Members OldGuy56 Posted September 30, 2016 Members Share Posted September 30, 2016 “Disclaimer: many times I ask questions that may seem extremely simple- apologies up front”Had an Ampeg B3 combo, the head died. The head had a 4 ohm input for the 15” driver and an 8 ohm for the 10”. I bought an Ampeg PF500 head to use with cab. The PF500 also has two inputs but no separate ohm marking. I believe the specs state nothing under 4 ohms for the PF500. I replaced the 15” 4 ohm with a 15” 8 ohm surmising it would total 4 ohms if I plug each into the PF500 (15" 8ohm & 10" 8ohm) but I think I’m burning up the 10”. Is this due to the two input in the PF 500? I thought that, even though there are two speaker inputs, it would still equal 4ohm total. I’ve read as much as I can take on the ohms, impedance and wiring and still in the dark. Should I wire the cab in serial and simply plug into one input? I’m off to work, so I’ll have to check responses later tonight. Thanks in advance for any assistance. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted September 30, 2016 Members Share Posted September 30, 2016 The Portaflex heads self adjust wattage depending on the load (I own the 350W version) Page 14 in the manual has your version and it says it produces 500W at 4 ohms or 300W with 8 ohms. http://www.ampeg.com/pdf/pf_heads_om.pdf You could probably run the head at a higher impedance but the wattage and tone drops off drastically. (You can even run the head with no load, but you'd want to turn the master volume down). In parallel two 8 ohm speakers give you 4 ohms total. Each speaker sees 250W whether they can take it or not. Power handling - as a rule of thumb, when mixing speaker types in a two speaker cabinet, maximum power handling is 2 x the lowest rated speaker (e.g. for 30-watt & 60-watt speakers, max power handling = 2x30-watt = 60-watt) For example. If you have a 400W speaker and a 100W speaker. Your 400W speaker will see 250W and be fine. The 100W speaker will see 250W cranked and blow. The total wattage you can run with in this case = 100W to each speaker which is 200W total Speaker wattages don't add up evenly unless the wattages of the speakers match. How much wattage is applied to the speakers is based on their impedance. All the amp sees is the load. It cannot and will not apply 400W to one speaker and 100W to the other for a total of 500W. You should bookmark this calculator. You can use it for series, parallel, Series/parallel etc and know exactly what your totals are. http://www.speakerimpedance.co.uk/?act=two_parallel&page=calculator If Both speakers have 8 ohms impedance, the current and voltage split evenly giving you 250W for each. If one speaker is rated for less then 250W it will fry. Over 250 you're fine. Think of it like this - The lower wattage speaker has a weaker coil and will simply fry before the other speaker. When it does - the Other speaker will see the full wattage. Because this head self adjusts to the load, The heads wattage will drop to 300W for a single 8 ohm speaker. The speaker needs to be rated 300 or higher to prevent it from being damaged. Of course all of this would be with the amp fully cranked. If you run the master volume low the speakers wouldn't see as much total wattage. [ATTACH=CONFIG]n31823837[/ATTACH] If the impedance of speakers were different, you had a 16 ohm and 8 ohm in parallel and you has a 100W head, the 8 ohm would see . The 16 ohm would see twice the wattage of the 16 ohm or be [ATTACH=CONFIG]n31823836[/ATTACH] I'm not sure what you're speakers are rated for but it may be a good time to check. Your 10" obviously must be too low of a wattage or it wouldn't be frying on you. Also single 250W 10's are not easy to find and very expensive. You'd be better off with a pair of 125W 10's in that cab that total 250W at 8 ohms (2X4 ohms in series or 2X16 ohms in parallel) then run that cab in parallel with your other cab and the wattage split between speakers will be safe. Because these will be in parallel to the other cab you'll be able to run the full power (if your other cab can take 250W. These can be which you put in parallel with your other cab to give you 8 oms Link to comment Share on other sites More sharing options...
Members OldGuy56 Posted October 1, 2016 Author Members Share Posted October 1, 2016 Thank you, WRGKMC! This clear the ohm/impedance mystery better than what I turned up on my searches.I appreciate your assistance. Now I can look around Harmony Central and see what else it has to offer, any suggestions about this site for a newbie? Link to comment Share on other sites More sharing options...
Members WRGKMC Posted October 3, 2016 Members Share Posted October 3, 2016 The form here is not super busy but it has regulars that have been here a long time. There are also some pros in the industry who will answer any reasonable questions you may have. The site does have some good expert and user reviews. When purchasing a piece of gear you may find its useful to look up and see what others thing of different pieces of gear. you may avoid the issues that had or discover things you may not have known form their first hand opinions. Link to comment Share on other sites More sharing options...
Members Perfessor Posted October 18, 2016 Members Share Posted October 18, 2016 I lubbs my PF 500 head. I had heard they had a problem with them dying early, but I think the company must have gotten that taken care of. Mine has been rock solid for two years. Link to comment Share on other sites More sharing options...
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