My latest project - KT88 SE tube amp

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Separate lines coming off the driver LC filter to the plate resistors:

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Seen here are the left and right coupling caps. The driver stage plate resistors are directly (out of view) behind them:

[weinstro]

Re: My latest project - KT88 SE tube amp

In the second picture you can see the two most important things you need to do when building a tube amp or any AC powered project, the ground wire directly to the chasis with a star washer and of course the properly sized fuse!

My opinion: this is still a low power circuit drawing around 50-60 watts each. I would ditch the chassis ground as this will invite hum and a single ended circuit has poor PSRR to begin with. All of my tube amps, and all of my solid state amps (up to 200 watts per channel) use a regular L-N (2-prong) connection.

Cool project. I'll be interested to hear how you like the Edcors.

Regards,

Rob

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Interesting point Rob. Certainly easy enough to try and I will. I'll report what I find as well as how I like the Edcors.

[AJ]

Re: My latest project - KT88 SE tube amp

My opinion: this is still a low power circuit drawing around 50-60 watts each. I would ditch the chassis ground as this will invite hum and a single ended circuit has poor PSRR to begin with. All of my tube amps, and all of my solid state amps (up to 200 watts per channel) use a regular L-N (2-prong) connection.

Cool project. I'll be interested to hear how you like the Edcors.

Regards,

Rob

Wrong. The chassis ground is there for safety. It has nothing to do with how much power the equipment draws.

Additionally, how you build your own projects is your business but do not advocate unsafe practices. With any DIY project powered by the mains, any exposed metal must be connected to earth ground.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Correct AJ, I would never disconnect the earth ground (3rd prong) connection from the exposed top plate chassis. I was thinking of trying the amp with and without the 10 ohm\\0.1 uF from chassis ground to signal ground.

[AJ]

Re: My latest project - KT88 SE tube amp

Right, ok.

If you do end up using the 10ohm//0.1uF ground lift be sure to use the back to back diodes across them to limit any voltage drop to a safe value. You can also separate the signal and power ground circuits the same way. Only one or the other is necessary to break a ground loop. I typically place the network in between the signal and power grounds, that way the power ground can be connected directly to the chassis.

Looking at Abdellah's schematic, I would try it between the 6N1P cathode ground connection and the 470K resistor.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Hi AJ,

Just so I'm clear on what you suggested. I believe you mean that I should add two diodes (1N4007 or the like) in parallel with the parallel RC. So the four devices are in parallel with one diode's polarity opposite of the other. This would limit the voltage difference between the chassis and signal (power supply) ground points to a single diode drop of ~0.7 V. Is that correct? Thanks.

[AJ]

Re: My latest project - KT88 SE tube amp

Hi AJ,

Just so I'm clear on what you suggested. I believe you mean that I should add two diodes (1N4007 or the like) in parallel with the parallel RC. So the four devices are in parallel with one diode's polarity opposite of the other. This would limit the voltage difference between the chassis and signal (power supply) ground points to a single diode drop of ~0.7 V. Is that correct? Thanks.

That's correct. The purpose is just to keep the voltage drop, and therefore the power dissipated in the resistor low so it doesn't burn up during a fault situation. There's some details about it here in Rod Elliots basic power supply design. There's also a different version using a bridge rectifier in place of the two diodes here (see figure 3). The reason for using the bridge rectifier over two diodes is that in a fault situation a lot of current could flow through those diodes, typically until the power supply fuse blows or a circuit breaker trips, and a high current bridge rectifier is usually easier to find and cheaper than two 5A or higher rated diodes. This is only the case if the ground loop breaker network is placed between the power supply ground and the chassis. If the network is placed between the signal ground and the power supply ground (which is connected directly to the chassis) the diodes can be small signal diodes like 1N4148.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Thanks for the info and links. I have a few of these in my junk box:

http://www.parts-express.com/pe/showdetl.cfm?partnumber=050-030

So I'll try that instead of the descrete diodes.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

I was able to get a few more parts soldered in after work:

[blair]

Re: My latest project - KT88 SE tube amp

Before you ditch or use a ground break, just try it with a proper chassis ground. Being a stereo amp, I suspect it will have no issues with hum.

If you have faint hum, elevate your heater reference to 1/4 B+.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Good news! Just got e-mail confirmation that my output transformers have shipped from Edcor!

[AJ]

Re: My latest project - KT88 SE tube amp

Out of curiosity, how long did it take for Edcor to ship your transformers?

[PWR RYD]

Re: My latest project - KT88 SE tube amp

5 weeks

Edit: looking at a calendar it was closer to 6 weeks