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Old 13th June 2011, 09:24 PM   #31
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Quote:
Originally Posted by simon7000 View Post
I found some of the Russian equivalents at buy it now for under $2 but 4 12AX7 Used Tubes, 1 no name, 1 Zenith, 1 GE and 1 EH | eBay is at $1 and
Vacuum Tube Lot 8lbs 6bq5 12au7 6686 5bk7 6as8 6al5 | eBay
$50! For your own use way too much! Old TV's are free and contain all the parts you will need!
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Old 13th June 2011, 09:41 PM   #32
bst is offline bst  United States
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Some random thoughts (yes, I know, working without the proper tools again...)

I'm setting aside the compactrons and loctals for now, so that this project can easily be duplicated outside the US. I'm also shelving the unusual filament voltages, to utilize more mainstream transformers. So at the risk of reinventing the wheel, I'll be using an octal pentode for the output, which is readily and inexpensively available in both 6.3 and 12.6 volt versions.

I've settled on a 6W6, which is available NOS for less than $5.00. It also sounds excellent when strapped as a triode, so I'll incorporate a switch to select between the two modes in the output stage. The SE output stage will also allow lots of tube rolling - 6K6s, 6Y6s, 6l6Gs, and so on.

I'll be using a variable gain control for the input stage, and a master volume for the entire circuit. Between the mode selection, tube variations, and stage adjustments, people will hopefully be able to get any sound they want. You can play the last stanza of Hendrix's 'Third Stone From the Sun' at deafening volume into your headphones, or when the wife gets home, softly play Duane Allman's 'Little Martha' to her through a speaker.

I'm leaning toward new-production power and output transformers from Edcor; I like the company and their products. Don't know if they're available through Farnell for our co-conspirators across the oceans.

My biggest unknown at this point is the driver stage, and whether it will be triode or pentode (or another switchable triode-strapped pentode?). It will be octal; I have a personal idiosyncracy of trying to keep tube basing uniform throughout a circuit. Octals are also easier to wire for less-experienced builders. I'd love to use some of my stash of 2C52s for the driver stage, but they've become rare and expensive. Too bad -- the mu of 90 matches many *AX7s, and would make the circuit implementation a snap.

Now to start adding up costs so far, and see how much I have left to flesh out the first stab at this design...

Last edited by bst; 13th June 2011 at 09:50 PM. Reason: correct typo
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Old 13th June 2011, 09:47 PM   #33
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A question for the musicians and instrument techs: how popular are pedals / effects boxes, and should I incorporate the ability to use them with this little practice amp?
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Old 13th June 2011, 09:49 PM   #34
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I'm leaning toward new-production power and output transformers from Edcor; I like the company and their products. Don't know if they're available through Farnell for our co-conspirators across the oceans..
Anything coming from the US arm of Farnell to Europe automatically involves a 25, (+vat), handling and shipping fee. Direct from Edcor may well be more reasonable.

:edit:

An effects loop would be nice, but by no means essential.
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Old 13th June 2011, 10:55 PM   #35
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Quote:
Originally Posted by bst View Post
A question for the musicians and instrument techs: how popular are pedals / effects boxes, and should I incorporate the ability to use them with this little practice amp?

Great Question,

Personally,
I always lean towards a fairly clean dynamic amp that distorts well at maximum volume. When I want bedroom levels I just plug in the old Pod multi-effects and go with it.

Most certainly I will never get the perfect Trainwreck or Dumble tones, but hey those are not bedroom amps either
(Not married with kids amps either)

Iron Options

Difficult part is naturally the iron cost and availability world wide. Music Power Supplies has gotten very popular on many guitar amp DIY sites due to having many low cost options as well.
I can honestly say that these transformers work and sound just fine.
Sales and Information - Musical Power Supplies Inc.- Quality Tube Amplifier Parts You CAN Afford

Unfortunately his PT's are a tad more than the minimalist units needed for this challenge. He does however have a few comparable OT's PP & SE Edcor units.

