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Old 4th June 2007, 02:44 AM   #1
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Default Quick And Dirty ~75W Guitar Amp

Hi, just asking the feasibility of the following amp. A fellow senior at my school just finished building a PIMPING guitar (maybe he'll send me a pic or two) and would like a pretty solid tube amp to accompany it. His preliminary idea was around 75 watts, with some basic controls such as overdrive, bass and treble. Here are my thoughts. I'm an amateur at guitar amps, so let me know if something is absolutely infeasible.

Output tubes - 2 KT88's. At guitar frequencies, 75 watts should be possible, right? I might use a single SS diode to tap off a negative bias.

Driving tubes - 2 12AX7's, 1 12AT7. The 12AX7's serve to jack up the gain, and at least one triode will feed a filter network. I have Duncan Amps' Tonestack Calculator and TubeCAD, and I have basic biasing knowledge. Each half of the 12AT7 will drive a KT88. The OPT would be one of Edcor's 5K 100W deals.

PSU: I'm thinking a 282X (200mA 1000VCT) feeding a 5U4, into a small filter cap, then a 193H choke (5H 200mA 65R) and a decent filter cap. RC's to feed the 12AT7 and 12AX7.

This is a pretty sketchy layout but does it seem feasible? His budget is around $600. Nothing amazing for the tubes, probably Sovtek. Does anyone happen to have a schematic lying around that looks like it could work? Thanks!
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Old 4th June 2007, 05:35 AM   #2
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Everything looks good, but I would use the following power transformer:

TRANSFORMER, POWER, HAMMOND, 400-0-400 V, 200 mA
P-T278X

The 1000vct unit is a little on the high side.

If you use a tube rectifier be aware that the power supply will sag when playing hot. This makes for a good sounding amp to play "blues" through but not so good for hard rock / metal. For that, I'd use SS diodes.

Here's a schematic on the "Dr tube" website:

http://www.drtube.com/schematics/marshall/1967u.gif

Basically you'd be using 2 output tubes instead of 4. With the 278x transformer, you will get 75 watts, with the 1000ct transformer it will be more like a 100 watts with 2 tubes.

It's do-able; just watch out for the high voltages, they will ZAP you!

Good luck... Daniel
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Old 4th June 2007, 05:55 AM   #3
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Thanks for the schematic and advice!

A couple of questions:

I understand that a really really high B+ isn't great, but wouldn't a small capacitor after a 5U4 both increase the PSU's stability (nudging it towards choke-input) AND decrease voltage to close to choke input? I like the idea of using a small 630V Solen. This amp would be in ultra-linear by the way, in case that makes a difference.

I'd also like this friend of mine to be able to substitute in different tubes, i.e. 6550's, 6L6GC's, EL34's, etc. to check out tone. I suppose that fixed-bias and a couple of small pots could let him dial in the right current for each?

One other thing. Since only 2 KT88's would be used, wouldn't the 5U4's sag be a lot less? I don't expect peak current to the 88's to ever exceed 180mA or so... I'll ask him what kind of music he prefers.
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Old 4th June 2007, 07:38 AM   #4
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B+ voltage sag occurs from a power supply that has (relatively) high impedence. Any simple power supply with a tube rectifier is going to have rather high impedence. As a good rule of thumb, the higher the voltage drop of a tube rectifier, the higher its impedence. Rectifier sag sounds sort of like tremolo effect. This goes good with blues where you can "play off" of this sag; not so good for hard rock, etc.
A capacitor after the 5U4 will make it a capacitor input power supply; you don't want to use any more than 40-50uf right after the 5U4, for tube (rectifier) life reasons.
I would build the amp like the above Marshall schematic but with 2 output tubes and see how that sounds. Then start changing things around a bit to see how it affects the sound, like to a tube rectifier.
With a solid state rectifier, you will be able to use several hundred uf's worth of filtering cap, which will give the amp serious PUNCH.
Basically the more you experiment, the more knowledge you will gain as to what affects what sound in the amp.
And don't fall into thinking that you have to use top quality parts to get good sound from a tube guitar amp, especially the output transformer. All the classic outputs were wound really cheaply, that is to say their bandwidth wasn't very good. What's good for hi-fi lots of times isn't good for guitar amps.
Daniel
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Old 4th June 2007, 07:49 AM   #5
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Go to:

http://frank.pocnet.net/sheets/084/k/KT88.pdf

for KT88 specs. Go to the line that says, "Push Pull Ultralinear, Fixed Bias". Using the 278x transformer will be pretty close to this data.
2 X KT88 uses 300ma at max signal, so the 5U4 will be wimping out a bit when power chords are being played.

Daniel
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Old 4th June 2007, 09:48 PM   #6
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Hi,

Welcome to the crash-course-in-guitar-amp club

I just had the same wiccaning in guitar amps recently, having only known HiFi.

A trick I learned to smooth overdrive from a pre tube (lots of bloom, low fuzz) is burn them as hot as possible current wise. 12AX7 is the easiest tube to deal with for guitar OD, 12AT7 the hardest with 5751 right in the middle. Too little current can increase gain, but make them sound like transistors fuzzing away.

Depending on the tone your client is looking for, you can "sag" the B+, or rock-solid the B+ and pretty much make a HiFi phase splitter and output and have all the tone coming from the pre. The latter is harder to do, but more effective with PP, IMO.

Just my

Cheers!
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Old 4th June 2007, 10:59 PM   #7
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Solid state it is then. I'm really looking for some punch and anything that reduces that is a hassle for me. How much current do KT88's take to bias? In other words, I'd like to know the ranges of the biasing pots. Is 2mA per tube at -40V or whatever a good estimate? I haven't looked at a KT88-UL curve sheet yet, but I'd like to bias for around 50mA of current per.
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Old 4th June 2007, 11:00 PM   #8
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Sounds good, Geek. I'll probably just use a clone of a circuit for now that uses two 12AX7's, and a 12AT7 driving.
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