Peavey 6505 Plus 112 Repair - Bad Preamp Heater Circuit Diodes

I bought an old used Peavey 6505 Plus 112 Combo several years ago. The amp was in decent shape and sounded great. But it recently died of a common failure. The rectified diodes in the 12AX7 heater voltage supply circuit burned up. Here is the relatively simple replacement procedure.

[@oktrabnotreve]

Isso aconteceu comigo em 2015... Uma semana antes de um show....

[@willisblair00]

Nice vid

[@isolirionband7300]

I man, i want to modify my amp adding it more gain ,i got a Rivera M100 and Peavey 6505+112 plus combo. Can you give me some advice?

[@6dbthreshold]

More gain? Which amp? The 6505 has too much (more than enough). The Rivera is a nice cleanish fender. Soldano added more gain by adding insane non-symmetrical bias at the third gain stage. Very smooth and creamy. But mushy in comparison to fender circuits. I always recommend playing with overdrive pedals first, before modding an amp. Way too much circuit analysis involved to give a simple answer. Rob Robinette has a great website for simple amp mods. Good luck.

[@isolirionband7300]

@@6dbthreshold thanks

[@richarddelrosario9572]

Nice video, very informative. Which diode to buy to prevent this? Cheers

[@6dbthreshold]

I used 1N4935 diodes (1 Amp) for the immediate repair. They have about 12 hours on them with no issues. I bought 1N5404 diodes in case of future failure. They are at least physically 4 times larger. 3 Amp 400 V diodes with 1.2v forward voltage drop. They will need to be mounted pretty high off the board or even remote mounted. I won’t install them until the next failure (probably 3-4 years from what I’ve seen on the message boards).

[@richarddelrosario9572]

Thank you for your reply. I’ve been searching for answers on how to prevent this problem, I have this amp almost new for a month now and Ioved it. V30 and footswitch are on it’s way, I will order those diodes, just in case. And would likely convert it to a head, eventually. Cheers from qatar 🇶🇦

[@gstube1]

Looking at buying one, found one for sale locally for 350. Worth it?

[@6dbthreshold]

No. It’s not worth it for a normal player. It does the 5150 heavy tone pretty well. It sounds great through my 4x12. For me, it’s worth it. But I like to tinker with old amps. The diodes WILL eventually fail. They are simply sized too small for the heater circuit. If you are comfortable with replacing the diodes in the heater circuit, it’s worth it. If not, don’t buy one. It’ll be a headache in service repairs. The cabinet is heavy and I don’t like the single 12. I’ll convert it into a head soon. And then it won’t be worth anything.

[@schwartzmagic]

How'd you get your logo to light up? Mine doesn't work?

[@6dbthreshold]

If your amp still works (makes sound), I suspect your logo light circuit is burned up. The bias low voltage tap from the power transformer not only power the bias voltage rails, but also has another 4 diode rectifier that feeds a couple solid state voltage regulators for the +\~ 15 vdc switching circuits and the “BACKLIGHT PCB”. If the amp still switches channels correct, the logo pcb is broke. I dunno. I never took the logo light assembly apart.

[@schwartzmagic]

@6dbthreshold okay yeah my amp still works and all the channels switch fine. Guess it's the logo PCB. I unplugged it and plugged it back in and same, no lights. Will look into getting a new logo PCB. I read somewhere someone fixed their by resoldering some of the leds. Guess I can also inspect the led solder joints.

[@6dbthreshold]

Maybe hook the logo light leads up to 2 9v batteries in series? Easy simple test to see if it the light circuit board works. I like how the amp sounds, but it’s definitely not designed with long term reliability in mind.

[@isolirionband7300]

@@6dbthreshold I man, i want to modify my amp adding it more gain ,i got a Rivera M100 and Peavey 6505+112 plus combo. Can you give me some advice?

[@viriatovillamil9825]

​@6dbthreshold so I hooked up 2 9v batteries it popped/blew one of the LEDs. I'm sure I did something wrong. Now I'm wanting to replace all the LEDs on the logo PCB just not sure which ones I need. If you happen to know please pass on the info. Thanks.