I checked my supplies and have everything I need except the chassis but I have been kicking around a twist on that as well.
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Old 13th June 2011, 11:25 PM   #36
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Just a random drive by thought... Anyone know what a Futterman sounds like when overdriven?
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Old 13th June 2011, 11:52 PM   #37
bst is offline bst  United States
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My proposed parts expenditures so far (all new parts from established vendors):

Edcor XPWR024-120 Power Transformer $20.01 (Edcor)
Edcor XSE-10-8-*K Output Transformer $17.22 (Edcor)
[4 x] UF4007 Fast-recovery diodes $ .44 (Newark)
3-Wire wire-in power cord $ 7.99 (AbTec)
Panel-mount 5x20 fuse holder $ 1.50 (Pacific TV)
Ceramic 250v GMA fuse $ .25 (Cascade Surplus)
[3 x] Chassis-mount octal sockets $ 6.00 (Cascade Surplus)
6W6 Pentode, NOS $ 4.10 (Pacific TV)
[2 x] 6SF5 Triodes, NOS $ 8.40 (Pacific TV)
[4 x] 1/4 Panel-mount jacks $ 3.72 (PartsExpress)
[2 x] Panel-mount SPST toggle switch $ 3.00 (PartsExpress)

Total so far $72.63

Still need to fire up PSUDII and TubeCad / SEAmpCad to figure out caps and resistors, and still need to add pots, chassis, and misc. terminal strips, grommets, knobs, etc.

It's going to be pretty tight, but I think I'll be able to come in at $100 for a complete amp using all-new, over-the-counter parts. Of course, if I scrounge through my existing stock, the only thing I MIGHT have to buy is the transformer set. I could probably build the design I'll be working up for nothing.

Therein lies a good part of the challenge can your final design be built by someone who has to purchase every component? And of course, it has to sound magnificent...
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Old 14th June 2011, 12:00 AM   #38
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Another annoying question. Anyone have a problem with using a mosfet follower in the signal chain like the PowerDrive circuits that I use in my HiFi amps? All active gain stages will be pure tube but a mosfet follower to drive the output tube grid can be used to eliminate blocking distortion. They are also useful to drive the tone stack. Just trying to maximize the bang for the buck equation.
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Old 14th June 2011, 12:08 AM   #39
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Quote:
All active gain stages will be pure tube
By all means! If you can build a better mousetrap...

Quote:
Just trying to maximize the bang for the buck equation.
And that's exactly what this exercise is all about. I have no delusions that my entry will
sound like anything but 'geese farts in a 55-gallon drum' (borrowed from Leo Kottke), but I'll have fun lashing it together. I'm really looking forward to the innovations that experienced and knowledgeable people like yourself will unveil.
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Old 14th June 2011, 12:42 AM   #40
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Quote:
I have no delusions that my entry will sound like anything but 'geese farts in a 55-gallon drum'
I never heard that one before. I have heard and built some stuff that I thought sounded terrible, but other people liked it. About 15 or 20 years ago I used to make custom guitar amps. All were sold by word of mouth and some of my weird stuff is what spiked sales, but once a lot of the local musicians have the same sound, no one wants it any more. I didn't have any new tricks and sales eventually died off. I think the last guitar amp that I actually completed was about 15 years ago. It's time to make another one, or two, or three, but first I got to find all the parts of my Stratocaster and put it back together.

I made several versions of the Turbo Champ. In fact I don't think that any two were the same and the schematic I posted was drawn from memory after the fact, but should be pretty close. I think the big "sound trick" on that amp was using the Hammond 125CSE OPT which is just a bit too small, and using a switch on the output to force a mismatched output. This allows the amp to distort at a lower volume, which may be useful here.

I also made a bunch of P-P amps that were in the 20 to 50 watt range. The "sound trick" on that one was to use two different output tubes, but set the idle current the same. Common combinations were 6L6GC and EL34, or 6L6GC and 6V6GT, or EL34 and KT88. This gives the power of a push pull amp, but the fat harmonics of a SE amp.

Just thought I would toss these out there for people to play with. I have posted both on this forum in the past.
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