OT/Speaker missmatch: what is better?

[rexindigo]

Im just about to build a Speaker cabinet with 4x 4Ohm Speaker that i have lying around.
The tube amp I want to use this cab with only has a 8 Ohm secondary on the OT.
Now i can wire the speakers together to get 4 Ohms or 16 Ohm.
Which will be less harmfull to the Tube Amplifier?

Thanx for Your Help!

[MWJB]

Knowing the primary Z would be useful, or knowing what amp the OT was designed for. Some brands run with a degree of mismatch in the first place, you can't always assume that doubling or halving the speaker load will be safe.

Assuming a typical 3.6K-4K:8ohms, per tube for 6L6s, 16 ohms will give the tubes a slightly easier life.

[rexindigo]

To be honest, I only know the following about the OT:
It was in an tube amplifier with a PP 2xEL34 Powerstage and was connected to an 8 Ohm Speaker.
I ripped it out of the dead amp and build a Marshall Plexi Clone around it.
I could probably measure primary and secondary DC-Resistance now.
But is there not a rule of thumb, like for example:tube amps prefer a higher resistance of the speaker if missmatched, solid state amps prefer a lower resistance, or so?

[MWJB]

I had always believed that a matched load yeilded best signal transfer, but recently taking measurements, this doesn't always appear to be the case, nor does a reliable trend emerge from one amp to another. However in these tests I wasn't necessarily looking at clean W RMS, rather power developed under overdrive/real world use, which may skew results somewhat from the ideal (as this is NOT how wattage is traditionally measured).

However, regarding reliability, it is largely accepted that a larger speaker (up to double) load gives the tubes an easier life. But if we don't know how well matched the original components were, we don't know for sure by how much we are mismatching. For example the OT's primary Z (into a matched load) might be anywhere from 2.5K to nearly 4K, if it's 2.5K then doubling the speaker load will be no problem. If it's 4K then you are effectively slightly more than doubling the load...but if this sounds good, you may feel that this outweighs any risks and still go ahead?

Bearing in mind that you already have speakers that you want to use, I would either just use 2 speakers wired for 8ohms. If wattage rating for 2 speakers is not sufficient, then I'd go with 4 speakers at 16ohms.

DC resistance isn't going to be of much use, transformers understand AC impedance, not DC resistance. Do a search on establishing turns ratio, you'll need a voltage source (0.5VAC) and 2 meters. Apply a known voltage (0.5VAC) to the OT secondaries (confirmed by your first meter) then measure the voltage developed accross the primaries with the second meter. Divide primary voltage by secondary voltage, square the result, then multiply by speaker load to get your primary Z.

[rexindigo]

ok, I found a way to do the measuring:
- I applied 10V AC to the secondary (speaker output) and measured 280V Ac across the Pins 3 of the Powertube sockets:
- according to the formula this makes 3,2k for a 4 Ohm load
(280/10)²=784, 784x4= 3,2k so this seemes to point to 4 Ohms being the right load (EL34 Datasheet says in AB PushPull Zaa is 3,4k)
- actually i found a hidden second secondary tap: with 10 Volts applied I measured 200 Volts Primary, wich points to this tap being the one for 8 Ohms [(200/10)²=400, 400x8=3,2k]

Thank You MWJB! Your Post was very helpful, i can now build the Cab with an Impedance of 4 Ohm without any missmatch. (as long as my calculations are right?)

[Bruce / Mission Amps]

Assuming it is a vacuum tube PA section, going one step lower is almost always safer then going higher.... ie., running a 4 ohm speaker load on your 8 ohm tap.
This does load the power tubes down a little more of course so they will maybe wear out sooner.
Doubling (8 on the 4 ohm tap) is dubious, depending on the quality of the amp and it's supporting components.
Quadrupling the speaker load (16 on a 4 ohm tap) is a very good way to blow your power tube sockets, power tubes and or the output tranny!!
Careful with this one as you don't really know the quality of the OT... it could be fine, as it will not make as much power running at 6800 ohms, but the AC voltage in the windings will be higher and higher as you mismatch up.

