Problem with 6505+ 112 combo

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Illumination
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Problem with 6505+ 112 combo

Post by Illumination » Fri Apr 09, 2021 5:55 pm

Hi folks, wondered if anyone might be able to help. I've got a 6505+ 112 combo that I've had for a few years. Honestly, it hasn't been used much - I bought it second hand and for various boring reasons, it has only seen a few hours of play since I got it. I actually decided to sell it on recently. I tested it twice and it seemed fine, but after a few days of use, the buyer contacted me to say it had packed in. I've got it back now, and when you switch it on, the front logo lights up for about a second, then it immediately goes out and the fuse blows (the 2A fuse in the little fuse-holder on the rear panel of the amp). Change the fuse, same happens. And again.

I don't know much about guitar amplifiers at all to be honest, but I have a fair bit of practical experience with hobby electronics. So, I did some googling and I came across a thread where someone had a similar issue and it was a blown flyback diode. So, I replaced the flyback diodes D25, 26, 27 and 28 on the schematic. I also noticed that the PCB trace looked like there was possibly a burn around the rectifier diodes D18, 19, 23 and 24. So, I replaced these as well. I also inspected all the tubes and didn't see any obvious signs of damage, although I note that both 6L6s and 4 out of the 5 12AX7s are all original stock tubes, so are probably getting a bit old now. The tube V1 on the schematic was an Electro Harmonix one so I assume that one has been replaced.

Given my lack of amp knowledge, and the lack of any further obvious things to try, I'm at a bit of a loss. The most obvious thing I can think of that I've not tried is replacing the tubes, but firstly it's obviously a fairly costly thing to try, and second, I have no idea if a bad tube could even cause this to happen? I'd appreciate any further ideas/input from you knowledgeable people. Thank you!

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JamesPaul
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Re: Problem with 6505+ 112 combo

Post by JamesPaul » Thu Apr 15, 2021 2:43 am

Yes, a bad tube usally causes a fuse to blow.

Have you pulled all the tubes, replaced the fuse and then powered on? Try it with Standby On and then Off. If the fuse blows without tubes, it is likely an issue with the power supply ciruits or a short somewhere on the board. Review the schematic to see which components would have power in each case and troubleshoot accordingly.

If the fuse holds without tubes, it is likely a bad tube, bias circuit issue or bad screen resistor. Check the screen resistors, bias and resolve any issues before trying new tubes so the new tubes aren't damaged.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
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Illumination
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Re: Problem with 6505+ 112 combo

Post by Illumination » Thu Apr 15, 2021 4:16 pm

Thanks for the reply. I've done some more troubleshooting since my post and the fuse still blows with all the tubes removed, so it doesn't seem to be a tube problem and it seems to be something in the power circuit. I actually thought it might be the power transformer but I measured all the output cable voltages and they seem to be as they should be. I've already replaced the rectifier diodes as I thought they might be the issue but no joy there. I'll take the board out again over the weekend and have another look for shorts.

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Enzo
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Re: Problem with 6505+ 112 combo

Post by Enzo » Thu Apr 15, 2021 8:00 pm

Check for shorted "flyback" diodes. They are wired from the plates of the power tubes to ground.

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JamesPaul
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Re: Problem with 6505+ 112 combo

Post by JamesPaul » Thu Apr 15, 2021 10:19 pm

You are on 230V AC power and not 120V?
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
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Illumination
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Re: Problem with 6505+ 112 combo

Post by Illumination » Fri Apr 16, 2021 12:51 pm

Yes 230V AC.

I've checked the flyback diodes with my multimeter and they seem to check out fine.

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JamesPaul
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Re: Problem with 6505+ 112 combo

Post by JamesPaul » Fri Apr 23, 2021 2:26 am

Please check on your schematic. My schematic is showing a 3A fuse for 230V. It shows a 2A fuse for 120V.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
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Illumination
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Re: Problem with 6505+ 112 combo

Post by Illumination » Sun Apr 25, 2021 7:21 am

I think you might have them the wrong way around, the schematic I've got says 2A for 230V and 3A for 120V. It also says 2A on the back panel of the amp.

I'm making very slow progress with troubleshooting, I thought it was the input transformer but I've tested it and it's fine. It's definitely something in the power circuit or bias circuit. I've ordered a few replacement parts but it's more than a week now and they've still not been sent out. All being well I'll have it fixed by Christmas.

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Re: Problem with 6505+ 112 combo

Post by JamesPaul » Mon Apr 26, 2021 6:03 pm

Yes, you are correct and my apology. I had asked a similar question, on another forum, and was working off my memory when I replied to this. That schematic is on an older desktop I don't often use, but I should have fired it up and confirmed what I was thinking. Now doing that I see I asked to confirm you were 230V and should be using the 2A.

This is a difficult issue, with it blowing the fuse so quickly after power on. You noted the power transformer tested good, so I would try either a Light Bulb Current Limiter approach or isolating the different power circuits (primary side, plate/screen/B+/etc, tube heater, bias & +/-15V) and testing them one at a time.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

Illumination
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Re: Problem with 6505+ 112 combo

Post by Illumination » Thu May 06, 2021 5:18 pm

Got it sorted, phew. I replaced all of the filter capacitors and that did the job, clearly one of them was the problem. Back up and running. Lightbulb limiter was a big help.

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JamesPaul
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Re: Problem with 6505+ 112 combo

Post by JamesPaul » Thu May 06, 2021 9:03 pm

Good to see that you have it up and running again.

My current amps mostly date back to the mid '90s and I have yet to experience issues with any filter caps. I will definitely keep those in mind should I experience a similar issue. I know most parts are inexpensive, but I am a fan of trying to identify the failing component(s) and replace them. Primarily because my desoldering/soldering skills are not what they used to be and it consumes a lot more of my time than it used to.
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

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