Impedance matching tube vs. solid state

[Super Turbo Deluxe Custom]

Impedance matching SS power sections is pretty straight forward.

What are the hazards of running a lower impedance cab with a tube powered section?  Specifically, a 4 ohm cab with 8 ohm head?

 

[Cagey]

Any time you connect lower impedance loads you draw more current. If the output transformer isn't designed to source that much current, it'll eventually burn. Conversely, if you connect higher impedance loads, the output transformer tends to reflect higher voltage spikes back into the output stage due to saturation issues, which can cause arcing in and eventual failure of your power tubes.

Neither of those things are guaranteed to happen, or where they are it may not happen immediately, so you'll hear all kinds of "success" stories about mismatching loads and life going on. But, it's poor practice and can end up costing you some real money and/or embarrassment.

 

[mayfly]

What he said.

Just to add - what's the amp?  Some brands (i.e. Marshall) have weaker output transformers than others (i.e. HiWatt) and are more prone to problems under mis-match.

 

[Super Turbo Deluxe Custom]

It's a Carvin TS 100 stereo power amp.

http://www.carvinguitars.com/products/TS100

My cab is an Avatar 2x12 bass cab.  It is 4 ohm with (2) 8 ohm speakers in parallel.  The bridge value of the amp is 8 or 16 ohm.  Each stereo side has a 4, 8, and 16 ohm switch.

Here are my options for proper impedance matching:
1.) Just use one 50w side of the amp, leave the cab alone and call it good.
2.) Wire the speakers in series for a 16 ohm load, use 100w bridge mode at 16 ohms.
3.) Use it in stereo (parallel feed from pre amp to both sides), and have separate inputs for each 8 ohm speaker from each 8 ohm side of the amp.
4.) Ignore impedance matching, bridge at 8 ohms to a 4 ohm cab.

 

[Cagey]

I used to have a TS100 and 2 1x12 8Ω cabs, and I went with your option 3 - run each channel set at 8Ω to its own speaker. Everything matches, and everybody's happy. I tried running it in mono set for 4Ω with the cabs in parallel for a 4Ω load, but didn't hear a significant difference so I figured since I have a stereo preamp and effects I may as well use the stereo capability of the power amp to make chorus/flange/delay effects sound richer.

 

[fdesalvo]

You can do it, but need to moitor the temp of your tranny.

 

[mayfly]

You can do it, but need to moitor the temp of your tranny.

Sadly, that wont stop a higher impedance load from causing voltage spikes on your plates at full power.

 

[Strat Avenger]

I love my TS-100.

As for your question; I would probably re-wire the cab for 16 ohms.

 

[Super Turbo Deluxe Custom]

I'm thinking I will wire each speaker with it's own jack, which will treat it like (2) 8 ohm cabs that just happen to be in the same enclosure. Outside of using this power amp, I have little need for a 16 ohm bass cab.  Using it as 4 ohm cab again with another amp, it just takes an external 1/4" speaker cable as a jumper.

Also, reading up on it, when in bridge mode, both presence knobs have to be roughly the same selection even though channel 1 is the master volume in bridge mode.  Running it in stereo, even with the inputs in parallel receiving the same signal from the preamp equally to both sides, I can play with the presence knobs of each output independently.

 

[Cagey]

That's the way to do it. It'll still get as loud as running it in mono at 100 watts, and it leaves options open. You lose a slight bit of clean headroom, but that's an unusually clean tube amp anyway, so it probably won't be an issue.

 

[Strat Avenger]

'Not sure why you'd use a TS-100 for bass.

 

[Cagey]

Does seem a bit low-powered for bass. But, it depends where you're playing and what gear is already there. You can always mic a smaller setup, and some players are even going without an amp at all these days. Let the PA handle it, and you can drive a much smaller car to gigs.

 

[Super Turbo Deluxe Custom]

'Not sure why you'd use a TS-100 for bass.

The EQ curves are different when using a guitar amp for bass, but I've got a Sansamp RBI as the pre, so that oughta take care of that.

