1971 50 Watt - Mains fuse keeps blowing

[Loop-Master Pedals]

Seeing if anyone can help:

Installed new caps, new bias caps, new tubes, and biased the amp in early 2018. (1971 50 Watt No Mods etc..)

I used this amp extensively after all of that without any problems throughout 2018.

In about November, I went into rehearsal and fired it up. Powered it on and on and let it sit in stand by mode. No playing or nothing. Just warming it up. Came back to the amp about 30 minutes later ready to play and the LED was off. The 2 amp fuse had blown.

I put in a new fuse and it blew as soon as I powered it on. LED didn't even blink. It blew fuse after fuse.

Pulled tubes, installed new fuse, still blew the fuse without tubes.

I have checked the mains tranny (resistance check across windings) and it is good. Checked the diode beside the bias caps and it is good.

Any other ideas? Thanks

[neikeel]

The commonest cause for this problem on this era of 50w is a blown PT. You say you have excluded this.
By saying you have a 2A fuse this means you are on Euro voltages and using T2A (Slo-blo) fuse.

Without looking at your amp - whether it has been modded, including the PT and rectifier and the quality of the parts used in the refurb.

Is it a stand up or laydown PT? Do you have single or double pole switches?

How is your bias feed wired? Did you correct the factory error that takes the bias from the 'cold' limb of the standby switch (or the diode block) or from the hot side of the standby - ie sees bias in standby.

Lots of questions but we can help.

[jnew]

Oh WOW! What timing. I mean no disrespect to the OP here and I really hope this doesn't end up being a transformer having gone bad. That would be SAD.

But while we are on this subject, I'm working on a 50W and have been trying to research the bias supply issues. Unlike a 100W amp, there isn't a bias supply tap off of the PT. I've read on some models, depending on year (notably 72's) that many OPT Transformers were lost because of how the bias supply came off the rectifier bridge essentially bypassing the standby switch. But in all the pics I've been able to see so far, it looks like when it is connected to the standby, it is on the side that directly hot wires it to one of the HV taps. So to me, this also bypasses the standby switch. NO? This has me a bit perplexed. Do we want the steady current when the power is on? Or do we want the standby switch acting as an on/off for the bias supply? Also, does it matter which HV leg is used? It doesn't seem like there should be any difference which leg but while I'm asking?

[danman]

Jnew...the issue with the older models was the lack of bias voltage until the standby switch was switched into playing mode. This causes a current surge when the standby is flipped to playing mode because the power tubes are running full tilt while the bias circuit charges and begins to develop the bias voltage. You want to place the bias tap on the same side of the standby switch that the PT secondary leads are attached to. This will ensure that the bias circuit charges up anytime the amp is powered up.

[jnew]

Got it. So it is the surging of current that we are eliminating from the standby switch functions, by wiring it straight to an HV source. I don’t suppose it matters which HV leg of the PT is used since they are both about the same voltage. Sound right?

[neikeel]

Got it. So it is the surging of current that we are eliminating from the standby switch functions, by wiring it straight to an HV source. I don’t suppose it matters which HV leg of the PT is used since they are both about the same voltage. Sound right?

Correct.

[jnew]

AWESOME! The Opt on this build looks to be an original. I don’t know all the #’s on Drake and Dagnall iron but this one is 784 139. Would hated to have put it at risk.

[Loop-Master Pedals]

The commonest cause for this problem on this era of 50w is a blown PT. You say you have excluded this.
By saying you have a 2A fuse this means you are on Euro voltages and using T2A (Slo-blo) fuse.

Without looking at your amp - whether it has been modded, including the PT and rectifier and the quality of the parts used in the refurb.

Is it a stand up or laydown PT? Do you have single or double pole switches?

How is your bias feed wired? Did you correct the factory error that takes the bias from the 'cold' limb of the standby switch (or the diode block) or from the hot side of the standby - ie sees bias in standby.

Lots of questions but we can help.

