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Old 8th May 2007, 01:40 AM   #1
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Default Cheapo Guitar Amp

I have some parts lying around and was wondering if anyone had any ideas for a pretty budget but functional guitar amp for a friend before we all disappear off to college. I have a few parts lying around that should provide a foundation:

1 35VA isolation transformer (can put out 320V into 70mA with a doubler)
1 3.33A 12V filament transformer

Various resistors, some film caps, etc.

2 unused Sov 6SL7's, 6 5763 radio tubes (EL84-shaped, probably hard to work with), 4 or so very unmatched 12B4's, 3 6SN7's

Any ideas? I was thinking about that $20 6.6K 20W Triode Electrics OPT, and a smallish choke for the power supply, but that's where my ideas run out. Does anyone have a pretty simple schematic I could adapt to work? Thanks!

Budget is under $200 - I think we can manage
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Old 8th May 2007, 06:21 AM   #2
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A couple of places to get you started or even finished.

Search the "AX84" project pages - a cooperative small guitar amp design project - lots of peoples experience with this little project.
www.ax84.com/

You could also do a search on "Trainwreck" project - about 9 variations on a theme (a Blues Amp) - it won't go into cranked Marshall territory (thank god say we old guitarists).
http://www.web-ee.com/Schematics/Tra...TrainWreck.htm

For a heap of guitar amp schematics then "Schematic Heaven" is the go - know an amp you really like, the schematics may well be here.

Cheers,
Ian
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Old 8th May 2007, 06:36 AM   #3
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Something like a champ with a 6SL7 instead of the 12ax7 and the 5763 as the output in single ended, or a pair in push pull, would be pretty easy. http://www.angela.com/catalog/how-to...incetonsch.gif
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Old 8th May 2007, 11:33 PM   #4
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Quote:
Originally posted by gingertube
www.ax84.com/
http://www.web-ee.com/Schematics/Tra...TrainWreck.htm
"Schematic Heaven"
Thanks for all these resources! I've looked at a bunch of the AX84 projects and Fender/Gibson/Mesa amps at Schematic heaven, and the Trainwreck amps, but am still a little clueless as to which designs to pick. I've been looking at low-powered versions, but push-pull - I'll explain why right below here.

Quote:
Originally posted by dsavitsk
Something like a champ with a 6SL7 instead of the 12ax7 and the 5763 as the output in single ended, or a pair in push pull, would be pretty easy. http://www.angela.com/catalog/how-to...incetonsch.gif
The schematic looks cool but I would love something that has a few more knobs to turn.. Is an amp like this pretty much all the isolation transformer can manage? Also, I'd prefer PP since I'd be running 6.3V filaments in series, and I'd want to put two of the same tube in series, for obvious reasons.

Thanks so much for the advice so far!
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Old 9th May 2007, 04:24 AM   #5
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If your using indirect heated tubes, seriesing heaters in a pair of parallel tubes in SE is also an option. Or a 12V power tube. Maybe 1626? Another heater option is just using a dropping resistor.

Id look at the 8K or 5K Edcors over the Triode Elec. 6.6K. Theres PP and SE versions both, <$20, 40% screen taps.

Triad makes suitable chokes cheap, see Allied.
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Old 9th May 2007, 05:06 AM   #6
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Thanks for the tipoff on the Allied chokes but Hammond stuff is also pretty cheap and easier to find.. In any case, the 1626 would be really cool, but I'd really like to go with something with a bit more power.

Most likely I'll just bias a pair of 6P14P/EL84's at 35mA idle. 190 ohms cathode resistance, and 180V on the plate should go nice and easy on them, with only 6.3W dissipation on each one. 12.6W plate dissipation shouldn't hassle one of those 15W open-reel 8K Edcor transformers, right? My PSU should be fine - even on a transient surge to 120mA current for the tubes, B+ only drops to 300. I think I'll give it a go.


I'll probably substitute a couple of 6N2P's for 12AX7's and just transplant the "rest" of a schematic onto this output section. I don't really know how tone controls work but I'll come up with something.

How does the above plan sound?
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Old 9th May 2007, 05:11 AM   #7
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The 8K Edcor is rated for 10 watt output, 15 watts of plate dissipation should be no strain at all.

The 5K Edcor is rated for 15watts output, 40 watts plate dissipation (~90ma) is no problem for it.

If your going PP class AB I recommend using a low Q choke and relatively low filtering capacitance. Better sag.

Tone stack calculator. The 6n2p may not have enough gain to drive a normal Fender stack. A lower impedance lower loss stack may be in order.


Do you know what speaker(s) you will be using? This has a huge effect on power, moreso than a few more watts will.
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Old 9th May 2007, 06:43 AM   #8
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I wouldnt worry at all in the case of the PP Edcors, as the primary limitations are thermal rather than magnetic.
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Old 9th May 2007, 10:01 AM   #9
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Default Re: Cheapo Guitar Amp

Hi sorenj07,


Quote:
2 unused Sov 6SL7's, 6 5763 radio tubes (EL84-shaped, probably hard to work with), 4 or so very unmatched 12B4's, 3 6SN7's
Any ideas? I
For a very simple, yet good sounding amp, you could use the SL7s and SN7 alone ... like in this "Octal Mojo" amp.

Regards,


Tom Schlangen
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Old 9th May 2007, 07:28 PM   #10
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Default Re: Re: Cheapo Guitar Amp

Quote:
Originally posted by Tweeker
The 8K Edcor is rated for 10 watt output, 15 watts of plate dissipation should be no strain at all.

The 5K Edcor is rated for 15watts output, 40 watts plate dissipation (~90ma) is no problem for it.

If your going PP class AB I recommend using a low Q choke and relatively low filtering capacitance. Better sag.

Tone stack calculator.

The 6n2p may not have enough gain to drive a normal Fender stack. A lower impedance lower loss stack may be in order.


Do you know what speaker(s) you will be using? This has a huge effect on power, moreso than a few more watts will.
That's reassuring about the Edcors, I'm using them in every amp I'm building, though I actually haven't finished a single one yet. Maybe today I'll finish one, with some luck. I'll also try a smaller choke, all it means is more convenience for me, and less cost for him. Besides, a bit of ripple in a guitar amp won't be a problem.

Thanks for the tone stack reference, I'll give it a look.

As for speakers, the guy does have a big crappy Crate amp with a radio-susceptible SS amp that he might cannibalize. I don't know how many speakers it has but hopefully it can work out.

Quote:
Originally posted by Tubes4e4
in this "Octal Mojo" amp.
Thanks, I'll check it out!
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