First try at PCB layout, feedback needed (single-ended EL84 tube amp) - Page 1
Forum section: electronics / projects

author: itsikhefez
author_rank: Newbie
timestamp: December 19, 2020, 09:27:21 am
content: Hi all,I've done a short online course on Eagle and am attempting my first PCB layout.My knowledge in circuit design and layout is the smallest increment above none at all, and I would appreciate your help!I chose the RH84 because it's an easy circuit and I wanted to build one anyway.The schematic can be found here:http://rh-amps.blogspot.com/2013/02/rh84-amplifier-revision-2_26.htmlThe PSU (rectifier, main filter cap and high voltage regulator) are off board in the chassis.For the PCB, all the traces on the bottom layer except GND. I tried to do a star ground, where each channel has one trace return to the decoupling cap at the B+ input.The tube sockets are mounted on the top side, and heaters wires be connected to the pads near the socket pins.FWIW, I plan on using regulated 6.3 VDC to avoid any hum from the heater wiring.A few high level questions--1) Are there any glaring issues with regards to a specific components placement or route?2) Ground -- did I implement the star ground correctly? Is it preferred on top or bottom? Should I add any ground planes/copper pours?Thanks!!

author: StuartA
author_rank: Regular Contributor
timestamp: December 19, 2020, 04:21:02 pm
content: I have a query more about the circuit rather than your PCB. What is plugged in to the input at J1?If you look at various pre-amp designs, some feed signal direct onto the grid, like your circuit, but many isolate the grid from the outside world via a capacitor and maybe even a so called "stopper resistor". You have to consider how likely it is that the pre-amp could go in to oscillation when there is nothing there to dampen input to the grid.To prevent that, some designers will put something like a 10k resistor physically wired as close as possible to the grid socket pin, maybe even with something like a ferrite bead on it. I wonder what others think?Other than the issue of the input on V1, the PCB design generally looks pretty good.

author: itsikhefez
author_rank: Newbie
timestamp: December 19, 2020, 06:05:39 pm
content: The input at J1 would be AC signal from your source (streamer, CDP, phono pre, etc).In this design, I'm not entirely sure why there isn't a grid stopper on the 12AT7.There are grid stoppers on the EL84's though (240R), maybe that is sufficient with the specific feedback arrangement of this amp.Regardless, I sent an email to the designer to ask about this.

author: StuartA
author_rank: Regular Contributor
timestamp: December 19, 2020, 08:21:40 pm
content: I guess that the argument may be that if the feed is a typical solid state device as you list, their outputs will be relatively low impedance and will 'tie things down'.But for the price of a couple of components, I'm inclined to think in terms of "better to have and not need, than need and not have".Let's see what the guy says.

author: itsikhefez
author_rank: Newbie
timestamp: December 19, 2020, 08:51:22 pm
content: Yes, curious to hear what others have to say as well.From the designer:"Grid stoppers are not needed on the input because that part of the circuit is outside of the feedback loop. The driver tube needs to have a sound reference to ground, and the amplifier should never be operated without the inputs connected to a source or shorted to ground, otherwise it will oscillate."

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 03:55:22 pm
content: Please find attached a PCB layout doc , hope is of use