Messing with an old bass guitar amplifier, need help - Page 1
Forum section: electronics / projects

author: Obisch
author_rank: Newbie
timestamp: August 04, 2023, 11:27:26 am
content: Hello all,I am experimentating with an old Bass guitar amplifier (Trace Elliot GP12 SMX). The power board is defective and i will try to replace it with a new aliexpress digital SMPS one (class D) : j115d-amp 1000WPreamp section and power amp are separatedI keep the original toroidal transformer for preamp supply voltages only (19v-0v-19v) since the new board has his own SMPS supply (connected to L&N 240v)Do you guys think it's ok like that and that there will be no stability issue with interconnected preamp and power ground and each section having his own supply ?The new digital board supply voltages for a preamp 15v, 0v, 15v but rated at only 0,5A which i think is too low for this particular preamp (this have a 12ax7 tube that draw 0,3 amp for heaters and a lot of light bulbs and other components)I think i will use this supply for an additional fan.Also i wonder about safetyThe chassis is a metal case with protective earthPreamp ground and speaker are insulated from chassis. The old power board provided an audio ground to chassis connection with a 10k resistor and a 100nf cap.The new board provided a 0v to chassis connection too but only with a blue ceramic cap, no resistor.Is it safe like that with a floating ground for a guirar bass amplifier or should i add a resistor and if so do you think 10k like the original board will be an ok value ? Instrument and musician body are linked with the amp groundAlso the SMPS supply of the new board has two Y cap connected to chassis and i worry these could add a potential to the chassis especially in places with no PE which can happen that could create annoying and even dangerous zap?What do you guys think ?Thank you for your help !

author: moffy
author_rank: Super Contributor
timestamp: August 04, 2023, 10:45:20 pm
content: Grounds/Earths on audio devices are tricky things to get right as you no doubt know. All the external metal work needs a solid ground/earth. What is the large heatsink of the J115D connected too? It is preferable to use the existing supply for the preamp and connect the 0V's together, use shielded cable for the audio connection. Safety is something you should look into I cannot offer much advice there as they are often country dependent.

author: Obisch
author_rank: Newbie
timestamp: August 05, 2023, 08:38:52 am
content: Thank you very much for your helpYes the metal case has a solid earth connection. The transistors heatsink is connected to 0v which is connected to chassis with a ceramic disc cap, that's why i wonder if a resistor should not be added here for safety? Directly connect the 0v to chassis could cause instability i thinkI will use a very short shielded cable for audio connection.When you say it's preferable to use the existing supply for the preamp, do you mean the existing toroidal one of the amp ?

author: moffy
author_rank: Super Contributor
timestamp: August 05, 2023, 10:18:27 am
content: You can put a resistor in parallel with the chassis cap it will tend to keep the voltages the same without the problem of a ground loop. Yes it is better to use the original toroidal power supply for the preamp.

author: Obisch
author_rank: Newbie
timestamp: August 05, 2023, 11:11:49 am
content: Ok, thank youDo you think 10k like the one on the old board is an ok value or should i go with something much lower like 10R ?

author: moffy
author_rank: Super Contributor
timestamp: August 05, 2023, 11:30:48 am
content: You can try it and see, lower value could induce a little hum, it's really not meant to carry current, just stop the 0v from floating too far from ground.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 05, 2023, 02:06:19 pm
content: 500mA seems like a lot for a preamp.

author: Obisch
author_rank: Newbie
timestamp: August 05, 2023, 02:45:15 pm
content: Ok, i will try and see, thank youThe preamp has a 12ax7 tube which need 300ma just for the heaters, that's why i worry the 500ma max rating might be problematic for this particular preamp.

author: moffy
author_rank: Super Contributor
timestamp: August 06, 2023, 12:33:35 am
content: Having looked at the photo of the J115d I think I see the cap you are talking about paralleling with a resistor between the transformer and the heatsink. On second thoughts I wouldn't try the resistor, since the ground of the J115d will be connected to the preamp and by that through the cap/10k the chassis, I would leave it a is. If you decide to go ahead make sure you measure both the DC and AC voltage across that cap on the J115d before deciding to parallel it.

author: Obisch
author_rank: Newbie
timestamp: August 06, 2023, 03:25:47 pm
content: Hi,The cap/10k 0v to chassis was on the old power board not the preamp. Preamp ground is totally floating, that's why i was wondering paralleling a resistor to the blue cap of the j115d.Yes that is the blue ceramic cap between the transformer and the heatsink. In fact, i made a mistake, on this board (j115d) the big heatsink is connected directly to the chassis, and the 0v is separated from chassis by this ceramic cap.Yes i will check AC and DC voltages across it, but what if i find something big here ? i think the board would be unsafe and unusable for this application at least ?Also there is something i don't understand about ground loop breaker citcuits. Some uses big diode rectifier (35A) for safety to allow the whole faulty current to be carried to chassis and to allow the main fuse to blow in case of a fault.Since the j115d has onboard protection(2 fuses), a big diode rectifier may not be necessary and only a small resistor just to keep voltages about the same (0V in relation to chassis) to avoid hazardous zap is ok, right ?Really that's not simple stuff lol, tubes amps are so much easier and forgivingAgain thank you for your help and time

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2023, 06:07:19 pm
content: Quote from: Obisch on August 05, 2023, 02:45:15 pmThe 12AX7 heater can be connected for 6.3 V at 300 mA (parallel) or 12.6 V at 150 mA (series), if that helps.

author: Obisch
author_rank: Newbie
timestamp: August 06, 2023, 09:54:17 pm
content: Yes it helps, thank youI will check the wiring of the tube, maybe i can use the new board supply and get rid of the toroidal transformer

author: moffy
author_rank: Super Contributor
timestamp: August 06, 2023, 10:36:35 pm
content: Quote from: Obisch on August 06, 2023, 03:25:47 pmWithout a reliable schematic I would only be guessing and that could be dangerous. If you find AC or DC voltages of any significance across the cap that is a red flag, as the robot in Lost in Space used to say "Danger Will Robinson."

author: Obisch
author_rank: Newbie
timestamp: August 07, 2023, 01:34:52 pm
content: 😊, yes it's true, i searched but unfortunately don't found any clear schematic for this board

author: moffy
author_rank: Super Contributor
timestamp: August 07, 2023, 04:57:30 pm
content: You could always reproduce the 10k/Cap parallel between the preamp 0V and the chassis and not worry about the J115d, as its 0V will connect to the preamp 0V.

author: Obisch
author_rank: Newbie
timestamp: August 08, 2023, 05:45:25 pm
content: Yes i will try that after checking ac and dc between the j115d 0v and the chassis.Thank you