[rexindigo]

Assuming that my calculations are right I realized that I had the negative feedback going of from the wrong tap (usually the 8 Ohm Tap in Marshall1987 schematics). So I added another plug for the (according to my measuring) correct 8 Ohm tap and redirected the negative feedback to (or from) this plug. But what happend: I had horrifing loud midfrequency oscilations coming from an 8 Ohm speaker connected to this tap.
So now I have rearranged everything like i had it before. I will not head for the 4 Ohm or 16 Ohms cab and just use the 8 Ohm single 12" cab I have attached to the (whatever Ohmage) output tap I used all the time without experiencing problems. If the OT fails someday, I can use one with known data, this won't hurt, because i got this one for free.

[kg]

I had always believed that a matched load yeilded best signal transfer, but recently taking measurements, this doesn't always appear to be the case, nor does a reliable trend emerge from one amp to another.

power TRANSFER is always most efficient with a conjugate load, however, with different reflected primary impedances, the amp may actually be producing more or less power. if the load lines for example run into cut off or grid conduction with a high degree of asymmetry then the actual output power developed by the final stage may very well drop, even if the power transfer efficiency goes up. so you're essentially transferring less power more efficiently, or vice versa.

DC resistance isn't going to be of much use, transformers understand AC impedance, not DC resistance. Do a search on establishing turns ratio, you'll need a voltage source (0.5VAC) and 2 meters. Apply a known voltage (0.5VAC) to the OT secondaries (confirmed by your first meter) then measure the voltage developed accross the primaries with the second meter. Divide primary voltage by secondary voltage, square the result, then multiply by speaker load to get your primary Z.

well the dcr CAN be very handy to determine how much current the opt was originally designed to handle, and generally it follows the trend of primary impedance (ie higher primary z generally means low primary current and relatively higher permissible dcr, and vice versa).

Assuming it is a vacuum tube PA section, going one step lower is almost always safer then going higher.... ie., running a 4 ohm speaker load on your 8 ohm tap.
This does load the power tubes down a little more of course so they will maybe wear out sooner.
Doubling (8 on the 4 ohm tap) is dubious, depending on the quality of the amp and it's supporting components.
Quadrupling the speaker load (16 on a 4 ohm tap) is a very good way to blow your power tube sockets, power tubes and or the output tranny!!
Careful with this one as you don't really know the quality of the OT... it could be fine, as it will not make as much power running at 6800 ohms, but the AC voltage in the windings will be higher and higher as you mismatch up.

bruce has got it here.

the big thing about going higher in primary reflected impedance is that it puts a LOT of stress on your screen grids because of the huge plate voltage swings. any time vp drops below vg2 you're going to see screens glowing red. most of my power tube failures have been g2 related, almost none of them plate dissipation failures.

also those high plate voltage swings can cause arcing, in tube, on socket, or punching through the tranny winding enamel.

for those reasons i prefer to mismatch low, ie 8r load on 16r tap, etc. i actually like the sound better anyway.

ken

[loudthud]

Although I agree with the screen grid argument, it may depend on which type tube is used. In the 70's I had a PA system and did bar gigs with a band that used blackface Bassmans into 16ohm Marshall cabinets. They sounded great because they were able to get a heavily distorted sound at a reasonable volume. As far as I know, they never had a failure and never replaced the tubes. Worked with them for about a year.

[kg]

Although I agree with the screen grid argument, it may depend on which type tube is used. In the 70's I had a PA system and did bar gigs with a band that used blackface Bassmans into 16ohm Marshall cabinets. They sounded great because they were able to get a heavily distorted sound at a reasonable volume. As far as I know, they never had a failure and never replaced the tubes. Worked with them for about a year.

as a matter of fact, the quality and attention to detail during the construction can make a huge difference in reliability.

if the screen grid is wrapped precisely aligned in the "shadows" of the control grid, the current decreases rapidly. you CAN get away with a lot more abuse and/or power output. screen stoppers were much lower too. and the amps sounded stiff!

screen dissipation is just something i've learned to watch, not having the luxury of good NOS.

ken