7704A
Well-known member
Sweet! Clips?
Could you clarify why? I've been trying to nail down an exacting explanation of what goes on, what breaks, etc.
FourT6and2
Well-known member
NOS 6550a are 42W tubes. Modern 6550 are 35W. A 2204's power supply is designed for 50W clean output (cranked). Biasing 6550a at 70% is asking the amp to produce more than 50W at idle, before you've even turned the volume up past zero.
So while the tube is happy enough to dissipate that kind of wattage, the amp's power supply probably isn't. Get the amp cranked a bit and you're stressing the PT.
Vintage Marshalls got away with this because they were biased with -63v on the grid instead of biased to some percentage of the tube's plate dissipation. Throw some 6550 in your amp with -63v bias voltage and see where they actually measure with regard to cathode current. Probably much lower than 70%...
The reason Marshalls in North America had 6550 was three-fold.
1. Cheaper. Marshall shipped amps across the pond without tubes, which reclassified the amps as incomplete products = lower import duties/tariffs.
2. Tubes were breaking in transit. Shipping without tubes = no more breakage = lower cost.
3. EL34 were apparently crapping out more and more.
So they shipped without EL34 and let the North American importer install 6550 once they came into the country.
Ultimately, it's up to you. As long as you're hot enough to eliminate crossover distortion but cold enough not to redplate the tubes or stress the PT, you're good. Anywhere within that range is fine.
Last edited: