Building my son a guitar Amp, Scratch built Full Deluxe Reverb AB763. - Page 1
Forum section: electronics / beginners

author: Donknowsnothing
author_rank: Newbie
timestamp: March 20, 2022, 09:39:15 am
content: "I have a little problem"I tried to build an indicator for the high Tension (Ht) On.I placed a LED across 420Vdc with a dropping resistor of 30K ohm 1Watt in series. and turned it on and it went POOF.I had calculated that I wanted 15-20ma to drive the LED."What I didn't caculate was the power this dropping resistor would require".It works out to be 6.4 WattsI do not want to stress my HT transformer."I now have a hole in the amplifiers front panel for this indicator that does not work.""I would like to fill it with a working HT ON indicator"So I know that a neon requires 70-90 strike voltage, but I have no idea what current they require."Is there a way of connecting a neon safely across 440Vdc ?"Help pleaseDonBy the way, Dave got my BM235 multimeter, "Happy"

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2022, 10:34:05 am
content: For a start, you should use a modern, efficient LED and a much lower current: 2mA is often enough, to achieve a fairly high brightness with a decent quality LED.A neon will work, but only one electrode will glow on DC.Why not connect a neon to the primary side of the transformer?

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 20, 2022, 10:44:30 am
content: His amp probably has a standby switch and he wants an indicator for when the high voltage is being applied to the tubes.

author: Donknowsnothing
author_rank: Newbie
timestamp: March 20, 2022, 10:48:43 am
content: Hi Zero999,thank you for the reply,I already have a mains on indicator installed and working.I would like to have a HT on indicating that the "HT is ON"On most Fender type guitar amplifiers have a mains switch as well as a Standby switch.I wanted to see that the HT was on, as a safety thing

author: Donknowsnothing
author_rank: Newbie
timestamp: March 20, 2022, 11:23:09 am
content: bsfeechannel,"Exactly"Thank you

author: eugene
author_rank: Frequent Contributor
timestamp: March 20, 2022, 12:10:17 pm
content: Hmm... how about using the 6.3V heater supply to light the diode (the way everyone else does it) but use the HT to switch a transistor so that the LED lights only when HT is present.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 20, 2022, 12:39:43 pm
content: Option 1: Same as original but with 2 mA or smaller LED. Power needed < 420 V * 2 mA = 0.84 W. Still bigger that typical signal resistors. A 2 W resistor would get warm, but not real hot. Be sure the voltage rating of the resistor is not exceeded.Option 2: As suggested earlier (Post #2 ), use a neon indicator but get its power directly from the transformer so it fully lights.Option 3: How is the HV switched? If it's a relay with a low voltage primary, connect the LED and resistor across the primary for the relay.Option 4: Use a "transformerless" style circuit from the unrectified HV to power the LED. Example:https://www.eleccircuit.com/transformerless-ac-to-dc-power-supply-circuits/

author: Donknowsnothing
author_rank: Newbie
timestamp: March 21, 2022, 01:58:17 am
content: Ok jpanhalt,Option 1: seems to be the best option-Option 1: Same as original but with 2 mA or smaller LED. Power needed < 420 V * 2 mA = 0.84 W. Still bigger that typical signal resistors. A 2 W resistor would get warm, but not real hot. Be sure the voltage rating of the resistor is not exceeded.Option 2: won't work with this amp Cct.Option 3: No relay in this Cct.Option 4: There is no unrectified HV after the HV switch.Are there 5mm 2mA Red LED's available?Fender AB763 Cct:-

author: Donknowsnothing
author_rank: Newbie
timestamp: March 21, 2022, 04:56:29 am
content: Found this post on this forum.Sort of relates to this post, but doesn't cover high voltages as high as 420-440Vdc."Solved - How to make LED indicator more efficient on high voltage?"

author: jpanhalt
author_rank: Super Contributor
timestamp: March 21, 2022, 06:02:48 am
content: Quote from: Donknowsnothing on March 21, 2022, 01:58:17 amI have not seen or searched for one.There is one at Digikey (not in stock):https://www.digikey.com/en/products/detail/lite-on-inc/LTL2R3KRD-EM/2675133You could also get a 5 mm or larger LED, drill out the insides (junction), and put your 2 mA one inside. The acrylic (?) package will act like a light pipe or diffuser. Of course, you can make your own from acrylic rod of almost any size. Light sanding with wet-or-dry paper will make it frosted.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2022, 06:45:35 am
content: Almost any modern high efficiency LED will be fairly bright on 2mA or less, this should not be an issue.The neon lamp is easy to do too, the steady state voltage of a standard NE-2 is about 60V at around 0.3mA. Only one electrode will glow on DC but it works fine.

author: Renate
author_rank: Super Contributor
timestamp: March 21, 2022, 01:15:17 pm
content: (Your green LED in the modified schematic is drawn in backwards.)I'd go for running the LED off the rectified filament supply through a transistor detecting B+.

author: Shock
author_rank: Super Contributor
timestamp: March 21, 2022, 03:08:43 pm
content: 500V Buck LED Driver driving an optoisolated fet switching the led from the heater supply? Or use a capacitive dropper voltage circuit or something to get a lower voltage one. I think you need to avoid running 500V all over the place and also make it isolated so you aren't presenting 500V to the front panel. As well as fuse, layout the circuit well for safety. Anyway something like that.Edit:Apparently London Power has a circuit for this scenario, not sure how old (out of date) or "tube amp vibe" it has though. I suggest you start trawling schematics for someone who uses a dual standby/on lamp/led on their amp.Maybe someone reading this knows of a better device to achieve the same purpose. Current/power dissipation is not really an issue if it's powering tubes in standby anyway. You just want to keep minimal loading on the supplies if possible.

author: Shock
author_rank: Super Contributor
timestamp: March 21, 2022, 03:52:11 pm
content: I was also watching some Psionic Audio amp videos a while back. This one is his design and just has a bright dull LED on the mute switch, so perhaps it switches the input to ground with a two pole switch which also switches dropper resistors for the led powered from the heater circuit. Or something like that, perhaps go through his videos, he has probably covered standby and on lamps at some stage.Oh and it reminds me if you are using a real neon jewel anyway, I think they are very tolerant to voltages so if you get it down below it's target AC or DC voltage it's fine but you just need to make sure you account for safety.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 21, 2022, 09:56:10 pm
content: Quote from: eugene on March 20, 2022, 12:10:17 pmThis is probably the solution with the lowest load on the 420V rail, just a high value base resistor, and since it's only switching 6.3V (or 9 if it's AC that needs rectified) any little NPN will do. If one really wanted to save microamps I guess a Darlington could be used, or a mosfet and a divider on the gate. In any event a simple enough circuit it could be hidden in a bit of heat shrink tubing.

author: eugene
author_rank: Frequent Contributor
timestamp: March 22, 2022, 02:12:11 pm
content: Quote from: BrokenYugo on March 21, 2022, 09:56:10 pmI've thought more about this. I like the idea of using the HT to switch a lower voltage, but using the heater supply *might* be problematic. The main source of trouble is the fact that the heater supply is referenced to ground through a pair of 100 ohm resistors (to simulate a center tap in the heater winding on the power transformer.) If a bridge rectifier (and additional load) is added in parallel it will make a mess of the supply to the heaters.It might work just fine if the two 100 ohm resistors are removed. The AC voltage across the heaters will no longer be centered around ground, and they will see some diode switching noise, but not a lot. With indirectly heated tubes it might not be a problem. The Deluxe Reverb is not a high gain amp.

author: SeanB
author_rank: Super Contributor
timestamp: March 22, 2022, 05:30:59 pm
content: 10mA pulled via a pair of diodes from each end, relying on the centre tap resistors to give current return, is not going to worry the 1A plus heater current at all. If you want put 100R resistors in series with the rectifier diodes, and a 10uF 35V electrolytic to ground, to give you a smoothed 4V or so supply when the LED is on, and it will not affect heater current balance at all, and will be almost noise free as well. Transistor off around 5V, and simply running the LED with another 220R resistor will provide plenty of light, especially with modern red LED's.you can even use a 2 colour red green LED, 3 lead, which is invariably common cathode, so you just use the transistor to short the red LED, and run both with 1k resistors. then power on is green, and standby is orange, with both lit at a low level.

author: eugene
author_rank: Frequent Contributor
timestamp: March 22, 2022, 07:26:14 pm
content: The deal is this: the heater circuit, even though not part of the signal circuit, needs to have some ground reference. The simplest thing to do is to connect one of the two sides to ground and be done with it. But, even with indirectly heated tubes this can cause some hum.To address this, a center tap is sometimes put in the heater winding on the power transformer. Grounding the center tap tends to quiet things. (The physics behind this is beyond the scope of this discussion.) If there is no CT on the transformer, it can be 'simulated' with a pair of resistors of identical value. I have seen cases where a pot was used so that the center point could be found by ear, minimizing hum. This is usually called a humdinger.Fender used a pair of 100 ohm 10% resistors with adequate results. Based on the schematic in post #7, I'm assuming that the OP did the same thing in his clone.If we add a bridge rectifier in parallel, and connect the negative end of the bridge to ground, then we have created two ground paths. Each side of the 6.3VAC winding will alternately be connected to ground through one of the diodes. The result is that the heaters will see only 6.3/2 = 3.15VAC plus or minus some diode drops and possibly the drop across a 100 ohm resistor due to LED current (I haven't analyzed it that far.)This can be remedied by removing the 100 ohm resistors thus removing the second ground path. But now the heaters will not have the benefit of the faux center tap, possibly resulting in added hum. That might not be objectionable to the OP, or maybe it will.The operation of tube amps is more sensitive to heater supplies than one might think, even with indirectly heated tubes. This is just an example of how problems can arise trying to use the heater supply for other things. As a point of interest, the original Fender Deluxe Reverb used an incandescent bulb powered directly by the 6.3VAC heater supply, but it had no effect on what the heaters themselves saw, so it worked fine.