The BA-3 as preamp build guide

[6L6]

Subbu- as always, please post a series of in-focus, well-lit photos as well as the exact schematic of your PSU.

 

[Subbu K]

Subbu- as always, please post a series of in-focus, well-lit photos as well as the exact schematic of your PSU.

I will take some pictures of the build tonight and post it. Thanks.

 

[ozorfis]

You could try around 250 Ohms 5W resistors as a load to test your PSU (normal BA-3 should draw around 100 mA or so...more with higher bias).

If with dummy load behaviour is the same, try a higher wattage bulb - maybe the 35 W bulb drops enough voltage without lighting up.

 

[Subbu K]

I measured the voltage / current with dummy load and the behavior was same as connecting the line stage.

I removed the 25w lamp after confirming no short, etc and repeated the tests. There was no voltage drop after connecting the load. Set the bias / offset again and let the line stage play for an hour. All voltages were stable. Will let it run for some more time and measure before putting into a case.

Thanks to all for the help.

 

[6L6]

Great news!!

 

[Chiptech]

This didn't go as planned

I was doing my first dim bulb test. With everything on, the bulb never brightened, but the back of the switch started smoking pretty good. Whew, that is an acrid smell.

Appreciate any help on sorting this out.

Using a F1AL250V 6X30MM 1A Fast Blow Fuse 1 Amp 250V F1AL Glass. The fuse did not blow.

 

[andynor]

You have a short in your switch.

Measure resistance between AC positive and neutral solder joint on the switch, and I think you will find your culprit.

Edit: I would think you have a short in/at the switch. The board traces are unknown to me, but I think you’ll solve it through a bit of troubleshooting ivo the switch.

 

[budwiser]

I was doing my first dim bulb test. With everything on, the bulb never brightened, but the back of the switch started smoking pretty good. Whew, that is an acrid smell.

Appreciate any help on sorting this out.

Using a F1AL250V 6X30MM 1A Fast Blow Fuse 1 Amp 250V F1AL Glass. The fuse did not blow.

I use the same switch. Here is my connection. I use a wire terminal block to terminate transformer leads to than run single wires to switch.

 

[ItsAllInMyHead]

Not disagreeing with Andy, but with a short at the mains switch (or neutral directly shorted to hot either from your IEC or on the transformer primaries), the fuse likely would have blown, and/or your bulb likely would have glowed brightly.

A potential cause for your bulb not glowing brightly (assuming a few things) is that wattage of the bulb may be too high. Still, it's a good idea to use a DBT, and even a bulb with too high a wattage should indicate a mains short.

Beep test (power off) from IEC through to the transformer primaries with the switch in all 3 positions. I can't be sure, but my guess is that you also only need one of those caps on the switch board. Doubtful that it'll hurt anything having the 2nd, but just mentioning.

With what I'm using now it's hard to see your wiring, but I think I see the secondaries hooked to your PSU board, but no V+ / V- connections to your BA-3 board, correct?

If so, if your beep testing goes well / matches the schematic, and if you have a Variac (or similar) you can slowly ramp up the mains AC voltage while measuring the voltage at the secondaries. You can do it with the DBT in place.

If you don't have a Variac, I'd disconnect the PSU from the secondaries. Then check AC voltage at the secondaries. If you get past that, then put them back.

The switch is rated for 120VAC and 0A6, so if you use a ~60W bulb (or less) in your DBT, you likely won't melt your switch even if you mis-beeped and do actually have a short in your switch. I typically use a 20W bulb or even a little candle shaped 5W bulb for a quick heart-saving DBT check on a PSU with no load.

I don't know the current draw under normal operation or the VA of the transformer you're using, but a 1A fuse seems too high to me for this circuit... again, not root cause, but just mentioning.

I'm sure others will chime in if I've missed something or have it wrong.

 

[Chiptech]

I use the same switch. Here is my connection. I use a wire terminal block to terminate transformer leads to than run single wires to switch.

I don't quite follow what you are describing, even with the photo. I don't see your terminal block in the photo. I think you are saying you use a block to run both red or black leads to combine and have just one wire connect to the switch?

 

[Chiptech]

Not disagreeing with Andy, but with a short at the mains switch (or neutral directly shorted to hot either from your IEC or on the transformer primaries), the fuse likely would have blown, and/or your bulb likely would have glowed brightly.

A potential cause for your bulb not glowing brightly (assuming a few things) is that wattage of the bulb may be too high. Still, it's a good idea to use a DBT, and even a bulb with too high a wattage should indicate a mains short.

