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Old 13th June 2011, 03:16 PM   #21
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I think you will need to set some required low frequency response (maybe 40 Hz for guitar?),
The lowest note on a guitar is around 80 Hz. Many guitar amp specific OPT's are specced from 80 Hz to 4000 Hz.

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And cheap 480 V industrial/control transformers off Ebay are usually a few hundred Watts at least.
True, you can find industrial control transformers cheap on Ebay. The supply and price is not constant and they are real expensive if purchased new.

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And the Watt limit should be taken as a minimum, since its easy to get a couple of $1 sweep tubes to do 50 Watts in P-P.
I thought about this for a minute. Even though I probably could make a screen driven sweep tube amp using some 98 cent 6BQ6GT's that rocked 50 watts or more for under $100, it would be a one trick pony. How often would it get used.

This thread started as an offshoot from another thread that was about low powered practice amps. I know that it is against my grain, but maybe this should be about making a great amp in the 2 to 5 watt range that might do 10 watts when driven to extreme saturation. The amp should be able of running all day long without melting down, or heating up the room. The bulk of the money could be spent on making the thing sound good.

There is actually a decent demand for these. Just look around at the botique guitar amps selling for $1000+ that make 5 watts or less. If they are used on stage or in the studio, they are miked. The 1.21 Giggawatt amps are being used by bar and club bands that need to look good and don't play through a house PA.

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The whole amp could be done with just one odd tube type, like 6LU8, 6LR8, 8AL9, 6AL11.
There is not enough gain in a 6LU8 or 6LR8, been there tried that. They are OK for HiFi spud amps when driven by a 1 volt CD player. A guitar puts out 50 to 400 mV.
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Old 13th June 2011, 03:24 PM   #22
Trout is offline Trout  United States
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Originally Posted by smoking-amp View Post
I think you will need to set some required low frequency response (maybe 40 Hz for guitar?), or the amp/OT will just be a tweeter amp. Real easy to do a 400 Hz and up amp. (use air core even) Similarly the HF end needs some minimum, or the amp will be 1000 Hz limited. And the Watt limit should be taken as a minimum, since its easy to get a couple of $1 sweep tubes to do 50 Watts in P-P. And cheap 480 V industrial/control transformers off Ebay are usually a few hundred Watts at least.
The OP was 2-5W Challenge
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The challenge is to build a low-powered practice amp for electric guitar. Power output should be in the range of 2-5 watts.
Being an avid guitar player, and reading multiple forums regularly, it is very common for even 5W to be considered " To Loud " for home practice amps.
This is especially true multi-family dwellings.

Lets not forget the el-cheapo 6AV6 or 6AT6 (Ala Tweed Harvard) for an addition gain stage if needed, those are available world wide cheap.

Edcor makes a tasty/hefty OP transformer very cheap that would easily accommodate low powered 6AQ5/6005. Though I am thinking PP as well.

examples
XPWR024-120 Power trans $20.00 (175-0-175) at 60mA center tapped and 6.3V (3.15-0-3.15) at 2A center tapped.
XSE10-8-5K SE output $17.22
GXPP10-8-8K PP output $25.10
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Old 13th June 2011, 03:28 PM   #23
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"There is not enough gain in a 6LU8 or 6LR8, been there tried that. They are OK for HiFi spud amps when driven by a 1 volt CD player. A guitar puts out 50 to 400 mV. "


Point taken on the 50 to 400 mV input. But how about the twin pentode tubes with a CCS loaded input pentode. 6AL11, 6G11, 6AD10, 6BY11, 6T10 Or there's the triple valve tubes like 6AF11, 6BD11, 6AS11, 11CF11
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Old 13th June 2011, 06:01 PM   #24
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A twin pentode compactron would definitely be a contender, although compactrons are not common outside the US.

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Edcor makes a tasty/hefty OP transformer very cheap that would easily accommodate low powered 6AQ5/6005. Though I am thinking PP as well.
Sounds like 2 of us are on the same track. I happen to have a big box of 6AQ5's and a bigger box of 6AU6's. Both are available world wide. I have some XPP10-8-8K OPT's too, if I don't find something else that works.
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Old 13th June 2011, 06:32 PM   #25
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Interesting,

I've been in Key West for the past week and just got back this morning at 4am (1000 mi barn-burner drive).

A couple more tubes to consider are the 6P1P(/-EV) which is a 6V6 derivative, along with the 6P2P(/-EV) which is similar to the 12AX7.

Creative use of transformers looks interesting.
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Old 13th June 2011, 06:38 PM   #26
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I expect 6J6 in phase splitter and a pair of 6BA6 in output will give nice Fender-style overdrive. Instead of 6BA6 I may use 6AQ5, but they would sound more Marshall-like than Fender-like. Also, 6P30B are good candidates for output, and they don't need tube sockets. In such case I may go with 6N16B in phase splitter.

For preamp I will use 6N17B, they are very like 12AX7, but tiny and don't require tube sockets.
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Old 13th June 2011, 07:00 PM   #27
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by ratbagp View Post
Last year our little group ran a similar competition for a 6V6 amp (stereo) housed in a container that came from Ikea.

ray
LMAO,
6V6 Entries

Brilliant

Just a thought, what about a mains Tx as Op Tx..toroid maybe/
Post #2 in this thread
Link:

Power Transformers in Output!

I suppose you could get a small cheap Op tx for the same money...might sound better.

Then again OTL .....only very low power use the speaker as cathode load.

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Old 13th June 2011, 08:21 PM   #28
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$100! Way too much money! Use say four 70V to 8 Ohm distribution transformers connected in series on the primary and series parallel for the output. That is under $11. Use an old fashioned dual triode 6N7 in a socket for $10. I would pull the transformer out of an old CRT TV set. That is free since they are not HD! If that isn't allowed use 3 surplus or Chinese 12 VCT 1 A or so transformers. The first one is a bit higher current and steps the voltage down for the filaments and also drives the other two to get from 12 volts back to 240 V ct. That is under $20!

Now to really save money use a 20K/600 ohm transformer as a phase splitter! $2 buys a cheapie!

Now a $.99 12ax7, and a few more parts under a dollar leave only the choke and filter capacitor. Skip the choke and use power resistors, a 10 uf 400 volt capacitor is under a $1 so the CRCRC may be your best filter value. At .125A 47 ohms is a nice value. The chassis should be an old cookie tin. Even if you include the price of the cookies around here that is $2.99!

So You might be a hair over $50! Of course this would be a 10 Watt amplifier!
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Old 13th June 2011, 09:14 PM   #29
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$100! Way too much money!.......So You might be a hair over $50!
Yes, I could build a $50 amp too, with all new parts. In fact I am ordering the parts to do just that. I am not sure that too many GOOD guitar players would keep it for long though. Of course you are welcome to build a $50 amp and prove me wrong.

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Now a $.99 12ax7
Show me where I can buy a 12AX7 for $1 and I will use them. I can get an output tube capable of 75 watts per pair for $1, but 12AX7's seem to be about $5 each.

Next question for pondering......

All things being equal (they are not really, but...) would it score points to use octal output tubes over miniature tubes? Size matters you know.....
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Old 13th June 2011, 09:22 PM   #30
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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
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