The Hundred-Buck Amp Challenge

[TheGimp]

Bass note saturation of the transformer?

[Cassiel]

Yeah, I think I'm going to junk it.

[tubelab.com]

This small OT can't cope with certain notes.

The Champ is a SE amp that runs the tube's plate current through the OPT. If the OPT isn't gapped or severely oversized there will be saturation induced distortion created in the OPT. Saturation sounds rather nasty. If your "OPT" has a center tap try wiring it like a P-P transformer with a load (even a resistor) on the other side to see if that is the issue. A resistor will waste power, but a CCS will not. In my amp I used a self split output stage but the undriven side is more for eliminating OPT saturation than anything else.

I am getting good sound out of both of my budget amps. One uses a 70 volt line transformer and the other uses an Antek power toroid. Both are P-P amps though.

If your OPT can be dissassembled it may be possible to restack the laminations where all the E's and I's line up and make a gap with a few thicknesses of scotch tape.

I apologize for not confirming the heater on the 50bm8...I swear I looked at it though

I just googled up a data sheet, it could have been wrong so double check. I kind of thought it was 150 mA too, but I chose not to use them because they weren't in large supply.

In the radio days the 50L6 was common for the octal output tubes. When miniature tubes appeared the 50B5 was the first output tube to appear. People kept getting zapped by touching the chassis of a radio with a broken knob so UL and others proposed safety standards that became a requirement for selling product. The practice of tying one side of the power line to the chassis became taboo and the 50B5 was repinned to create the 50C5 to satisfy the creepage requirements in effect at the time. It then became common practice to connect a capacitor from the line to chassis ground.

Safety standards have evolved and today we refer to this capacitor as the "death cap". The name is for a good reason since old caps tend to be leaky and leaky caps can fry guitar players. Yes the schematics for these old amps are all over the net, but I would not play one without an isolation transformer between me and the wall outlet. The action of placing ones hand around the neck of the guitar would make it impossible to let go of when a short occurred.

Somewhere back in this thread you can find the schematics of my two designs that are line powered with series heater strings. Both run the tubes off of the B+ supply on DC for zero hum. The little guy makes about 2 watts and the big guy makes 20 watts but it requires a 100 VA isolation transformer.

[Printer2]

Nothing to do with this contest but...

Speaking of SE transformers and saturating transformers, I could use a bit of direction. I have an old line powered amp, 12AX7, 35W4, 50C5. The transformer core that the amp uses is a bit small 2" x 1.5" x 0.5", I am guessing it is a 2W mind you I do not know what size a 4W would be for guitar. I want to use the transformer but with a different tube and was thinking of using one of two tubes with it. Either a 6AQ5, lower secondary impedance than the tube is used to but running it at a lower voltage and higher current than would be normal but still within dissipation limits. Using a 25L6, more in line with the secondary impedance but the 50C5 normally puts out 2W where the 25L6 is around 4W but is also rated at 2W at 110V. Not sure which way to go. May use a 12AU6 for the input with a Mosfet follower on it and a Tweed volume-tone arrangement. Just something low power to distort at a low volume.

[Minussss3db]

I don't think I can afford Vintage Bumblebees, or Black Cats here...so
I like PartsExpress Polyproplene Film and Foil Caps @ about $1.oo each in the few stocked values , they sound excellent smooth, detailed, and slightly warm.
However I need eight signal caps here, and I like to bypass the power supply caps with these caps also, in the usual 1% of the bypassed cap ratio. Plus bypassing the Cathode Caps. This means 7 more caps, so 15 Caps total at $1 each...$15.00 is too much...

So, what I have found is Ted Webers shop of Weber speaker fame, has some of the cheapest Caps, and magnetics I have seen. I am ordering a Tweed Bassmand power Transformer for $50 + $9 for top mount end bells, and this compares to $90 or more usually...
and their Polypropylene Film Al Foil caps are...yes!!....$.40 cents each!!!

They say these caps are a 716P style equivalent, I don't know I have not seen or heard them, however for this project they are right in the BallPark!!! ok so 15 Caps @ .40 each for $6.00 total, Ok I can handle...

Also I am seperately using a copy of the Sear Silvertone 1484 output transformer 4k 40w from them (TedWeber)...this is an Ultra Fat Stack transformer for $50, and I doubt that many of the $200+ Fat Stack transformers from Mercury Magnetics are any better...so anyway here is the link ...Enjoy...( and obviously I am not currently getting paid by Weber to say these things)

https://taweber.powweb.com/store/capord.htm

Thanks for all your work, Ladies & Gentlemen,
Sincerely,
-3db

[Minussss3db]

I do a small bit of business with them, and Steve runs a pretty good surplus shop there....In the language of this contest, no parts from vendors with " severly limited supplies "....so I think generally of Steve ( ApexJr) as having hundreds of piece parts in any given value...not thousands, but also not ten....

so if I were to use Steve as a source for an amp that I would make available as a kit, generally I would think I would be ok...if something sells out he typically will get a very similar item within a couple months, no not two days, but not a year either...

ApexJr would be a vender of what I'd call " moderately limited supplies", but what is the ruling for the purpose of this contest???

comments???

thanks,
sincerely,
-3db

[Printer2]

Or as a third option ab 12AE10 compactron that is rated at a.1.25W out. The tube might limit before the transformer.

[Cassiel]

If the OPT isn't gapped

I don't know if it's gapped or not, it should be since it was in a SE output stage the firt time we met. It really depends on how I play: if I play gentle no problem, if I play agressively like I did last night (madman on) it runs out of steam. It really isn't distortion anymore - it's butchering time. Chops the head off notes in an uncanny fashion. What I'm going to do is inject a dosis of negative feedback (it really needs it), and maybe, if I become desperate enough, turn it into a triode. Anyway, the lack of clean headroom is obvious and there's nothing much to do about that.

Some people say the Champ is a one trick pony, well, this is a 1/2 trick pony. Too short legged. It comes with the price, I shouldn't be looking at its teeth.

Ready for a morning shock? Here are some very unprofessional recordings. These were recorded with a cheap Nikon camera. The clips recorded with a good mike are yet to be compressed - files can't be attached here.

[Cassiel]

You can clearly hear the limitations of this amp on these audio clips - despite the bad quality of the recording.

Off to fix it or to junk it.

[Cassiel]

Still shocked? Yeah, that sounded really bad. Wonder how the youtube guys do it, well some of them. There are videos that sound as bad as mine. The real challenge is not about making a cheap amp, the real challenge is to make a decent recording. What a bore!

OK, the reason of my post is that I bought two old tube radios a week ago, Telefunken's, and today I opened the package and ....what a shocker.....it turns out the OT'S fit into my chassis. I thought finding another OT this size was next to impossible. I love all this strange coincidences, the amp was in the death row but now it gets an extension. Not dead yet.

Oh, and don't forget to learn from my failure - if you're going to use a cheap OT do a push pull amp!