JCP61 wrote:I have heard the same thing, that they were loud that is.
the mkII is basically the same amp as the yb1a but the B+ is kicked up to 525v making 2 el34's into 90 watts
so that ought to be interesting when I get some new caps and tubes into it.
looks like it hasn't been on since at least 75'
think i'll put a PPIV master volume in the 50 watt
that might tame it and get some decent sounds to boot.
From Wik:
In common with all 'E' prefix tubes, using the Mullard-Philips tube designation, the EL34 has a heater voltage of 6.3V. According to the data sheets found in old vacuum tube reference manuals, a pair of EL34s with 800V plate voltage can produce 90 watts output in Class AB1 in push-pull configuration. However, this configuration is rarely found. One application of this type was in "Australian Sound" public address amplifiers commonly used in government schools in Australia in the 1950s, using four EL34s for ~200 watts. More commonly found is a pair of EL34s running Class AB1 in push-pull around 375-450V plate voltage and producing 50 watts output (if fixed bias is used), while a quad of EL34s running Class AB1 in push-pull typically run anywhere from 425-500V plate voltage and produces 100 watts output. This configuration is typically found in guitar amplifiers, especially Marshalls and is quite hard on the valves.
From me:
The other problem with that is the increased plate curent it would take to get a good 100watts. Another the output transformer is not wound sufficiently or in a manner to support the changes. Not all transformers are created equal. Why don't big amplifiers have small transformers? The big ones have them. Is it to save on size and copper? Nope. It is because the transfer of of energy from the plate to the speaker requires the creation of a magnetic field and the collapse of that magnetic field to induce sufficient current and voltage to drive the speaker. It is similar to assuming that you could start your car with a set of flashlight batteries wired up to produce 12 volts. Not likely to happen.
At best you could expect an increase of distortion and a rapidly decreased life of the tubes. For that reason 4 tubes and a heavier transformer are used to create a "100 watt" amplifier. And even at that, it is not a clean 100 watts. Just a few thoughts about that mod.