A great find today

[Bajaguy]

I was on my way home this evening and spied a head unit sitting on a table at a garage sale. I pulled in and looked at it, and when the gal told me it worked and the price was $35, it followed me home :tu:

It's a Harmony 535 (tube), and is a loud 35 watts. The only thing I can say about it is it's way to clean, no crunch at all. Anyone know what forum might be the best to ask a few questions about hotrodding it a bit? I don't want to litter the SS forum up with a bunch of tube talk, but I trust the opinions on here and thought the question was worth asking.

Thanks,

Baja

[J M Fahey]

Not much "sleeping" gain there, waiting to awaken.
I'd use it as a clean but overdrivable basic tube amp base and blast it with a clean preamp.

A MXR Microamp is easy to make.
It's basically a Distortion +  without the clipping diodes, so I suggest you build one and add a switch so you can use or pull the diodes.

It will be a very versatile pedal, and easy to build.

A direct transfer iron-in PCB is available in the usual Pedal sites.

[Bajaguy]

Thanks Juan... Believe it or not, I have a distortion plus in the "box of doom". It's a great pedal, and I love it even more since I did the Wampler mods to it. I was just thinking that for only having a few bucks in it it might be cool to dial it up to where a guy has some nice crunch later in the dial. I know I can do it with a pedal, but it would be nice to do it without :tu:

Baja

[Roly]

After a fair bit of research it seems that Harmony is a house brand that covers stuff from a number of different suppliers.

It seems that the Harmony 535 head (pix appended) is also a Wards Airline Model GVC-9074a or 9064s and a Silvertone 1484 for which I was able to find a circuit (appended).

It actually looks like it does have a spare gain option in that the stage following the tonestack has an unbypassed cathode resistor, so something to try is to stick a 25uf of say 6V or more across this cathode resistor (-ve to ground, R22 @ V2 for channel 1, R23 @ V2 for channel 2) and see it it gives the desired "crunch".

There are three possible outcomes, just right, too much, not enough.  Too much is okay because it can be tweeked by placing a small trim pot, say 10k, in series with the new bypass cap to allow the gain to be set; but not enough ... well we'll deal with that if it arises.

HTH

[Bajaguy]

Just got back! That's definitely the monster, good looking until you see all the spagetti inside. I'm going to run down to the parts store this afternoon and pick up some parts to give your idea a shot. Thanks Roly!

Baja

[Bajaguy]

Well, I got my cap and opened it up. I'm now positive that the schematic posted is not the same at all. After tracing the inputs back a bit, there's no 68K in the circuits at all, and the placement of the the 12AX7's are different fron this amp to the one in the schematic. I looked online for an hour or two and the only schematic I can come up with so far is a GIF that is so blurry that I can't make out the component values. The schematic does however follow the circuit covention to a tee, so I'm positive that there's a good schematic out there.

Anyone got an idea where I might come up with a readable version?

Baja

[J M Fahey]

Well, if you have a blurry but accurate schematic, redraw it, and then add the parts values as you see them in the actual amp thet's on your bench.

[phatt]

Arrh of course,,Very good thought :dbtu:.
Not only is Jaun good looking He is clever also.
Phil.

[Bajaguy]

Yea, I had a feeling that was where I was headed.... looks like I'm gonna need an eye loupe and a bottle of asprin :grr

This point to point is kind of a pain to follow, but I should be able to trace it out and get a schematic up. Thanks for all of the help guys!

Baja

[Bajaguy]

Well, I spent some time looking last night and ran across a guy that made a post about having a schematic. I emailed him and got a much clearer copy in my inbox this morning. I'll still have to add in a few values that aren't real clear, but it's way better than starting off with the GIF image I had.



Baja

[Roly]

You will notice that the cathode of V4 isn't bypassed, so you still have the option of adding a cathode bypass to increase the gain.

[Bajaguy]

  Sweet! I had some time this morning so I dumped in the 25uF cap across the cathode resistor. I slid the amp back together so I can fire it up and do a test run tonight, so I'll report back with the changes as soon as that happens.

  It was interesting to see that the 12AX7's had different values of cathode bypass caps here and there, from .2uF to 35uF. It'll be interesting to see the difference in the gain. Thanks again Roly, you da man!

Baja

[Roly]

The low values of cathode bypass are normally used to give a lead channel top end tilt.

I'm not at home ATM but I did have a look at the difference of unbypassed to bypassed in LTSpice and you should get some significant extra gain, perhaps 20dB or x10.  If this turns out to be too hot then you can put a trimpot, say 10k in this case, in series with the bypass cap and set the gain as required.

[joecool85]

Let us know how it goes/went.  I know I'm curious.

[Bajaguy]

I'll be sure to post up the results tonight when I get home and fire it up. I was all set to give it the once over last night, and realized that I left my light bulb limiter at the shop. It wouldn't be a big deal, but I had all the tubes out and replaced the power cord while I was in there. I figure another 24 hours is better than finding out the hard way that I missed something.

Baja