Hammertone fiddle amp

[dhuebert]

This is an amplifier I built with a couple of transformers from a Diytube ST35 that got parted out and sold. It uses a quartet of 6BQ5 in ultra-linear mode with no other global negative feedback. The amp made 35 watts into a 10 ohm resistor which was a surprise to me as I was expecting more like 20 watts. I delivered it to the customer in prototype form with the intention of making a nice front panel and puting it in a more professional looking box once he had approved the design. So far I have been unable to retrieve tha amp to do the finish work. He loves the particle board box and the somewhat ugly front panel. As a concession to my vanity he did let me stencil Hammertone LFA onto the box so poeple would know what they were looking at. I have seen him at a couple of gigs with it and believe me it gets some looks. I designed it to make clean power right to its maximum rating which it does and the owner loves the sound it gives his violin.





Here it is on the bench getting measured:




Don

[jonnyeye]

I have to say I dig it, if only because that's how my builds end up looking...

Do you fold your own chassis? What gauge is it? How strong are they structurally? I got a small brake and want to give it a go.

[Geek]

Reminds me of an MDF cab I made once that the client wouldn't let me Tolex. In a panic, I hit it with primer, chalk-board paint and then a quick shot of semi-gloss clear coat. He thought I did Tolex it

The chassis looks like the ones I make on my hand brake. Cool!

Cheers!

[dhuebert]

I did fold that one up myself. It is plenty strong for a small amp. It took a couple of tries to get it right, the sequence of bends is critical to a good outcome. The aluminun is 0.050", whatever guage that is. I would love to have a 24" brake and shear for this kind of thing.
I brought the speaker and chassis to my cabinet maker and we went in his scrap bin and knocked the thing together in about 40 minutes. Usually he uses biscuits for a strong joint but this time we just used glue and brads. I tried to explain this to the customer but he won't give it back so I'll just have to wait until it breaks apart.

Don

[Casey4s]

What does the schematic look like?

[dhuebert]

[Casey4s]

Thanks for posting that.

It's a very interesting design indeed.If I am not mistaken ( I am no expert obviously) you are using a modified passive Baxandall. Is that your idea for the midrange control? Is it functioning as designed (intended) ?
I tried an active Bax variation (only on paper) and I always wound up with basically two levels of Bass.

Nice job!

You must be "old school"... I thought I was the only person who still draws schematics by hand LOL, and even I succumbed to CAD a few months ago.

Nice Job on the schematic.

I haven't used "tag Strips" since High School (circa 1963-67) but I know the HiFi guys use them often.

Nice job on the build.

[dhuebert]

Ya, "Baxandall". Alot of discussion over the years about what actually constitutes Baxandall, so who knows really. The mid range control does work somewhat. Gar Gillies showed me that one.

I spent many hours drawing schematics with Orcad years ago. I just haven't bothered to find and learn to use a CAD package for my Mac yet. So until I do it is hand drawn. It is kinda relaxing to sit in front of the TV and draw and draw. Gives you something to think about while the shite is on.

Tag strips are great for prototyping, allowing you to think with your hands.

Don

[Casey4s]

I just tried that ExpressSCH CAD and love it because there is almost no learning curve.

I was a draftsman for a defence contractor right out of HS and guess what ? 1968 and I got drafted myself. LOL

So good hand drawn schematics always get my attention.

I tried to add an active midrange to two RCA model active Baxandall type circuits that I am sure everyone here is familiar with and they were both a bust even though I never actually bread boarded the ideas. I moved on...

[Geek]

Q: How come the "suicide bias"? If your wiper lifts, your tubes are toast.

Cheers!

[dhuebert]

Suicide bias, cute. The bias pot is just beside the bottom pot on the chassis. It is a bourns 3006P in an H83P pot holder. These are the most expensive, reliable trim pots you can buy. In 25 years of using them I have yet to have a single failure. The only failure I have ever seen is if a lead breaks off. To address this, I solder them very carefully and put heat-shrink on them. If I use a pot and have to remove it for any reason it goes in the garbage and a new one is installed. I have had a couple of other failures on my amps but never yet the Bourns.

Don

[EWBrown]

I still hand-draw all my schematics, though they are not as artful and neat as yours. I use tube CAD and SE CAD, but that is as close to "computer design" as I get, and that is generally just for circuit parameters verification after the fact. I'm also "old school", took drafting in 1966-67 and also got "drafted" in 1969. I actually dod a lot of electrical and electro-mechanical drafting for a rather quirky little company named Acumeter Laboratories, in the mid to late 70s, with the drafting boards, pencils, vellum paper (anything from A to E size) , etc.

Re the suicide bias

To remove the "death-wish factor" from the bias circuit, just connect a 100K-220K resistor between the pot's wiper and the 10K resistor to its left, that way, in the unlikely event that the wiper lifts or the pot otherwise fails, the grids will then receive a more negative bias voltage and simply draw less current.

I use good quality pots for biasing, typically Bourns, Allen-Bradley or Clarostat. I leave the cheap imports for service as volume and tone controls.

/ed B

[Geek]

It is a bourns 3006P in an H83P pot holder. These are the most expensive, reliable trim pots you can buy.

Yup! I replaced one in a Dumble clone

Cheers!

[dhuebert]

The Hammertone LFA 30 is finally finished











A nice pair:

[Geek]

Those turned out sweet!

Cheers!