New 5f6a homebrew build blows fuse right after flipping standby

[Frisco]

Thanks for taking the ime to read this. Consider me a beginner.

The title explains the problem. I turn the amp on and all tubes light up and look good. Flip the standby and the fuse blows immediately.

I am building a light bulb limiter to start some troubleshooting, but in the meantime please chime in about any useful multimeter readings or other tests I should be doing.

[xtian]

Hi! Light bulb limiter is an EXCELLENT idea. Take a step back, pull all tubes, and read Paul Ruby's advice for first time power up procedures:

Paul Ruby Amplifiers

[pdf64]

Once you've sorted it, bear in mind that the 5F6A standby arrangement is a dreadful clodge, having no technical benefit, but, by hot switching the reservoir, causing surge current that likely exceeds the rectifier plates' limit.
If determined to retain and use that that arrangement then a suitable silicon diode in series with each rectifier plate is a good idea, to accommodate / protect / maintain operation when it shorts plate-plate or plate-cathode.

[Frisco]

Hi! Light bulb limiter is an EXCELLENT idea. Take a step back, pull all tubes, and read Paul Ruby's advice for first time power up procedures:

Paul Ruby Amplifiers

Thanks for the useful link. I will do the bulb limiter tests and report back.

[Frisco]

So I did the bulb limiter test, following the sequence in Robinette's website:

All tubes in, power switch on, amp on standby: dim bulb.

All tubes in, power switch on, flip standby to play: bright bulb. This indicates a short somewhere in the amp as I understand.

All tubes in, power switch on, standby on play, take one power tube out: light stays bright.

Put the power tube back, remove the other one: Light stays bright.

Remove both power tubes: Light stays bright.

Flip the standby to standby mode, the light dims.

At this point, I understand that the short is in the filter cap assembly.

I did a continuity test between the cap assembly and the circuit board and they are connected properly. I also checked proper grounding of the caps and that is good too.

I don't have a capacitor tester, so this brings me to my questions:

Should I buy new caps and install them? Is there any other test that would help me confirm the bad cap diagnosis?


Thanks in advance for your advice.

[Frisco]

Hi! Light bulb limiter is an EXCELLENT idea. Take a step back, pull all tubes, and read Paul Ruby's advice for first time power up procedures:

Paul Ruby Amplifiers

So I did the bulb limiter test in the following sequence:

Turned the on switch with all tubes in, and the bulb is dim.

Then, flipped the standby to the play position, and the light shines bright (this means I do have a short somewhere).

Then I take the power tubes out one at a time, then both together. Light stays bright each time.

Then put the amp on standby, the light dims.

At this point, the diagnosis should be a short in the filter caps. Do you agree?

I checked for proper grounding and proper continuity to and from the filter caps and most everything checks out (see below).

My next step will be to replace the caps, as I don't own a cap tester. What do you think?


One thing that I found odd when doing connectivity tests is that there is a faint connectivity between the A and B connections from the filter caps to the circuit board (very slight intermittent beep from the multimeter).

[pdf64]

There could be various shorts / wiring errors that would cause this; as you're not an experienced builder, that may be more likely than the issue being a faulty part, with new parts made by a reputable manufacturer and bought from a good vendor generally having a vanishingly small failure rate.
A cap tester is not needed for this type of basic fault finding, eg try lifting one leg of each cap, test the amp again with the limiter, then move on to the next.

[R.G.]

Are the caps wired into the circuit backwards? Wrong polarity?

Before buying new caps, you might try opening/lifting one lead of each cap and then re-doing the LBL test. It could be a short in the rest of the wiring after the caps, too.

Edit: simulpost. Yes, what he said!

[Jazz P Bass]

One thing that I found odd when doing connectivity tests is that there is a faint connectivity between the A and B connections from the filter caps to the circuit board (very slight intermittent beep from the multimeter).

It appears that you are using the 'diode test' function of your meter.
Perhaps to locate gross faults?

I think more could be learned by using the resistance scale.

[Justin Thomas]

There's always here:

New Page 1

Still my go-to spot. In the top left corner, "tube amp debug page." Very detailed instructions for "blows fuses" within.

Justin

[Frisco]

All, thanks for bearing with me.

I re did the bulb limiter tests, this time lifting each cap one at a time, then rewiring after the test. I still got a short each time.

The caps are wired with the correct polarity. So I am thinking that unless it is more than one cap that's shorting at the same time, it might be something after the cap assembly.

Any ideas about what to look for next will be appreciated. To my untrained eye it looks like a shorted OT or choke would do this, among many other things.

In the meantime, I am going to conscientiously look at the layout to see if I did an obvious rookie mistake in that section of the amp. Otherwise I await your input.

[pdf64]

Can you measure a steady resistance between HT and chassis 0V after the standby switch (amp de-energised and isolated, standby switch in standby mode)?

[Frisco]

Can you measure a steady resistance between HT and chassis 0V after the standby switch (amp de-energised and isolated, standby switch in standby mode)?

I am sorry, can you explain? I know 0V is one if the wires on the PT going to ground. Are the HT THE 325V wires from the PT?

[Chuck H]

For reference purposes:

[Chuck H]

Divide and conquer. Lift the OT CT. Still shorted? If yes it's not the OT. Lift the choke input. Still shorted? If yes it's not the choke or anything down stream of the choke. I can't think of any way to wire the rectifier so it behaves until the standby is disengaged. So if the OT and the choke check out that would only leave the main filters or the way they are wired.