I'll try to get a pic posted up asap. It is a stand up transformer and there are double pole switches. The bias feed thing I am unsure of at the moment.

[Loop-Master Pedals]

Here ya go guys:

[neikeel]

So, as you say the PT has been replaced/grafted in.
The switches have all been replaced, as has the pilot lamp, snubber caps removed.
HT fuse holder is new too, as are the filter caps, bias caps and bias range resistor.

The preamp and the rest looks really neat and tidy.

Looking at the pics I am guessing the red bias feed wire is connecte to the same lug as one of teh red wires from the PT?
If that is the case that is good, if is is on a lug with blue wire (that goes to where the snubbers were and then on to the diode block).



Do it in the order I suggest as it avoids smoking too many fuses unnecessarily.

If the amp is plugged into the mains with tubes out (no need for load) and HT fuse removed and standby off. If you look at the schematic you will see that the PT secondary powers the heaters (orange and black wires to V5 and the brown wire centre tap) for 3.15vac ground to orange and same black to ground with black to orange 6.3vac. Because the standby is off the HT is only going to the switch and bias circuit these are the only things PT heaters and HT) where a short can occur to blow your mains fuse.

You can temporarily lift the bias feed wire to eliminate that aspect before testing (unusual but to be considered).

If your mains fuse goes with standby (HT) out of circuit and the bias out of curcuit and no tube in V5 you can safely say the PT is dead (cue Python's Parrot sketch etc).

If it does not then resolder the bias wire on and repeat.

If still good throw the HT switch with HT fuse out, if fuse goes now it is either your diode block or mains filter can.
If still ok switch off and replace HT fuse, try again if mains or HT fuse go now then problem is in the B+ power line (shorted OT, shorted choke).

If still good drop in known good preamp tubes, if still good some output tubes with amp hooked up to a matching load (cab?) to your selector.

Make sure you are using slo BLO T2A for mains and slo blo T500mA for HT.

Post back your findings

[jnew]

Well, probably serves me right for chiming in on this thread about a separate issue because now I’m having the exact same problem blowing means fuses. The procedure posted above sounds awesome. I’ll be putting it to use ASAP.

[jnew]

I had the diodes backards.

Amp sounds like the quintessential Marshall. It's killer. I don't know if anyone keeps track or has any interest in PT sag but I'm seeing 425V's on the plates and a big hard strum on an open E chord pulls the voltages down to about 310 or so.

[neikeel]

I had the diodes backards.

Amp sounds like the quintessential Marshall. It's killer. I don't know if anyone keeps track or has any interest in PT sag but I'm seeing 425V's on the plates and a big hard strum on an open E chord pulls the voltages down to about 310 or so.

Write 1,000 lines "I must check diode polarity before soldering" and you will not do it again.
Sag is pretty typical.

Waiting for OP to chime back in.
Forgot to mark the schematic above up that the bias feed should be on the other side of the standby switch to that shown above so the bias circuit sees juice when on standby.

[jnew]

Well fortunately I had built a full wave bridge like 100W's use. So I didn't have to unsolder any of the diodes. Only had to move one B+ wire to the other side of the bridge, so to speak, where those diodes were properly oriented. Was a very quick and easy fix. I'll try and post a pic of that little gizmo I built.

[jnew]

I had the diodes backards.

Amp sounds like the quintessential Marshall. It's killer. I don't know if anyone keeps track or has any interest in PT sag but I'm seeing 425V's on the plates and a big hard strum on an open E chord pulls the voltages down to about 310 or so.

Write 1,000 lines "I must check diode polarity before soldering" and you will not do it again.
Sag is pretty typical.

Waiting for OP to chime back in.
Forgot to mark the schematic above up that the bias feed should be on the other side of the standby switch to that shown above so the bias circuit sees juice when on standby.

In the pics from above, are you suggesting that red wires from the PT and bias tap be placed on the bottom lugs of the switch while moving the blue wires to the rectifier to the top lugs?