#002 - my journey begins

[Richard1001]

Your voltages are very strange. The voltage on the 10uF should not come down slow but should read around -380 instantly. And since there is allways the load of the 220k cathode resistors in series with the 470k resistors on the bias supply voltage it should not be able to come down as high as - 510 volt either.

What is the voltage without the CF tube installed? (Also remove the powertubes for this measurements since the CF tube controls the grid voltage)

It is also very odd that you have high voltage on the preamp with the amp on standby.

I would suggest to take some time checking the powersupply step by step.

With the amp off you can measure the HV center tap to ground (i mean on the powertransformer) It should read 0 ohm to ground. You have a fuse on this ground wire, replace it or make absolutely sure it is ok. (Measure)

Check the rectifier board by measuring each diode string. If your meter has a diode setting, use this to measure the voltage drop over the diodes. Each should read 0,4 - 0,7 V

If all checks out, you can measure the AC voltage from each PT coil to ground. They should read around 310V on each side.

The standby switch you put in could be measured by disconnecting the side comming from the rectifier/47uF caps and checking if there is still voltage on the preamp. If there is no voltage anymore after disconnecting the wire, check the voltages on the 47u caps.

Take your time, maybe sleep on it and look again another day. Sometimes this helps.to see things that where overlooked earlyer.

[erwin_ve]

Rafael: do you have the protection diode installed? If so check this one too.

[Bombacaototal]

Good morning Richard,

Thanks for the help. I have gone through the steps you mentioned below.

With the amp off I have measured the HV center tap to ground (on the power transformer). On one side of the fuse closer to the PT is 9M and on the other closer to ground is 0Ohm. I have replaced the fuse and now get 2ohms before it and 1.2ohms after. For sure the fuse was gone. Now I am wondering how an user would have been able to identify this and also the reliability of having such type of fuse.

My B+ and plates is now fine at 440V, so the issue was the fuse there.

I have measured now the voltages on the 10uF at the bias and strats at -70V and goes up slowly still. I have removed the CF tube, continuing without the power tubes. Same result but started at -80V instead

AC voltage from each of the 2 PT HT coils are at 318V into the bias board

As far as the wiring, pic attached, of the supply board, I have STDBY1 (white lead) going to one side of the standby switch, the lead just behind it comes from the bias board OUT. STNDBY2 is my yellow lead going to the other side of the standby switch. I think this is correct

I have measured the 6 diodes at the bias board with my Tenma 72-8150 using the diode testing function. I get a reading of around 620 in all of them, I tested an unused diode and I get 614, so it should be fine, although the meter does not give me the unit of measure, neither it is detailed in the manual (page 15) https://datasheet.octopart.com/72-8150- ... 540258.pdf

I removed the 47uF lead from the standbyswitch and now I have no voltages on any cap after the standby. The 47uF cap has 445.5V. The 10uF at the bias board now starts at -110V and still going up slowly.

I have just replaced the standby and it works now. With it engaged there is no voltage, but once you it is turned off then I get the correct voltages going down the stream of caps. Probably busted the two previous ones. The voltage on the 10uF at the bias is still an issue, starts at -110V still. Probably worth replacing that cap? Just fyi, my 10K resistor in parallel with the pot is reading around 5.6K

Hi Erwin, yes the Diode was installed at the supply board, although I am not sure how to access it to measure as it is tucked in between the large caps.

[erwin_ve]

You could unsolder the bias wire that goes to the PI board. If your bias voltage is still low you know it is around the PT and bias board, If it is high you have to look around the PI stage incl the protection diode.

[Bombacaototal]

You could unsolder the bias wire that goes to the PI board. If your bias voltage is still low you know it is around the PT and bias board, If it is high you have to look around the PI stage incl the protection diode.

Thanks for the suggestion Erwin. I removed the lead going to the PI board (called Bias on the pcb). I still get around -110V at start up and it slowly goes up

I measured the diode before the 10uF bias cap and I get 615 which should be fine

Maybe time to replace the 10uf cap? I have run out of stock on the 22uF as well. Will get an order in for a couplde 10uF and 22uF and will replace it. Could the issue be elsewhere?

[Richard1001]

It just occured to me that you probably installed the (to me somewhat questionable) biaspot / variable series resistor that is not original in the schematic and which could explain the slow rising negative bias voltage and low bias supply voltage on the 10uF cap.

Slow rising negative bias supply voltage could also be the cause of the fuse blowing since turning the amp on and of a couple of times with the mains switch, testing different tubes, means the tubes are allready warm and ready with the B+ instantly applied, while the bias supply is not there yet.

The amp Will indeed hum from the huge current draw and the fuse will blow.

[erwin_ve]

It just occured to me that you probably installed the (to me somewhat questionable) biaspot / variable series resistor that is not original in the schematic and which could explain the slow rising negative bias voltage and low bias supply voltage on the 10uF cap.

The bias pot is in parallel with the bias resistor on the first batch of pcb. Off course one could remove the bias pot and replace the 10k resistor on the bias board with a 3k3 or 4k7 resistor.