I was hesitant for a while, because of the relatively low wattage.  However, the old Ampeg SVTs were 100 watts, Haichikid powers a 410 with a 100w tube guitar Carvin, Mesa Dual rectifiers power plenty of bass rigs, I know a guy using a 65w Dr. Z as a bass amp, and when we mic the kick drum, I always have the option of PA support.  As is, I've seldom been told to turn up, and being a tube power section, I'd hope it'd sound better cranked.  Besides, it's tube watts.  I know putting the tester on it will tell the truth as far as output comparisons with SS, but tube watts are supposedly louder with how the human ears hear things and what not.  I don't have a steady band as such anymore, my bread and butter lately have been sub gigs with guitar players' rigs usually being 30 watts max.  What's the old formula?  Twice the watts of the guitar player's amp plus 50?  It oughta be plenty loud.  Also, theres the whole 4 times the power to double the output formula, so a 200 watter is just a hair louder.

As for space, I drive a mini van and am not hauling an 810 around.  The TS 100 is 2 rack spaces, so was the DCM 1000 it's replacing.

 

[Cagey]

The TS 100 is 2 rack spaces, so was the DCM 1000 it's replacing.

Interesting. I did just the opposite, replacing my TS-100 with a DCM-1000. We should've talked <grin>

As it works out, I'm not using the DCM-1000, either. I use a DCM-200L, and sometime in the near future I'm going to move away from that as well in favor of a powered FR/FR monitor.

 

[Super Turbo Deluxe Custom]

I have (2) DCM 1000, (1) DCM 2000, and an HD 1500.  Very fine power amps.  They can't be beat for the price, weight, and performance.  3 of them are in my PA - mains, monitors, and subs.  It blows peoples' minds when one person can move a self contained PA with board, crossover, and 3 poweramps by themself in one trip.

However, neither the HD 1500 or DCM 1000 can handle the bass from my Sansamp set at 3 o'clock.  The sound level just drops out in waves when boosting the bass and treble and cutting mids.  Take the bass back to flat, or cut it, it's fine again.  4 ohms bridged on the DCM, cuts out altogether 2 secs at a time, 5 times a night.  What's strange, a boosted bass kick drum signal crossed over and sent to the same 4 ohm bridged amp, fine.  Bridged at 4 ohms on a bass rig, not happening.  A Google search and a call to Carvin later reveals it's not an isolated incident and no one, including Carvin, knows why.

 

[Super Turbo Deluxe Custom]

Here's what I ended up doing with the cab rewire. 

Originally, the (2) 1/4" jacks and speakon were all in parallel.  I disconneted one of the 1/4" jacks and made it a dedicated line to one of the speakers.  Here is one of the forseeable problems.  Even full range bass cabs do not receive full range signals.  They do have crossovers that cut some of the highs above a set point.  This is an Avatar cab, and their cabs have 2 available crossovers.  One that has a the point set up to work with a 4 ohm load, the other crosses over at the same point with an 8 ohm load.  I found this out when I requested a 2 ohm cab a while back.  Dave at Avatar said he'd be happy to do it, though he didn't have a 2 ohm crossover, he'd have to use one for a 4 ohm cab.  He said had he not told me, I never would've heard the difference.  He probably can't hear a 4 ohm cab and think it's crossing over at the wrong frequency, must be an 8 ohm crossover.  He said he's convinced that crossed over at the wrong frequency is still preferable than not crossed over at all.  Ultimately I went with the 4 ohm cab and corresponding crossover based on the DCM 1000 bridged at 4 ohms I'd be using.

Now, with the rewire I did so I could properly impedance match with the new power amp, one speaker is receiving full range signal and the other is an 8 ohm load with a 4 ohm crossover.  It may be fine, as GK cabs I've had in the past had no horn or crossover, then again, maybe I just didn't know any better.  If it just sounds horrible, here's my options:  order an 8 ohm crossover and (2) 16 ohm speakers, run them in parallel, bridge the amp at 8 ohms.  Order (2) 8 ohm crossovers and just wire each one to it's own speaker and run in stereo like I'm doing.

Lastly the wiring explanations with pics:


Speakon and (1) 1/4" jack still in parallel going to (1) 8 ohm speaker, bottom 1/4 jack going to other 8 ohm speaker.

Not actually speaker cables, but instrument cables illustrating each isolated jack getting power from each side of the power amp.

This one is probably the most confusing, and why you don't want to borrow gear from me.  The 1/4" jumper puts all the jacks back in parallel and makes it's a 4 ohm cab again where I can use it with my DCM, BX 500, or most any other bass amp.

Oh yeah, the amp in it's new home:

 

 

[Strat Avenger]

+1 on the RBI .  :glasses10: