shooting for 100 watts


Poster Extraordinaire
Feb 1, 2007
If I were thinking of building myself a 100-watt tube amp (probably for bass, but let's speak generically), what would be factors to choose between a 4x6L6GC/EL34 output section as opposed to a 2xKT88/6550 section? The tube costs are about the same, the transformer costs would be about the same, I'd save a few bucks in tube sockets and chassis work (fewer holes to punch, right?)

About the only thing I can think of is that if 1 tube fails in my 2-tube setup, the amp goes down, whereas a 4-tube setup could continue to function, albeit poorly.

And don't spend a ton of time working on this, it's a largely theoretical question on my part at this stage. Thanks

2L man

Nov 23, 2020
Obviously main reason two KT88/6550 produce almost double the power four EL34/6L6GC is because they can operate higher voltage. Their plate dissipation is not double.

KT120 price is only little more and using that opens possibility to use lower impedance Output Transformer but also bit higher impedance using higher voltage.

Among HiFi is where you find more OTs which can produce low frequenzy power. Toroidal transformers are lighter than EI-cores. Toroidy in Poland make reasonably priced HiFi frequency rated OTs which can be bought also without "potting" to save cost and weight. I have test KT88-PP on EL34 amp and when it did sound good I plan to use it in SLO driving using pair of 6550 for more power.

Russian GU50 or FU50 is budget option for a bass amplifier. I have read multi page thread of a bass amp using four or six GU50 on other forum which you should be able to find...

If you plan to get 4 tube reliability I think you should build each pair plate and screen power behind own fuses. Sometimes when tube fails suddenly it burn HV fuse or screen resistor and then other pair might continue operating.
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