Amps that can supposedly use a variety of power tubes?

rpf · « **on:** 16 Feb 2011, 06:35 pm »

There are a number of tube amps out there that are advertised as being able to use any variety of pentode power tubes: EL34/6CA7s, KT66/6L6s, and KT88/90/6550s.

A friend brought one over, an auto biasing Prima Luna Dialogue Two that comes stock with KT88s. It sounded fantastic with both the stock tubes and a NOS quad of GE 6550As I have. It sounded distorted with both NOS 6L6GCs and (broken in) Black Treasure 6CA7s.

I have a Rogue Cronus (non-magnum), which I was told can also use any pentode. It sounds fantastic with the stock EL34/6CA7 family (of which I've tried several, including the BT 6CA7s above) as well as both the NOS 6L6GC used above and Genalex KT66s. It sounds distorted with the NOS GE 6550s (even after adjusting the bias both up and down).

So what gives?

I was told by a friend that the output impedance of an amp's transformers must match the input impedance of the tubes for optimized sound production and that there is a wide input impedance variance between the two tube families (around 3k for the EL34/KT66 family and around 6-7k for the KT88/6550 family?). Is this the reason for the variance in sound quality with the different tube families? Or is it something else?

I'm really curious about this.

TIA

Rob

Niteshade · « **Reply #1 on:** 16 Feb 2011, 08:43 pm »

Your friend had part of the answer. An output transformer's primary impedance has allot to do with how well it will accept rolling various tubes. A primary impedance of 6K and under for a push-pull 6L6 family amp should work decently.

NOTE: Some plate voltage/primary impedance setups do not work well together with various power tubes and this is another reason for the hit & miss performance issues.

Ericus Rex · « **Reply #2 on:** 17 Feb 2011, 12:39 am »

I don't think the power supply in the non-mag Cronus can handle the 6550/kt88/kt90 tubes. My guess is that's where the distortion is coming from.

How did you fit the KT88 through the small holes on the Cronus amp top?

rpf · « **Reply #3 on:** 17 Feb 2011, 02:13 am »

Quote from: Ericus Rex on 17 Feb 2011, 12:39 am

I don't think the power supply in the non-mag Cronus can handle the 6550/kt88/kt90 tubes. My guess is that's where the distortion is coming from.

How did you fit the KT88 through the small holes on the Cronus amp top?

The GE 6550s, like the BT 6CA7s and Genalex KT66s, require socket extenders. They work well w/o affecting the sound (I also auditioned the tubes with the cover off).