Output Transformer substitute question

[mresistor]

I need to sub a known good OT into '64 Deluxe which has a 6.6K OT. All I have on hand is a 4K Super OT. Will it be suitable for test purposes.  I have 4 8 or 16 0hm cabs.

[Ed_Chambley]

Sure, 8000K reflected is good for 6V6.

[VMS]

I think yes, and if the 4k ot has got 8ohm secondary i would use that with the 16ohm cab

[mresistor]

Thank you guys- yes it has 4 8 and 16 ohm secondarys  -   no 2 ohm as it's not a true Super replacement   rather one to build a Super and run it with  2x10 or 1x15 load. 

[mresistor]

Ed (or anyone else)   your answer was considering the OT had a 2 ohm tap. But it doesn't.  Lowest is a 4 ohm tap.  Would your answer change?  The OT is a Classic Tone 40-18001 Super or Bassman 4.2K primary I should have been more specific, sorry.

[VMS]

I think Ed was thinking what i was thinking that when you connect your 16ohm cab to 8ohm secondary the 4k ot changes to 8k OT.


i'm not sure if 8000K reflected is good for any tube 

[mresistor]

Wow  I missed that       good one 

[tubenit]

TRANSFORMER MATCHING

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 ridiculous 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 objective.

Design approaches are dealt with in TUT4.

From Kevin O'Connor's  London Power FAQ

[mresistor]

Thanks Tubenit   

[Ed_Chambley]

Tubenit is correct and a lot of times with guitar amps having a higher impedance than your datasheet suggests may give you a sound you want. Datasheets show how the tube is designed and what it was designed for.


Greeter amp builders usually are not looking for max clean, if we were we would not run screens at basically the same conditions as plates. Audio power tubes in guitar amps are worked hard in a lot of cases, especially today when everyone wants a 18 watt to sound like a 100 watt super lead.


It is fine to expirament. I was suggesting connecting 8 ohm to 16 speaker would make the tubes see 8k which is very common in push pull 6v6 and a lot of other small bottles.


This being said and me knowing you have a 40 watt transformer it will be just fine. Should pass lower frequencies better.


NOW reverse the senerio and use a 15 watt transformer on a 40 watt amp and you may smell an electric train, have a fantastic compressed tone just before it decides to take a smoke break. Wonder how I know this.

[Ed_Chambley]

I think Ed was thinking what i was thinking that when you connect your 16ohm cab to 8ohm secondary the 4k ot changes to 8k OT.


i'm not sure if 8000K reflected is good for any tube 

No it would be. Good catch. 8k is the one and she is the only one when the wolves come howling at the moon.

[PRR]

> for test purposes.

Yes, a healthy 4K or 8K will test a whole lot better than a sick 6K.

There may or may not be much difference in "tone" versus a healthy 6K. But you seem to doubt the 6K in your hand. If the known-good 4K or 8K plays better than the dubious OT, you have your clue.

Depending on the amp, it may not be wise to play a whole stadium concert with a 4K where a 6K should be- excess current. But a few bars won't hurt nothing.