6505MH: Guts & Biasing

Discussion in 'Gear & Equipment' started by Petar Bogdanov, Feb 4, 2016.

  1. Petar Bogdanov

    Petar Bogdanov SS.org Regular

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    Some say it's made of metal... After marketing for rock didn't work.

    Sadly, like most amps, the tubes are taller than the transformers. It has to be propped up to be worked on. At least the transformers are similar height.
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    Tube layout - the preamp tubes seem to use some kind of a shielding socket. Nice.
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    Here's the general view as seen from the back. A lot of through-hole components for the preamp section, which is nice, because it's moddable without learning to SMD. Sadly, a lot of the switching components are SMD and so is the FX loop. Better than Blackstar's approach to mini amps, but not by a huge margin. On the big daughter board (bottom centre) are attenuator and dummy load. They seem to use ceramic resistors, one might see an improvement by replacing them with metal oxide. As stock, this amp sounds best in full power mode.
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    Bias adjustment is much easier with the chassis out of the shell. Without EL84 bias probes, it's the only way to go. In either case, you will need a flathead miniature screwdriver.
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    This is where you would measure the bias if you had no bias probes. You don't want to go to a hospital. Get it to a tech, or do it safely. Don't be a clown.
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    Also note the remaining soldering flux on the tube socket pins. Those joints aren't great either. Someone in China isn't taking pride in their work. Maybe they get Undercover Boss in China.

    The voltage drop across R26 must be measured, in millivolts. However, it's not possible to probe its legs directly because it's SMD (thanks Peavy). It's been a few months since I've done this, but I think I measured between the left leg of F104 and R66. (as pictured)

    Do not EVER think of sticking your probes into a live amp. If your multimeter has croc leads, use them! If you don't, make yourself some croc cables. Croc clips are sold in every electronics store, and you should be able to make your own cables. This is an example of a croc cable:
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    Make sure the croc tips aren't touching anything. Wrap them in electrical tape, to be sure. Make sure you're not touching any live metal while measuring the voltage. Turn on the amp and wait until the TSI leds go green. Definitely do not touch the amp now. The amp starts at a low bias and gradually increases it. Wait until it stabilizes. This amp works at 353V, so the "proper" bias range is between 24mA and 17mA. My amp was running 28mA stock, which was obviously causing crossover distortion. Not a fan. I biased it at 22mA. Now my cleans are clean and buttery.


    This resistor is socketed. If anyone knows why, please tell. Also shown is a fuse, of which there are 3 or 4 throughout the amp.
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    Phase inverter tu--- I mean MOSFETs. Very Blackstar. I wish the positive and negative MOSFETs matched in appearance. I hope they are matched in performance, but judging by the lack of markings, they are probably not. This might be an opportunity for upgrade, or it might be a wasted afternoon.
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    "Spring" reverb, as indicated on the PCB. You mean I could have had plate?! What the hell, Peavy?! Spring reverb is the worst for metal. This is the worst choice you could have made.
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    Someone was very proud of this transformer. It's for the mains.
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    Power supply capactitors are not drainable without taking the PCB out. Working this amp is a bit sketchy. Supposedly, all 6505s have capacitor bleed resistors. I wouldn't want to find out.
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    MSDI daughterboard. Note that Peavey claims the standalone MSDI is all-passive, so the chip must be for USB.
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    Hey, wanna hear a joke? Welded-shut plastic footswitch without rubber feet. What a piece of ..... I took off a whole star from the "quality" rating of the amp just for this thing. Adding insult to injury, it doesn't footswitch all the footswitchable features! If you want to switch all 4 things, you have to buy another of these ....s. For the lazy, the following features are footswitchable: FX Loop, Reverb, Channel, Crunch. If you want to footswitch Channel and Reverb with the same footswitch, you're out of luck.
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  2. schizoidasylum

    schizoidasylum SS.org Regular

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    Removing that lining in the edges of the chassis will allow more hot air to get out of the amp. Seems minor but works. Just peel them of. Its going to get messed up eventually every time you take the amp out.
     
  3. jrui

    jrui SS.org Regular

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    the tansformers were made in taiwan . I believe that's the same trannies like 112's
     
  4. AkiraSpectrum

    AkiraSpectrum SS.org Regular

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    Thanks for the inside look at the 6505MH!

    I agree. I have no idea why Peavey are still making those garbage footswitches. Probably the worst footswitch I've ever used.
     
  5. accidental

    accidental SS.org Regular

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    When you say "measured between the left leg of F104 and R66? Where should I clip onto for voltage? This is my first time biasing an amp where legs aren't exposed.
     
  6. Petar Bogdanov

    Petar Bogdanov SS.org Regular

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    You can clip onto both the cap's leg and the jumper, you just need alligator clips that are small enough. Obviously, make sure the amp is off when you do that and you shouldn't be touching both legs of the capacitor.

    A safer alternative is bias probes. ;)
     
  7. ESPImperium

    ESPImperium @ESPImperium

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    I ditched my Peavey POS Footswitch for a Boss FS-7 Dual Footswitch and a Stereo Cable, better bold quality. However mine wasn't for quality reasons for ditching it, mine was pedalboard space, the FS-7 was all i had space for.

    I love my 6505MH, its got a brilliant tone for me, however i am in the market for a Mesa 25W and Fender Bassbreaker 007, both of which can accept my footswitch of choice easily. for my main pedalboard as my alternative pedalboard will be set up for everything but the 6505MH.
     

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