The Hundred-Buck Amp Challenge

[scott17]

Originally Posted by benb

I presume the oscillation is because of the close proximity and capacitance between the different plates and other elements.

That is my understanding as well, but under control it sounds darn nice!

[tubelab.com]

I presume the oscillation is because of the close proximity and capacitance between the different plates

There are several reasons for the tendency for oscillation. As tubes became smaller the size of their internal elements shrank. This increased their natural high frequency capability. As the technology improved the transconductance increased. When multi section tubes appeared it became possible to stick 50+ dB of gain into one glass bottle and the frequency response went to tens or hundreds of MHz.

Early vacuum tubes had low gain. The voltage gain was between 2 and 10 and the transconductance was around 1000. It takes a 1 volt change on the grid to get a 1 mA change on the plate. You had to work to MAKE these tubes oscillate. The pentode in the 6AF11 has a transconductance of over 10000. It takes a 1/10 volt change on the grid to get a 1 mA change on the plate. These newer tubes have a higher maximum frequency, and a much more sensitive grid. If there is any external path that allows sufficient RF energy to couple from the plate to the grid with sufficient phase shift, the criteria for oscillation are satisfied.

Multiple section tubes allow for more gain in a small space, and if the sections are used together in a signal chain, guarantees a phase shift. The tendency for osicllation is great. In my case I tested the P-P output stage first. it was stable and worked great. I added the LTP made with the lower gain triodes, and with a little tweaking it worked. I also got the two gain stages working with the higher gain triodes, but any attempt to connect the two circuits together created an oscillator as the controls were turned. I believe that this was caused by the close proximity of all the circuits, the high input impedances involved, and the multiple paths that went between the two tubes.

This was made during my mother in laws end days where I spent a lot of time sitting around in hospitals, her house, and the passenger seat of a car during the twice daily 90 mile commute for her treatments. During this time I took wild ideas from my head and committed them to a PCB layout on my laptop computer. Some worked, some didn't. This one didn't!

I wonder why there were never shield electrodes between the functions of multifunction tubes such as dual-triodes and such. Maybe there were on some tubes, but I'm not familiar with all the different types.

Many dual section tubes did indeed have a shield between the sections IF the manufacturer intended the two sections to be connected together in the same circuit. Some tubes were made with and without the shield. The 6FQ7 has NO shield, the 6CG7 has a shield. They are otherwise the same tube.

Most multi section compactrons were designed for TV sets. The intended use generally had each section performing a different unrelated task. In tubes where interaction was a possibility, a shield was used. None of the tube manufacturers could have dreamed that we would be inventing new and unusual uses for their tubes long after the manufacturing plants were closed.

[flieslikeabeagl]

Inspired by Scitizen17's creation and the original Squirrel Monkey amp, I was up late last night thinking about Compactron guitar amps.

Following a Google search for "6AF11 guitar amp" I found this video of the "Super Squirrel Monkey" guitar amp on YouTube: 6af11 super squirrel monkey - YouTube

Here's a little write-up by the designer/builder of the Super Squirrel Monkey here:
Introduction the Super Squirrel Monkey 6AF11 amp

The Super Squirrel schematic is here:
http://dl.dropbox.com/u/14312589/Super Squirrel Monkey.pdf

As you can see from the schematic, this guy stuck and additional gain stage (half a 12AX7 as a common cathode stage, the other half as a cathode follower driving a tone stack) in between the three tube sections already in the 6AF11. He says in his write-up that despite the warnings he received about the 6AF11's tendency to oscillate, he didn't encounter the issue.

How strange that there is so much variation between 6AF11's! It seems some oscillate uncontrollably at the drop of a hat, and others are stable even when you stick an extra 34 dB or so of gain in between the stages of the Compactron!

-Flieslikeabeagle

[flieslikeabeagl]

It looks like the forum software was confused by the white spaces in the link to the Super Squirrel Monkey schematic. The link above is not clickable, even though you can read it correctly and cut and paste into your browser.

Let's see if I can fix the URL problem by typing in the formatting code manually. the Super Squirrel Monkey schematic is here:
http://dl.dropbox.com/u/14312589/Sup...rel Monkey.pdf


-Flieslikeabeagle

[Printer2]

Still needs a few screws little tweaking but not too shabby.

[jazbo8]

Cool looking amp you got. Sound clips please...

Jaz

[tubelab.com]

I thought this thread had died....I guess not.

I still have the same two amps I designed for this thread. the whole thing bogged down when I started making the cabinets.

The box for the small amp was first. The plan was for a clear Lexan front plate to make the tubes visible. That was a big mess that ended in frustration. I tried to use spray adhesive to attach the Tolex to the tiny cabined. I now have an ugly box with glue all over it and the Tolex is peeling off in the corners and edges.....

The box for the larger amp is still uncovered. I think I will try the spray on bedliner trick. I have seen speaker boxes made for pickup trucks that were sprayed by a company that does trucks with a paint spray booth. They wanted $75 to spray my tiny box so I walked....

I went to the Orlando hamfest a few weeks ago where I bought 5 dead guitar amps for $5 each. The plan was to gut them and use the cabinets / speakers for these amps. Well the Fender G-DEC Junior intrigued me enough to fix it, now I find myself playing with this little toy. It's kind of a guitar version of a Karaoke machine....cool idea. The Fender Frontman 15, a mid size Crate and a Line 6 stereo head will all be used for their cabinets. I am not interested in fixing them. All have been "tweaked by an expert".

There is a local hamfest for the next two Saturdays, so no amp work will happen. After that, I will finish one of them I promise......Oh yeah.....I put some SAND in the small amp (right in the signal path) and it sounds BETTER.........

[Cassiel]

They wanted $75 to spray my tiny box so I walked....

LOL. I wonder what they charge for a big box. Yeah, frustrating trying to make an amp look cool with a low budget. I really hate to depend on outsiders to finish my amps. They always want to rip me off. Besides, it's like going to the hairdresser's: you're never sure if they're going to get it right (despite my detailed instructions). Too many morons out there.

So ugly amps for the moment. Some day I will buy proper tools and take my time designing the chassis and maybe, just maybe end up with some kind of professional looking job.

My amp is dead BTW. It didn't deserve to live so I killed it.

[Printer2]

Originally Posted by tubelab.com

I thought this thread had died....I guess not.

I am a little slow.

[GloBug]

Heck maybe I will build one, whens the deadline?
I need a little bench/practice amp.
It's a shame, I did notice the thread before, but did not post(subscribe) so I forgot about it.