DIY Audio Projects Forum

Hi, Since the heaters are in series....just insulate it and not use it. Remember if you rectify the 12VAC ....with typical filtering it will produce about 2-3 volts higher DC voltage. It can be brought down to the 12VDC needed with a resistor . The actual value varies a bit, but is in the 1-3 ohm range . Use a 5 watt or 10 watt size as it dissipates a bit of heat. Also remember not to ground any part of the heaters as it will defeat the reference DC voltage applied to it that protects the driver tube from heater to cathode failure.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Thanks Bruce. I thought that would be the case but just wanted to check. Don't the three diodes in series prior to the smoothing cap. drop the voltage by just over 2V? Is a resistor still needed if these are present?

Rog

Hi, Yes that is a viable way to drop the voltage as well. Be aware that they will get hot. I used some a long while ago in a power supply for two KT88 Oddblocks. Everything was fine until the music faded out. When I checked the PS I found that one of the rectifiers (10 amp size used) had unsoldered itself. I was using fairly low temp solder then. Now if I use that method I fasten them to screw type terminal strips. Power resistors can also unsolder themselves BTW.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Hi everyone, after reading this entire topic (yes really !) I now feel confident enough to start building. It won’t be my first tube amp, I’ve built several but my theory is shaky, to say the least.
I have a few questions : I’ve found a source for the Edcor transformer set here in Europe (horrendously expensive though) and would like to hear your opinion : I don’t much like the looks of those very blue transformers and would like to use some simple black steel covers.
However, somewhere in this forum I read that the power transformer gets quite hot, would this be an issue when it is encased ?
I will be building the “Type D” version which has only one biasing pot, what should be the power rating of this pot ? (Rheostat ? Not sure of the difference)
Also I’ve seen the PSU schematics without the LR8 regulator, would that be a good choice ?

Hi,
I also bought Edcor set in Europe, will send you a PM with the details.
As to the blue covers, I have sanded them to the bushless metal (by sanding paper and brush), what was worthy, because unfortunately I found some rust beneath blue paint... Then re-painted using a spray primer and then widely used black matt spray paint. I think it looks pretty well (see picture). I also replaced the screws by black ones.
That's true, that after few hours running power transformer gets warm, but you still can touch it, so the temperature is not very high.
As to the bias pot, I have used rheostat, however Bruce replied in some post, that this can be also low power potentiometer.

I will happily share my experience, if you have any other questions

Thank you. I'm sure I'll have a ton of questions when I start building. That could be a while, though.

could you post a picture of the inside of your amp ? I am going to use the same tube lay-out and I'm curious how you routed the heater connections.

Here you go.
It's no so pretty inside as outside, however this was my first built, so I learnt on the fly.
Heater wires are these twisted blue/brown, directly from the transformer (AC).
Generally I directly followed Bruce's design, adding anode voltage delay switch (red box) and main power switch soft start circuit (green box).

Thanks Michal!

So I made a concept design of the layout that appeals to me. I would like some feedback from this learned community. Would this be acceptable ? And what would be the best spot for the star earth ?