Hey All,
I've got a bug to make a low power EL84 amp. The idea is to have a B+ of 300v or slightly less and make it fixed bias. I'm not going for the Vox sound or anything like that (obviously). I just want a nice low power p-p amp that sounds good clean, but breaks up nicely when pushed.
So regarding the XPP series. They come in 10, 15, and 25 watt and are rated for a frequency response of 70 to 18K Hz.
Just starting off, I'm thinking an 8K primary impedance and use the 10 or 15 watt. Probably the 10 watt would be fine if I only played clean, since the lowest note on a guitar is around 82 Hz IIRC.
Which one would you pick if you also wanted to really crank it?
Final note: I see the 25 watt only comes in a 7.6K primary.
I've got a bug to make a low power EL84 amp. The idea is to have a B+ of 300v or slightly less and make it fixed bias. I'm not going for the Vox sound or anything like that (obviously). I just want a nice low power p-p amp that sounds good clean, but breaks up nicely when pushed.
So regarding the XPP series. They come in 10, 15, and 25 watt and are rated for a frequency response of 70 to 18K Hz.
Just starting off, I'm thinking an 8K primary impedance and use the 10 or 15 watt. Probably the 10 watt would be fine if I only played clean, since the lowest note on a guitar is around 82 Hz IIRC.
Which one would you pick if you also wanted to really crank it?
Final note: I see the 25 watt only comes in a 7.6K primary.
The OT is the foundation on the amp's tone, and you should never skimp on it. So i would personally recommend the 15W OT. You may want to look into some the Marshall 18W clone transformers that are around. The original Marshall 18W used a budget hifi OT, and therefore gave excellent clarity and string definition.
BTW why do want such a low B+ voltage? Most Class AB EL84 amps run at 330V and above. I once repaired a Marshall 20/20 stereo power amp, which ran its EL84s with fixed bias at around 400V. It had a very sweet, clear tone.
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BTW why do want such a low B+ voltage? Most Class AB EL84 amps run at 330V and above. I once repaired a Marshall 20/20 stereo power amp, which ran its EL84s with fixed bias at around 400V. It had a very sweet, clear tone.
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I don't really need to skimp at all. The most expensive one (25 watt) is only about $34 IIRC. But it seems you get to the point with bigger and more power capable OPTs where it just stops making any difference, given the output of the amp remains the same. So it would just be silly to buy the 25 watt OPT if it would sound and respond just the same as the 15 watt model given you have an amp that doesn't do much more than about 12 watts clean. OTOH, if the 10 watt model over saturates in a nice way when cranked, it might be the way to go. But yeah, I hear what you're saying. Without any more info I would go with the 15 watt.
I was thinking along the lines of not abusing the tubes without getting into regulating a lower voltage for the screens. I wanted to keep it simple. I have heard (only internet rumours - I have no direct knowledge) that the tubes didn't last too long in those Marshalls, however wonderful they may have sounded.
Edited to add: I was also entertaining the thought of trying out untralinear, so going way over 300v might not work for tube longevity.
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The 15W OT will hit the sweet spot. Going below that will tend to muddy your tone.
Although their specs appear to be somewhat over-conservative, i wouldn't recommend running regular EL84s at that kind of voltage, only the tougher 7189, 7189A or EL84M variants, that are rated for higher voltages. And this type of amp tends to offer a lot of clean headroom, with little breakup.
You are somewhat on the right track when you talk about screen grid voltage, and you can go a lot higher on the anodes, if the screen voltage is kept in check. There was a Seymour Duncan 50W amp that ran a quad of EL84s at 560V, with the screens at 280V. In the case of the Marshall stereo amp I repaired, found the screen voltage was also really high, with only 100 ohm Rg2 resistors to protect the screens! I changed them to 1k without any hesitation.
I would point out that you don't need to regulate the screen village in a guitar amp, and in fact it may be counter-productive. All you need to do is drop the voltage a little with a resistor and filter cap, and then feed that to each screen via a 1k resistor. This approach allows the screen voltage to fall when the tubes are working harder and also helps limit screen current extremes. The tubes will last for a good many years this way, plus you will also get some nice touch-sensitive distortion tones with the dynamically shifting operating point.
That's a good call. There's also very little or no reason you would want UL in a guitar amp.
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So then what is the voltage at the screen grid (relative to the anode voltage) after putting in the 1KΩ resistor?
Going from my experience you would typically expect to see 10 to 15V lower voltage at idle on the screen grids than at the anodes with this kind of arrangement. For example a typical Marshall 18W will have around 330V on the anodes and 320V on the screens, and they aren't known to suffer from any over-voltage issues.
However, the real point of the 1k resistor is to limit screen grid power dissipation, so they don't get melted. In most cases, power dissipation is a more significant danger than just voltage. A regulated screen supply will unfortunately tend to hold up the screen voltage, rather than allowing it to sag on signal peaks, so that wouldn't be recommended, nor would it be beneficial toward good tone and dynamics.
However, the real point of the 1k resistor is to limit screen grid power dissipation, so they don't get melted. In most cases, power dissipation is a more significant danger than just voltage. A regulated screen supply will unfortunately tend to hold up the screen voltage, rather than allowing it to sag on signal peaks, so that wouldn't be recommended, nor would it be beneficial toward good tone and dynamics.
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