Beep test (power off) from IEC through to the transformer primaries with the switch in all 3 positions. I can't be sure, but my guess is that you also only need one of those caps on the switch board. Doubtful that it'll hurt anything having the 2nd, but just mentioning.

With what I'm using now it's hard to see your wiring, but I think I see the secondaries hooked to your PSU board, but no V+ / V- connections to your BA-3 board, correct?

If so, if your beep testing goes well / matches the schematic, and if you have a Variac (or similar) you can slowly ramp up the mains AC voltage while measuring the voltage at the secondaries. You can do it with the DBT in place.

If you don't have a Variac, I'd disconnect the PSU from the secondaries. Then check AC voltage at the secondaries. If you get past that, then put them back.

The switch is rated for 120VAC and 0A6, so if you use a ~60W bulb (or less) in your DBT, you likely won't melt your switch even if you mis-beeped and do actually have a short in your switch. I typically use a 20W bulb or even a little candle shaped 5W bulb for a quick heart-saving DBT check on a PSU with no load.

I don't know the current draw under normal operation or the VA of the transformer you're using, but a 1A fuse seems too high to me for this circuit... again, not root cause, but just mentioning.

I'm sure others will chime in if I've missed something or have it wrong.

My BA-3 board is not connected.

My beep test in all three position provides only OL.

I will use a 25W bulb going forward and I have a Variac.

Should I move to a 0.5 A fast blow fuse?

Should I retest with the dim bulb and Variac? Or is this switch dead?

thanks

 

[flamethrower1]

pot setting

Just wondering if these are the correct values for the initial setting of P1 and P2?

 

[Zen Mod]

you must check that in situ, as 0R reading across C1,C2

 

[ItsAllInMyHead]

@Chiptech

traveling ... sorry for the delay. Hopefully others will also chime in with help. I have spotty internet until ~ the 17th.

You need to be more specific, please. OL could either be really good ... or not so good. OL from where to where? Walk us through it.

What I'm suggesting is that for two wires (hot and neutral) going from the IEC to the switch (hot via a fuse), it's not too challenging for a veteran builder such as yourself to get it wired properly. Plus, from what little I could see, it looks right.

Since the DBT didn't light up ... it's unlikely (to me) that you had a short at the switch or IEC. Depending on the wattage of the bulb you used in your DBT, that may have prevented the fuse from blowing even if you did have a short ... It would/should not blow unless you were drawing over an amp. So, if you had anything less than ~120W bulb... it won't draw over an amp. Since the bulb didn't light... I lean toward no short. Just need you to confirm with measurements.

Once you are confident that you don't have a short and that the two positions of the switch that you need (off and AC-1 on) are working properly, you can proceed to check the PSU circuit itself. Where I am cautious is that you said the smell came from the switch. I didn't see any charring or part damage, but something in the switch itself may have gotten toasty that pics don't reveal. Even though a short seems unlikely to me... It's best to rule everything out before moving on. Make sense?

re: fuse values. I don't know the actual expected current draw of the circuit (I just suspect that it's well below 1A). Also the transformer doesn't look power-amp huge based on relative size, so...

You can read back in the thread to find the expected current draw or use the transformer VA / Mains voltage rule of thumb. I go with the highest, and pick the closest fuse value.

Again... this what I do... and I screw up a lot. The surest way to get you a good answer quickly is for me to post bad advice and have someone correct me.

Hope you get it sorted quickly.

 

[Chiptech]

ItsAllInMyHead

Thanks for your input.

I read OL when testing the switch at input and output with no power and testing the 3 switch positions.

I was using a 100W bulb. Now will use a 25W.

The photos suggest the switch didn't short, but I saw the smoke coming from it yesterday, which gives me pause.

I guess I can try a new DBT with the 25W and be ready to turn it off if it smokes?

 

[Zen Mod]

that switch for mains???????

get something plasticky

that Drek isn't even made for mains voltage

 

[Chiptech]

Well, this might be a good place to use Mark's soft-start PCB and on/off push button.

I went with the AC switch from Glassware as I had seen other's use it.

 

[myleftear]

or look up at glassware what switch they recommend for the jig?
(FWIW, for rotary power-switches, I used lorlin: RMS – Lorlin Electronics — quite nice...)

 

[Chiptech]

thanks for the tip. I will add the RMS to my Mouser order list.

The Glassware Audio switch I used is rated: offers a 125Vac and 0.6A rating.

So doesn't look like a good choice for a mains switch.

 

[Zen Mod]

I have enormous respect for Broskie's work and brain, but that switch used for mains is simply dangerous