[Bombacaototal]

It just occured to me that you probably installed the (to me somewhat questionable) biaspot / variable series resistor that is not original in the schematic and which could explain the slow rising negative bias voltage and low bias supply voltage on the 10uF cap.

Slow rising negative bias supply voltage could also be the cause of the fuse blowing since turning the amp on and of a couple of times with the mains switch, testing different tubes, means the tubes are allready warm and ready with the B+ instantly applied, while the bias supply is not there yet.

The amp Will indeed hum from the huge current draw and the fuse will blow.

Hi Richard, I think you found the source of my issue, as this was something that always puzzled me, after having the amp on a lot o times I got the standby ON then OFF I had this weird light saber type of sound/hum for a few seconds. Which I think is what caused the HT fuse to blow and probably the Standby! I think in the end my socket was fine, but good to have changed for peace of mind

What is the solution to fix this bias board without the parallel resistor/pot? Why is the addition of the pot causing the slow increment on the C-?

Just saw Erwins post. Mine is now reading 5.5K but I biased it a bit hot. Should I just replace by a 4.7K and have no bias adjustment?

[erwin_ve]

I think there are 2 possibilities: either the bias resistance is off or the bias cap is leaking. My guess is the latter.

[Bombacaototal]

I think there are 2 possibilities: either the bias resistance is off or the bias cap is leaking. My guess is the latter.

Thanks for the input Erwin. I should have replacements caps by Friday latest, I will replace it and update accordingly.

What could have been the root cause of my fuse, standby and potentially the bias cap going and how can I prevent this from happening again?

[erwin_ve]

I think there are 2 possibilities: either the bias resistance is off or the bias cap is leaking. My guess is the latter.

Thanks for the input Erwin. I should have replacements caps by Friday latest, I will replace it and update accordingly.

What could have been the root cause of my fuse, standby and potentially the bias cap going and how can I prevent this from happening again?

If your bias cap fails(production failure?), your bias voltage drop and the powertubes will pass more current. Too much current will cause heat in fe. the standby switch which may have melted some supporting plastics or put a similar conducting path as on your tube socket. Off course too much current will also take out the fuse.

[Richard1001]

I think there are 2 possibilities: either the bias resistance is off or the bias cap is leaking. My guess is the latter.

Thanks for the input Erwin. I should have replacements caps by Friday latest, I will replace it and update accordingly.

What could have been the root cause of my fuse, standby and potentially the bias cap going and how can I prevent this from happening again?

The cap you got should be very good and long life. 22000 hours at 125 degrees Celcius. In your amp it should work for 30 years. Like Erwin pointed out, it could just be bad luck you got a bad cap with an existing defect. Still, there could be other things wrong. Bad connection or other paths bleeding current to ground lowering the voltage. Dirt / solder pieces on the rectifier board or leaking current through the bias pot to ground for instance. Even the diode could be leaking.

The root cause is the bias not reaching the needed voltage fast enough (or not at all). To prevent future fails this should be fixed. By switching the amp on and off a couple of times using the mains switch, the slow rising bias voltage blew the fuse. This was caused by hot tubes ready to conduct and B+ instantly available after switching the mains on while the bias voltage was not there yet making the powertubes draw to much current.

The fuse blew and doing so prevented further damage. The fuse did its job and it it is a good thing it was there. You know now to allways measure and never check fuses just by looking at them.

The standby switch failure could be related but also could be unrelated. Thing happening at the same time (or things you notice at the same time) do not necessarily have a relation. It could already have a failure. For instance: a piece of solder or cut off wire could easely fall in to an open Carling switch. You could have damaged the switch soldering on it, etc. I would suggest opening up the defect switch and find out. It is another opertunity to learn.

[Bombacaototal]

Caps arrived. I replaced the KEmet to a F&T 10uF 500V. Now I get -345V (both positions of the standby) which is good.

Time to get the power tubes in, any tips in regards to checking if the power tubes are good beforehand

EDIT: 2 power tubes test is looking good, all voltages are fine and there is sound, no noise or hiss. Time for the 4 tubes

EDIT2: all fine as well with the 4 tubes. I am wondering if I should leave the amp out of the shell for a while to monitor the C-. Any idea why the bias cap failed and how I can prevent this from happening again?

[Bombacaototal]

OK, not so good. I turned on and off and when I turned on again, I head a hum and saw the PT HT fuse go. I was monitoring C- and had -356V, so not there the issue

Standby Switch is gone again. Back with 2 power tubes and seems fine. Will test further. Probably a bad power tube (one of the two I removed?). Bias seems fine at 36/37mA on the two tubes. Voltages seem good across as well

EDIT 1: So I turned on and off again and heard the hum again (turned off imediately) but C- was in rage at -350V. Any idea of what else may be causing the PT to pull so much current?

EDIT 2: Power tubes all out again, and all voltages look fine. To me its clear that on first start it is fine, but if I shut down and bring the amp back up, the tubes already have the B+ and this is where there is the issue

[erwin_ve]

So the new bias cap is giving the right voltage, but when the previous one failed the excess of current likely took out something else . Tube, screenresistor? Also the things Richard mentioned should be worth checking.