New amps that accept different output tubes

[Ed_Chambley]

There is a great advertising on new tube amps where you can use many different Power Tubes.  For instance, Egnater says in is its Tewaker you can use any octal power tube.  How is this possible without an OT mismatch.  They are not the only ones, Victoria has a couple now as well.  There are many.  If you use EL-34's you need 3.4k or so, but on 6V6 you need 6600 to 7000.  How are they doing this without causing a mismatch?

[sluckey]

The OT has no impedance of it's own. It is merely a matching device and it's purpose is to match the high impedance (high voltage, low current) output tubes to the low impedance (high current, low voltage) speaker. The two things that need matching are the tubes and the speaker, not the tubes and the OT, nor the OT and the speaker. The OT simply has an impedance ratio.

When changing tubes that have a different impedance, you should also change the impedance of the speaker. For example, you have EL34s perfectly matched (by the OT) to an 8Ω speaker. Now you put 6V6s in the amp. The primary impedance is now doubled that of the EL34s, so, to maintain a perfect match, you'll have to also double the speaker impedance, ie, now use a 16Ω speaker.

That's the perfect solution. Impedance matching is mostly about transfer of power. If you have a perfect match, you will transfer maximum power. If the match is not perfect, you will not transfer maximum power.

In the EL34/6V6 example, you could ignore the speaker impedance when changing tubes, but you will not transfer maximum power so the amp becomes less efficient.

[tubenit]

The source for this is Kevin O'Connor's FAQ on his LondonPower website. I think he has some excellent insights and knowledge.

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objecti

I change between 6V6 and 6L6 and 5881 tubes in my TBM and TOS amps and never have experienced any concern or issue. And all the tubes sound good to me in either amp. I don't adjust or change anything.

With respect, Tubenit

[HotBluePlates]

If you take in all of what O'Connor says, then boil it down to its essence, you get:

There is an optimum load impedance at which a given set of tubes, produces maximum power with a given available B+. With any other load impedance, the tubes produce something less than maximum power.

You can light a 120v bulb with 12v. It just won't burn as brightly as with 120v. You could light it with 180v, but it won't last as long as with 120v.

[TIMBO]

Great Topic, As this has confused me greatly. I was concerned about the hookup for the SLOW DRIVE 84 using the 125d OT http://www.el34world.com/Forum/index.php?topic=13167.0 .I would like to keep using these tubes because they are a great size for my use.
But @ $40/OT i have found a cheaper option ( thanks to Darryl from Aussie Guitar) he uses 100v line transformers as PP OTs but as i understand it they are not SO perfectly match with the power tubes but must have a better ratio   (this is where i looses me ) between the tubes and speaker ???

With the 125d i can hit close to the mark with the universal hookup to get the right match between tubes,OT and speaker but the line transformers seem to work well in Darryls IMPACTII as well as others.
IS this a trail and error thing (if it sounds good, its OK)or is there more math to it,that will protect tubes etc. from damage. Thanks

[jim]

This may be outside the question but can we take this one step further to alleviate some uncertainty for me?  What is we want to use 4 6V6GT power tubes in our amp and we wanted optimized tone and power.  These tube impedences are connected in parallel with regard to the OT so...these are treated like resistances: ie ....the 7K pairs are presenting a 3.5K or so load for the OT enabling us to utilize a OT optimized for a pair of EL-34's?  This is what I have assumed to be true.  Amps with 4 6V6's are more unusual than 6L6 or EL-34 quads that have OTs commonly available.   Jim

[HotBluePlates]

... want to use 4 6V6GT power tubes ... connected in parallel with regard to the OT so...these are treated like resistances

Yes, basically.

If with a given B+ and class of operation, if 8k was a good loading for a pair of output tubes, when you move to a quad, 4k will be a good loading. So move up from a OT meant for 6V6 push-pull pair to an OT meant for 6L6/EL34 push-pull pair.

....the 7K pairs are presenting a 3.5K or so load for the OT ...

Be sure you know the tubes do not present a "7k load" to the OT. The speaker is the load, and the value of its impedance is being reflected (like a funhouse mirror) by means of the transformer's turns ratio to appear as a 7k load for the tubes.

In fact, the tubes' impedance, since they are pentodes or beam power tubes, is very high and on the order of several-hundred kilohms to several megohms. This is implied by the nearly horizontal slope of the plate curves of these tubes.

[Ed_Chambley]

My question had 2 sides to it.  I do understand matching equals efficiency as there are many OT for say 6L6's with different impedance.  I see them varying from 4k to 5.9.  These variables allow for different setups in various builds.  Also, speakers are never what they say they are in wattage and ohms.  The Celestian Gold 12 in 40 watts comes as a 15 ohm, not 16 however the blue (same speaker except for the paint and cone) is 16 ohms.  If I measure them they are identical within reason.

So if what we are discussing is true, then why do people concern themselves with damaging their tube amps if they plug a 16 ohm cabinet into an amp with the selector set to 4 ohms.  Efficiency aside, why do the scream about flyback and killing the OT if a mismatch is nothing more than loss of efficiency?

[sluckey]

So if what we are discussing is true, then why do people concern themselves with damaging their tube amps if they plug a 16 ohm cabinet into an amp with the selector set to 4 ohms.

It is true. People do need to be careful of their amps. Most of these amps are designed on the bleeding edge of 'power', many times pushing or exceeding design limits. I don't mind a 2X mismatch, but a 4X mismatch makes me nervous. It's probably more critical in high power fussy amps like an SVT.

why do the scream about flyback and killing the OT if a mismatch is nothing more than loss of efficiency?

A lot of people doing the screaming don't know what they're talking about. Some are simply echoing what they've heard on the net without understanding and often injecting their own misconceptions.

Flyback can cause burned tube sockets and dead OTs. Flyback is mostly associated with OPEN speaker connections while driving an amp hard.

[jjasilli]

An amp can be designed to handle different types of power tubes, but will not be optimal for all of them.  Modding this to optimize ea tube type will complicate the design and use of the amp.  Manufacturers may shy away from such complexities. But it can be done especially in the DYI world. 

Things to make switchable:  B+ voltage; Bias Voltage or cathode R values; multi-OT leads for impedance matching.

OTOH, there's something to be said for simplicity.  The B+ voltage for a 6V6 will not fully power a 6550.  But it's still a viable option which will probably sound good, and different, from the 6V6.

[Ritchie200]

Flyback can cause burned tube sockets and dead OTs. Flyback is mostly associated with OPEN speaker connections while driving an amp hard.

That and UL designs driven to distortion into crappy OTs....  Not that I've had that experience...lately.

Jim