Attention amph gurus! Ohm mismatch with t00b amphs

[marnold]

Hi all. On the H-C forums there is a Jet City amph owner's club thread that I check out periodically. My JCA20H head has one 16 ohm tap and two 8 ohm taps. The cab has one 16 ohm speaker. I have it plugged into the 16 ohm tap currently. In that thread, someone said they always recommend plugging the cab into an 8 ohm tap. They claim it gives much better low end. Can the output transformer on the head handle a mismatch like that? I can't remember which way is "good" and which way is "bad".

IIRC, if you have two cabs Jet City tells you to plug them into the two 8 ohm taps even though they are both 16 ohm cabs. I think I'm safe--I just don't want to release the magic smoke.

 

[Ch0jin]

I remember posting about this in detail here a while ago, but I can't remember what the original thread was actually about

The quick version of my opinion is "Match your impedance".

Tube amps are designed (unless things have changed recently) to have the output impedance of the amp match the impedance of the speakers to ensure maximum power transfer and frequency response. Mis-matching up or down should (theoretically) yield less power transfer and a different frequency response. Also, if the amp and cab are well designed 4-4 8-8 and 16-16 should all sound pretty much the same and result in the same power output.

I'd be extremely surprised if an amp that size would smoke an output transformer by the mismatch you mention, most amps are built to handle at least that much of a mismatch. Technically, mis-matching up or down are equally "bad" as far as power transfer goes, but tube amps loathe very high impedance and can often self destruct with no impedance as in the case of no speakers connected. A severe mis-match lower (say a 16 Ohm tap into a 2 Ohm speaker) is better for a tube amp than vice versa. Basically the exact opposite of a transistor amp.

That said, I'd say it's safe to try as your curious, but try it at sensible power levels and see how it sounds. Theory tells me it shouldn't sound "better", but in the music world "better" is pretty massively subjective.

Anyway, I'm sure the other guys will jump in too with more practical advice.

 

[duhvoodooman]

From what I've read, a "one-step" mismatch on the speaker impedance (i.e. either double or half) will not hurt any decently designed & manufactured tube amp. But it's still best to match impedances....

 

[tunghaichuan]

I agree with Ch0jin, you're best bet is to always match the tap to the proper impedance.

http://www.geofex.com/tubeampfaq/taffram.htm:

... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

[emphasis mine]

BTW, when you plug two 16-ohm cabs into the 8 ohm jacks, you are effectively running the 16 ohms in parallel for an 8 ohm load. FWIW.

 

[marnold]

BTW, when you plug two 16-ohm cabs into the 8 ohm jacks, you are effectively running the 16 ohms in parallel for an 8 ohm load. FWIW.

Aha! That makes sense. I may try it for kicks and report back. If you hear a scream, you'll know what happened

BTW I'm sending this from my cellphone via Tapatalk.

 

[ET335]

Tung....I was just wondering if this may also work....2= 16 ohm. cabs...one cab to the 16 ohm. tap and the other to the 8 ohm. tap?

 

[tunghaichuan]

Tung....I was just wondering if this may also work....2= 16 ohm. cabs...one cab to the 16 ohm. tap and the other to the 8 ohm. tap?

That's probably not the best way to hook up your amp to speakers. Ideally, you would want to run both 16 ohm cabs in parallel for 8 ohms and use the 8 ohm tap. I built a box to do this with a Valve Junior head that only has one each 4/8/16 ohm taps. The box itself is pretty straightforward, using one input jack to connect to the amp and two output jacks to connect to the speaker cabinets. The output jacks are wired in parallel, of course. However, I only grounded the input jack. The output jacks are connected to ground, but not physically connected to the metal box; I used isolation washers. This keeps ground loops out of the signal.

One feature I really like on the Blackheart amps is that there are two jacks for the 8 ohm and 4 ohm taps. That way two 16 ohm cabs can be plugged into the 8 ohm tap and two 8 ohm cabs can be plugged into the 4 ohm tap.

 

[kiteman]

On my Alchemist 112 above the 8 ohm jack says 16 ohm ok. Since then I wanted to try my Blackheart 16 ohm 112 cab to see how it would sound. Of course the internal 8 ohm speaker have to be unplugged to do that.

