Newbie: Cab Ohm question

Discussion in 'Amps and Cabs [BG]' started by Thomas C, Jan 4, 2020.


  1. Thomas C

    Thomas C

    Jan 4, 2020
    I love this forum, and I love getting lost in some of the threads...

    Did not see this specific item - I have an old Peavy 1516 cab (not the PVM). Just picked it up from a national music store. Item is rated at 4 Ohms, and I need it to run an all-tube amp that requires 4 or 2 Ohms.

    I took readings at each input jack (see pic) the bi amp inputs are 3.8 Ohms and the full range inputs are about 7.0 ohms. I believe this cabs should be about 4 ohms each input. The cab sounds fine on all the inputs with my solid state amp that runs at 4 or 8 Ohms.

    The tube amp sounds fine through the biamps, but not through the full range, so I turned it off right away. I don't want to run the tube amp at anything other than the rating - (the manufacturers says not to,) and don't want to be a cowboy about the thing.

    Anyway, why is the full range running at different than spec, and can it be fixed? I wonder if the problem is the crossover, and whether that can be fixed or replaced.
     

    Attached Files:

  2. bholder

    bholder Affable Sociopath Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    When you "take readings" do you mean you're using a DC resistance meter? That only measures the DC resistance portion of the speaker's impedence, the other part being the reactive or inductive load. Those measurements sounds like they might match up with 4 ohms and 8 ohms respectively. Not sure I can help with the rest, need more information / better pics.
     
  3. I BELIEVE what you're seeing is: the pair of 8's is presenting a 4ohm load. And so is the single 15. You see (nominally) 8ohms at the Full Range jack because there, the pair of 8's and the 15 are wired together in series. The solution in your case would be to change this wiring configuration to parallel. This will provide a (nominally) 2ohm load at the Full Range jack-this is safe for your amp.
     
    HolmeBass and alaskaleftybass like this.
  4. Regarding the 'spec:' It CANNOT be the same for the full range input vs either of the others. Peavey probably called it '4 ohms' simply because most people run SS amps and this is safe for them. A bit irresponsible though, as you've learned as it is not safe for some tube amps.
     
  5. Thomas C

    Thomas C

    Jan 4, 2020
    Thanks. Are each bi amp inputs (one marked hi and the other low - I think, one drives the 15, and the other the two 8s) completely separate circuits?

    So, if I run the two speaker outputs from the amp into the two bi-amp inputs, will I have the amp operating at 2 ohms?
     
  6. I'm gonna have to say: I don't know. I will have to defer to those with better knowledge. I don't want to tell you the wrong thing and your amp blows up. I'm pretty sure you CAN make it work, based on what I've said so far. But I don't know that cab and am just inferring things.
     
  7. I'll say this:. NEVER run both amp outs to a common circuit. In other words never run both outs to one cab unless you ABSOLUTELY know what you're doing. No. Don't.
     
    BassmanPaul likes this.
  8. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    This seems unlikely. Unfortunately I can't find a manual for the cabinet on the Peavey web site, but if the "Full range" config was full range to the 8s and the 15 I bet you'd blow a shedload of 8s. Also the text on the back mentions a crossover frequency. It seems more likely to me that the full range inputs should run to the crossover, and the biamp ones bypass the crossover and go direct to the drivers.

    I know very little about crossover design, certainly not enough to predict what DC resistance should be, but it would not enormously surprise me if the DC resistance reading were valid for nominal 4ohm impedance. On the other hand it wouldn't surprise me if that were an inappropriate figure either. You need a circuit diagram and someone who knows. However my [limited] understanding is that its quite feasible to have 4ohm overall impedance to the cab, and also 4ohm impedance to the individual drivers

    The big question is whether the box internals are the same as when it left the factory. I would be minded to take a good look at the internals and see whether they look as if they have been modified. The DC readings are also not inconsistent with someone having bypassed the crossover.

    What is the problem experienced with running the tube amp into the full range input?

    BTW I suspect you would get 2ohms if you ran the amp into both biamp inputs, but I reckon you'd get blown 8in mids in pretty short order. I suggest not.
     
    Last edited: Jan 5, 2020
  9. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    I seemed to recall that both high and low were 4 Ohms and when going through the full range jack (because of the crossover) it was also 4 Ohms. The information I pasted in below seems to suppoprt that. It is also referenced in this TB Amps and Cabs post which has a lot of credible information IMO:

    Peavey 1516
    Discussion in 'Amps and Cabs [BG]' started by iriegnome, Apr 9, 2014.

