Members Chrisjd Posted November 28, 2012 Members Share Posted November 28, 2012 I got a new amp today and moved my 6505 onto another cab. I had the 6505 set to 4 ohms, and plugged it into a 16 ohm cabinet. I realized this after playing for about 10-15 minutes. Did i likely do any damage or shorten the life expectancy of my amp??? How safe or unsafe was this? Link to comment Share on other sites More sharing options...
Members guitarbilly74 Posted November 28, 2012 Members Share Posted November 28, 2012 no big deal. Link to comment Share on other sites More sharing options...
Members AdrianPOA Posted November 28, 2012 Members Share Posted November 28, 2012 If it was the other way around (16 ohms from the head going into cab set to 4 ohm), that would be bad. Link to comment Share on other sites More sharing options...
Members guitarbilly74 Posted November 28, 2012 Members Share Posted November 28, 2012 If it was the other way around (16 ohms from the head going into cab set to 4 ohm), that would be bad. not really... the 6505 can handle all these mismatches really well. The only real dangerous thing you could do to it is to run without a load completely.. other than that, these amps are tanks. Link to comment Share on other sites More sharing options...
Members Chrisjd Posted November 28, 2012 Author Members Share Posted November 28, 2012 thanks guys, I would love to get some more opinions. Link to comment Share on other sites More sharing options...
Members HKSblade1 Posted November 28, 2012 Members Share Posted November 28, 2012 not really... the 6505 can handle all these mismatches really well. The only real dangerous thing you could do to it is to run without a load completely.. other than that, these amps are tanks. The only thing we had notes about 6505 6505+ etc was a PS cap going bad (opening) sooner when using lower impedance cabs. I have no real explanation for that other than the cap in question is of poorer quality (cheaper Illinois) and is rated at its voltage spec limit as well as a low temp filter cap (85 vs 105 rating or higher) in circuit which is never a good idea. Perhaps the lower impedance cab id'd for premature failure of that part was a survey from Peavey/Auth Svc techs. Not sure either since I've seen that cap go bad regardless of cab speaker makeup, brand, or impedance used. OP next time you are in there, Change out C37 or have it done at next time of service. They will likely recommend it anyhow. Link to comment Share on other sites More sharing options...
Members wrongnote85 Posted November 28, 2012 Members Share Posted November 28, 2012 it would be worse on the amp if you took it and threw it out the back of a semi going 80 down the interstate. and even then it would probably still be fine. Link to comment Share on other sites More sharing options...
Members Chrisjd Posted November 28, 2012 Author Members Share Posted November 28, 2012 it would be worse on the amp if you took it and threw it out the back of a semi going 80 down the interstate. and even then it would probably still be fine. They're tough amps for sure. Link to comment Share on other sites More sharing options...
Members guitarbilly74 Posted November 28, 2012 Members Share Posted November 28, 2012 The only thing we had notes about 6505 6505+ etc was a PS cap going bad (opening) sooner when using lower impedance cabs. I have no real explanation for that other than the cap in question is of poorer quality (cheaper Illinois) and is rated at its voltage spec limit as well as a low temp filter cap (85 vs 105 rating or higher) in circuit which is never a good idea. Perhaps the lower impedance cab id'd for premature failure of that part was a survey from Peavey/Auth Svc techs. Not sure either since I've seen that cap go bad regardless of cab speaker makeup, brand, or impedance used. OP next time you are in there, Change out C37 or have it done at next time of service. They will likely recommend it anyhow. are you trying to keep him up at night?? he mismatched for 5 minutes, the amp probably did not even notice it Link to comment Share on other sites More sharing options...
Members Chrisjd Posted November 28, 2012 Author Members Share Posted November 28, 2012 Thanks billy, bump. Link to comment Share on other sites More sharing options...
Members NinjaRaf Posted November 28, 2012 Members Share Posted November 28, 2012 Yeah, I wouldnt worry about it at all. Link to comment Share on other sites More sharing options...
Members Zozobra Posted November 28, 2012 Members Share Posted November 28, 2012 If it was the other way around (16 ohms from the head going into cab set to 4 ohm), that would be bad. I really wish people would stop perpetuating this. The safest mismatch is to run into a lower impedance (e.g. 16 ohm head into an 8 ohm load) as at the worst you will over dissipate the plates or pop a fuse but in practice this doesn't happen all that often so long as the poweramp is reasonably conservative and wasn't designed by a dick hole (never mismatch an old marshall, it will break no matter which way you mismatch). However a mismatch of two notches is too far and eventually it will kill something. I'm not going to explain it fully but if you plug into a higher impedance (e.g. 8 ohm head into 16 ohm load) you effectively double the OT primary impedance which rotates the load line counter-clockwise. This has two worrying results:1) The load line passes significantly below 0V grid curve knee which means as you approach overdrive or full power conditions you pull excessive screen grid current. The effect of this is that the screen will over dissipate which at best will take out a fuse. The screen will eventually fail and short if the fuse doesn't pop. On top of this you could blow a screen grid resistor and have to for a trip to the techs and a bad enough short will take out the power transformer. $$$!2) The class A portion of the load line will be at very high plate voltage at 0 plate current. In some cases this means you will exceed the maximum plate voltage of the tube and you'll have internal flash overs which will kill the tube. EL34s can take 2kV but 6L6s, KT88s are much less robust. The real killer is that the plates are fed from the OT so at full signal swing you have one half of the winding at a very low voltage and the other half at a very high voltage and the insulation on the winding can only take a certain potential difference before it is compromised and its punched through resulting in a shorted OT which can take out the tubes and PT with it. $$$$$$$$$$$$$$! Edit: If its at real low volume for 5 minutes its probably fine. If it still works it still works although if you were running loud then the OT winding insulation may have had a rougher time than it would like. So yeah, potentially a blown OT, blown PT and a dead set of power tubes. I've never given it much thought but I guess you could blow the choke too leaving you with an amp thats just not worth repairing. Link to comment Share on other sites More sharing options...
Members hcprimerib Posted November 28, 2012 Members Share Posted November 28, 2012 yes, upon doing this terrible impedance mismatch, you've rendered your amp completely useless. therefore send it to me so you can move forward in your life (ya newb!) Link to comment Share on other sites More sharing options...
Members Chrisjd Posted November 28, 2012 Author Members Share Posted November 28, 2012 Ok, Here's what i am taking away from what you're all saying.... The amp still works fine, so the amp still works fine. I may have damaged, i may not have damaged it. There's really no way of knowing. Link to comment Share on other sites More sharing options...
Members hcprimerib Posted November 28, 2012 Members Share Posted November 28, 2012 Does it behave oddly? No? then you didn't damage anything. Don't go looking for something that isn't there Link to comment Share on other sites More sharing options...
Members HKSblade1 Posted November 28, 2012 Members Share Posted November 28, 2012 are you trying to keep him up at night?? he mismatched for 5 minutes, the amp probably did not even notice it It gets boring around here when I'm on graveyard ! Need some company Really though. if C37 has never been changed do it by next service. That cap has had about a 35% failure rate according to Peavey. Jerry at FJA and tons of others say C37 at the first sign of any amp issue with a 6505 series. Was more an FYI for future reference. Link to comment Share on other sites More sharing options...
Members Zozobra Posted November 28, 2012 Members Share Posted November 28, 2012 If it works is works but you may have run some of the components above their specification which may shorten their working life. Every transformer will fail eventually! All those 70's marshalls that have been dimed for mos their lives are approaching their deathbeds! Link to comment Share on other sites More sharing options...
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