6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

[Zoea]

Hi there

I'm new to this forum and a bit of a newb at repair so please be kind

I have a class A valve amp bought from ebay based on the 6p3p and 6N1 valves. I believe it was based on a kit from SiliconRay and I bought it from UK ebay prebuild.

Here is a video and here is the thread that seems to be authoritative for the kit.

The circuit diagram is there in the thread, though apparently there was a second version of the circuit as I recently saw another ebay listing mentioning this.

Long story short, I had a small problem with the amp and was wary of running an amp randomly built on ebay so I gave it to my local hi-fi shop for an expensive once over. They fixed all problems and suggested I only use the 4 ohm taps on the back of the amp for all speakers. It worked fine for some weeks.

Never one to do the sensible thing, I read that the amp is working with KT66 somewhere (not true) so I tried a couple of JJ KT66 in place of the 6p3p. The amp sounded better for a while but soon broke at high volume (as high as 5-8w class A gets anyway) with a little smoke from inside. Since then I have kept it and it's been retired - I've put a lot of money into it so I am loathe to chuck it.

I want to repair this amp and get it working again with the 6p3p valves or perhaps modify it to use KT66 which is apparently quite simple. I have had a look at it and there are no obviously burnt components or smoky bits on the inside. The circuit diagram is available in the linked thread (hopefully)...

... if I figure out how to modify the amp for KT66 maybe these will be the same components to swap out as those that failed already. I'm quite the newb though I can solder and I'm hoping to learn amp repair and valve amp repair in particular.

I can add pictures if that helps. When I gave it to the shop I'm not sure if they massively changed the amp from the original siliconray spec but I hope not. (the shop tech was quite vocal about how unsafe the amp was before he fixed it up!). It looks quite simple inside. I would like to have it as a backup or perhaps swap it into the main system for a change once in a while.

Any suggestions? Maybe which caps to check and replace and how to do so? Maybe not caps? How would this amp be modified for KT66 and would that likely be the fix if this is all that happened so far?

Thanks
Zoea / N

[eccerr0r]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

Well what failed? What broke? What smoked? That's where the forensics should start.

It's audio frequencies and the caps, unless they're too close to a hot tube, should last a while.

Design defect methinks.

[Zoea]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

I am yet to turn it on again to check for smoke lol. I probably will be doing this soon (preferably after I buy a variac)

thank you for the reply

[speter]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

The first thing to do is take the bottom off the amp and look for burned parts.
I will look over the schematics and get back to you, as the likely damaged parts

Edit: Having trouble finding the schematic....

[Zoea]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

Here, the yellow transformer seems to be a little burnt out, I think that may have gone but hard to tell as it looks mainly intact just there was a little black discharge from it that chipped off.

Circuit diagram should be this one: https://www.diyaudio.com/forums/sili...st5488829.html

[budm]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

Can you just upload the schematics here so we do not have to join the DIY site to get to see the schematic?

[Zoea]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

Sure

[budm]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

So it has no Bias adjustment (it has fixed Bias by that 330 Ohms Cathode resistor) to be able to really set the Bias to be true Class A since not all the output tubes will have exact same spec, so when you put in the difference tubes, the new set may be conducting too much idling current than the original tubes.
This is Chinese kit?

[Zoea]

Re: 6p3p*2 Class A valve amp - put the wrong valves in - it's broken...

Yes, this kit originated in China. The seller is no longer contactable apparently. I bought it after someone successfully put it together (there was sound when I got it).

I'm just wondering if it will be a cheap fix but I suspect that transformer and it's likely going to be hard to replace.... I am still unsure.