    This is from the Peavey forum (and was refernced in the TB post above), but I can not vouch for it...however it seems legit:

    Re: 1516 owner's manual
    Posted: Wed Jan 08, 2014 6:56 pm
    by FatMixG
    I wrote to Peavey and here is the response.

    There wasn't a manual but I've attached some promo literature.

    All inputs are 4 ohms.
    The low input is for the 1502-4 BW woofer only and it is rated at 350 watts continuous and 700 watts program.
    the High input is 4 ohms and rated at 180 watts continuous 320 watts program.
    I normally don't recommend using more then the continuous rating.
    The full range input is 4 ohms and rated at 200 watts continuous and 400 watts program.
    List price in 1994 was $499.99
    Weight - 87 pounds.

    If the high or low speakers are dropping out when you use the full range input then it's possible that the bi-amp high &/or low input jacks are intermittent. or, " open ".
    Because even when you don't plug directly into them the signal from the full range input still goes through them to get to their respective speakers.
    If you plug directly into the bi-amp high or low inputs and they work then this is most likely what is happening.
    In this case the jacks need to be cleaners or replaced.

    The literature they attached is just promo pictures.
     
    srayb and JimChjones like this.
  10. Thomas C

    Thomas C

    Jan 4, 2020
    Yes, that Peavy info sounds right, but I just am not convinced the full range circuit is 4 Ohms now based on my circuit tester. That model Peavy cab had problems with the crossover.

    Well, there is a "common wisdom" (pick one or the other, both is very rare,) that says you can run an amp rated at 4 Ohms into an 8 ohm circuit, with just a loss of watts. The manufacture's FAQs say "DON'T DO THAT".

    So, I am worried if I run the amp set at 4 Ohms into a circuit with 8 Ohm resistance, I am going to cook the amp eventually. So, without knowing that the full-range is a 4 Ohm circuit, I'm not going to do that.

    Most cabs are 8 Ohm, (I have a very nice 4x10 I use for outdoor gigs,) but it seems that nowadays the 4 Ohm cabs are an unusual thing. Plus the ones at the in the range I need - 400 watts plus - weigh as much as a volkswagon, so even if I see a cheap one, shipping is a major cost.

    For now, running into the low bi-amp input at about 1/2 power (master at 5) has a very nice tone, doesn't sweat the 15", and I might stick with that until I get it figured out. Next step is going to be to bring it to an amp guy, check the wiring and speakers, and see if the crossover is doing what it is supposed to. I am deep in the country though, and have to drive for about an hour to get to a decent guy.

    Thanks for everyone's help![/QUOTE]
     
  11. BassmanPaul

    BassmanPaul Inactive

    It sounds like this is a used cabinet. As such you really don’t have any idea what might have been done to it in the past. You need to verify that the wiring to the jacks is as original. If you know someone with some technical proficiency perhaps he/she will help you check out the cabinet wiring.
     
    Al Kraft likes this.
  12. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    The common wisdom you mentioned on running an amp at 8 Ohms is fine if it is rated to run at anything less than that (e.g 4 Ohms, 2 Ohms) only safely applies to solid state amps. On a tube power amp, the common wisdom is to read the manual carefully (a good recommendation for SS too) and do what is necessary to match cab load and amp output impedance as closely as possible.

    I suspect that the crossover may be messed up and/or the 8" speaker wiring (and possibly the 8" speakers themselves) might have been replaced/altered in way that the cab no longer meets theh original specs.
     
  13. I don't know exactly what happens when you apply a DC voltmeter to a crossover connected to its output speakers ( ie the full range cab input ). It sends a small voltage and sees what comes back.

    If I remember correctly it should show a value mostly representative of only the DC resistance of the low side output of the crossover.

    This disagrees with your findings.

    Either I remember wrongly or your cab has been modded.
     
  14. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    I just took a look at some crossover circuits with a view to figuring out what one might expect for DC values, and it seems to me that for a basic crossover the DC resistance ought to be the sum of the DC resistance of the woofer and the DC resistance of the series inductor(s) in the low pass section of the crossover. A bit more checking suggests that the DC resistance of the inductors should not be significant, so in turn that implies that something is not right.

    Time to open her up and take a good look I submit.
     
  15. adds up to my recollections.
     
  16. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Jun 15, 2021

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