I'm leery about mismatching myself.

 

[Ch0jin]

On my Alchemist 112 above the 8 ohm jack says 16 ohm ok. Since then I wanted to try my Blackheart 16 ohm 112 cab to see how it would sound. Of course the internal 8 ohm speaker have to be unplugged to do that.

I'm leery about mismatching myself.

Does the Alchemist 112 have a 4 ohm jack as well? If so, you could run the internal 8 ohm speaker and the external 16 ohm cab in parallel (5.1 Ohms) This is how I'm set up at home right now with the 12" in my combo and my 4x12" cab in parallel. Granted I'm using maybe 1 of the available 60 watts when I'm at home, but it works just fine.

 

[kiteman]

Here's a pic. What do you thing of the setup?

 

[Ch0jin]

If it was my amp I'd try plugging the internal speaker into one of the 4 ohm jacks and the 16 ohm into the other 4 ohm jack and see how it sounds. 16 ohm and 8 ohm in parallel is 5.1 ohms. Real close to the 4 ohm load the amp is expecting and extremely unlikely to cause any damage. As I mentioned, this is exactly how I run my Peavey at home.

Anyone else?

 

[kiteman]

Thanks. Anyone agree?

 

[marnold]

Well, I tried it and recorded some clips. I play a chord progression then some lame single note sweeping. The first time through it is with the cab plugged into the 8 Ohm tap. The second time it is in the 16 Ohm tap. Unfortunately, the recording is a bit of an epic fail. The 16 Ohm tap was noticeably louder, something I'm not sure that the recording caught because I checked the levels at the beginning but didn't re-check them. You can hear it clipping on the 16 Ohm version. Basically the 8 Ohm tap was slightly quieter and slightly duller. Not worth the swap, IMO.

Ohmage test

This was just my DK2M plugged right into the head. No pedals or anything else.

The recording does remind me of how much I like this amph and what a nice job my phone does of recording it (with the levels set properly, anyway).

 

[Katastrophe]

Completely OT.... But it sounds like you've been putting in some work on the arpeggios! Good stuff, Matt.

 

[marnold]

Completely OT.... But it sounds like you've been putting in some work on the arpeggios! Good stuff, Matt.

Thanks. Actually, I've played them much better than that. Almost sounds like I know what I'm doing!

 

[Ch0jin]

Well, I tried it and recorded some clips. I play a chord progression then some lame single note sweeping. The first time through it is with the cab plugged into the 8 Ohm tap. The second time it is in the 16 Ohm tap. Unfortunately, the recording is a bit of an epic fail. The 16 Ohm tap was noticeably louder, something I'm not sure that the recording caught because I checked the levels at the beginning but didn't re-check them. You can hear it clipping on the 16 Ohm version. Basically the 8 Ohm tap was slightly quieter and slightly duller. Not worth the swap, IMO.

Ohmage test

This was just my DK2M plugged right into the head. No pedals or anything else.

The recording does remind me of how much I like this amph and what a nice job my phone does of recording it (with the levels set properly, anyway).

So if I'm reading you right, the theory actually holds up then. With the matched 16 ohm speaker and 16 ohm tap sounding louder (more power) and not suffering from a high frequency roll-off. It's awesome to hear examples of the theory proven in the real world!!!

 

[deeaa]

I have my 4x12" wired as stereo 2x8 ohms, but I have a Y cord that basically joins the two sides into one 16ohm loop, and I can use it either with that in mono 4x12" from the 16 ohm out or I can use it as a 2x12" from the 8 ohm out on my amp, in which case I can use two heads on the same cab with an A/B switch, which was the original idea I toyed with. Meaning, I would have my 36W Ceria on usually into two of the speakers and a JCM800 into the other two, that way I could have had completely different (and much louder) leads for instance.

But, I would always match the impedances. My 15W dual-ended tube thing had a single output, though, and it was recommended to use any speaker between 6-20 ohms, depending slightly whether you loaded it with what tubes, KT, EL, 6L6 etc...it accepted pretty much any tubes and there was this chart on calculating what the power and best impedances would be for a given tube combo (I liked an EL34 and a 6L6 simultaneously just fine)