Questions regarding first build - K-502

[nistur]

Hi all!

I've been trying to build a K-502 amp kit off and on for the past 3 years (found it difficult to get time to make the case). I think it's finally nearing completion, which means it's time to share and get feedback.

I hope people don't mind me uploading the photos on a 3rd party site, but it's easier for me to send the link to friends. (if it's an issue, I can re-upload them here)
Build photos from the last few days can be found here.

I'm slowly getting the PCB wired up to the inputs, outputs, switches etc, but I was going over the circuit diagram (which someone else has very kindly uploaded here) and I noticed that the input transformer steps the 115V AC up to 250V AC. As I'm building this in the UK, and our mains voltage is 240V AC, I was wondering if I could remove this transformer entirely, and possibly add a small step down transformer for the 12V rails (which I can't figure out what they're used for...)

On a marginally different point, I have one of these switches, which it says is rated up to 24V, and I have a relay (can't remember exactly which, will update later when I dig it out) I assume I can run this off the 12V, once I figure out where I'm getting it from. I'm pretty sure I know how this works, but just to stop me blowing stuff up, or electrocuting myself, does anyone have a circuit they would suggest (I also have a tiny 240V AC->5V DC adaptor I ripped out of a USB charger which I can use to drive the LED...)

Anyway, thoughts? Suggestions? As this is my first build, I'm not expecting amazing sound quality (although it would be nice!) so I don't think I want to replace any components at this stage... maybe on my next build... or if this one doesn't turn out so good...

 

[Soonerorlater]

I noticed that the input transformer steps the 115V AC up to 250V AC. As I'm building this in the UK, and our mains voltage is 240V AC, I was wondering if I could remove this transformer entirely, and possibly add a small step down transformer for the 12V rails (which I can't figure out what they're used for...).

A power transformer must be used for safety reasons, it isolates the amplifier from the mains power supply. Have you read our sticky thread on safety ?

http://www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

I presume your power transformer is suitable for UK mains if not you'll need a step down transformer. I think later K502's had dual inputs for American and Eu/UK mains voltages


The 12Vac is for the 11BM8 heaters and will drop to about 11Vac under load.


Whats the relay going to be used for?

 

[nistur]

Thanks for the reply Soonerorlater.

I was pretty sure that this was the case, I just wanted to check. I have also had a look over the sticky, but will make some more time to read it thoroughly.

The transformer I have which came with the kit only has one set of input wires, and all documentation only mentions US mains voltage. I have a UK->US transformer available, but I don't really want to just be sticking random parts into this, so I was wondering, if the transformer I have won't support UK mains, does anyone have any suggestions for (relatively cheap) power transformers I could replace it with?

 

[Soonerorlater]

If you have a UK->US transformer available then it might be best to use it. That's what I did. Worked well for me.

IIRC a UK transformer would need to have a 2 amp 10.7V ac heater supplies and around 150 mA's of 240V ac. The heater voltage is the main problem as the norm is 6.3V ac


Plenty of mods to do on this amp. Found mine got better after a week of "burning in". Best upgrade IMHO was changing the output transformers. Currently I'm changing my power tranformer to 6.3V ac as I've got a quad of NOS Mullard 6BM8's ( probably make no difference )

 

[Soonerorlater]

and around 150 mA's of 240V ac

Correction - IIRC my final B+ was around 210V dc so that 240V ac secondary would have to be a lot lower.

( C13/14 power supply reservoir caps only rated at 250V )

Best for me to go and measure again to be sure......