

I tried the most negative voltage (-21vdc) earlier today with no success.

[g1]

I haven't tried everything disconnected yet. The initial indication is the fuses are blowing on the violet taps for that secondary power supply.

Not everything disconnected, everything connected except the reds from the PT.
If there is some fault related to the power tubes, it's possible that pulling the violets dims the bulb because you are taking away the heaters.
Checking with violets connected but reds open will help verify.

[mikeboone]

Not everything disconnected, everything connected except the reds from the PT.
If there is some fault related to the power tubes, it's possible that pulling the violets dims the bulb because you are taking away the heaters.
Checking with violets connected but reds open will help verify.

Understood. Will check mañana. Already shut down the ship for the night.

Thanks guys!

[52 Bill]

Just a quick note of something to check, is the board marked for the fuse ratings? The schematic shows T1.5A rating for those fuses.

[Jazz P Bass]

T 1.5A is a far cry from "I am using 1A fast acting fuses just like the blown factory fuses. "

[J M Fahey]

The actual plate voltage will have to be measured unloaded. The limiter has an affect on that. I can't measure it plugged in standard power, it blows those fuses.

I am using 1A fast acting fuses just like the blown factory fuses.

For now I wouldn't worry about the mains fuse because the ones that do are the violet/violet low voltage ones.
And the schematic says "1.5A" .... what was actually there? 1A?????

EDIT: just saw later posts.

Maybe the fuses blew once for an unknown reason and from there on faster and smaller ones were substituted.

FWIW regulators and low voltage supply in general seem to be behaving , so at least we have no shorts or similar gross stuff.

Maybe things are normal, someday somebody fit 1A fuses there ... and it worked, although marginally.

One day they blew and later "same spec" ones didn't take it.

I'd get a couple 1.5A ones and try them, progressively:
1) turn amp on, without 12AX7
2) same but with 2 of them, one "above" , one "below" , if that makes sense to you. (say V1 and V3 or V2 and V4 so we load symmetrically 0>15 and 15>30 supplies)

Very klunky, noob EE student design.

Unelegant, works only if everything is perfect but does not "fail safe".

In other amps a problem may kill reverb or the loop or one preamp or distortion or chorus or unplugging one tube lets others work ... here any failure is catastrophic.
Oh well.

Edit 2

WARNING, ENTERING RANT AREA:
another example against shotgunning parts: Tech could have replaced :
> 2 regulators
> 6 diodes
> 11 caps (of which 4 relatively large 2200x25V electrolytics)
> 5 resistors (of which 2 large 7W ones)
and maybe:
> 4 12AX7
> 4 Noval sockets
> assorted wiring and connectors.
> PCB murdering (resoldering pads, pulling, reinserting and soldering parts legs and cables, cutting traces to isolate sections, then scratching and resoldering)
AND HE STILL WOULD NOT HAVE SOLVED THE PROBLEM

[mikeboone]

Recapping from original post, the schematic is for a 20W. Can't find one for the 40.

The only fuses that are blowing are in the low voltage supply. Perhaps that's what had me fixated on that part of the amp. The reference designators and parts are the same values in the amp as they are on the schematic. My hope was that part of the circuit is similar enough for me to follow to the issue.

The fuses are supposed to be 1A as it is silkscreened on the board.

I'm not following the schematic as gospel but as a reference since the circuit architecture is similar.

[Jazz P Bass]

Use a time delay fuse.

[g1]

The lead 40 is supposed to be like the lead20 with 4 power tubes instead of 2.
The schematic posted shows 4, with "no" labelled around the stuff that wouldn't be in the 20 version.
It's quite odd they would use smaller fuses there for more power tube heaters.

All that being said, it's possible it's just the turn on surge of the heaters killing the fuses, in which case slow-blows should solve it.
But have you tried asking VHT about that fuse value, or for a proper schematic?

[J M Fahey]

The lead 40 is supposed to be like the lead20 with 4 power tubes instead of 2.
The schematic posted shows 4, with "no" labelled around the stuff that wouldn't be in the 20 version.
It's quite odd they would use smaller fuses there for more power tube heaters.

It dioes not matter, both (20 and 40) have the same preamp, and low voltage supply only feeds their filaments.

All that being said, it's possible it's just the turn on surge of the heaters killing the fuses, in which case slow-blows should solve it.

Guess the same, *maybe* 1A is fine, but only if T1A (the timed/slow ones) .

It's funny because those filaments only need 300mA DC (4 x 12AX7@12V series parallel) so simple logic says 1A fuses should be more than enough, but probably the combination of cap charging surge and cold filaments surge are enough to kill them.

[g1]

It does not matter, both (20 and 40) have the same preamp, and low voltage supply only feeds their filaments.

Missed that part, thanks!

[mikeboone]

The lead 40 is supposed to be like the lead20 with 4 power tubes instead of 2.
The schematic posted shows 4, with "no" labelled around the stuff that wouldn't be in the 20 version.
It's quite odd they would use smaller fuses there for more power tube heaters.

All that being said, it's possible it's just the turn on surge of the heaters killing the fuses, in which case slow-blows should solve it.
But have you tried asking VHT about that fuse value, or for a proper schematic?

The fuses in questions aren't the heater fuses. They are the fuses for the low voltage supply, which feeds a multitude of circuits on the amp. The heater fuses are 6.3A and are not blown.

All that said, something has changed. Dont know what or why but it did. I disconnected the HT taps (red) from the board with everything else in circuit. No bright glow on limiter. Every thing else looks good, the low voltage supply is kicking at 15vdc and 30vdc, the bias is fine with a range of -9vdc to -26vdc. The heaters for the power tubes are at 7vac. Plug the HT taps back in and it burns brightly like the sun. No tubes are installed and I also took the OT PRI off of the power tubes to rule it out.

I am terribly confused.

Now, I am more than positive that I have never unplugged the HT taps until now. And All those other symptoms and measurements I made were made with the HT taps in place. HT fuse is in tact, has not blown.

So, with that said, I checked the rectifiers with my diode checker and they show .5VDC and OL just as they should. None of the filter caps are bloated or leaking, nor are any of them shorted to the 0VDC reference. The amp is not grounded to chassis, there is a 10K resistor isolating it.

Short of shot gunning and changing all the filter caps, is there any other way to test them?

EDIT: VHT has not replies to my request for a Local Service Center to have it repaired. I thought I'd ask for that instead of asking for a schematic and getting no reply. I am looking at this as a favor to a friend.

[52 Bill]

The fuses in questions aren't the heater fuses. They are the fuses for the low voltage supply, which feeds a multitude of circuits on the amp. The heater fuses are 6.3A and are not blown.

The low voltage supply also supplies the heaters for the 4-12ax7 tubes, the 6.3 amp fuses are for the power tube filaments only.